Endocrine Abstracts

SEMDs are a heterogeneous group of skeletal disorders characterised by defective growth and modelling of the spine and long bones. Genetic defects in two inherited SEMDs have been identified and these involve abnormalities of the collagen type II gene located on chromosome 12q12-q13.2, and an ATP sulfurylase/APS kinase gene located on 10q23-34. These are not the cause of the Missouri variant (SEMD_MO), which occurs as an autosomal dominant trait in a unique four-generation kindred. In order to identify the gene causing SEMD_MO a genome wide linkage search was performed, using chromosome-specific sets of fluorescently labelled microsatellite markers at an average intermarker distance of 10 cM. Linkage between SEMD_MO and loci from an approximate 17 cM region on chromosome 11q14.3-q23.2 was established (peak LOD score = 4.92 at 0% recombination with D11S898). The ENSEMBL database revealed 234 genes within this region including a cluster of 9 genes coding for matrix metalloproteinases (MMPs) at 11q22.3. Of these, MMP13, which is known to degrade collagen type II and is specifically expressed in foetal chondrocytes and osteoblasts, appeared the best candidate for SEMD_MO. Sequence analysis of the 1416 bp coding region of the MMP13 gene from an affected family member revealed a T to C transition at residue 56 (TTC to TCC), predicting a phenylalanine to serine missense mutation (Phe56Ser), which co-segregated with SEMD_MO and was absent in 110 unrelated normals. This mutation resides in the pro-region domain and substitution of the hydrophobic phenylalanine for a hydrophilic serine is likely to affect processing and activity. This was studied by transient expression of wild type and mutant MMP13 in HEK293 cells, which revealed secretion of both forms with intracellular degradation of the mutant protein. Thus, our results are the first to show that a molecular defect of MMP13 causes abnormalities of human skeletal development.