👤 Martine Le Merrer

Bardet-Biedl syndrome (BBS) is a multisystemic disorder characterized by postaxial polydactyly, progressive retinal dystrophy, obesity, hypogonadism, renal dysfunction, and learning difficulty. Other manifestations include diabetes mellitus, heart disease, hepatic fibrosis, and neurological features. The condition is genetically heterogeneous, and eight genes (BBS1-BBS8) have been identified to date. A mutation of the BBS1 gene on chromosome 11q13 is observed in 30%-40% of BBS cases. In addition, a complex triallelic inheritance has been established in this disorder--that is, in some families, three mutations at two BBS loci are necessary for the disease to be expressed. The clinical features of BBS that can be observed at birth are polydactyly, kidney anomaly, hepatic fibrosis, and genital and heart malformations. Interestingly, polydactyly, cystic kidneys, and liver anomalies (hepatic fibrosis with bile-duct proliferation) are also observed in Meckel syndrome, along with occipital encephalocele. Therefore, we decided to sequence the eight BBS genes in a series of 13 antenatal cases presenting with cystic kidneys and polydactyly and/or hepatic fibrosis but no encephalocele. These fetuses were mostly diagnosed as having Meckel or "Meckel-like" syndrome. In six cases, we identified a recessive mutation in a BBS gene (three in BBS2, two in BBS4, and one in BBS6). We found a heterozygous BBS6 mutation in three additional cases. No BBS1, BBS3, BBS5, BBS7, or BBS8 mutations were identified in our series. These results suggest that the antenatal presentation of BBS may mimic Meckel syndrome. Show less

Spondyloepimetaphyseal dysplasias (SEMD) represent a heterogeneous group of conditions composed of at least 15 well-defined entities. The classification is based on clinical, radiological and molecular findings. Among them, several conditions also include a mental retardation (MR) syndrome, namely Wolcott-Rallison syndrome, Dyggve-Melchior-Clausen syndrome (DMC) and lysosomal storage disorders. Here, we report on a novel form of SEMD with MR in two Pakistani sisters born to first-cousin parents. SEMD, MR, microcephaly, ataxia, facial dysmorphism and hirsutism of back and legs were noted in the two children. Skeletal findings included flat vertebral bodies with irregular vertebral plates, irregular and flared metaphyses with vertical striations, small and irregular epiphyses, small carpal bones and narrow iliac wings without lacy pelvis iliac crest. Similarities with DMC prompted us to test and eventually exclude the DMC gene, dymeclin, by direct sequencing. Similarly, we excluded the PAPSS2 gene (3'-alpha phosphoadenosine 5'-phosphosulphate synthase 2) responsible for SEMD Pakistani type. The combination of features observed in the two sisters does not fit with any previously reported SEMD and represents therefore a novel form of autosomal recessive SEMD with MR. Show less

Dyggve-Melchior-Clausen (DMC) is a rare autosomal-recessive disorder characterized by the association of a progressive spondylo-epi-metaphyseal dysplasia and mental retardation ranging from mild to severe. Electron microscopy studies of both DMC chondrocytes and fibroblasts reveal an enlarged endoplasmic reticulum network and a large number of intracytoplasmic membranous vesicles, suggesting that DMC syndrome may be a storage disorder. Indeed, DMC phenotype is often compared to that of type IV mucopolysaccharidosis (Morquio disease), a lysosomal disorder due to either N-acetylgalactosamine-6-sulphatase or beta-galactosidase deficiency. To date, however, the lysosomal pathway appears normal in DMC patients and biochemical analyses failed to reveal any enzymatic deficiency or accumulated substrate. Linkage studies using homozygosity mapping have led to the localization of the disease-causing gene on chromosome 18q21.1. The gene was recently identified as a novel transcript (Dym) encoding a 669-amino acid product (Dymeclin) with no known domains or function. Sixteen different Dym mutations have now been described in 21 unrelated families with at least five founder effects in Morocco, Lebanon, and Guam Island. Smith-MacCort syndrome (SMC), a rare variant of DMC syndrome without mental retardation, was shown to be allelic of DMC syndrome and to result from mutations in Dym that would be less deleterious to the brain. The present review focuses on clinical, radiological, and cellular features and evolution of DMC/SMC syndromes and discusses them with regard to identified Dym mutations and possible roles of the Dym gene product. Show less

Dyggve-Melchior-Clausen syndrome (DMC) is a rare autosomal-recessive disorder, the gene for which maps to chromosome 18q21.1. DMC is characterized by the association of a spondylo-epi-metaphyseal dysplasia and mental retardation. Electron microscopic study of cutaneous cells of an affected child showed dilated rough endoplasmic reticulum, enlarged and aberrant vacuoles and numerous vesicles. As the etiology of the disorder is unknown, we have used a positional cloning strategy to identify the DMC gene. We detected seven deleterious mutations within a gene predicted from a human transcript (FLJ20071) in 10 DMC families. The mutations were nonsense mutations (R194X, R204X, L219X, Q483X), splice site or frameshift mutations (K626N+92aa to stop). The DMC gene transcript is widely distributed but appears abundant in chondrocytes and fetal brain. The predicted protein product of the DMC gene yields little insight into its likely function, showing no significant homology to any known protein family. However, the carboxy terminal end comprises a cluster of dileucine motifs, highly conserved across species. We conclude that DMC syndrome is consequent upon loss of function of a gene that we propose to name Dymeclin, which may have a role in process of intracellular digestion of proteins. Show less

Hereditary multiple exostoses (HME) is a genetically heterogeneous autosomal dominant disorder characterised by the development of bony protuberances mainly located on the long bones. Three HME loci have been mapped to chromosomes 8q24 (EXT1), 11p11-13 (EXT2), and 19p (EXT3). The EXT1 and EXT2 genes encode glycosyltransferases involved in biosynthesis of heparan sulphate proteoglycans. Here we report on a clinical survey and mutation analysis of 42 HME French families and show that EXT1 and EXT2 accounted for more than 90% of HME cases in our series. Among them, 27/42 cases were accounted for by EXT1 (64%, four nonsense, 19 frameshift, three missense, and one splice site mutations) and 9/42 cases were accounted for by EXT2 (21%, four nonsense, two frameshift, two missense, and one splice site mutation). Overall, 31/36 mutations were expected to cause loss of protein function (86%). The most severe forms of the disease and malignant transformation of exostoses to chondrosarcomas were associated with EXT1 mutations. These findings provide the first genotype-phenotype correlation in HME and will, it is hoped, facilitate the clinical management of these patients. Show less

Hereditary multiple exostoses (EXT) is an autosomal dominant disorder characterized by the presence of multiple cartilage-capped exostoses in the juxta-epiphyseal regions of the long bones. EXT is heterogeneous with at least three different locations currently having been identified on chromosomes 8, 11 and 19. We have tested a series of 29 EXT families for possible linkage to the three disease loci and estimated the probability of linkage of the disease to each locus in our series, by using an extension of the admixture test, which makes modelling of heterogeneous monogenic disease feasible. The maximum likelihood was obtained for proportions of 44%, 28% and 28% of families being linked to chromosome 8, 11 and 19, respectively. The a posteriori probability of linkage of the disease to EXT1, EXT2 and EXT3 was greater than 80% for 8/29, 5/29 and 3/29 families, respectively, and did not give evidence of a fourth locus for the disease. The present approach can be generalized to the investigation of genetic heterogeneity in other monogenic diseases, as it simultaneously estimates the location of each disease gene and the proportion of families linked to each locus. Show less