Many human diseases are caused by mutations in collagen genes including Schmid metaphyseal chondrodysplasia (SCMD). Patients with SMCD exhibit dwarfism, coxa vara, and a waddling gait. Collagen X is synthesized specifically in hypertrophic chondrocytes of long bones. Transgenic mice that express the mouse equivalent of a collagen X SMCD mutation, a 13 bp deletion within the NC1-encoding domain [13 del], in the growth plate show shortened limbs and bone overgrowth. This mutation in the NC1-encoding domain, which is a domain crucial for collagen trimerization, apparently impairs the collagen assembly process in long bones. It is hypothesized that the mutant protein failing to assemble into helices generates abnormal stress to the cells. The stress due to the accumulation of mutant protein in the ER is hypothesized to cause dedifferentiation of chondrocytes, which may be triggered indirectly by a phenomenon known as the unfolded protein response (UPR). The UPR alleviates stress caused by misfolded or unfolded proteins accumulated in the ER by repressing protein translation. There are three UPR pathways known in mammals to date with all three UPR mutants present in C. elegans viz.
ire-1,
atf-6 and
pek-1. To address what impact the 13del mutant collagen or UPR in general has on worm development, a heterologous expression system was generated, with full length and truncated 13del cDNA and
ram-4 or
dpy-13 promoters to see if expression of this mammalian gene has any impact on body morphology and male tail morphogenesis. Our results show that the full length 13DEL driven by the
ram-4 promoter show 54% with a blunt tail phenotype and the RAM-4-13DEL shows 45% of ray 1-2 fusion. Therefore, the mutant proteins are expressed in C. elegans and affect morphogenesis of the male tail. No abnormality was observed for 13DEL driven by the
dpy-13 promoter in the body hypodermis, suggesting that the male tail may be more sensitive to aberrant protein accumulation. The 13del transgene has also been expressed in the
atf-6 mutant to establish the relevance of a specific pathway with handling of the mutant 13DEL protein. In a parallel study, when stress is applied at different stages of worm development, different phenocopied features occur. Phenocopy experiments will be conducted in the UPR mutants to ascertain if the absence of these UPR components would enhance or attenuate the phenocopy effect. (This study is supported by Research Grants Council, Hong Kong).