AN RNA-SPLICING MUTATION (G(-II COLLAGEN GENE (COL2AI) IN A FAMILY WITH SPONDYLOEPIPHYSEAL DYSPLASIA CONGENITA(51VS20)) IN THE TYPE)

Defects in type II collagen have been demonstrated in a phenotypic con tinuum of chondrodysplasias that includes achondrogenesis II, hypochon drogenesis, spondyloepiphyseal dysplasia congenita (SEDC), Kniest dysp lasia, and Stickler syndrome. We have determined that cartilage from a terminated fetus with an inherited form of SEDC contained both normal alpha 1(II) collagen chains and chains that lacked amino acids 256-27 3 of the triple-helical domain. PCR amplification of this region of CO L2A1, from genomic DNA, yielded products of normal size, while amplifi cation of cDNA yielded a normal sized species and a shorter fragment m issing exon 20. Sequence analysis of genomic DNA from the fetus reveal ed a G-->T transversion at position +5 of intron 20; the affected fath er was also heterozygous for the mutation. Allele-specific PCR and het eroduplex analysis of a VNTR in COL2A1 independently confirmed the una ffected status of a fetus in a subsequent pregnancy. Thermodynamic cal culations suggest that the mutation prevents normal splicing of exon 2 0 by interfering with binding of U-1 small-nuclear RNA to pre-mRNA, th us leading to skipping of exon 20 in transcripts from the mutant allel e. Electron micrographs of diseased cartilage showed intracellular inc lusion bodies, which were stained by an antibody to alpha 1(II) procol lagen. Our findings support the hypothesis that alpha-chain length alt erations that preserve the Gly-X-Y repeat motif of the triple helix re sult in partial intracellular retention of alpha 1(II) procollagen and produce mild to moderate chondrodysplasia phenotypes.