Major contributions of haematology to genetics - British Society for ...

Major contributions of haematology to genetics
Introduction
Genetics is the study of origins: the origins of life, but also the origins of disease. Like many
branches of science, our desire to unravel the aetiology and pathogenesis of disease has
driven our understanding of genetic theory and the development of DNA technologies. In this
regard, haematological pathologies have perhaps been the greatest contributors. The
timeline of major events in the history of genetics reveals numerous instances of
breakthroughs and advances that arose from the study of haematological disease. The
concept of sex-linkage traces its roots back to observations regarding the heritability of
haemophilia. Investigation of sickle cell disease (SCD) provided the first demonstration that
a single mutation in a single gene could give rise to a heritable human disease. Discovery of
the Philadelphia chromosome marked the first cytogenetic abnormality to be implicated in
the genesis of human malignancy.
Consideration of the contributions of haematology to the field of genetics provides a
historical perspective on how some of the fundamentals of the discipline arose. It also serves
to demonstrate how the study of pathology frequently enhances our understanding of human
biology, a concept embodied by the English physician William Harvey when he wrote:
“Nature is nowhere accustomed more openly to display her secret mysteries
than in cases where she shows traces of her working apart from the beaten
path; nor is there any better way to advance the proper practice of medicine
than to give our minds to discovery of the usual law of nature by careful
investigation of cases of rarer forms of disease.”
- quoted by Garrod, A. in The Lesson of Rare Maladies, Lancet (1928) 1:1055
In this essay, I will concentrate on four main areas: molecular medicine, inheritance theory,
linkage and the genetic basis of malignancy. In each case, I will discuss how haematological
disease provided the catalyst for the development of genetic theory and techniques and how
these developments have subsequently been advanced.
Haematological Disorders – exemplars in excelsis
The vague, centuries-old notion of the passage of traits “in the blood” suggests that the link
between haematology and genetics was forged long before the latter field even existed.
Although such sanguine transmission is now only alluded to in a strictly metaphorical sense,
the association between the two disciplines still has a scientific basis. Above all other
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tissues, blood has two key properties which render it the ideal research subject. Firstly, it is
readily accessible via venepuncture and secondly, it is constantly replenished by the bone
marrow. Thus, it is unsurprising that new techniques and ideas have often been applied to,
or arisen from, investigation of blood physiology and pathology before other tissues. As new
knowledge is generally founded on old, some haematological disorders have found
themselves de facto paradigms for our understanding of disease as it has evolved from
simple phenotypic observations to include histological, cellular, proteomic and genetic
aspects. SCD is one such disorder and elucidating its origin heralded the birth of molecular
and genetic medicine.
Blood was one of the first tissues to be put under to the microscope by its inventor,
seventeenth century Dutch scientist Antonie van Leeuwenhoek. In 1840, Hünefeld used a
(much improved) version of van Leeuwenhoek’s instrument to observe the first crystals of a
protein, haemoglobin (Hb) 1 . In 1910, the American cardiologist James Herrick described
“peculiar elongated and sickle-shaped red blood corpuscles in a case of severe anaemia”
(figure 1) 2 . Irving Sherman later observed birefringence of such sickled red cells under
polarised light demonstrating that the cause of such peculiar corpuscles was the
crystallisation of haemoglobin 3 . In his seminal 1949 paper, “Sickle Cell Anaemia, a Molecular
Disease” Linus Pauling demonstrated a difference in electrophoretic mobility between
haemoglobin from SCD patients (HbS) and from healthy controls (HbA) 4 . He suggested that
this was due to a difference in the number of ionisable amino acids in the globin portion – the
concept of molecular medicine was born. Eight years later, Vernon Ingram finished what
Pauling had started and identified the single amino acid substitution in β-globin responsible
for the sickling of red blood cells – glutamate to valine at position 6 (figure 2) 5, 6 . Ingram’s
work demonstrated that a difference of just one amino acid (and he postulated, one DNA
base – a missense mutation) was enough to cause a disease, earning him the moniker, the
“Father of Molecular Medicine”.
2

Direction of Chromatography →
Figure 1: Original drawings from James
Herrick’s 1910 paper describing “peculiar
elongated and sickle-shaped red blood
corpuscles in a case of severe anaemia” 2 .
Sickle Cell Haemoglobin (HbS)
Wild-type Haemoglobin (HbA)
Figure 2: These haemoglobin “fingerprints” demonstrate the presence of an amino
acid difference between sickle cell and wild-type haemoglobin 5 . Vernon Ingram used
trypsin to digest both types of haemoglobin into approximately 30 peptide fragments
(each containing ~10 amino acids). Using a combination of electrophoresis and
chromatography the fragments could be separated to produce the molecular
fingerprints shown above. The shaded (sickle cell) and stippled (wild type) spots
belong to the peptide fragment showing the difference between the two
haemoglobins. This fragment was then isolated and its amino acids sequenced to
reveal the specific, single amino acid substitution 6 .
Since Ingram’s discovery, thousands of single-gene disorders have been identified
and in many, such as cystic fibrosis, the relevant gene and molecular basis are known. In
addition, Ingram’s work contributed to unravelling the genetic code. At the time, it was known
that a triplet of DNA bases coded for a single amino acid, but early models employed an
overlapping code, with one base contributing to more than one codon. Ingram had proved
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that such a code was impossible as a substitution of one or more bases would have affected
several adjacent amino acids, not just one as he had found.
It was fitting that the study of a haematological disorder should usher in the medical
genetic era because we now appreciate that haematological diseases comprise the second
largest group of inherited single-gene disorders (after neurological and neuromuscular
disorders) and that haemoglobinopathies are the commonest Mendelian diseases
worldwide. The very prevalence of haemoglobinopathies is in itself intriguing and uncovering
this mystery made a significant contribution to genetics – specifically, to inheritance theory.
Inheritance Theory – Passage of Traits in the Blood
Like many diseases, the aetiology of SCD was originally woven into a fabric of mythology
and local medical understanding. Until recently the Igbo people of Nigeria believed that a
second child born with SCD was a reincarnation – or ogbanje – of a previous sibling who
had died from the disease 7 . A genetic basis was suspected following the investigation of a
family demonstrating the sickling phenomenon and with a history of multiple deaths from
severe anaemia 8 . Although asymptomatic sickle cell trait had been identified, early theories
proposed that SCD was caused by a single dominant gene, expressed strongly in some
(SCD) and weakly in others (trait). Strong evidence that the inheritance was autosomal
recessive emerged in 1949 when prospective analysis of the parental blood of known SCD
patients demonstrated the sickling phenomenon in all cases 9 . This was also indicated by
Pauling’s aforementioned work, in which patients with the trait phenotype had an equal
proportion of both HbS and HbA 4 . Even then there was confusion as the prevalence of SCD
amongst African populations was lower than expected from the trait prevalence. The
subsequent realisation that children were dying undiagnosed before they could be included
in prevalence surveys was a sobering solution to the discrepancy. Although Mendel’s laws
were well known, their practical application to human disease, away from the controlled
Drosophilia-based laboratory environment, proved difficult. SCD was a learning curve and
working through these difficulties provided the academic keys to unlock the genetic
mysteries of other disorders.
