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Meckel-Gruber syndrome (MKS) is an autosomal recessive lethal malformation syndrome characterized by renal cystic dysplasia, central nervous system malformations (typically, posterior occipital encephalocele), and hepatic developmental defects. Two MKS genes, MKS1 and MKS3, have been identified recently. The present study describes the cellular, sub-cellular and functional characterization of the novel proteins, MKS1 and meckelin, encoded by these genes. In situ hybridization studies for MKS3 in early human embryos showed transcript localizations in agreement with the tissue phenotype of MKS patients. Both MKS proteins predominantly localized to epithelial cells, including proximal renal tubules and biliary epithelial cells. MKS1 localized to basal bodies, while meckelin localized both to the primary cilium and to the plasma membrane in ciliated cell-lines and primary cells. Meckelin protein with the Q376P missense mutation was unable to localize at the cell membrane. siRNA-mediated reduction of Mks1 and Mks3 expression in a ciliated epithelial cell-line blocked centriole migration to the apical membrane and consequent formation of the primary cilium. Co-immunoprecipitation experiments show that wild-type meckelin and MKS1 interact and, in three-dimensional tissue culture assays, epithelial branching morphogenesis was severely impaired. These results suggest that MKS proteins mediate a fundamental developmental stage of ciliary formation and epithelial morphogenesis.

Meckel-Gruber syndrome is a severe autosomal, recessively inherited disorder characterized by bilateral renal cystic dysplasia, developmental defects of the central nervous system (most commonly occipital encephalocele), hepatic ductal dysplasia and cysts and polydactyly1,2,3. MKS is genetically heterogeneous, with three loci mapped: MKS1, 17q21-24 (ref. 4); MKS2, 11q13 (ref. 5) and MKS3 (ref. 6). We have refined MKS3 mapping to a 12.67-Mb interval (8q21.13-q22.1) that is syntenic to the Wpk locus in rat, which is a model with polycystic kidney disease, agenesis of the corpus callosum and hydrocephalus7,8. Positional cloning of the Wpk gene suggested a MKS3 candidate gene, TMEM67, for which we identified pathogenic mutations for five MKS3-linked consanguineous families. MKS3 is a previously uncharacterized, evolutionarily conserved gene that is expressed at moderate levels in fetal brain, liver and kidney but has widespread, low levels of expression. It encodes a 995–amino acid seven-transmembrane receptor protein of unknown function that we have called meckelin.

Isolated (non-syndromic) congenital cataract may be inherited as an autosomal dominant, autosomal recessive, or X-linked recessive trait. Considerable progress has been made in identifying genes and loci for dominantly inherited cataract, but the molecular basis for autosomal recessive disease is less well defined. Hence we undertook genetic linkage studies in four consanguineous Pakistani families with non-syndromic autosomal recessive congenital cataracts. In two families linkage to a 38 cM region 9q13-q22 was detected. Although a locus for recessive congenital cataracts had not been mapped previously to this region, the target interval encompasses the candidate region autosomal recessive adult-onset pulverulent cataracts (CAAR). The CAAR was mapped previously to 9q13-q22, and may therefore be allelic to non-syndromic autosomal recessive congenital cataracts. The other two families did not demonstrate linkage to 9q, but both had a region of homozygosity at 16q22 containing the heat shock transcription factor 4 (HSF4) gene. The HSF4 mutations have been reported in four families with autosomal dominant cataracts and, recently, in a single kindred with autosomal recessive congenital cataract. Mutation analysis of HSF4 revealed homozygous mutations (p.Arg175Pro and c.595_599delGGGCC, respectively) in the two families. These findings confirm that mutations in HSF4 may result in both autosomal dominant and autosomal recessive congenital cataract, and highlight the locus heterogeneity in autosomal recessive congenital cataract.

