A nonsense mutation in the carboxyl-terminal domain of type X collagen causes haploinsufficiency in schmid metaphyseal chondrodysplasia.
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.
Chan
D
D
Orthopedic Molecular Biology Research Unit, Department of Pediatrics, University of Melbourne, Parkville, Melbourne, Australia 3052.
Weng
Y M
YM
Graham
H K
HK
Sillence
D O
DO
Bateman
J F
JF
D000818:Animals; D001483:Base Sequence; D002356:Cartilage / Q000473:pathology; D002417:Cattle; D002648:Child; D003094:Collagen / Q000235:genetics*; D005260:Female; D006132:Growth Plate / Q000473:pathology; D006579:Heterozygote; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D010009:Osteochondrodysplasias / Q000235:genetics*; D017354:Point Mutation; D012150:Polymorphism, Restriction Fragment Length
D003094:Collagen