Having established the inheritance pattern of SCD, the question remained as to why
the HbS gene was so frequent in certain, largely African, populations when the lethality of
the homozygous state severely curtailed its vertical transmission? The incidence of novel
HbA mutation was not high and could not explain the intriguing geographic distribution of the
HbS gene described by Anthony Allison. He found high frequencies of sickle cell trait carriers
(20-30%) along the cost of Kenya and near Lake Victoria but low frequencies in the
intervening highlands (

parasite, providing a selection pressure to maintain its frequency in these populations. This
hypothesis was backed up by population studies and an ethically dubious investigation of
parasitaemia levels following malaria inoculation of individuals with and without sickle cell
trait 10 . Allison concluded that:
“Genetically speaking, this is a balanced polymorphism where the heterozygote
has an advantage over either homozygote.”
Allison’s ground-breaking work made a major contribution to the theoretical basis of
population genetics and how we understand gene-environment interactions.
Linkage and the Royal Disease
Just as SCD is perhaps the archetypal autosomal recessive disorder and provided the
disease model from which the concept of heterozygous advantage arose, so the study of
haemophilia was invaluable for the development of theories of linkage. Christian Friedrich
Nasse first commentated on sex-linkage in his 1820 treatise on hereditary bleeding
disorders 11 . Nasse observed that the hereditary disposition to fatal bleeding (the term
haemophilia did not arise until 1828) only occurred in males and that these males did not
transmit the disease themselves, rather it was transmitted by their unaffected female
relations. An experimentally validated theory of sex-linkage arose from Thomas Hunt
Morgan’s observations in 1910 that the white eye phenotype of Drosophilia followed patterns
of sex chromosome inheritance 12 . A year later the genes causing both colour blindness and
haemophilia were mapped to the X chromosome, resulting in the development of one of the
major techniques in human genome analysis – genetic linkage (the tendency for certain
alleles to be inherited together). In 1937, Bell and Haldane reported the first genetic linkage
in humans, between haemophilia and colour blindness 13 . This linkage was of practical value
in itself as it allowed the prediction of carrier status in females of families displaying both
traits. In addition, Bell and Haldane’s research methodology paved the way for the
identification of other genetically linked traits which facilitated early attempts at antenatal
diagnosis and is still employed in the localisation of genes for specific genetic disorders.
In the mid-19 th century, haemophilia was described as “the most hereditary of all
diseases” and its prevalence amongst the European monarchy served to frame haemophilia
as the exemplary hereditary disorder amongst both experts and laypeople 11 . Given such
status, it was natural that questions of reproductive control concerning haemophilia should
arise following the popularisation of eugenics in the early 1900s. The principal debate
surrounded whether the reproduction of females from haemophilia families should be
prohibited due to their carrier status. Although opposed to sterilisation, Haldane was a
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proponent of such negative eugenics 14 and it is ironic that it should be a study by Haldane
himself that undermined the notion that haemophilia could be eradicated by preventing
carriers from reproducing. In an insightful paper, Haldane calculated the mutation rate for the
haemophilia gene, providing the first such (accurate) estimate for a human gene 15 . The
haemophilia gene was found to have a high rate of mutation and Haldane concluded that
approximately one third of haemophilia cases arise de novo, from spontaneous mutation.
Haldane then remarked that haemophilia in Queen Victoria’s children most probably arose
from a gene mutation,
“in the nucleus of a cell in one of the testicles of Edward, Duke of Kent,
Victoria’s Father, in the year 1818.” 11
While the exactitude of his statement remains unknown, we do know that with haemophilia
as his muse, Haldane’s insights into X-linked inheritance, genetic linkage and mutation rates
made lasting contributions to genetic theory and were a rational force against fundamentalist
eugenic principals.
Molecular Carcinogenesis – the Cytogenetic Revolution
Although genetics as a discipline originally arose from the study of heredity it was not long
before we began to appreciate the effects of acquired genetic mutation. One of the first
proposals of a genetic basis for cancer came from biologist Theodore Boveri. In 1914, he
hypothesised that human malignancies originated from mitotic abnormalities that resulted in
aneuploidy or other, more subtle genetic abnormalities that did not involve the entire
chromosome 16 . Definitive proof of Boveri’s theories necessitated the development of more
sophisticated molecular genetic techniques and had to wait 45 years for Nowell and
Hungerford’s oft-cited Science abstract describing the presence of “A minute chromosome in
human chronic granulocytic leukaemia” 17 . This distinctive chromosome was only observed in
neoplastic leukocytes and was designated the Philadelphia chromosome, after the city in
which it was discovered (figure 3a). This landmark “paper” is recognised as the first
example of a consistent chromosome abnormality in a human neoplasm. Further support for
the cytogenetic basis of cancer came from studies demonstrating increased spontaneous
breakage of chromosomes prepared from normal circulating lymphocytes from patients with
a hereditary leukaemic disposition, such as Fanconi anaemia. Despite this evidence, other
consistent cytogenetic abnormalities associated with tumours failed to materialise and the
general consensus was that these anomalies were the result, rather than the cause, of
malignancies. The development of chromosome banding in 1970 allowed the accurate
identification of individual chromosomes and the recognition of small translocations and
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other rearrangements. Application of banding techniques to human malignancies led to a
rapidly expanding list of specific tumour-associated cytogenetic alterations, with the
Philadelphia chromosome sitting proudly at the top. Rowley et al. demonstrated that the
“minute chromosome” is actually a truncated chromosome 22, resulting from a reciprocal
translocation between chromosomes 22 and 9 (figure 3b) 18 . Further work in the mid-80s and
early 90s identified the resultant fusion gene as BCR-ABL. The product of this genetic hybrid
was an aberrant protein tyrosine kinase that gave leukaemic myeloid cells a survival
advantage over their non-leukaemic counterparts.
A
B
Figure 3: A, shows a karyotype from one of the
early studies on chromosome abnormalities in
CML 19 . The minute, Philadelphia (Ph)
chromosome is labelled. Note that the Ph
chromosome was first assigned to chromosome
21 and later changed, by convention, to 22. B,
depicts the reciprocal translocation between
chromosomes 9 and 22. The similarity between
the additional material at the end of the long
arm of chromosome 9 (arrow) and that missing
from the long arm of chromosome 22 suggested
the translocation.
Chronic myeloid leukaemia (CML) is another example of a haematological disorder
whose pathophysiology serves as the archetype for other diseases. The discovery and study
of the Philadelphia chromosome and its tumourigenic product proved the genetic origin of
cancer and laid the foundation for our current understanding of molecular carcinogenesis.
