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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.

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Integrins play a pivotal role in organogenesis, by mediating the interactions between differentiating cells and the extracellular matrix. We analyzed the expression of integrins and their ligands during human liver organogenesis. The expression of ␤1, ␤3, and ␤4 integrins and the distribution of several extracellular matrix proteins were studied by immunoperoxidase in fetal liver samples from 5 to 40 weeks' gestation. Hepatoblasts expressed only the ␤1, ␣1, ␣5, ␣6, and ␣9 integrin chains. Fetal hepatocytes, emerging at the 8th week of gestation, initially retained the same combination of integrins, but presented a progressive decrease in their expression levels. After 15 weeks' gestation, the expression levels of ␤1, ␣1, ␣5, and ␣9 reached levels comparable to those observed in the adult state. ␣6 expression became undetectable after 30 weeks' gestation. As compared to hepatoblasts, intrahepatic biliary epithelial cells, differentiating at the 8th week of gestation in the ductal plate, were characterized by the progressive loss of ␣1, the marked induction of ␣6, and the de novo acquisition of the ␤4, ␣2, and ␣3 integrin chains. The disappearance of integrin receptors for laminin on hepatocytes was associated with the rarefaction of laminin in the perisinusoidal matrix, whereas their induction on biliary epithelial cells was associated with laminin deposition at the point of contact with the ductal plate. In conclusion, integrins likely play an important role in the differentiation of the epithelial and endothelial cell populations of the liver. (HEPATOLOGY 1998;26:839-847.)Integrins are membrane receptors for extracellular matrix proteins, which play an essential role in mediating the structural and functional interactions between cells and the extracellular matrix. 1 Previous studies have shown that markedly different combinations of integrins are expressed by the two populations of epithelial cells present in the adult liver, hepatocytes and biliary epithelial cells. [2][3][4] Like other simple epithelial cells, intrahepatic biliary epithelial cells express a large combination of integrin receptors, including the ␣2␤1, ␣3␤1, ␣5␤1, ␣6␤1, ␣9␤1, ␣V␤1, and ␣6␤4 dimers. 2,5 In striking contrast, the integrin repertoire of hepatocytes is restricted to the ␣1␤1, ␣5␤1, and ␣9␤1 dimers. This combination is highly distinctive. 3 The expression of ␣1␤1 integrin, a receptor for collagens and laminin, 6 is very unusual for an epithelial cell, since this integrin receptor is characteristically expressed by fibroblasts, muscle cells, and endothelial cells. Moreover, the absence of ␣6␤4, a characteristic component of hemidesmosomes, 7,8 is unique among the epithelial cells tested so far.It is likely that the differences in the combinations of integrins expressed, respectively, by hepatocytes and biliary epithelial cells are related to the differences in the constitution of their pericellular environments. Like all other lining epithelial cells, intrahepatic biliary epithelial cells are surrounded by a typical basement membran...