Matrix deposition of tryptophan-containing allelic variants of type IX collagen in developing human cartilage.

Genetic polymorphisms that encode a tryptophan (Trp) residue in the triple-helical domain of the alpha2 (Trp2) or alpha3 chain (Trp3) of human type IX collagen have been linked to risk of degenerative intervertebral disc disease. To determine whether these two allelic variants express protein that may affect the extracellular matrix of cartilage in vivo, we examined the properties of resident type IX collagen in an anonymous collection of embryonic and fetal human cartilage samples screened for Trp genotypes. No difference was found in the yield and electrophoretic properties of pepsin-solubilized type IX collagen between Trp2, Trp3 and non-Trp cartilage samples. On Western blot analysis, a polyclonal antiserum raised against a synthetic peptide matching the immediate Trp-containing sequence of the Trp3 allele reacted specifically with the alpha3(IX) chain prepared from Trp3 cartilage samples. Two-dimensional peptide mapping of type IX collagen in CNBr-digests of whole tissue gave indistinguishable fingerprints for Trp2, Trp3 and control tissues, including the yield of cross-linked peptides. Analysis of one cartilage sample that was homozygous for the Trp2 allele also gave a normal yield of collagen IX, including its alpha2 chain and a normal profile of cross-linked peptides. Together, the findings indicate that both Trp2 and Trp3 allelic products are incorporated into the cross-linked fibrillar network of developing human cartilage apparently normally. Any pathological consequences are likely, therefore, to be long-term and indirect rather than from overt misassembly of matrix.