Research Report 2000 - MDC

Gene Mapping and
Identification in
Monogenic and
Complex Diseases
André Reis
Positional cloning is now widely used
for the identification of gene defects
that underlie inherited diseases. A
necessary first step for positional
cloning is the mapping of the gene
locus that co-segregates within
families with a particular disease or
trait, which allows allocation of a
specific chromosomal position to the
responsible gene. Although mapping
was initially developed for monogenic
traits, it is now possible to locate
genetic factors involved in the
aetiology of complex diseases. The
most powerful technique currently
available is linkage analysis with
highly polymorphic microsatellite
markers, which involves an
examination of the entire genome with
a set of evenly spaced markers. This
type of study is usually referred to as
a whole genome scan.
The Gene Mapping Centre is a
specialised laboratory carrying out
such high throughput genotyping for
gene mapping in monogenic as well
as complex diseases. We have
developed various sets of well
established markers from the
Généthon reference genetic map with
different marker densities to
accommodate the requirements of
special study designs. The laboratory
is mainly funded through grants from
the German Ministry of Science,
Research and Technology (BMBF)
and, since January 1997, we have
participated in the German Human
Genome Project. Additional funding is
provided through a strategy-fund
project, “genetics of complex
diseases”, from the Helmholtz Society
of National Research Centres. The
laboratory is also available for
mapping projects by other groups.
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Mapping of complex diseases
The main focus of the Gene Mapping
Centre is mapping genetic factors in
complex diseases. This type of study
involves the analysis of large numbers
of phenotypically well characterised
families. Hundreds of markers are
used for genotyping and sophisticated
biostatistical analyses are subsequently
required to identify the genetic loci
that contribute to a complex disease.
For this purpose we have established
suitable techniques with an emphasis
on automation of the experimental
procedures. In 2000 we expect to
reach an annual capacity of 2.000.000
genotypes. Currently, mapping is
based on highly-informative
microsatellite markers but in the
future analysis will shift more towards
single nucleotide polymorphisms
(SNPs). Two scientists are involved in
project management, genotyping and
technology development. One
scientist concentrates on laboratory
information management (LIM) which
involves the integration of genotype
and phenotype data and the
biostatistical analysis of these data.
This is done in close collaboration
with the bioinformatics group (Dr. K.
Rohde) and the University of Bonn
(Prof. T. Wienker) where data analysis
is carried out.
A total of four genome scans for
complex diseases have been
completed and two further studies are
under way. In a German collaborative
study on the genetics of asthma (Wjst
et al. 1999) 100 families with two
affected siblings each, totalling 400
subjects were investigated.
Furthermore, in a study to identify
genetic factors for susceptibility to
psoriasis, 32 extended families with
three or more psoriasis patients with
in all, 500 subjects were investigated
(Lee et al., in preparation). Each study
required a total of approx. 200.000
genotypes. Besides confirming
already known loci in both studies, we
have identified novel susceptibility
loci, which are currently being
investigated further with refined
mapping and testing of positional
candidate genes.
Data analysis of two studies, for
which we recently completed
genotyping, is well advanced. In a
study of the genetics of a subtype of
schizophrenia we were able to identify
new susceptibility loci and determine
the mode of inheritance, at least for
this subtype. The largest study in our
laboratory to date is a European
collaborative study of the genetics of
juvenile myoclonic epilepsy. In total,
we have investigated 130 families
comprising 700 subjects. This is the
first comprehensive genome-wide
study of this type of epilepsy and we
expect to gain important insights into
the aetiology of both disease groups.
Ongoing studies include two affected
sib-pair studies, one on atopic
dermatitis from a European
consortium headed by the Charité
Hospital (Prof. Wahn) and a second
on juvenile obesity in collaboration
with the University of Marburg (Prof.
Hebebrandt). Finally, genotyping of a
study of the genetic factors involved
in hypertension, in collaboration with
the Franz-Volhard-Clinic on the
campus (Prof. Luft), is scheduled for
the year 2000. The study design is
based on isolated populations and
takes advantage of the restricted
genetic heterogeneity in these
populations. Running costs for all the
studies are funded through additional
external grants.
Mapping of monogenic diseases
In contrast to complex diseases,
mapping of monogenic traits requires
much less genotyping and, usually, it
is sufficient to analyse 30-50 subjects.
The statistical evaluation is different
and often requires skilled
interpretation e.g. haplotyping. In the
four years since the lab opened, a total
of 31 monogenic traits have been
mapped in humans. For several of
these the underlying gene defect has
already been identified, completing
the process of positional cloning. For
instance, the gene for an autosomal
recessive condition, Nijmegen
Breakage syndrome, was found to be
caused by mutations in a protein
involved in DNA double-strand repair
(Varon et al. 1998). This important
finding has solved a long-standing
puzzle and pointed research in this
field in a new direction. In addition,
we have also initiated mapping of
monogenic traits in animal models,
mainly the mouse. Several
spontaneous and ENU-induced
mutants have been mapped and, in
two cases, the underlying mutations
have also been identified. The
majority of these projects originated
in external laboratories in Germany
but also from England, The
Netherlands, Canada, United Arab
Emirates and Australia.