Nowell’s work also left a legacy that occupies researchers to this day - the concept of cancer
stem cells, the discovery of countless cytogenetic abnormalities and the appreciation of the
crucial role of kinases in malignancy. Interestingly, his work also gave rise to one example of
a growing phenomenon – the contributions of genetics to haematology.
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Concluding Remarks on a Symbiotic Relationship
Blood is often seen as the essence of an individual which, along with its accessibility,
rendered it the subject of much early scientific enquiry. Consequently haematological
disorders have found themselves pathophysiological exemplars in numerous fields, but
particularly genetics. Sanguine disease has been a foil for genetics; contributing to many of
its sub-disciplines, including molecular medicine, inheritance theory, eugenics and
carcinogenesis. However, genetics has now started to repay its debt through the
development of novel diagnostic tests and targeted therapies. Techniques such as
polymerase chain reaction have allowed the prenatal diagnosis of a variety of disorders,
including haemoglobinopathies, and have significantly reduced the number of children born
with thalassaemia in certain parts of the world. Recombinant human factor VIII has obviated
the need for haemophiliacs to use allogeneic blood products, dramatically improving
haemophilia therapy. Following the identification of the BCR-ABL protein, rational drug
design led to the development of a specific tyrosine kinase inhibitor – imatinib – which
revolutionised the treatment of CML and increased 5-year survival from 70 to 89% 20 . We can
even dare to hope that gene therapy may offer a cure for monogenic disorders. The
reciprocal contributions between haematology and genetics serve as a prime example of
what can be achieved by cooperation between disciplines and the translation of basic
science to clinical medicine. It seems likely that such symbiosis will continue to benefit both
parties well into the distant future.
Word Count: 2,494
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References
1. Hünefeld FL. 1840. Die Chemismus in der Thienschen Organization, Leipzig,. 160
[Taken from Reichert and Brown, 1909].
2. Herrick JB. 1910. Peculiar elongated and sickle-shaped red blood corpuscles in a
case of severe anaemia. Archives of Internal Medicine. 6:517-21.
3. Sherman IJ. 1940. Bulletin of Johns Hopkins Hospital. 67:309.
4. Pauling L, Itano HA, et al. 1949. Sickle cell anemia, a molecular disease. Science.
110:543-8.
5. Ingram VM. 1956. A specific chemical difference between the globins of normal
human and sickle-cell anaemia haemoglobin. Nature. 178:792-4.
6. Ingram VM. 1957. Gene mutations in human haemoglobin: the chemical difference
between normal and sickle cell haemoglobin. Nature. 180:326-8.
7. Nzewi E. 2001. Malevolent ogbanje: recurrent reincarnation or sickle cell disease?
Social Science and Medicine. 52:1403-16.
8. Emmel VE. 1917. A study of the erythrocytes in a case of severe anaemia with
elongated and sickle-shaped red blood corpuscles. Archives of Internal Medicine. 20:586-98.
9. Neel JV. 1949. The Inheritance of Sickle Cell Anemia. Science. 110:64-6.
10. Allison AC. 2004. Two lessons from the interface of genetics and medicine. Genetics.
166:1591-9.
11. Pemberton S. 2011. The Bleeding Disease: Hemophilia and the Unintended
Consequences of Medical Progress. 1 ed: The Johns Hopkins University Press.
12. Morgan TH. 1910. Sex-limited inheritance in Drosophilia. Science. 32:120-2.
13. Bell J. 1937. The Linkage between the Genes for Colour-Blindness and Haemophilia
in Man. Proceedings of the Royal Society of London Series B, Biological sciences. 123:119-
50.
14. Sarkar S. 1992. Science, philosophy, and politics in the work of JBS Haldane, 1922–
1937. Biology & Philosophy. 7:385-409.
15. Haldane JBS. 1946. The mutation rate of the gene for haemophilia, and its
segregation ratios in males and females. Annals of Human Genetics. 13:262-71.
16. Bovari T. 1914. Zur Frage der Entstehung maligner Tumoren. Gustav Fischer Jena,
Germany. 64 pp.
17. Nowell PC, Hungerford D. 1960. A minute chromosome in human chronic
granulocytic leukemia. Science. 132:1497.
18. Rowley JD. 1973. Letter: A new consistent chromosomal abnormality in chronic
myelogenous leukaemia identified by quinacrine fluorescence and Giemsa staining. Nature.
243:290-3.
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19. Tough IM, Court Brown WM, Baikie AG, et al. 1961. Cytogenetic studies in chronic
myeloid leukaemia and acute leukaemia associated with monogolism. Lancet. 1:411-7.
20. Sherbenou DW and Drucker BJ. 2007. Applying the Discovery of the Philadelphia
Chromosome. Journal of Clinical Investigation. 116: 2067-2074.
10

Major
Contributions
of
Haematology
to Genetics
Arun Arujun Bhaskaran
Word count: 2,494 excluding
references and figures
Ask anyone to name a common genetic disorder and their answer would probably be
sickle cell anaemia. Indeed, sickle cell anaemia was the first diagnosed disease to be
genetically characterised when Guthrie and Huck of John Hopkins University Medical
School, analysed pedigree charts from two families in 1923. However, sickle cell
anaemia is only one of many blood disorders with a genetic basis. Haematology has
made profound contributions to genetics, furthering our understanding of the field. This
essay aims to explore how haematology has helped in our quest to make sense of the
‘stuff of life’ using various examples from haemophilia to thalassemia.

HAEMATOLOGY AND AUTOSOMAL DISORDERS
Figure 1 – Gregor Mendel; an
Austrian monk considered the
‘father of genetics’
Genetics is arguably the ‘baby’ of modern day science, still in
its developing stages. However, we have made overwhelming
medical advances with what little we know. It began with
Gregor Mendel who proposed all organisms possess 2
hereditary ‘factors’ for any given characteristic; one of which is
‘dominant’ and overrides the effect of a ‘recessive’ factor on
the organism’s physical traits. These factors come from the
organism’s parents and are passed down through generations
in a randomised fashion 1 . The discovery of sickle cell
anaemia and various other blood disorders provided further
evidence for this and helped shape our understanding of
various inheritance patterns.
Erythrocytes contain haemoglobin, which transports oxygen. Normal adult haemoglobin
consists of 2 α and 2 β globin chains. In sickle cell anaemia, a glutamate residue is
substituted for a valine on one of the β-globin chains forming haemoglobin S. This is prone
to polymerisation. Polymerised haemoglobin distorts erythrocytes, making them rigid and
sickled (crescent shaped). These rigid and sickled cells can occlude vessels and block blood
flow causing problems like gall stones, hypertension, retinopathy and renal failure. The allele
responsible is located on chromosome 11. Because it is recessive, only homozygotes
(individuals carrying 2 copies of the mutated allele) are affected 2 .
