Functional expression and cellular distribution of diastrophic dysplasia sulfate transporter (DTDST) gene mutations in HEK cells

Defects in sulfate transport in chondrocytes lead to undersulfation of the cartilage extracellular matrix proteoglycans. Mutations in the diastrophic dysplasia sulfate transporter (DTDST) gene have been linked to four chondrodysplasias of varying severity. To characterize disease-causing mutations of DTDST, we expressed DTDST-mediated sulfate transport in mammalian HEK-293 cells and determined that the wild-type protein is glycosylated and localized to the cell plasma membrane. Four mutations, A715V, C653S, Q454P and R279W, stimulated sulfate transport at rates only 39-62% of wild-type DTDST. These four mutations were expressed on the plasma membrane of the cell, but the amount of expressed protein was reduced when compared with wild-type DTDST. The Q454P mutant is unique in that it is not properly glycosylated in HEK cells. There was no difference in sulfate transport activity between cells transfected with either the DeltaV340 or the G678V mutations and control HEK cells. Furthermore, the G678V mutation is not expressed along the plasma membrane, but is trapped within the cytoplasm. When comparing the sulfate transport capacity of each DTDST mutation with the chondrodysplasia in which it has been identified, we find that individuals with severe achondrogenesis 1B phenotype have null mutations on both DTDST alleles. Heterozygotes for both a null mutation and a partial-function mutation result in either atelosteogenesis type 2 or DTD, whereas the milder, recessive multiple epiphyseal dysplasia phenotype is homozygous for partial-function mutations. In contrast to previous studies in Xenopus laevis oocytes, we find a strong correlation between the severity of the phenotype and the level of residual transport function in mammalian cells.