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The COL5A1 gene, a member of the clade B fibrillar collagen gene family, was recently shown to contain two alternatively spliced exons (64A and 64B) that encode 23 amino acids in the carboxyl-terminal propeptide. The two are identical in length, very similar in sequence, and used in a mutually exclusive fashion because of the small intron that separates them. Each COL5A1 allele uses both exons, but a given transcript will contain only one of the two exons. The sequences in other species are highly conserved at the amino acid level. The expression profile of the two isoforms was determined from analysis of RNA levels in a panel of murine tissues. While both isoforms were found in all tissues studied, actively proliferating tissues (liver, lung) used isoform B more often, while a less mitotically active tissue, brain, had a higher proportion of exon 64A. The high degree of conservation between the two exons is consistent with a regional genomic duplication. The presence of the two isoforms as far back as pufferfish (tetraodon) implies an important functional significance. The exact role determined by the two sequences is not known, but involvement in the determination of chain composition of mature type V collagen or regulation of cell activity is possible, given the differences in tissue distribution.

Attempts to identify genetic contributors to human meiotic nondisjunction have met with little, if any, success. Thus, recent reports linking Down syndrome to maternal polymorphisms at either of two folate metabolism enzymes, methylenetetrahydrofolate reductase (MTHFR) and methionine synthase reductase (MTRR), have generated considerable interest. In the present report, we asked whether variation at MTHFR (677C-->T) or MTRR (66A-->G) might be associated with human trisomies other than trisomy 21. We analyzed maternal polymorphisms at MTHFR and MTRR in 93 cases of sex-chromosome trisomy, 44 cases of trisomy 18, and 158 cases of autosomal trisomies 2, 7, 10, 13, 14, 15, 16, 18, or 22, and compared the distributions of genotypes to those of control populations. We observed a significant increase in the MTHFR polymorphism in mothers of trisomy 18 conceptuses but were unable to identify any other significant associations. Overall, our observations suggest that, at least for the sex chromosomes and for a combined set of autosomal trisomies, polymorphisms in the folate pathway are not a significant contributor to human meiotic nondisjunction.