Figure 2 – Electron micrographs comparing normal and sickled erythrocytes
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Figure 3 – Original pedigree chart created by Guthrie and Huck illustrating the recessive
nature of sickle cell anaemia
Sickle cell anaemia also exhibits overdominance. Since it was first described, clinicians
noted that the disease ran most frequently in families of African descent. Indeed, the
prevalence of sickle cell anaemia is greatest in tropical and sub-tropical regions of the world
such as Sub-Saharan Africa. 3
Figure 4 – Figure illustrating the prevalence of sickle cell anaemia worldwide (darker colour
indicates greater prevalence)
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It was later revealed that the geographical distribution of sickle cell anaemia corresponded
with that of malaria. Studies found that the sickle cell trait is actually protective against
malaria though exact mechanisms are yet to be determined. It may be that the Plasmodium
falciparum parasites which cause malaria are unable to grow and reproduce effectively
inside HbS-containing erythrocytes. However, a recent study by Williams et al. at the Kenya
Medical Research Institute found that while children with the sickle cell trait have enhanced
immunity to the parasite, the level of which increases with age, suggesting a role for
acquired immune processes too 4 .
So what’s all this got to do with genes? Heterozygotes (carriers of sickle cell anaemia)
produce small amounts of HbS erythrocytes, though not enough to produce symptoms.
However, they are still resistant to malaria, meaning they have a survival advantage over
both homozygous dominant individuals (with no malaria resistance) and homozygous
recessive individuals (with sickle cell anaemia). This phenomenon where homozygotes ‘get
the best of both worlds’ is known as overdominance and is described using the Gillespie
model, which is used widely in the field of population genetics 5 .
HAEMATOLOGY AND SEX-LINKED DISORDERS
Haematology has helped us understand sexlinked
disorders, as well as autosomal ones.
Haemophilia A is a sex-linked disorder caused
by a loss of function mutation in the gene
encoding factor VIII near the tip of the q arm of
the X-chromosome. Commonly patients have
deletions in the factor VIII gene but some severe
cases exhibit a ‘flip-flop’ inversion where the
gene is disrupted by an inversion at the end of
the X chromosome. These mutations result in
little or no synthesis of factor VIII, which is
needed in the blood coagulation cascade.
Without it, clots cannot form effectively following
blood vessel damage and profuse bleeding
occurs. 6
Figure 5 – Diagram illustrating the so called
‘flip-flop’ inversion in some severe cases of
haemophilia A
Abu al-Qasim al-Zahrawi, in the 10 th century AD, was the first physician to document men
bleeding to death following minor injuries 7 . It was not until sometime later when the genetics
of the disorder became clear. It was frequently observed that males were more commonly
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affected by this bleeding disorder than women, even in members of the European royal
family.
Figure 6 – Pedigree chart showing the inheritance of haemophilia A in the British Royal Family
In X-linked disorders like haemophilia, men are more affected than females. This is due to a
process known as X inactivation. Females possess 2 copies of the X chromosome; during
development, in utero, females silence one copy in all body cells. This process is completely
random. Usually though, copies with mutations such as factor VIII deletions or inversions are
‘inactivated’ giving rise to no or very mild symptoms. In some exceptional cases this doesn’t
work out well and the ‘good’ X chromosome is turned off giving rise to symptoms - skewed
X-inactivation. Females with Turner’s syndrome with only 1 X-chromosome are also at risk.
Figure 7 – Young girl with Turner’s Syndrome; symptoms
include short stature, infertility, skin folds, cardiac
malformations and characteristic facial features.
Sufferers only have 1 X chromosome. Thus, those which
carry mutations suffer from X-linked disorders like
haemophilia
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HAEMATOLOGY, MULTIPLE ALLELES AND CODOMINANCE
As mentioned, Mendel believed all phenotypes are controlled by a pair of alleles, the
dominant of which is actually expressed. However, we now know this is not completely true
thanks to the heritability of blood type. There are 4 major blood groups: A, B, AB and O,
which are defined by the carbohydrate antigens expressed on the surface of an individual’s
erythrocytes and antibodies in their blood. Hirszfeld discovered blood type was determined
by, not 2 but, 3 alleles – I A , I B and I O . This phenomenon is now known as ‘multiple alleles’
and is seen with other phenotypes including coat colour in certain animals 8 . Relatively few
physical traits are in fact ‘di-allelic’. Blood group heritability also highlights another flaw in
Mendel’s work. In some instances, alleles at specific loci are equally dominant and one does
not overpower the other. In such circumstances, which one determines the actual
phenotype? The answer is both - for example, in some plant species, pink flowers result
from a cross between red and white parents. I A and I B are co-dominant alleles. Thusly,
heterozygotes have the AB blood group 9 .
Figure 8 – Table illustrating the antigens and antibodies present in the blood of individuals
with each of the 4 blood groups
HAEMATOLOGY AND MUTATIONS
Thalassemia is a common blood disorder where there is a reduced synthesis of the globin
chains needed to make haemoglobin. Thalassemia has helped promote our understanding
of mutations. Mutations are spontaneous changes in an organism’s DNA, which can affect
transcription and translation. Sickle cell anaemia has only one known cause – a glutamate to
valine substitution at position 6 of the β-globin sequence. Thalassemia, however, can arise
as a result of many different types of mutation. Examples of mutations that produce β-
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thalassaemia range from deletions and insertions of several bases right through to
polyadenylation-signal mutations that result in production of unstable mRNA. 10
Figure 9 – Diagram depicting some of the many mutations that can potentially give rise to β-
thalassemia
Understanding mutations have improved our knowledge
of selection and evolution and helped us make sense of
protein synthesis. Base triplets in genes code for amino
acids, which join to form polypeptides. Mutations cause a
change in the base sequence which can produce
different amino acids. Mutations can be harmful in the
case of thalassemia but not always. Studies have
revealed marked variations in haemoglobin amino acid
sequences of different mammals. Storz et al. identified 4
mutations in the haemoglobin genes of deer mice that
dwell in mountainous areas, which increase their affinity
for oxygen, and is extremely advantageous at high
altitudes where oxygen is scarce 11 . Another interesting
study found mutations in mammoth haemoglobin that
allowed them to extract oxygen from the air even at
below freezing temperatures, enabling them to survive
during the Ice Age 12 .
Figure 10 – Artist’s depiction of a
woolly mammoth, whose
haemoglobin was adapted to
extract oxygen from the
atmosphere in extremely cold
conditions
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HAEMATOLOGY AND EPISTASIS
Mendel also proposed that genes encoding different characteristics were passed down
through generations independently of one another (independent assortment). We now know
this is not entirely true, thanks to haematology. We now know that genes at different loci can
and do interact with each other. This phenomenon is known as epistasis. One of the first
examples of epistasis to be described was the Bombay phenotype. As mentioned, there are
4 major blood groups. Individuals with the Bombay phenotype may have A and/or B alleles
but appear to be blood group O, due to the absence of a H protein needed to form A and B
antigens. 13
HAEMATOLOGY AND GENE TECHNOLOGY
Gene technology is a rapidly evolving field dealing with understanding how genes work and
using this knowledge to our advantage. A new exciting branch of gene technology is genetic
modification, in particular gene therapy. Gene therapy involves editing faulty base
sequences. Haemophilia A and B have been extensively researched in this field and we are
now closer than ever in finding appropriate treatment. As discussed, in haemophilia A
excessive bleeding occurs following injury due to a lack of factor VIII. In haemophilia B
inappropriate haemorrhage also occurs but due to the absence of factor IX.
