A nonsense mutation in the carboxyl-terminal domain of type X collagen causes haploinsufficiency in schmid metaphyseal chondrodysplasia

• Published in: The Journal of clinical investigation Vol. 101; no. 7; pp. 1490 - 1499
• Main Authors: Chan, D, Weng, Y M, Graham, H K, Sillence, D O, Bateman, J F
• Format: Journal Article
• Language: English
• Published: United States 01.04.1998
• Subjects: Animals; Base Sequence; Cartilage - pathology; Cattle; Child; Collagen - genetics; Female; Growth Plate - pathology; Heterozygote; Humans; Mice; Molecular Sequence Data; Osteochondrodysplasias - genetics; Point Mutation; Polymorphism, Restriction Fragment Length
• ISSN: 0021-9738
• DOI: 10.1172/jci1976

Type X collagen is a short-chain homotrimeric collagen expressed in the hypertrophic zone of calcifying cartilage. The clustering of mutations in the carboxyl-terminal NC1 domain in Schmid metaphyseal chondrodysplasia (SMCD) suggested a critical role for this type X collagen domain, but since no direct analysis of cartilage has been conducted in SMCD patients, the mechanisms of type X collagen dysfunction remain controversial. To resolve this problem, we obtained SMCD growth plate cartilage, determined the type X collagen mutation, and analyzed the expression of mutant and normal type X collagen mRNA and protein. The mutation was a single nucleotide substitution that changed the Tyr632 codon (TAC) to a stop codon (TAA). However, analysis of the expression of the normal and mutant allele transcripts in growth plate cartilage by reverse transcription PCR, restriction enzyme mapping, and a single nucleotide primer extension assay, demonstrated that only normal mRNA was present. The lack of mutant mRNA is most likely the result of nonsense-mediated mRNA decay, a common fate for transcripts carrying premature termination mutations. Furthermore, no mutant protein was detected by immunoblotting cartilage extracts. Our data indicates that a functionally null allele leading to type X collagen haploinsufficiency is the molecular basis of SMCD in this patient.