Philippe Parent

Pleiotropic Effects of CEP290 (NPHP6) Mutations Extend to Meckel Syndrome

Meckel syndrome (MKS) is a rare autosomal recessive lethal condition characterized by central nervous system malformations, polydactyly, multicystic kidney dysplasia, and ductal changes of the liver. Three loci have been mapped (MKS1-MKS3), and two genes have been identified (MKS1/FLJ20345 and MKS3/TMEM67), whereas the gene at the MKS2 locus remains unknown. To identify new MKS loci, a genomewide linkage scan was performed using 10-cM-resolution microsatellite markers in eight families. The highest heterogeneity LOD score was obtained for chromosome 12, in an interval containing CEP290, a gene recently identified as causative of Joubert syndrome (JS) and isolated Leber congenital amaurosis. In view of our recent findings of allelism, at the MKS3 locus, between these two disorders, CEP290 was considered a candidate, and homozygous or compound heterozygous truncating mutations were identified in four families. Sequencing of additional cases identified CEP290 mutations in two fetuses with MKS and in four families presenting a cerebro-reno-digital syndrome, with a phenotype overlapping MKS and JS, further demonstrating that MKS and JS can be variable expressions of the same ciliopathy. These data identify a fourth locus for MKS (MKS4) and the CEP290 gene as responsible for MKS.

A deep intronic mutation in the RB1 gene leads to intronic sequence exonisation.

Familial forms of retinoblastoma, an embryonic neoplasm of retinal origin, are caused by constitutional mutations of the RB1 gene. In this paper, we describe a family with retinoblastoma affecting two brothers with no previous family history of cancer. Complete RB1 mutational screening including point mutation and large rearrangement screening failed to demonstrate any mutation. The whole coding sequence was therefore investigated at the cDNA level, demonstrating a 103u2009bp intronic insertion between exons 23 and 24, leading to subsequent frameshift and premature termination of translation. This intronic exonisation was caused by a deep intronic mutation in intron 23 generating a cryptic 3′ splice site. This is the first report of a deep intronic mutation in RB1 and is a proof of concept that some undetected RB1 mutations should be investigated at the cDNA level, particularly in hereditary forms of retinoblastoma.

Failure to detect an 8p22-8p23.1 duplication in patients with Kabuki (Niikawa-Kuroki) syndrome

Kabuki syndrome (KS) is a rare MCA/MR syndrome with an estimated frequency of 1/32u2009000 in Japan. This syndrome is characterized by postnatal growth retardation, distinctive facial features, dermatoglyphic anomalies, skeletal dysplasia, and mental retardation. The molecular basis of KS remains unknown. Recently, Milunsky and Huang reported on six unrelated patients with a clinical diagnosis of KS and an 8p22–8p23.1 duplication using comparative genomic hybridization and BAC-FISH studies. Also, they suggested that a paracentric inversion may contribute to the occurrence of KS. In the present study, 24 patients with a clinical diagnosis of KS based on Niikawa–Kuroki criteria have been collected. They were tested for the presence of an 8p duplication using the same clones as described by Milunsky and Huang. Our results do not confirm the previously described association between KS and an 8p22–8p23.1 duplication.

Use of a set of highly polymorphic minisatellite probes for the identification of cryptic 1p36.3 deletions in a large collection of patients with idiopathic mental retardation.

Editor—Mental retardation is a component of a large number of syndromes, most of which qualify individually as rare genetic diseases. Altogether, mental retardation affects 2-3% of the population and is unexplained in 40% of cases. According to Knight et al ,1 subtle telomeric chromosomal rearrangements are responsible for approximately 1% of unexplained mental retardation (with the proportion being highest, 7.4%, in the subclass of unexplained moderate to severe mental retardation). The identification of the genes responsible will require the precise delimitation of minimum deletion regions, which relies upon the collection of a large number of cases. Because of the low expected frequency of each telomeric deletion in mental retardation, the procedure to be applied should allow the screening of many patients at a low cost.nnIt is not yet clear whether all chromosome ends are associated with mental retardation syndromes with similar frequency. Distal chromosome 1p36 deletions were initially detected cytogenetically because of an associated segmental imbalance.2-5 This syndrome results from both interstitial and terminal deletions of varying sizes and different breakpoints6 and the severity of the phenotype is related to the extent of the deletion.7 Clinical examination can efficiently detect a large proportion of cases, so that the number of reports of 1p36 deletions has increased significantly in the past few years which may give the false impression that this is a relatively frequent syndrome.nnWe show here how highly polymorphic minisatellites located within a short region can provide an efficient prescreening of samples without the need for parental samples at the initial stage. The procedure was tested here using 1p36 minisatellites and provides an estimate of the frequency of 1p36 deletions in mentally retarded patients. A selection of five highly polymorphic minisatellite probes was used to search for 1p36.3 deletions in a …

