Mechanistic insight into point mutations in sedlin that result in spondyloepiphyseal dysplasia tarda

Spondyloepiphyseal dysplasia tarda (SEDT) is the only osteochondrodysplasia which has been shown so far to be due to a defect in trafficking, via mutations in a gene (SEDL) that encodes sedlin, involved in ER-Golgi transport. A number of mutations have previously been identified in patients with SEDT including nonsense mutations, missense mutations, deletions, insertions and splicing errors. Here, we investigate the functional properties of four point mutants of sedlin, D47Y, S73L, F83S and V130D, and compare to the wild type protein. On expression in E. coli in a number of conditions, only WT and D47Y were found to express in the soluble fraction, whilst the other three point mutants entered inclusion bodies. D47Y and WT sedlin were found to be very similar by circular dichroism, steady-state fluorescence and chaotrope induced unfolding, suggesting that D47Y only caused a local perturbation to structure compared to all other mutants which led to misfolding. We also tested expression of all mutants in a variety of mammalian cell lines. All point mutants expressed at significantly lower levels compared to wild type and localization was investigated. We were able to correlate specific mutations with severity of mild clinical symptoms. Our data provides a mechanistic understanding at the protein level of the varying clinical severities of spondyloepiphyseal dysplasia tarda and lays a foundation for further work into the specific protein-protein interactions of sedlin. This work was funded by the Hong Kong UGC under the Area of Excellence Programme in Developmental Genomics and Skeletal Research