2009 Vienna - European Society of Human Genetics

Concurrent Symposia
s11.2
clinical, biochemical and genetic aspects of the methylmalonic
acidaemias
M. R. Baumgartner;
Division of Metabolism, University Children’s Hospital, Zürich, Switzerland.
Methylmalonic acidurias (MMAurias) are a heterogeneous group of
inborn errors of metabolism biochemically characterized by the accumulation
of methylmalonic acid (MMA) in body fluids and tissues.
They result from deficiency of methylmalonyl-CoA mutase apoenzyme
(MCM) or by a defect in the synthesis of its cofactor adenosylcobalamin
(AdoCbl). MCM catalizes the conversion of L-methylmalonyl-CoA
to succinyl-CoA thus linking the final catabolic pathways of L-isoleucine,
L-valine, L-methionine, L-threonine, odd-chain fatty acids, and
cholesterol side chains to the tricarboxylic acid cycle.
Several mutant genetic classes that cause isolated MMAuria are
known, based on biochemical, enzymatic and genetic complementation
analysis. The deficiencies of the apomutase locus are further subdivided
into defects without (mut 0 ) and with residual activity (mut - ). The
cblA, cblB and the variant 2 form of cblD complementation groups are
linked to processes unique to AdoCbl synthesis. The cblC, cblD and
cblF complementation groups are associated with defective methylcobalamin
synthesis as well resulting in combined MMAuria and homocystinuria.
Mutations in the genes associated with most of these
defects have been described. Finally, a few patients have been described
with mild MMAuria associated with mutations of the methylmalonyl-CoA
epimerase gene or with neurological symptoms due to
SUCL mutations.
The clinical presentation of affected patients is variable. The majority of
patients present during the newborn period or infancy with life-threatening
metabolic crises resulting in multi-organ failure or even death.
These crises are often precipitated by catabolic stress, e.g. induced
by febrile illness. Severe keto- and lactic acidosis, hypo- or hyperglycemia,
neutropenia, hyperglycinemia, and hyperammonemia are the
most common laboratory findings. In a subgroup of patients chronic
progressive disease, psychomotor retardation, and failure to thrive
are the leading symptoms. Although the overall survival has improved
during the last two decades, long-term outcome still remains disappointing.
Neurological outcome is often impaired by extrapyramidal
movement disorder and developmental delay. Furthermore, chronic
renal failure is frequently found. Patients with mut 0 and cblB have an
earlier onset of symptoms, a higher frequency of complications and
deaths, and a more pronounced urinary excretion of methylmalonic
acid than those with mut - and cblA defects. Reliable classification of
these patients is essential for ongoing and future prospective studies
on treatment and outcome.
s11.3
Homocysteine/folate and neural tube defects
H. J. Blom;
VU University Medical Center, Department of Clinical Chemistry, Metabolic
Unit, Amsterdam, The Netherlands.
No abstract received as per date of printing. Please see www.eshg.
org/eshg2009 for updates.
s12.1
Using naturally-occurring mutations to identify stem cell niches,
trace cell lineage and the origins of cancer in humans
N. Wright;
Histopathology Lab, London Research Institute, Cancer Research UK, London,
United Kingdom.
There have been considerable advances in our understanding of the
organisation of stem cells in epithelial tissues. The identification of reliable
stem cell markers has allowed lineage tracing in some tissues,
such as the intestine. But these observations are currently confined
to experimental animals, and the genetic manipulation required is not
available in humans. We have used a series of naturally-occurring genetic
alterations in humans to infer what we think are interesting observations
about human stem cells and the origins of human cancer.
It is widely accepted that tumors are monoclonal in origin, arising from
a mutation or series of mutations in a single cell and its descendants.
