Human COL6A1: genomic characterization of the globular domains, structural and evolutionary comparison with COL6A2.

The alpha1(VI) and alpha2(VI) chains of type VI collagen (nonfibrillar) are highly similar and are encoded by single-copy genes in close proximity on human Chromosome (Chr) 21q22.3, a gene-rich region that has proved refractory to cloning. For the alpha1(VI) chain, only the regions encoding the triple-helical and the promoter have been characterized hitherto.To facilitate our study of the role of this gene in the phenotype of Down syndrome, we have cloned and sequenced the amino- and carboxyl-terminal globular domains of COL6A1. The amino-terminal domain consists of seven exons and the carboxyl-terminal globular domain of nine exons. Together with the exons of the triple-helical domain, COL6A1 is encoded by a total of 36 exons spanning approximately 30 kb. Comparison of the genomic organization of COL6A1 and COL6A2 revealed that despite the similarity within their triple-helical domains, the intron-exon structures of their globular domains differ markedly. Conservation is limited to the exons encoding amino acids immediately adjacent to the triple-helical region, including the cysteine residues essential for the structure of mature collagen VI. The intron-exon structures of these two genes are highly similar to the collagen VI genes of chicken. These data suggest that COL6A1 and COL6A2 arose from a gene duplication before the divergence of the reptilian and mammalian lineages.