European Human Genetics Conference 2007 June 16 – 19, 2007 ...

Concurrent Symposia 1
phomagenesis. I will also draw attention to pitfalls of the conditional
gene targeting approach, which can lead to serious misinterpretations
of experiments.
S38. On principal & modifying genes in Hirschsprung disease
A. Chakravarti;
Center for Complex Disease Genomics, McKusick - Nathans Institute of Genetic
Medicine, Johns Hopkins University School of Medicine, Baltimore, MD,
United States.
Hirschsprung disease (HSCR), or congenital aganglionosis, is a classical
multifactorial disorder that has continued to teach us important
lessons in non-Mendelian inheritance and the genomics of complex
disorders. To date, we know of 9 genes that harbor rare mutations, all
incompletely penetrant but with greater effects in males than females.
One of these genes, the RET tyrosine kinase, also harbors a polymorphism
in an enhancer leading to reduced RET transcription and high
association with short segment HSCR (S-HSCR). Interestingly, the enhancer
polymorphism modifies the genetic effects of rare mutations
not only for RET in HSCR but many other HSCR-related traits such
as Down syndrome, Congenital Central Hypoventilation syndrome and
Bardet-Biedel syndrome. Thus, a single gene can have a diversity of
mutations with differential effects but the same mutation can also have
differential effects depending on its interactions with other (mutant)
genes. In HSCR, the genetic modifiers can be both allelic and nonallelic.
I will describe the genetic tests needed to distinguish between
these scenarios.
S39. Why use the candidate gene approach to find CF
modifiers?
M. Drumm1 , M. Knowles2 ;
1 2 Case Western Reserve University, Cleveland, OH, United States, University of
North Carolina, Chapel Hill, NC, United States.
Genes that do not cause, but modify, a clinical phenotype are of medical
interest as these genes provide information about the biology causing
the phenotype and will potentially suggest therapeutic strategies to
treat the disease phenotype. There are various strategies to identify
these modifying genes and one of the key design features is the choice
of variants selected. Technologic advances are making genome scans
more feasible and affordable, and these approaches allow one to test
hypotheses about candidate genes, as well as identify genes not previously
considered to contribute to the phenotype. However, these
scans are still costly and therefore candidate gene approaches are
still useful. The candidate gene approach relies on knowledge of the
pathophysiology of the disease to be effective, as genes whose products
lie in relevant pathways will be assessed. Consequently, when
a candidate gene is found to associate with the phenotype, some information
is already known about the gene’s role in the phenotype.
Therefore, this approach is most likely to verify a pathway’s involvement
in the disease phenotype, rather than to identify new pathways.
We have taken the candidate gene approach to identifying modifiers
of cystic fibrosis. In doing so, we have incorporated several strategies
to identify candidates. One approach has been to examine genes in
pathways thought to contribute to the pathophysiology of the disease,
such as epithelial ion transport mediators, inflammatory cascade components,
endocrine pathways and innate defense, to name a few. A
second approach has been to examine genes that have been reported
to modify or cause related disorders, such as asthma and chronic obstructive
pulmonary disease (COPD), as CF, asthma and COPD are
likely to have overlapping biology. Using this candidate approach, we
have tested apparent associating genes in at least two populations of
CF patients to verify associations. These approaches have identified
transforming growth factor beta1, and genes in inflammatory pathways
and airway function as genes contributing to disease severity.
S40. Genes that modify iron loading in mice
N. Andrews1,2 ;
1Childrens Hospital, Karp Family Research Laboratories, RM 8-125, Boston,
MA, United States, 2Harvard Medical School, Boston, MA, United States.
