The human COL9A3 gene:structure of the gene for the α3 chain of type IX collagen and its role in human cartilage and intervertebral disc diseases

Abstract The nucleotide sequence of the entire COL9A3 gene, coding for the human α3(IX) chain, was determined. The gene was approximately 23 kb in length and consisted of 32 exons. The polymerase chain reaction (PCR)-based procedure of conformation-sensitive gel electrophoresis (CSGE) was used to screen the gene for sequence variations and mutations in 83 unrelated patients with generalized primary osteoarthritis (OA), 31 with rheumatoid arthritis (RA), 171 with intervertebral disc disease (IDD), and 50 with various osteochondrodysplasias. The frequencies of certain sequence variations in healthy individuals were also determined. The COL9A3 gene was analyzed for mutations in two unrelated families with multiple epiphyseal dysplasia (MED). The analysis revealed a splice site mutation leading to skipping of exon 3 and an in-frame loss of 12 amino acid residues in the COL3 domain, the first diseasecausing mutation to be identified in the COL9A3 gene. Sequencing also indicated a 9 bp deletion in one allele in the second MED family that removed a Gly-Pro-Pro triplet. Surprisingly, the deletion did not co-segregate with the MED phenotype in the family. A similar 9 bp deletion, was also found in an unrelated family with no obvious phenotype, suggesting that the two 9 bp deletions represent neutral sequence variants. A construct with the deletion was then made in order to produce a recombinant protein, and the mutant type IX collagen was analyzed under reducing conditions by SDS-PAGE. The results indicated that the recombinant type IX collagen proteins consisted of three α chains, α1(IX), α2(JX), α3(IX), in a 1:1:1 ratio. To study the triple helix stability, pepsin treatment followed by SDS-PAGE was performed on normally folded and denatured recombinant type IX collagen samples. The results demonstrated that the recombinant type IX collagen containing the Gly-X-Y deletion in the a3(IX) chain is secreted as a correctly folded triple-helical molecule. CSGE analysis of exon 5 of the COL9A3 gene identified two nucleotide variations in the same codon, and thus three alleles: CGG (Arg), CAG (Gln), and TGG (Trp). The frequency of the Trp for Arg substitution, the Trp3 allele, was 0.244 among the probands with the IDD, while its overall frequency in the combined group of all non-IDD cases was 0.093. This difference was significant, with a p-value of 0.000013. The Trp3 allele increases the relative risk of IDD by a factor of 2.6 (95 percent confidence interval, 1.6 to 4.3). COL9A3 mutations are shown to be associated with mild cartilage and intervertebral disc diseases