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous autosomal recessive disorder characterized by variable obesity, pigmentary retinopathy, polydactyly, mental retardation, hypogonadism and renal failure. In order to identify novel BBS loci we undertook autozygosity mapping studies using high-density SNP microarrays in consanguineous kindreds. We mapped a BBS locus to a 10.1 Mb region at 12q15-q21.2 in a large Omani BBS family (peak lod score 8.3 at theta = 0.0 for marker D12S1722) that contained the recently described BBS10 locus. Mutation analysis of candidate genes within the target interval, including the BBS10 gene, revealed a homozygous frameshift mutation in FLJ23560 and mutations were also detected in four smaller consanguineous families with regions of autozygosity at 12q21.2. These findings (a) confirm a previous report that FLJ23560 (BBS10) mutations are a significant cause of BBS, and (b) further demonstrate the utility of high-density SNP array mapping in consanguineous families for the mapping and identification of recessive disease genes.

Joubert syndrome (JS) is an autosomal recessive disorder characterized by cerebellar vermis hypoplasia associated with hypotonia, developmental delay, abnormal respiratory patterns, and abnormal eye movements. The association of retinal dystrophy and renal anomalies defines JS type B. JS is a genetically heterogeneous condition with mutations in two genes, AHI1 and CEP290, identified to date. In addition, NPHP1 deletions identical to those that cause juvenile nephronophthisis have been identified in a subset of patients with a mild form of cerebellar and brainstem anomaly. Occipital encephalocele and/or polydactyly have occasionally been reported in some patients with JS, and these phenotypic features can also be observed in Meckel-Gruber syndrome (MKS). MKS is a rare, autosomal recessive lethal condition characterized by central nervous system malformations (typically, occipital meningoencephalocele), postaxial polydactyly, multicystic kidney dysplasia, and ductal proliferation in the portal area of the liver. Since there is obvious phenotypic overlap between JS and MKS, we hypothesized that mutations in the recently identified MKS genes, MKS1 on chromosome 17q and MKS3 on 8q, may be a cause of JS. After mutation analysis of MKS1 and MKS3 in a series of patients with JS (n=22), we identified MKS3 mutations in four patients with JS, thus defining MKS3 as the sixth JS locus (JBTS6). No MKS1 mutations were identified in this series, suggesting that the allelism is restricted to MKS3.

Meckel syndrome (MKS) is a rare autosomal recessive lethal condition characterized by central nervous system malformations (typically occipital meningoencephalocele), postaxial polydactyly, multicystic kidney dysplasia, and ductal proliferation in the portal area of the liver. MKS is genetically heterogeneous and three loci have been mapped respectively on 17q23 (MKS1), 11q13 (MKS2), and 8q24 (MKS3). Very recently, two genes have been identified: MKS1/FLJ20345 on 17q in Finnish kindreds, carrying the same intronic deletion, c.1408-35_c.1408-7del29, and MKS3/TMEM67 on 8q in families from Pakistan and Oman. Here we report the genotyping of the MKS1 and MKS3 genes in a large, multiethnic cohort of 120 independent cases of MKS. Our first results indicate that the MKS1 and MKS3 genes are each responsible for about 7% of MKS cases with various mutations in different populations. A strong phenotype-genotype correlation, depending on the mutated gene, was observed regarding the type of central nervous system malformation, the frequency of polydactyly, bone dysplasia, and situs inversus. The MKS1 c.1408-35_1408-7del29 intronic mutation was identified in three cases from French or English origin and dated back to 162 generations (approx. 4050 years) ago. We also identified a common MKS3 splice-site mutation, c.1575+1G>A, in five Pakistani sibships of three unrelated families of Mirpuri origin, with an estimated age-of-mutation of 5 generations (125 years). © 2007 Wiley-Liss, Inc.

Primary cilia have a broad tissue distribution and are present on most cell types in the human body. Until recently, they were considered to be redundant organelles, but progress over the past 5 years has led to an understanding of their role in normal mammalian development. The class of inherited disorders that involve aberrant ciliary function are known as ciliopathies, and although their range of severity can vary, they share some common and unexpected clinical phenotypes. The aim of this review is to assess recent insights into the structure, function and formation of primary cilia, and relate this to the pathology, molecular genetics and cell biology of the ciliopathies.