Safe, long-term expression of factors VIII and IX has already been demonstrated in murine
models using different gene transfer strategies. Initially adenovirus vectors were used.
Despite being effective, they were associated with hepatotoxicity. Recently attention has
turned to lentiviral vectors. Unlike adenoviruses, these contain RNA. Using the enzyme
reverse-transcriptase, lentiviruses can create a cDNA strand that can be inserted into host
DNA using ligases. When the cell undergoes protein synthesis, it transcribes the viral DNA
too. For retroviruses to have effect, the cell must be actively dividing. This is perfect for
haematological disorders because haematopoietic stem cells divide rapidly. However,
insertional mutagenesis is a problem. Inserting viral DNA into the genome can disrupt other
gene sequences and increase susceptibility to various cancers. Clinical trials on healthy
volunteers and patients are yet to be done too. 14
HAEMATOLOGY AND THE GENETIC BASIS OF CANCER
Cancer is a family of around 200 diseases affecting virtually every organ in the human body.
In cancer, cells divide uncontrollably forming tumours. Some tumours, known as malignant
neoplasms, can invade neighbouring tissues and spread around the body via blood or
lymphatics wreaking havoc. Cancer has been around for a long time – it was even observed
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y the Ancient Greeks but the aetiology was widely unknown. It was not until 1914 and the
ingenuity of German biologist Theodor Boveri that the genetic basis of cancer was unveiled.
Since Boveri first proposed that ‘malignant tumours might be the result of a certain abnormal
condition of the chromosomes, which may arise from multipolar (abnormal) mitosis’, a lot has
been discovered. Studies of various haematological malignancies have helped us better our
understanding of the aetiology of cancer and the role genes play 15 .
Figure 11 – Illustration from Theodor Boveri’s original work on malignant tumours and their
cause; diagram shows his chromosome studies with eggs of the roundworm Ascaris
Two major families of genes have been implicated in cancer: oncogenes and tumoursuppressor
genes. Proto-oncogenes are needed for normal cell function. However, gain of
function mutations can produce oncogenes that promote excessive cell division and failure
of cell apoptosis (programmed cell death). Tumour suppressor genes control the rate of cell
division. They repair damaged DNA or encourage apoptosis of affected cells. Loss of
function mutations can result in inactivation of such genes, increasing the risk of abnormal
division.
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Figure 12 – Figure to illustrate the
role of oncogenes and tumour
suppressor genes in tumour
formation
There are various haematological malignancies that arise as a result of defective oncogenes
including leukaemia. Leukaemia is a groups of disorders in which white blood cells divide
uncontrollably and become malignant, invading the bone marrow (myeloid) and lymphatic
system (lymphoid). It can be acute or chronic. Chronic myeloid leukaemia (CML) is
associated with various mutations including mutations of the c-kit proto-oncogene. This
normally codes for a tyrosine kinase receptor on haematopoietic stem cells used in cell
signalling but if defective can increase cell proliferation. The evidence for its involvement in
carcinogenesis is undeniable. In fact, tyrosine kinase inhibitors such as Imatinib are
mainstay treatment for CML and have proven efficacious in all patients at just 400 mg per
day 16 .
Another characteristic feature of CML is the Philadelphia chromosome. Part of the protooncogene,
ABL1, on chromosome 9 is transferred to the BCR gene on chromosome 22
forming the BCR-ABL1 fusion protein. This is believed to modulate tyrosine kinase activity
and is responsible for promoting rapid cell division. Tumour cells of CML patients express
high levels of this protein and detection of the Philadelphia chromosome on a karyotype aids
diagnosis. Fusion proteins also crop up in other malignancies such as Burkitt’s and follicular
lymphoma. In Burkitt’s, promotor regions of genes encoding the heavy chains on antibodies
join with the transcription factor gene myc forming c-myc. High levels of the c-myc protein
can promote excessive B-lymphocyte proliferation. In follicular lymphoma, a t(14;18)
translocation augments bcl-2 activity. This encodes anti-apoptotic factors that are handy for
tumour cells trying to evade bodily controls 17 .
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Figure 13 – Formation of the Philadelphia chromosome – there is a 9;22 translocation resulting
in the formation of a fusion protein
The table below (adapted from Essential Haematology, 6 th Edn © A.V. Hoffbrand & P.A.H.
Moss; published 2011 by Blackwell Publishing Ltd.) shows some of the oncogenes involved
in various other haematological cancers:
Disease Mutations Oncogene
Acute myeloid leukaemia Translocations [t(8;21)] ETO/AML1
Myelodysplasia Deletions on 5q and 7q RPS 14
N RAS
B-acute lymphoblastic
leukaemia
Translocations [t(12;21),
t(9;22), t(4,11)]
TEL/AML1
BCR-ABL1
AF4/MLL
Myeloproliferative Point mutations JAK-2
TET-2
Chronic lymphoid
leukaemia
Deletions on 17p and 11q
P53
ATM
Malfunctioning tumour-suppressor genes are also seen in various haematological
malignancies. p53 is a protein essential to the running of the cell cycle. It is produced in
response to DNA damage during interphase. It ‘pauses’ the cycle until the DNA is repaired
or if it cannot be repaired, it promotes cell suicide. Mutations of p53 are seen in conditions
such as Li-Fraumeni where sufferers have an abnormally high risk of developing cancers
such as acute leukaemias 18 .
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Now that we know more about the causes of various cancers we can go about finding
suitable treatments. As mentioned earlier, tyrosine kinase inhibitors are extremely valuable
in the treatment of CML. Recently interest has shifted to epigenetic alterations in certain
cancers like myelodysplasia (MDS) and acute myeloid leukaemia. Epigenetics is concerned
with how genes are transcribed. Various transcription factors are needed to initiate the
process and produce stable mRNA. However, in conditions like MDS and AML processes
are at work, which hinder this. Common mechanisms include DNA methylation or
deacetylation of the histones that support DNA. These mechanisms interfere with the binding
of transcription factors and ultimately block gene transcription. In AML and MDS,
transcription of tumour suppressor genes is inhibited in this way leading to dysregulated cell
growth and tumour formation. Pharmaceutical companies have developed demethylating
agents which reverse these epigenetic changes and have been very effective in AML and
MDS sufferers. 19
HAEMATOLOGY, GENETIC MARKERS AND DIAGNOSIS
By learning so much about genetics from haematology, we are now able to test individuals
for various abnormalities. Fluorescence in situ hybridisation analysis uses fluorescently
labelled gene probes to detect mutated base sequences. It is commonly used to diagnose
CML. Flow cytometry is used commonly in B-lymphocyte malignancies to count and
examine leukemic cells. It involves using different fluorescent labels (flourophores) that
attach to antigens on the surface of either normal or malignant cells. Other diagnostic tools
include karyotype analysis, immunohistology and DNA microarrays. New diagnostic methods
are been discovered all the time. MicroRNAs are short, non- coding RNA sequences which
are thought to be involved in carcinogenesis. The microRNA mir-17-92 was shown to
behave as an oncogene in individuals with B-cell lymphoma. Analysis of over 300
candidates found that microRNA profiles could be of use in cancer diagnosis. These tools
are not only used for cancers. Carriers of haemophilia can be detected with gene probes.
