cDNA cloning and chromosomal mapping of the mouse type VII collagen gene (Col7a1): evidence for rapid evolutionary divergence of the gene.

Type VII collagen is the major component of anchoring fibrils, critical attachment structures at the dermal-epidermal basement membrane zone. Genetic linkage analyses with recently cloned human type VII collagen cDNAs have indicated that the corresponding gene, COL7A1, is the candidate gene in the dystrophic forms of epidermolysis bullosa. To gain insight into the evolutionary conservation of COL7A1, in this study we have isolated mouse type VII collagen cDNAs by screening a mouse epidermal keratinocyte cDNA library with a human COL7A1 cDNA. Two overlapping mouse cDNAs were isolated, and Northern hybridization of mouse epidermal keratinocyte RNA with one of them revealed the presence of a mRNA transcript of approximately 9.5 kb, the approximate size of the human COL7A1 mRNA. Nucleotide sequencing of the mouse cDNAs revealed a 2760-bp open reading frame that encodes the 5' half of the collagenous domain and a segment of the NC-1, the noncollagenous amino-terminal domain of type VII collagen. Comparison of the mouse amino acid sequences with the corresponding human sequences deduced from cDNAs revealed 82.5% identity. The evolutionary divergence of the gene was relatively rapid in comparison to other collagen genes. Despite the high degree of sequence variation, several sequences, including the size and the position of noncollagenous imperfections and interruptions within the Gly-X-Y repeat sequence, were precisely conserved. Finally, the mouse Col7a1 gene was located by interspecific backcross mapping to mouse Chromosome 9, a region that corresponds to human chromosome 3p21, the position of human COL7A1. This assignment confirms and extends the relationship between the mouse and the human chromosomes in this region of the genome.