Genetic and clinical specificity of 26 symptomatic carriers for dystrophinopathies at pediatric age

The molecular basis underlying the clinical variability in symptomatic Duchenne muscular dystrophy (DMD) carriers are still to be precised. We report 26 cases of early symptomatic DMD carriers followed in the French neuromuscular network. Clinical presentation, muscular histological analysis and type of gene mutation, as well as X-chromosome inactivation (XCI) patterns using DNA extracted from peripheral blood or muscle are detailed. The initial symptoms were significant weakness (88%) or exercise intolerance (27%). Clinical severity varied from a Duchenne-like progression to a very mild Becker-like phenotype. Cardiac dysfunction was present in 19% of the cases. Cognitive impairment was worthy of notice, as 27% of the carriers are concerned. The muscular analysis was always contributive, revealing muscular dystrophy (83%), mosaic in immunostaining (81%) and dystrophin abnormalities in western blot analysis (84%). In all, 73% had exonic deletions or duplications and 27% had point mutations. XCI pattern was biased in 62% of the cases. In conclusion, we report the largest series of manifesting DMD carriers at pediatric age and show that exercise intolerance and cognitive impairment may reveal symptomatic DMD carriers. The complete histological and immunohistological study of the muscle is the key of the diagnosis leading to the dystrophin gene analysis. Our study shows also that cognitive impairment in symptomatic DMD carriers is associated with mutations in the distal part of the DMD gene. XCI study does not fully explain the mechanisms as well as the wide spectrum of clinical phenotype, though a clear correlation between the severity of the phenotype and inactivation bias was observed.

Acral dysostosis dyserythropoiesis syndrome

The term congenital dyserythropoietic anaemia (CDA) designates a group of rare but well defined erythrocytic disorders. Type I is defined by macrocytosis and megaloblastic changes of the bone marrow cells. Two unrelated children with CDA are described with associated defects: absence phalanges, polysyndactyly of the fourth metacarpal. One of them had also areas of depigmentation.ConclusionThe association of congential dyserythropoietic anaemia with morphological defects of hands and feet is suggested to constitute a new syndrome caused by a single morphogenetic gene.

Molecular diagnosis of hypophosphatasia and differential diagnosis by targeted Next Generation Sequencing

Hypophosphatasia (HPP) is a rare inherited skeletal dysplasia due to loss of function mutations in the ALPL gene. The disease is subject to an extremely high clinical heterogeneity ranging from a perinatal lethal form to odontohypophosphatasia affecting only teeth. Up to now genetic diagnosis of HPP is performed by sequencing the ALPL gene by Sanger methodology. Osteogenesis imperfecta (OI) and campomelic dysplasia (CD) are the main differential diagnoses of severe HPP, so that in case of negative result for ALPL mutations, OI and CD genes had often to be analyzed, lengthening the time before diagnosis. We report here our 18-month experience in testing 46 patients for HPP and differential diagnosis by targeted NGS and show that this strategy is efficient and useful. We used an array including ALPL gene, genes of differential diagnosis COL1A1 and COL1A2 that represent 90% of OI cases, SOX9, responsible for CD, and 8 potentially modifier genes of HPP. Seventeen patients were found to carry a mutation in one of these genes. Among them, only 10 out of 15 cases referred for HPP carried a mutation in ALPL and 5 carried a mutation in COL1A1 or COL1A2. Interestingly, three of these patients were adults with fractures and/or low BMD. Our results indicate that HPP and OI may be easily misdiagnosed in the prenatal stage but also in adults with mild symptoms for these diseases.

