2009 Vienna - European Society of Human Genetics

Molecular and biochemical basis of disease
P16. molecular and biochemical basis of disease
P16.01
molecular diagnosis for Alström syndrome
A. Palmeiro, R. Cerqueira, L. Lameiras, H. Gabriel, P. Rendeiro, M. Tavares;
CGC Genetics (www.cgcgenetics.com), Porto, Portugal.
Introduction: Alström Syndrome (AS) is a monogenic recessive disorder
featuring an array of clinical manifestations: systemic fibrosis and
multiple organ involvement, retinal degeneration, hearing loss, childhood
obesity, diabetes mellitus, dilated cardiomyopathy, and pulmonary,
hepatic, and renal failure. Currently, 300 individuals with AS have
been identified in 44 different countries. AS is caused by mutations
in the ALMS1 gene, which comprises >224 kb of genomic DNA, 23
exons and encoding a predicted 461.2 kDa protein with unknown function.
CGC Genetics is the reference laboratory for Alström Syndrome
International. Here we report our experience and contribution for the
identification of new Alström patients.
Method: In 2004-2008, we received 52 samples for ALMS1 gene analysis
from 8 different countries, from patients with clinically established
diagnosis and also from patients clinically suspected for AS. Our approach
for the molecular genetic testing was the complete sequence
analysis of exons 10 and 16, plus a portion of exon 8.
Results: Causative mutations (missense) were identified in 13 patients
in both alleles and also 6 new nonsense mutations. In 6 patients we
found a causative mutation only on one chromosome.
Conclusion: Being a rare disorder and not commonly known, AS is
most probably underdiagnosed, has delayed diagnosis or is even misdiagnosis.
Genetic test allows an earlier diagnosis of the disease. Efforts
are being made to increase the detection rate by detecting large
deletions.
P16.02
Analysis of the candidate gene PDE6B in families with Bardet-
Biedl syndrome
L. De Jorge1 , I. Pereiro2 , T. Piñeiro-Gallego2 , M. Baiget3 , D. Valverde2 ;
1 2 Instituto de Investigación Biomédica de Bellvitge, Barcelona, Spain, Universidad
de Vigo, Vigo, Spain, 3Hospital Sant Pau, Barcelona, Spain.
Bardet-Biedl syndrome (BBS, MIM 209900) is a rare multiorganic disorder
which patients manifest a variable phenotype that includes retinal
dystrophy, polydactily, mental delay, obesity and also reproductive
tract and renal abnormalities. Until now 12 genes (BBS1-BBS12) have
been involved in 70% of the families, indicating that additional mutations
in known BBS genes and new BBS genes remain to be identified.
Previous studies* have pointed out, by different methods (homocigosity
mapping, comparative genomics and gene expression analysis), a
total of 19 potential candidate BBS genes. One of them was PDE6B
(Rod cGMP-specific 3’,5’-cyclic phosphodiesterase beta-subunit), this
gene (4p16.3) codifies for the primary effector enzyme in the phototransduction
cascade in the rods, and it is implicated in congenital stationary
night blindness and recessive retinosis pigmentaria.
We analyzed the coding sequence of the PDE6B gene in 16 BBS families.
In the probandus of the families, we found 7 sequence variants,
one in homozygous state (p.V320I, c.958A>G), and six in heterozygous
state, including a novel missense variant (p.G352V, c.1055T>G),
a nonsense variant (p.T305T, c.915G>A) and several intron sequence
variants with no consequences in the splicing process (IVS9-117 C>T,
IVS18+63 G>A. IVS11+21C>T and IVS22+11A>G)
The familial segregation and the nature of the variants showed that
they are no disease causing mutations. So, we concluded that PDE6B
is not, at least in the studied BBS families, a gene implicated in the
Bardet-Biedl syndrome.
Supported by grants from FIS PI060049
*Nishimura DY et al. 2005. Am J Hum Genet. 7(6):1021-33.
P16.03
Identification of 30 novel mutations in the Bardet-Biedl
syndrome (BBs) genes: the burden of private mutations in
extensively heterogeneous diseases
J. Muller 1 , C. Stoetzel 2 , V. Laurier 2 , J. Danse 2 , S. Hellé 2 , V. Green 2 , M. Cossé 2 ,
M. Vincent 3 , C. Thibault 4 , P. Bork 1 , J. Mandel 4 , H. Dollfus 2 ;
1 EMBL, Heidelberg, Germany, 2 Laboratoire EA3949, Strasbourg, France, 3 Laboratoire
de Diagnostic Génétique, Strasbourg, France, 4 IGBMC, Strasbourg,
France.