Testing can be done in utero too simply using a sample of chorionic villus. As our knowledge
of haematology and genetics grow, we could potentially use these tools to screen healthy
individuals and identify those who are predisposed to certain diseases. In the UK, there is a
big drive to prevent disease rather than treat it and such screening programmes could aid
this reform. 6
CONCLUSION
Haematology has contributed massively to the field of genetics. Thanks to haematology, we
have built upon Mendel’s foundations and have better insight into how certain traits are
P a g e | 11

passed down from generation to generation. We recognise when things go wrong and are
taking steps to correct mistakes in the inheritance process. Haematology has helped get
genetics from the laboratory to the bedside and actually helping patients, including those
with cancer – the world’s biggest premature killer 20 . Nonetheless, haematology will continue
to inspire the world of genetics and we can hope to see even more future advances.
REFERENCES & FIGURES
All figures other than those taken from Essential Haematology, 6 th Edn © A.V. Hoffbrand &
P.A.H. Moss; published 2011 by Blackwell Publishing Ltd are courtesy of Google Images
1. Westerlund, J. F. and Fairbanks, D. J. (2010), Gregor Mendel's classic paper and the
nature of science in genetics courses. Hereditas, 147: 293–303. doi: 10.1111/j.1601-
5223.2010.02199.x
2. Steinberg, M.H. In the clinic: sickle cell anaemia. Ann Intern Med September 6, 2011
155:ITC3-1
3. WHO. Sickle cell anaemia. 59 th World Health Assembly. Provisional agenda item 11.4.
24 April 2006
4. Relation between falciparum malaria and bacteraemia in Kenyan children: a
population-based, case-control study and a longitudinal study
Dr J Anthony G Scott FRCP,James A Berkley MD,Isaiah Mwangi MMed,Lucy Ochola
PhD,Sophie Uyoga MSc,Alexander Macharia MSc,Carolyne Ndila MSc,Brett S Lowe
MPhil,Salim Mwarumba MSc,Evasius Bauni PhD,Kevin Marsh FRCP,Thomas N
Williams MRCP
The Lancet - 8 October 2011 ( Vol. 378, Issue 9799, Pages 1316-1323 )
DOI: 10.1016/S0140-6736(11)60888-X
5. Gillespie, John (2004). Population Genetics: A Concise Guide, Second Edition. Johns
Hopkins University Press. ISBN 0-8018-8008-4.
6. Essential Haematology, 6 th Edn © A.V. Hoffbrand & P.A.H. Moss; published 2011 by
Blackwell Publishing Ltd
7. Cosman, Madeleine Pelner; Jones, Linda Gale. Handbook to life in the medieval world.
Infobase Publishing. p. 528–529. ISBN 0816048878.
8. Crow J (1993). "Felix Bernstein and the first human marker locus". Genetics 133 (1): 4–7
9. http://anthro.palomar.edu/blood/ABO_system.htm - written by Dennis O’Neil; last
updated on Saturday, August 20, 2011
10. Olivieri, N.F. The β-thalassemias. The New Journal of Medicine. Volume 341. Number 2.
July 8, 1999.
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11. Genetic differences in hemoglobin function between highland and lowland deer mice,
Jay F. Storz, Amy M. Runck, Hideaki Moriyama, Roy E. Weber, and Angela Fago
12. Campbell, K.L., J.E.E. Roberts, L.N. Watson, J. Stetefeld, A.M. Sloan, A.V. Signore, J.W.
Howatt, J.R.H. Tame, N. Rohland, T-J. Shen, J.J. Austin, M. Hofreiter, C. Ho, R.E.
Weber† and A. Cooper†. 2010. Substitutions in woolly mammoth hemoglobin confer
biochemical properties adaptive for cold tolerance. Nature Genetics, 42(6):536-540.
13. Bhende YM, Deshpande CK, Bhatia HM, Sanger R, Race RR, Morgan WT, Watkins
WM. (May 1952). "A "new" blood group character related to the ABO system". Lancet. 1
(6714): 903–4
14. Murphy SL, High KA. Gene therapy for haemophilia. Br J Haematol. 2008;140:479–487.
15. Boveri, Theodor (2008). "Concerning The Origin of Malignant Tumours". Journal of Cell
Science 121 (Supplement 1): 1–84
16. Deininger MW, Druker BJ (September 2003). "Specific targeted therapy of chronic
myelogenous leukemia with imatinib". Pharmacol. Rev. 55 (3): 401–23
17. Kurzrock, R.; Kantarjian, H. M.; Druker, B. J.; Talpaz, M. (2003). "Philadelphia
chromosome-positive leukemias: From basic mechanisms to molecular therapeutics".
Annals of internal medicine 138 (10): 819–830
18. http://www.ncbi.nlm.nih.gov/books/NBK22268/ The p53 tumour suppressor protein.
Created: March 28, 2011; Last Update: August 11, 2011
19. Epigenomics of leukemia: from mechanisms to therapeutic applications. Cristina Florean,
Michael Schnekenburger, Cindy Grandjenette, Mario Dicato, and Marc Diederich.
Epigenomics. October 2011, Vol. 3, No. 5, Pages 581-609
20. Office for National Statistics Mortality Statistics: Deaths registered in 2009, England and
Wales (PDF 798KB) 2010, National Statistics: London
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Christopher Tang – BSH Essay Prize 2011
British Society for Haematology Essay Prize 2011: Major
Contributions of Haematology to Genetics
Christopher Tang, King’s College London
Word count: 2475
1

Christopher Tang – BSH Essay Prize 2011
Major Contributions of Haematology to Genetics
Modern genetics arises from the work of Mendel in the 19 th century, which detailed
the inheritance patterns of specific traits in pea plants. The initial study of genetics, known as
classical genetics, therefore focussed on exploring the inheritance of physical traits between
generations. With the discovery of chromosomes and DNA, research within genetics shifted
towards exploring the concepts of mutations and chromosomal abnormalities, and in recent
years the field of molecular genetics has grown increasingly important; this field pertains to
the study of genetics at a molecular level, and is focussed on understanding the
mechanisms of gene expression.
