Genetic Skeletal Disorders with Defects in Glycosaminoglycan Biosynthesis

Proteoglycans are a major component of the connective tissue matrix, which consists of a core protein and covalently attached glycosaminoglycan (GAG) chains, which are highly sulfated polysaccharides with a tetrasaccharide linker for the core protein attachment. Impaired synthesis or degradation of GAG causes genetic disorders. In the 1950s, deficient lysosomal GAG degradation was discovered in mucopolysaccharidoses. In the 1990s, a defective enzyme for GAG synthesis was implicated in a variant of Ehlers-Danlos syndrome and an impaired GAG sulfation in diastrophic dysplasia. Newer studies have uncovered that abnormal GAG synthesis causes a large group of genetic skeletal disorders with joint and skin abnormalities.

The prototype of this group includes diastrophic dysplasia and Desbuquois dysplasia. The former is attributed to abnormal GAG sulfation, while the latter to impaired GAG chain elongation. Defective linker formation causes distinctive phenotypes termed linkeropathy. Moreover, there remain many disorders with defective GAG synthesis, in which the phenotypes are poorly documented and thus the clinical suspicion and even interpretation of molecular findings are challenging. Here, we attempt to review the skeletal manifestations of abnormal GAG synthesis disorders, based on our own experiences and previous reports. Each disorder has distinct clinical and radiological features, but they share some common skeletal manifestations, such as distal humeral hypoplasia, misshapen proximal femora, accelerated carpal ossification, and malsegmentation of the short tubular bones.

Awareness of the phenotypic similarities and differences among this group of disorders facilitates our clinical and genetic practices for affected individuals.