The clonal origin of colonic adenomas and uninvolved intestinal
mucosa from an XO/XY mosaic individual with familial adenomatous
polyposis (FAP) was examined directly by in situ hybridization with Y
chromosome probes. In this patient, the crypts of the small and large
intestine were clonal, showing that each was derived at some stage
from a single cells. However, at least 76 percent of the microadenomas
were polyclonal in origin (Science 272:1187-90). Previously studies
using X-linked genes such as glucose-6-phosphate dehydrogenase
have been handicapped by the need to destroy the tissues to study the
haplotypes of glucose-6-phosphate dehydrogenase, but we were able
to directly visualize X-inactivation patches in human females heterozygous
for the G6PD Mediterranean mutation (563 C-T). The results
showed again that crypts were clonal but crypts formed large families
(Proc Natl Acad Sci USA 100 3311-3314)
We now describe a technique for detecting the expansion of a single
cell’s progeny that contain clonal mitochondrial DNA (mtDNA) mutations
affecting the expression of mtDNA-encoded cytochrome c oxidase
(COX). Since such mutations take up to 40 years to become
phenotypically apparent, we believe these clonal patches take origin
in stem cells. Dual-color enzyme histochemistry was used to identify
COX-deficient cells and mutations confirmed by microdissection of
single cells with polymerase chain reaction sequencing of the entire
mtDNA genome. These techniques have been applied to human intestine,
liver, pancreas and skin. Our results suggest that the stem cell
niche is located at the base of colonic crypts and above the Paneth cell
region in the small intestine, in accord with dynamic cell kinetic studies
in animals. In the pancreas, exocrine tissue progenitors appeared to
be located in or close to interlobular ducts, and in the liver, we propose
that stem cells are located in the periportal region. In the skin, the origin
of a basal cell carcinoma appeared to be from the outer root sheath
of the hair follicle. We propose that this is a general method for detecting
clonal cell populations from which the location of the niche can be
inferred, also affording the generation of cell fate maps, all in human
tissues. The technique also allows analysis of the origin of human tumors
from specific tissue sites (Proc Natl Acad Sci USA; 103:714-9;
Stem Cells (in press).
s12.2
mapping mRNA expression QtLs in hematopoietic stem cells
and their progeny
G. de Haan;
Department of Cell Biology, Section Stem Cell Biology, University Medical Center
Groningen, University of Groningen, Groningen, The Netherlands.
A fundamental problem in biology is how a single genome can lead to
widely different cellular phenotypes. An illustrative and clinically relevant
example is the generation of all mature blood cells types from
a small population of hematopoietic stem cells. Identification of gene
networks specifying stemness or lineage commitment is of major relevance
for the emerging fields of tissue engineering and regenerative
medicine. We developed a genetical genomics approach as a tool to
dissect networks of interacting genes that specify cellular function in
four developmentally distinct hematopoietic cell stages. We evaluated
genome-wide RNA transcript expression in highly purified Lin - Sca-1 + ckit
+ multilineage cells, committed Lin - Sca-1 - c-kit + progenitor cells, erythroid
Ter119 + and myeloid Gr1 + precursor cells isolated from a large
pedigree of genetically related and fully genotyped BXD recombinant
mouse strains. Variation in transcript abundance across all strains and
in all cell types was assessed by Illumina Sentrix Mouse-6 chip technology,
interrogating ~47,000 probesets mapping across of the genome.
For each variably expressed transcript genetic linkage analysis
identified a quantitative trait locus that affects variation in expression
levels of the corresponding gene (eQTL). These eQTLs map in the
vicinity of their target gene (cis-regulation), or map elsewhere in the
genome (trans-regulation).
Complex transcript profiles for each cell type could be dissected into
more simple individual gene networks, each consisting of transcripts
whose variation in expression levels are regulated in trans by a single
genomic locus. We identified 1316 regulatory loci with cell-stage-specific
activity, including 140 loci that controlled gene expression predominantly
in the most primitive hematopoietic cells. We performed hierarchical
clustering of all genes that were consistently downregulated
or upregulated during erythroid or myeloid development and detected
multiple modules of co-varying transcripts for each set of consistently
downregulated or upregulated transcripts. To reveal whether such
modules of transcripts were under common genetic control, we performed
eQTL clustering of 52 transcripts that were consistently down-