Haematology is the study of blood and blood-related diseases, and is relevant to all
aspects of genetics, given that many haematological disorders have a genetic basis. In
particular, it is arguable that haematology has made its greatest contributions towards
cytogenetics and the growing field of molecular genetics; indeed, in striving to understand
the underlying pathological basis of various haematological disorders, we have also greatly
aided our understanding of genetics in general. Both fields are therefore closely intertwined,
and are likely to remain so in future, with many issues and questions still to be resolved. This
essay will therefore examine the key contributions of haematology to our knowledge of
genetics, beginning with contributions to cytogenetics, before exploring contributions to
molecular genetics.
Contribution to cytogenetics
Cytogenetics refers to a field of genetics which is primarily concerned with the study
of chromosomes. Chromosomes were initially discovered in 1842 via microscopic
observations of plant cells, and much of our understanding of cytogenetics is derived from in
vitro experiments as well as the study of congenital disorders such as Down’s syndrome.
Nevertheless, the study of haematology has been necessary for elucidating the concepts of
X-inactivation and chromosomal translocations.
X-inactivation
X-inactivation describes the phenomenon occurring in female mammals, whereby
one X chromosome is randomly inactivated in each cell; this process therefore means that,
like males, females only have expression of one active X chromosome per cell. The idea of
2

Christopher Tang – BSH Essay Prize 2011
X-inactivation was initially proposed by Ohno et al in 1959, with the observation that one X
chromosome appeared to be more condensed and heterochromatic than the other. 1 In 1961,
Mary Lyon formally proposed the theory of random X-inactivation in order to explain the
mottled coat colour of mice, 2 however it was not until the work of Ernest Beutler investigating
glucose-6-phosphate dehydrogenase (G6PD) deficiency that random X-inactivation was
shown to occur in human females.
G6PD deficiency is a condition whereby patients experience haemolytic anaemia due
to an inability to process toxic oxidative metabolites. The G6PD enzyme is involved in the
pentose phosphate pathway, ultimately acting to sustain levels of reduced glutathione, which
functions to remove oxidative metabolites. Unfortunately, erythrocytes are uniquely
dependent on the G6PD pathway for protection from oxidative metabolites, and therefore
people deficient in G6PD accumulate these metabolites under conditions of high oxidative
stress, resulting in erythrocyte damage and haemolysis. G6PD deficiency is inherited in X-
linked pattern, and the disease state is highly variable in severity: patients can range from
only suffering anaemia when challenged with stressors to being chronically and severely
anaemic. Beutler proposed that females affected by G6PD deficiency possessed two
populations of erythrocytes – one containing normal G6PD function, and one containing
deficient G6PD – concluding that the variability in disease phenotype was due to random
inactivation of X-chromosomes containing either normal or deficient G6PD. This hypothesis
was validated by experiments using glutathione to measure the stability of erythrocytes from
heterozygous females, which clearly indicated separate erythrocyte populations, thereby
confirming the concept of X-inactivation in humans. 3
Moreover, in addition to aiding the discovery of X-inactivation, G6PD deficiency has
proven to be useful in demonstrating the monoclonal nature of malignant tumours. This
principle was first applied to leiomyomas, with A and B variants of G6PD being used as
markers of X-inactivation, thus allowing researchers to derive the origins of malignant cells. 4
The monoclonal origins of other tumours such as leukaemias and lymphomas have also
been detailed using this method. 5
Chromosomal translocations
Chromosomal translocation describes the rearrangement of material between two
nonhomologous chromosomes. An exchange of material between two nonhomologous
chromosomes is known as a reciprocal translocation, and it is towards the understanding of
this concept that haematology has contributed greatly.
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Christopher Tang – BSH Essay Prize 2011
The Philadelphia chromosome is a prime example of the consequences of reciprocal
chromosomal translocation, and is closely associated with chronic myelogenous leukaemia
(CML), a myeloproliferative disease causing proliferation of myeloid cells. Originally
discovered in 1959 via microscopy of chromosomes taken from the blood cultures of CML
patients, the Philadelphia chromosome was the first example of a specific genetic
abnormality being linked to cancer. 6 Further investigation has since characterised the
chromosome as containing a t(9; 22) abnormality, resulting in the formation of a fusion
oncogene caused by the juxtaposition of the Abl1 and BCR genes. This BCR-ABL fusion
protein possesses constitutively activated signalling, leading to aberrant cell signalling which
ultimately enables cancer development. Indeed, the importance of this finding is such that
inhibitors of BCR-ABL, such as imatinib, form the mainstay of current CML treatment.
This concept of chromosomal translocation is further clarified when considering the
development of acute promyelocytic leukaemia (APL), a myelodysplastic disorder
characterised by the proliferation of abnormal promyelocytes. Cytogenetic studies of this
disorder revealed a t(15; 17) translocation, 7 and further experiments later revealed that this
translocation enabled the fusion of the retinoic acid receptor α (RARα) gene to the
promyelocytic leukaemia (PML) gene, thereby creating a PML-RARα fusion protein which
interferes with pathways responsible for granulocyte differentiation. 8 It is now known that
various translocations may occur in APL, but all share the common mechanism of fusion
protein creation. As with CML, understanding of this disease mechanism has not only
illustrated the pathogenic potential of translocations and fusion gene creation, but has also
provided clinical benefits, enabling the use of targeted therapy to address the defect; in the
case of APL, all-transretinoic acid is used to counteract the effect of the aberrant fusion
protein, promoting cellular differentiation.
Contribution to molecular genetics
A central concept of molecular genetics is that of gene expression – this refers to the
process by which information stored in genes in DNA is expressed in the form of gene
products. These products are traditionally thought of as proteins, however recently it has
been appreciated that non-protein coding genes are also significant. During gene expression
of protein coding genes, the relevant DNA is transcribed into messenger RNA (mRNA), and
this is in turn processed and then translated into the protein gene product. Regulation of
gene expression is understandably vital for normal cell function, and consequently there are
regulatory mechanisms at each level of gene expression. Understanding the overall picture
4

Christopher Tang – BSH Essay Prize 2011
from genetic information contained in DNA to cellular and tissue function is therefore vastly
complex, and this is a reason why the study of haematology has been so important. Indeed,
haematological disorders have helped to clarify the mechanisms involved at various levels of
gene expression.
DNA mutations and their relation to phenotype
Gene expression is dependent on the genetic information contained in the host DNA,
and mutations in the original DNA template will therefore cause the expression of an altered
gene product, which may have implications for cellular function and disease. The field of
haematology has been especially important in illustrating this mechanism of disease, with
many haematological disorders being caused due to DNA mutations.