Adult onset hemochromatosis, an iron overload disorder affecting the
liver, heart and pancreas, is usually caused by mutations in HFE. However,
only a fraction of patients homozygous for disease-associated
mutations develop clinical hemochromatosis. A wide range in the severity
of iron loading and its complications can be explained by both ge-
netic factors (modifier genes) and environmental factors (e.g., alcohol
intake, dietary iron consumption, and menstruation). Iron physiology in
mice closely resembles that in humans, making the mouse a valuable
genetic model. We undertook a quantitative trait locus (QTL) analysis
in mice to identify modifier genes that might influence the severity of
hemochromatosis. We identified a strong QTL on mouse chromosome
9 that differentially affected macrophage iron burden in C57BL/10J and
SWR/J mice. A C57BL/10J missense allele of an evolutionarily conserved
gene, Mon1a, co-segregated with the QTL in congenic mouse
lines. We present evidence that Mon1a is a cytoplasmic protein involved
in trafficking of ferroportin, the major mammalian iron exporter,
to the surface of iron-recycling macrophages. Differences in amounts
of surface ferroportin correlate with differences in cellular iron content.
Mon1a is also important for trafficking of cell surface and secreted molecules
unrelated to iron metabolism, suggesting that it plays a fundamental
role in the mammalian secretory apparatus.
S41. Genetic Modulation of Sickle Cell Anemia
M. H. Steinberg;
Center of Excellence in Sickle Cell Disease, Boston Medical Center; Dept. of
Medicine, Boston University School of Medicine, Boston, MA, United States.
Sickle cell anemia, a Mendelian disease caused by homozygosity for
a beta-globin gene mutation (HBB, glu6val)), has notorious phenotypic
variability. We are conducting candidate gene and genome-wide association
studies (GWA) to understand the relationships between genetic
heterogeneity and the phenotype of disease.
Fetal hemoglobin (HbF) is the most powerful modulator of sickle cell
anemia. HbF levels are regulated by at least three quantitative trait
loci (QTL) on 8q, Xp and 6q and by elements linked to HBB. When
panels of single nucleotide polymorphisms (SNPs) were used to study
the association of variability in these QTLs in two independent sickle
cell anemia patient groups, SNPs in TOX ( 8q12.1) were associated
with HbF. TOX belongs to a high mobility group box protein family that
binds DNA with high sequence specificity. Many potential TOX binding
sites, including one in the HBG2 promoter are found near the HBB
gene cluster. GWA using pooled DNA confirmed the association on
SNPs in 6q and 8q associated with HbF and identified promising new
areas for further study.
Stroke is a common complication of childhood sickle cell anemia. Using
Bayesian network modeling to evaluate the interactions between
many candidate gene SNPs and the risk of a stroke, we developed a
prognostic model for stroke. SNPs in 11 genes and four clinical variables,
interacted in a complex network of dependency to modulate the
risk of stroke. Case-control association studies examining candidate
genes in other subphenotypes of sickle cell anemia showed associations
with several genes of the TGF-beta/BMP pathway.
To study the genetic association with a global estimate of disease severity,
we developed a model predicting which patients with sickle cell
disease are at risk for near-term death and validated this model in two
independent patient groups. Using this severity score as a phenotype
of disease, SNPs in EDN1, ECE1, KDR, EGF and NOX3 were associated
with overall disease severity.
Understanding the genetic modulation of the hemolytic, vascular and
inflammatory components of this disease could provide important
prognostic information and suggest novel approaches to treatment.
S42. Inborn errors of mitochondrial fatty acid beta-oxidation:
From newborn screening to diagnosis and treatment
R. J. A. Wanders;
University of Amsterdam, Academic Medical Center, Lab Genetic Metabolic
Diseases, F0-224, Amsterdam, The Netherlands.
The mitochondrial fatty acid (FA) beta-oxidation deficiencies constitute
an expanding group of clinically and genetically heterogeneous
disorders. Originally, diagnosis of patients suffering from a fatty acid
oxidation (FAO) disorder was difficult, but the introduction of (tandem)
mass-spectrometry and in particular its use for the analysis of acylcarnitines
in plasma from patients, has revolutionized the diagnosis of
FAO deficiencies. In fact, since tandem mass spectrometry has turned
out to be so robust and reliable, existing neonatal screening programs
have been extended in many countries around the world and now include
different FAO disorders, using tandem mass spectrometric analysis
of acylcarnitines in blood spots. The best known FAO disorder
is medium-chain acyl-CoA dehydrogenase (MCAD) deficiency with an