Bardet-Biedl syndrome (BBS) is an autosomal recessive ciliopathy
defined by progressive retinal degeneration, obesity, cognitive impairment,
polydactyly and kidney anomalies. BBS is genetically heterogeneous
with 14 genes identified to date, which account for ~75% of
affected families. BBS1 and BBS10 each account for ~20% of the mutational
load, BBS12 for about 8% whereas each of the other genes
accounts for ≤5% of the cases (or even for BBS11, 13 and 14 each in
a single family).
The genetic heterogeneity is a burden for identifying mutations as the
full sequencing of the BBS coding sequences implies >150 amplicons
and is time-consuming with routine techniques of diagnostic laboratories.
We analyzed a cohort of 174 families, using various strategies, including
SNP array for initial homozygosity mapping of candidate loci. The
latter can in some cases detect consanguinity unknown to the family.
Mutations have been identified in 135 families (78%) whereas 39 families
(22%) have no mutation detected and are explored for undetected
mutations (deletions, promoter) and new gene identification. We have
recorded 83 mutations in 11 BBS genes, of which 30 mutations are
novel, confirming the high level of private mutations in this very heterogeneous
condition and highlighting the difficult task of routine mutation
identification in the perspective of genetic counseling for the families.
We pinpoint the absence of BBS11 mutations, leaving this gene linked
to BBS through a single missense mutation reported in one family,
while several other mutations are responsible for the very different
phenotype of limb girdle muscular dystrophy (LGMD2H).
P16.04
A homozygous mutation in BBs2 is responsible for Bardet-Biedl
syndrome in the Hutterite population
J. S. Parboosingh 1 , K. M. Boycott 2 , T. Gillan 3 , D. Redl 1 , C. Beaulieu 1 , E.
Puffenberger 4 , R. Perrier 1 , A. Wade 5 , M. Innes 1 ;
1 Department of Medical Genetics, Calgary, AB, Canada, 2 Department of Genetics,
Children’s Hospital of Eastern Ontario, ON, Canada, 3 Department of Pathology
and Laboratory Medicine, Vancouver, BC, Canada, 4 Clinic for Special
Children, Strasburg, PA, United States, 5 Department of Paediatrics, Calgary,
AB, Canada.
Bardet-Biedl syndrome (BBS) is a genetically heterogeneous rare
autosomal recessive ciliopathy characterized by retinopathy, obesity,
genitourinary malformations, polydactyly and cognitive impairment.
At least 14 BBS genes have been identified to date. A Hutterite boy
was identified in infancy with polydactyly and renal cysts. Now age
15 years, he has typical features of BBS: learning disabilities, night
blindness with a rod-cone dystrophy and obesity. The Hutterites are a
genetically isolated population of 40,000 individuals derived from 89
common founders. Thus, we assessed the known BBS loci for evidence
of identity-by-descent from a common ancestor. Genotypic information
from his three unaffected sibs was used to rule out regions of
homozygosity in the patient present due to chance. Initially, microsatellite
markers flanking the known BBS genes were used and segregation
analysis made them unlikely candidates. Genome-wide analysis using
a 10K SNP micorarray identified two regions of homozygosity present
in the patient but absent in his sibs. One of the regions included the
BBS2 gene on 16q21, initially ruled out using microsatellite analysis.
Sequence analysis identified the splice variant c.472-2A>G; RNA analysis
confirmed aberrant splicing. These findings demonstrate a pitfall
in using microsatellite markers in homozygosity mapping: homozygosity
at a locus can not be excluded in the presence of heterozygosity at
a closely flanking microsatellite marker. The misleading marker in this
case was approximately 207 kb from the BBS2 gene.
P16.05
the natural history of primary arrhythmia syndromes
E. A. Nannenberg1 , E. J. G. Sijbrands2 , I. Christiaans1 , I. M. van Langen1 , A. A.
M. Wilde1 ;
1 2 Academic Medical Centre, Amsterdam, The Netherlands, Erasmus MC, Rotterdam,
The Netherlands.
Introduction: Whereas for most hereditary arrhythmia syndromes the
natural history is unknown, physicians face an increasing need for
such data, when decisions on treatment options have to be taken for
the rapidly increasing number of asymptomatic gene carriers in various
disorders with a definite but ill defined risk of sudden cardiac death.