Sickle cell disease has proven to be a useful disorder for understanding the
functional consequences of DNA mutations. This disorder is characterised by abnormal
erythrocytes which possess a sickle shape, and is caused by a mutation in the β-globin gene
of haemoglobin. In normal human adults, haemoglobin A is composed of 2 α-globin chains
together with 2 β-globin chains, and forms ~97% of haemoglobin in the blood. Upon
mutation of the β-globin in sickle cell disease, the host is said to possess HbS rather than
HbA; people homozygous for HbS are said to have sickle cell anaemia whereas
heterozygotes have sickle cell trait. Linus Pauling first postulated the mechanism for sickle
cell disease in 1949; he observed that haemoglobin from affected subjects had altered
electrophoretic mobility compared to normal haemoglobin, and thus suggested that the
pathological difference was due to different amino acid residues. 9 Remarkably, the mutation
for sickle cell disorder is down to just a single nucleotide change (A T) in the β-globin
gene, resulting in the sixth amino acid of the globin chain changing from glutamic acid to
valine: this point mutation and ensuing amino acid substitution is sufficient to cause sickling
of erythrocytes, thus highlighting how genetic mutations may be intimately linked to the gross
pathology of disease. 10
The thalassaemias are also useful for explaining the clinical consequences of genetic
mutations. β-thalassaemia is a condition affecting the β-globin chain of haemoglobin,
characterised by either a loss of β expression (β°), or reduced β expression (β + ). The
disorder ranges in severity from β-thalassaemia major (both β alleles are affected) to β-
thalassaemia minor (only one β allele is affected), and therefore the disease is also useful
for emphasising the link between genotype and phenotype. Compared to sickle cell disorder,
β-thalassaemia illustrates how mutations can cause disease by mechanisms other than
5

Christopher Tang – BSH Essay Prize 2011
affecting amino acid sequences. Numerous mutations have been described for β-
thalassaemia, and these can be divided into three groups based on their mechanism of
action: they may prevent transcription of mRNA from the gene, prevent adequate processing
of the mRNA, or prevent translation of the mRNA transcript into protein. 11 Thus whereas in
sickle cell disease DNA mutation causes an amino acid substitution, in β-thalassaemia
mutations act to prevent and hinder gene expression at various levels.
In a similar manner as sickle cell disease, it has recently been discovered that the
disease phenotype of myeloproliferative neoplasms such as polycythaemia vera is caused
by the defective products of mutated genes. Modern sensitive PCR-based techniques
resulted in the discovery, in 2005, of a mutation to the tyrosine kinase JAK2 which was
frequently present in patients with myeloproliferative neoplasms. 12 As mutations to this gene
have been further characterised via sequencing methods, it has become clear that they
serve primarily as gain of function mutations, resulting in production of an enzyme that is
constitutively active, thus promoting myeloproliferative activity. 13 JAK2 mutations are
therefore another example of how DNA mutations may influence cell function and
consequently disease development. Furthermore, knowledge of this genetic defect has
prompted the development of novel drug therapies which have been targeted at the aberrant
JAK2 enzyme – these are still under investigation in clinical trials, and it remains to be seen
what role they may play. 14
Epigenetic control of gene expression
An important paradigm shift in our understanding of molecular genetics is an
appreciation of the critical role of epigenetic mechanisms in regulating gene expression.
Epigenetics refers to heritable changes in gene expression which are independent of
alteration to the DNA sequence itself – these may include changes to chromatin or
methylation of DNA, as well as regulation of gene transcripts via the use of regulatory RNAs
such as microRNAs (miRNA). Epigenetic modulation underpins a variety of processes
including X-inactivation and imprinting as well as haematopoiesis and tumourigenesis, and it
is regarding the role of epigenetics and tumour development that the study of haematology
has proven most informative, with haematological malignancies serving as valuable models
and examples of epigenetic effects.
Chromatin modification and DNA methylation both enable the regulation of gene
expression by modulating the initiation and elongation phase of DNA transcription. It is
hypothesised that these processes are relevant during tumour development, as imbalances
6

Christopher Tang – BSH Essay Prize 2011
in these mechanisms of control may permit silencing of tumour suppressors or
overexpression of oncogenes. Moreover, as these modifications are reversible – unlike
mutations to the DNA sequence – it is hoped that they may be amenable to drug treatment
in a way that could address the underlying defect in disease. 15 Evidence supporting this
theory has been provided by the study of haematological malignancies. For example,
methylation specific PCR has been used to show that methylation and therefore silencing of
the p15 CDKN2B tumour suppressor gene is associated with leukaemic progression of
myelodysplastic syndrome to acute myelogenous leukaemia (AML). 16 Regarding the role of
chromatin modification, it has been demonstrated that there is overexpression of the LAZ3/
BCL-6 protein in non-Hodgkin lymphoma, with this protein functioning to recruit further
enzymes to modify chromatin. 17 These findings have not only contributed to understanding
of gene expression methods, but they have also enabled the development of novel
therapeutics aimed at targeting the enzymes responsible for aberrant epigenetic
modifications. These drugs have so far been promising, and are being examined in ongoing
trials. 15 miRNAs represent one of the most exciting and rapidly progressing fields of genetics,
and we have only recently begun to discover their importance for gene regulation. Since the
discovery of the first miRNA, lin-4, in 1993, 18 it has become evident that miRNAs serve to
control gene expression primarily by binding to mRNA transcripts and regulating translation.
As with chromatin and DNA modifications, haematological malignancies have proven to be
extremely useful in terms of providing models to validate the genetic theories. Notably,
miRNA expression profiling has been performed in patients with chronic lymphocytic
leukaemia (CLL): these experiments have indicated that the miRNAs miR-15a and miR-16-1
are both frequently downregulated in CLL, and further in vitro studies have suggested that
these miRNAs function as tumour suppressors, regulating BCL2 expression, and therefore
the induction of apoptosis. 19 Conversely, certain miRNAs such as miR-155 have been
identified as being upregulated in lymphomas, suggesting potential oncogenic activity, and
this knowledge emphasises the principle that miRNAs may have multiple roles in
tumourigenesis. 20 Nevertheless, the study of miRNAs is still in its infancy, with many
unresolved issues, and it is clear that haematological disorders will be useful to aid in
answering these questions.
Conclusion
Haematological disorders have a strong genetic basis, and it is therefore unsurprising
that the fields of haematology and genetics have contributed to each other as developments
7

Christopher Tang – BSH Essay Prize 2011
have been achieved. Above all, I would argue that haematology has made great
contributions to cytogenetics and molecular genetics. Regarding the former, the disorders of
G6PD deficiency, CML and APL have been instrumental in elucidating key genetic principles
of X-inactivation and chromosomal translocations. In terms of the latter, the
haemoglobinopathies such as sickle cell disease and β-thalassaemia serve as prime
examples of how subtle DNA mutations may cause clinically significant phenotype changes.
However, it is perhaps the developing field of epigenetics that is most exciting, and it is to
this field that haematology will undoubtedly continue to contribute, as more questions and
uncertainties are posed in future. In particular, it is likely that the process of haematopoiesis
and haematological malignancies will provide invaluable models and examples to aid the
understanding of epigenetic regulatory mechanisms. Ultimately, it is hoped that advances in
both fields can facilitate progress in treatment and understanding of relevant diseases, and
therefore improve patient care.
8

Christopher Tang – BSH Essay Prize 2011
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19 Cimmino A et al. miR-15 and miR-16 induce apoptosis by targeting BCL2. Proc Natl Acad
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10