Clustered cadherin genes: a sequence-ready contig for the desmosomal cadherin locus on human chromosome 18.

We describe the assembly of a cosmid and PAC contig of approximately 700 kb on human chromosome 18q12 spanning the DSC and DSG genes coding for the desmocollins and desmogleins. These are members of the cadherin superfamily of calcium-dependent cell adhesion proteins present in the desmosome type of cell junction found especially in epithelial cells. They provide the strong cell-cell adhesion generated by this type of cell junction for which expression of both a desmocollin and a desmoglein is required. In the autoimmune skin diseases pemphigus foliaceous and pemphigus vulgaris (PV), where the autoantigens are, respectively, encoded by the DSG1 and DSG3 genes, severe areas of acantholysis (cell separation), potentially life-threatening in the case of PV, are evident. Dominant mutations in the DSG1 gene causing striate palmoplantar keratoderma result in hyperkeratosis of the skin on the parts of the body where pressure and abrasion are greatest, viz., on the palms and soles. These genes are also candidate tumor suppressor genes in squamous cell carcinomas and other epithelial cancers. We have screened two chromosome 18-specific cosmid libraries by hybridization with previously isolated YAC clones and DSC and DSG cDNAs, and a whole genome PAC library, both by hybridization with the YACs and by screening by PCR using cDNA sequences and YAC end sequence. The contigs were extended by further PCR screens using STSs generated by vectorette walking from the ends of the cosmids and PACs, together with sequence from PAC ends. Despite screening of two libraries, the cosmid contig still had four gaps. The PAC contig filled these gaps and in fact covered the whole locus. The positions of 45 STSs covering the whole of this region are presented. The desmocollin and desmoglein genes, which are about 30-35 kb in size, are quite well separated at approximately 20-30 kb apart and are arranged in two clusters, one DSC cluster and one DSG cluster, which are transcribed outward from the interlocus region. The order of the genes is correlated with the spatial order of gene expression in the developing mouse embryo, and this, and previous transgenic experiments, suggests that long-range genetic elements that coordinate expression of these genes may be present. The complete bacterial clone contig described in this paper is thus a resource not only for future sequencing but also for investigations into the control of expression of these clustered genes.

Exon-intron organization of the human type 2 desmocollin gene (DSC2): desmocollin gene structure is closer to "classical" cadherins than to desmogleins.

The cadherins are a superfamily of calcium-dependent glycoproteins that are cell adhesion molecules. Two families of cadherins, the desmocollins (Dsc) and desmogleins (Dsg), are found only in the desmosome type of cell-cell junction. They are each present in at least three different isoforms with differing spatial and temporal distributions and are specified by two clusters of closely linked genes on human chromosome 18q12.1. The human DSC2 gene, coding for the most widely distributed form of the desmocollins, has been found to consist of more than 32 kb of DNA. By using PCR we have determined the exon-intron organization. The gene is arranged into 17 exons ranging in size from 46 to 258 bp; exon 16 is alternatively spliced, giving rise to the a and b forms of the protein. This has revealed a remarkable degree of conservation of intron position with other cadherins. The desmocollin exon-intron organization is more similar to the so-called classical cadherins than to the desmogleins, especially in the cytoplasmic domain. Intron 1 is the largest in DSC2, as it is in the desmogleins, in contrast to the classical cadherins, where intron 2 is extremely large; this latter intron is missing from the desmogleins.