Aberrant signal peptide cleavage of collagen X in Schmid metaphyseal chondrodysplasia - Implications for the molecular basis of the disease

Schmid metaphyseal chondrodysplasia results from mutations in the collagen X (COL10A1) gene. With the exception of two cases, the known mutations are clustered in the C-terminal nonhelical (NC1) domain of the collagen X. In v itro and cell culture studies have shown that the NC1 mutations result in i mpaired collagen X trimer assembly and secretion. In the two other cases, m issense mutations that alter Gly(18) at the -1 position of the putative sig nal peptide cleavage site were identified (Ikegawa, S., Nakamura, K,, Nagan o, A., Haga, N,, and Nakamura, Y. (1997) Hum, Mutat. 9, 131-135). To study their impact on collagen X biosynthesis using in vitro cell-free translatio n in the presence of microsomes, and cell transfection assays, these two mu tations were created in COL10A1 by site-directed mutagenesis. The data sugg est that translocation of the mutant pre-alpha1(X) chains into the microsom es is not affected, but cleavage of the signal peptide is inhibited, and th e mutant chains remain anchored to the membrane of microsomes, Cell-free tr anslation and transfection studies in cells showed that the mutant chains a ssociate into trimers but cannot form a triple helix, The combined effect o f both the lack of signal peptide cleavage and helical configuration is imp aired secretion. Thus, despite the different nature of the NC1 and signal p eptide mutations in collagen X, both result in impaired collagen X secretio n, probably followed by intracellular retention and degradation of mutant c hains, and causing the Schmid metaphyseal chandrodysplasia phenotype.