CDNA cloning of chick cartilage chondroitin sulfate (aggrecan) core protein and identification of a stop codon in the aggrecan gene associated with the chondrodystrophy, nanomelia.

Abstract

We previously reported the cloning and sequencing of a 1.5-kilobase cDNA which encoded a portion of the chondroitin sulfate domain from the chick cartilage proteoglycan core protein and the localization of a species-specific monoclonal antibody epitope. Using polymerase chain reaction amplification and primer extension, cDNA clones which code for the entire proteoglycan core protein have now been obtained from a 10-day chick embryo cDNA library. The composite sequence is 6464 nucleotides long, coding for a protein of 2109 amino acid residues with a calculated M(r) = 223,500. The overall arrangement of globular and carbohydrate-attachment domains is similar to human and rat chondrosarcoma aggrecan, but there are significant differences in detailed homology between chick and mammalian core proteins. Most significantly a highly repetitive region (19 repeat units of 20 residues each), not found in either human or rat, enlarges one of the characteristic serine-glycine containing regions (designated CS-2) while the other serine-glycine containing domain (designated CS-1) is approximately one-fourth the length of the mammalian CS-1. Analysis of a polymerase chain reaction-amplified fragment encoding the chick-specific repeat region revealed a single base mutation at position 4553 (G to T transversion) that converted the codon GAA for glutamate at amino acid 1513 to TAA, a stop codon, in nanomelic chondrocytes. Genomic DNA from nanomelic liver was also digested with restriction enzyme BsaBI to verify the G to T transversion. This single mutation leads to a shortened core protein precursor with a calculated M(r) = 158,300. The resulting phenotype, nanomelia, arises because the truncated core protein is neither processed to a mature proteoglycan, nor secreted from the chondrocyte.