2008 Barcelona - European Society of Human Genetics

Concurrent Symposia
s13.1
Dysregulated RAs signaling in Noonan syndrome and related
disorders
M. Tartaglia;
Department of Cell Biology and Neurosciences, Istituto Superiore di Sanità,
Rome, Italy.
In the last few years, mutations in genes coding for transducers with
role in the RAS-MAPK signaling pathway have been identified as the
molecular cause underlying a group of clinically related developmental
disorders with features including reduced postnatal growth, facial
dysmorphia, cardiac defects, ectodermal anomalies, cognitive deficits
and variable predisposition to certain malignancies . Noonan syndrome
(NS), which is the most common condition among these Mendelian
traits, is caused by heterozygous mutations in PTPN11, SOS1, KRAS
and RAF1 in approximately 65% of affected individuals . Missense
PTPN11 and RAF1 mutations also account for the vast majority of
LEOPARD syndrome (LS), while defects in KRAS, BRAF, MEK1 and
MEK2, and a bunch of missense changes in HRAS occur in 60-80%
of cardiofaciocutaneous syndrome (CFCS) and in Costello syndrome
(CS), respectively .
The RAS-MAPK signaling pathway controls cell proliferation, survival
and differentiation, and represents the most common target for somatic
activating mutations in cancer . NS-, LS-, CFCS- and CS-causing
alleles encode for proteins with aberrant biochemical and functional
properties, mostly resulting from impaired catalytic autoinhibition, that
promote increased signal flow through the MAPK cascade.
The available structural, molecular and biochemical data support the
view that, besides its crucial role in oncogenesis, dysregulation of
RAS-MAPK signaling has profound consequences on development .
These findings also provide evidence that germline transmitted mutations
causing developmental disorders define a novel allele series
that have distinctive perturbing role on signaling, and offer a model in
which distinct gain-of-function thresholds of the activity of individual
transducers are required to induce cell-, tissue- or developmental-specific
phenotypes, each depending on the transduction network context
involved in the phenotype .
s13.2
the molecular dissection of Joubert syndrome and allied
ciliopathies
H. H. Arts 1 , D. Doherty 2 , S. E. C. van Beersum 1 , S. J. F. Letteboer 1 , T. A. Peters
3 , I. A. Glass 2 , N. V. A. M. Knoers 1 , R. Roepman 1 ;
1 Dept. of Human Genetics, Radboud University Nijmegen Medical Centre, Nijmegen,
Netherlands, 2 Dept. of Pediatrics, University of Washington School of
Medicine, Seattle, WA, United States, 3 Dept. of Otorhinolaryngology, Radboud
University Nijmegen Medical Centre, Nijmegen, Netherlands.
Joubert syndrome is a disorder that is primarily characterised by a typical
hind brain malformation known as the “molar tooth sign”, as seen
on CT and MRI images . Retinal degeneration, cystic kidneys, mental
retardation and abnormal breathing patterns (episodic hypernea and
apnea) are also common features . To date, four genes are known to
be involved in Joubert syndrome, and three of the protein products
localize to primary cilia .
We examined the function of the RPGRIP1-like protein (RPGRIP1L),
encoded by RPGRIP1L on chromosome 16q12 .2 . This is the homologue
of RPGRIP1 (RPGR interacting protein 1), a ciliary protein
involved in congenital blindness (Leber congenital amaurosis) . RP-
GRIP1L is ubiquitously expressed and its protein product localizes to
basal bodies of cilia in brain, retina and kidney. We identified homozygous
frameshift and splice site mutations in two families with typical
Joubert syndrome and compound heterozygous nonsense and missense
mutations in a third family . All mutations disrupt the interaction of
the C2-domain of RPGRIP1L with nephrocystin-4, encoded by NPHP4
which is mutated in Senior Løken syndrome patients (retinal dystrophy
and cystic kidneys) . Interestingly, one of the patients had postaxial
polydactyly and encephalocele resembling the Meckel-Gruber
syndrome phenotype, suggesting that RPGRIP1L could be involved
in disorders that represent different spectra of the same underlying
defect .
