Human Molecular Genetics Advance Access published online on April 2, 2007
Human Molecular Genetics, doi:10.1093/hmg/ddm067
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COL10A1 nonsense and frameshift mutations have a gain-of-function effect on the growth plate in human and mouse metaphyseal chondrodysplasia type Schmid
1 Department of Biochemistry, The University of Hong Kong, Pokfulam, Hong Kong, China 2 Hospital for Sick Children, Toronto, M5G 1X8 Canada 3 Hospital for Children and Adolescents, University of Helsinki, Helsinki, Finland 4 Department of Clinical Genetics, The Children's Hospital at Westmead, Westmead, NSW 2145, Australia 5 Wellcome Trust Centre for Cell-Matrix Research, Faculty of Life Sciences, University of Manchester, Manchester M13 9PT, United Kingdom 6 Bone and Joint Center, ER21015, Henry Ford Hospital, Detroit, MI 48202, USA 7 Department of Orthopaedics and Traumatology, The University of Hong Kong, Pokfulam, Hong Kong, China
* To whom correspondence should be addressed. Tel: + 852 28199482; Fax: 852 28551254; E-mail: chand{at}hkusua.hku.hk
Received October 31, 2006; Revised March 14, 2007; Accepted March 14, 2007
Missense, nonsense and frameshift mutations in the collagen X gene (COL10A1) result in metaphyseal chondrodysplasia type Schmid (MCDS). Complete degradation of mutant COL10A1 mRNA by nonsense-mediated decay in human MCDS cartilage implicates haploinsufficiency in the pathogenesis for nonsense mutations in vivo. However, the mechanism is unclear in situations where the mutant mRNA persist. We show that nonsense/frameshift mutations can elicit a gain-of function effect, affecting chondrocyte differentiation in the growth plate. In a MCDS proband, heterozygous for a p.Y663X nonsense mutation, the growth plate cartilage contained 64% wild-type and 36% mutant mRNA and the hypertrophic zone was disorganized and expanded. The in vitro translated mutant collagen X chains, which are truncated, were misfolded, unable to assemble into trimers, and interfered with the assembly of normal 
1(X) chains into trimers. Unlike Col10a1 null mutants, transgenic mice (FCdel) bearing the mouse equivalent of a human MCDS p.P620fsX621 mutation, displayed typical characteristics of MCDS with disproportionate shortening of limbs and early onset coxa vara. In FCdel mice, the degree of expansion of the hypertrophic zones was transgene-dosage dependent, being most severe in mice homozygous for the transgene. Chondrocytes in the lower region of the expanded hypertrophic zone expressed markers uncharacteristic of hypertrophic chondrocytes, indicating differentiation was disrupted. Misfolded FCdel 1(X) chains were retained within the endoplasmic reticulum of hypertrophic chondrocytes, activating the unfolded protein response (UPR). Our findings provide strong in vivoevidence for a gain-of function effect that is linked to activation of ER-stress response and altered chondrocyte differentiation, as a possible molecular pathogenesis for MCDS.
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