Investigations of the Ufm1 pathway and its association with a familial form of hip dysplasia

Beukes Hip Dysplasia (BHD) is an autosomal dominant disorder where the abnormal shape of the hip joint leads to secondary osteoarthritis. The locus of BHD has been previously mapped to 4q35 and screening of candidate genes within this region revealed a mutation in the gene encoding the Ubiquitin-fold modifier 1 specific protease 2 (Ufsp2). The mutation prevents Ufsp2 from cleaving its target, Ufm1. Ufsp2 and Ufm1 are both components of a novel ubiquitin-like protein modification system which involves Ufm1 being processed via the E1, E2 and E3 enzymes (Uba5, Ufc1 and Ufl1, respectively) and attachment to target protein(s) one of which has been identified (Ddrgk1). The aim of this study was to investigate the link between the UFSP2 mutation and the BHD phenotype by: (i) examining the expression of components of the Ufm1 system in vivo and in vitro; (ii) development of an in vitro Ufm1 conjugation system; and (iii) generation and analysis of transgenic mice overexpressing the Ufsp2 gene with the BHD mutation.The expression of Ufsp2 was determined by radioactive RNA in situ hybridisation of mouse tissue sections. The analysis revealed Ufsp2 expression predominantly in the bone of the hip joint and in the bone and secondary ossification centres of the knee of 10 day old mice. Real-Time PCR analysis showed increased expression of components of the Ufm1 system during in vitro osteogenic and chondrogenic differentiation which coincided with induction of ER stress evidenced by upregulation of Bip. These components were also upregulated in response to chemically induced ER stress in vitro. Analysis of the promoter regions of Ufm1 system genes identified unfolded protein response elements in the upstream sequences of Uba5, Ufl1, Ufm1 and Lzap genes and the elements in Uba5 and Lzap were found to be required and responsive to ER stress using luciferase promoter assays.A Tandem Affinity Purification method was developed for isolation and identification of Ufm1 conjugation targets from cell lines expressing modified forms of Ufm1. Mass spectrometry analysis of Ufm1 conjugates purified from HEK293T cell line identified Uba5 and Ufc1 but no new Ufm1 targets. Western blot comparison of Ufm1 conjugated proteins purified from the HEK293T and 2T3 osteoblast cell lines identified putatative Ufm1 conjugation targets and increased conjugation in osteoblasts in response to ER stress. One of these targets was identified as Ddrgk1 but the remaining putative targets remain to be identified by mass spectrometry.Transgenic mice overexpressing the mutated Ufsp2 gene were generated and subjected to phenotypic analysis. No significant differences were found between transgenic and wild type mice following X-ray, histological and weight analysis.Higher expression of Ufsp2 in bone and secondary ossification centres as well as upregulation of components of the Ufm1 system in response to ER stress suggests that the molecular pathway between the UFSP2 mutation and the BHD phenotype may relate to abnormal ER stress responses during osteoblast differentiation. Further studies are however required to determine how the Ufm1 system modulates ER stress responses and how disruption of these processes caused by the UFSP2 mutation causes BHD.