Extensive alternative splicing within the amino-propeptide coding domain of alpha2(XI) procollagen mRNAs. Expression of transcripts encoding truncated pro-alpha chains.

Heterogeneity in type XI procollagen structure is extensive because all three alpha(XI) collagen genes undergo complex alternative splicing within the amino-propeptide coding domain. Exon 7 of the human and exons 6-8 of the mouse alpha2(XI) collagen genes, encoding part of the amino-propeptide variable region, have recently been shown to be alternatively spliced. We show that exon 6-containing mRNAs for human alpha2(XI) procollagen are expressed at 28 weeks in fetal tendon and cartilage but not at 38-44 days or 11 weeks. In the mouse, exon 6 is expressed in chondrocytes from 13.5 days onward. We recently identified conserved sequences within intron 6 of the human and mouse alpha2(XI) collagen genes, containing additional consensus splice acceptor and donor sites that potentially increase the size of exon 7, dividing it into three parts, designated 7A, 7B, and 7C. We show by reverse transcription polymerase chain reaction and in situ hybridization that these potential splice sites are used to yield additional alpha2(XI) procollagen mRNA splice variants that are expressed in fetal tissues. In human, expression of exon 7B-containing transcripts may be developmental stage-specific. Interestingly, inclusion of exon 7A or exon 7B in human and mouse alpha2(XI) procollagen mRNAs, respectively, would result in the insertion of an in-frame termination codon, suggesting that some of the additional splice variants encode a truncated pro-alpha2(XI) chain.