De Novo Disruption of the Proteasome Regulatory Subunit PSMD12 Causes a Syndromic Neurodevelopmental Disorder

Degradation of proteins by the ubiquitin-proteasome system (UPS) is an essential biological process in the development of eukaryotic organisms. Dysregulation of this mechanism leads to numerous human neurodegenerative or neurodevelopmental disorders. Through a multi-center collaboration, we identified six de novo genomic deletions and four de novo point mutations involving PSMD12, encoding the non-ATPase subunit PSMD12 (aka RPN5) of the 19S regulator of 26S proteasome complex, in unrelated individuals with intellectual disability, congenital malformations, ophthalmologic anomalies, feeding difficulties, deafness, and subtle dysmorphic facial features. We observed reduced PSMD12 levels and an accumulation of ubiquitinated proteins without any impairment of proteasome catalytic activity. Our PSMD12 loss-of-function zebrafish CRISPR/Cas9 model exhibited microcephaly, decreased convolution of the renal tubules, and abnormal craniofacial morphology. Our data support the biological importance of PSMD12 as a scaffolding subunit in proteasome function during development and neurogenesis in particular; they enable the definition of a neurodevelopmental disorder due to PSMD12 variants, expanding the phenotypic spectrum of UPS-dependent disorders.

Intragenic deletion of the WDR45 gene in a male with encephalopathy, severe psychomotor disability, and epilepsy

1 Laboratoire de génétique moléculaire et dhistocompatibilité, CHRU Morvan, Brest, France 2 INSERM U1078, Brest, France 3 Etablissement Français du Sang, Brest, France 4 Faculté de Médecine, UBO, Brest, France 5 Service de pédiatrie et de génétique médicale, CHRU Morvan, Brest, France 6 Service de radiologie et dimagerie médicale, CHRU Morvan, Brest, France Correspondence Pr Claude FEREC, INSERM UMR 1078-EFSCHRU 46, rue Félix le Dantec CS51819, 29218 BREST, Cedex2. Email: claude.ferec@univ-brest.fr

Progressive Atrial Conduction Defects Associated With Bone Malformation Caused by a Connexin-45 Mutation

BACKGROUNDnInherited cardiac conduction disease is a rare bradyarrhythmia associated with mutations in various genes that affect action potential propagation. It is often characterized by isolated conduction disturbance of the His-Purkinje system, but it is rarely described as a syndromic form.nnnOBJECTIVESnThe authors sought to identify the genetic defect in families with a novel bradyarrhythmia syndrome associated with bone malformation.nnnMETHODSnThe authors genetically screened 15 European cases with genotype-negative de novo atrioventricular (AV) block and their parents by trio whole-exome sequencing, plus 31 Japanese cases with genotype-negative familial AV block or sick sinus syndrome by targeted exon sequencing of 457 susceptibility genes. Functional consequences of thexa0mutation were evaluated using an inxa0vitro cell expression system and inxa0vivo knockout mice.nnnRESULTSnThe authors identified a connexin-45 (Cx45) mutation (p.R75H) in 2 unrelated families (a de novo French case and a 3-generation Japanese family) who presented with progressive AV block, which resulted in atrial standstill without ventricular conduction abnormalities. Affected individuals shared a common extracardiac phenotype: a brachyfacial pattern, finger deformity, and dental dysplasia. Mutant Cx45 expressed in Neuro-2a cells showed normal hemichannel assembly and plaque formation. However, Lucifer yellow dye transfer and gap junction conductance between cell pairs were severely impaired, which suggested that mutant Cx45 impedes gap junction communication in a dominant-negative manner. Tamoxifen-induced, cardiac-specific Cx45 knockout mice showed sinus node dysfunction and atrial arrhythmia, recapitulating the intra-atrial disturbance.nnnCONCLUSIONSnAltogether, the authors showed that Cx45 mutant p.R75H is responsible for a novel disease entityxa0ofxa0progressive atrial conduction system defects associated with craniofacial and dentodigital malformation.