Undersulfation of Cartilage Proteoglycans Ex Vivo and Increased Contribution of Amino Acid Sulfur to Sulfation In Vitro in McAlister Dysplasia/Atelosteogenesis Type 2

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Abstract
Mutations in the diastrophic dysplasia sulfate transporter gene cause a family of chondrodysplasias including, in order of increasing severity, diastrophic dysplasia, atelosteogenesis type 2 and achondrogenesis type 1B. McAlister dysplasia is a lethal chondrodysplasia considered on the basis of minor radiographic features to be a disorder different from atelosteogenesis type 2. Here, we demonstrate that McAlister dysplasia arises from mutations in the diastrophic dysplasia sulfate transporter gene and that this disorder essentially coincides on molecular and biochemical grounds with atelosteogenesis type 2. The fetus affected by McAlister dysplasia we have studied is a compound heterozygote for mutations leading to R279W and N425D substitutions in the diastrophic dysplasia sulfate transporter. Proteoglycan sulfation was studied in epiphyseal cartilage and in chondrocyte cultures of the patient by high performance liquid chromatography of chondrotinase digested proteoglycans; a high amount of non-sulfated disaccharide was observed as a consequence of the alteration of the transporter function caused by the mutations. However, sulfated disaccharides were detectable even if in low amounts, both in cultured cells and tissue. Functional impairment of the sulfate transporter was demonstrated in vitro by reduced incorporation of [35S]sulfate relative to [3H]glucosamine in proteoglycans synthesized by chondrocytes and by sulfate-uptake assays in fibroblasts. Parallel in vitro studies in a patient with achondrogenesis 1B indicated that the severity of the clinical phenotype seems to be correlated to the residual activity of the sulfate transporter. The capacity of fibroblasts to use cysteine as an alternative source of sulfate was evaluated by double-labeling experiments. Relative incorporation of [35S]cysteine-derived sulfate in the glycosaminoglycan chains was increased in the patient's cells, indicating that, in vitro, the catabolism of sulfur-containing amino acids can partially compensate for intracellular sulfate deficiency. Residual sulfation observed in proteoglycans extracted from cartilage suggests that this mechanism may be operating also in vivo.