We and others have found that these disorders with overlapping phenotypes
in the retina, the brain and the kidney, can result from perturbation
of individual components of shared functional modules, regulating
distinct processes that are based at (primary) cilia . Functional
dissection of these modules has yielded important information of the
molecular pathogenesis of the associated disorders, the “ciliopathies”
disease family . They point towards a role of the cilia in regulating a
wide variety of basic cellular processes, such as vesicle transport, Wnt
signalling and Hedgehog signalling . Furthermore, similar to the identification
of RPGRIP1L, they have provided us with a valuable collection
of novel “ciliopathy candidate genes”, that will even expand the ciliary
factor in various disease processes .
s13.3
Alterations of FGF signalling in LADD syndrome
B. Wollnik;
Institute of Human Genetics, Cologne, Germany.
Mutations in different components of the FGF signalling pathway
cause the lacrimo-auriculo-dento-digital (LADD) syndrome, an autosomal
dominant disorder mainly characterized by anomalies of the
lacrimal system, ears and hearing, teeth, and distal limb development .
Notably, all LADD mutations identified so far in FGFR2/3 are located
within the tyrosine kinase (TK) domains of the receptors within loops
that play an important regulatory function in the control of receptor
activity . Our functional studies of FGFR2 LADD mutants indicated a reduced
tyrosine kinase activity of the receptor itself as well as reduced
FGFR2-mediated substrate phosphorylation and reduced downstream
signalling . Moreover, the timely and precisely ordered dynamics and
patterns of autophosphorylation are changed in FGFR2 mutants as
shown by biochemical investigations and crystal structure analysis .
While FGFR2 LADD mutants exert a putative dominant-negative effect
on normal FGFR2 protein, FGF10 LADD mutations cause haploinsufficiency.
Beside novel mutational mechanisms of known LADD
genes, we found an autosomal recessive inheritance in a severely affected
LADD patient caused by the homozygous p .R579W in the TK
domain of FGFR2 . Interestingly, we also observed molecular overlaps
of LADD-like phenotypes with p63-related disorders . Our data shed
light on pathophysiological mechanisms underlying LADD syndrome
und expand the spectrum of disorders associated with altered FGF
signalling .
s14.1
molecular mechanisms of cellular senescence
F. Fagagna;
IFOM-IEO Campus, Milan, Italy.
Early tumorigenesis is associated with the engagement of the DNAdamage
checkpoint response (DDR) . Cell proliferation and transformation
induced by oncogene activation are restrained by cellular
senescence . It is unclear whether DDR activation and oncogene-induced
senescence (OIS) are causally linked . Here we show that the
expression of an activated oncogene (H-RasV12) in normal human
cells, results in a permanent cell cycle arrest caused by the activation
of a robust DDR . Experimental inactivation of DDR abrogates OIS and
promotes cell transformation . DDR and OIS are established after a
hyper-replicative phase occurring immediately after oncogene expression
. Senescent cells arrest with partly replicated DNA and with DNA
replication origins having fired multiple times. In vivo DNA labelling
and molecular DNA combing reveal that oncogene activation leads
to augmented numbers of active replicons and to alterations in DNA
replication fork progression .Therefore OIS results from the enforcement
of a DDR triggered by oncogene-induced DNA hyper-replication
. Senescence is also associated with a global heterochromatinization
of nuclear DNA . These senescence associated heterochromatic
foci (SAHFs) are enriched in histone H3 di-tri methylated on lysine 9
(H3K9m) and HP1 proteins and High mobility group A (HMGA) proteins
are also known to be essential structural components of SAHFs .
Our most recent results on the interplay between DDR activation and
oncogene-induced heterochromatinization will be presented .
s14.2
cornelia de Lange syndrome and the cohesinopathies:
Developmental Repercussions of cohesin Dysfunction
I. D. Krantz;
Division of Human Genetics and Molecular Biology, The Children’s Hospital of
Philadelphia, Philadelphia, PA, United States.
The cohesin proteins compose an evolutionarily conserved complex
whose fundamental role in chromosomal cohesion and coordinated