Skeletal dysplasia, global developmental delay, and multiple congenital anomalies in a 5-year-old boy-report of the second family with B3GAT3 mutation and expansion of the phenotype.

As a major component of the extracellular matrix, proteoglycans influence the mechanical properties of connective tissue and play an important role in cell-cell and cell-matrix interactions. Genetic defects of proteoglycan biosynthesis lead to multi-system disorders, often most prominently affecting the skeletal system and skin. Specific deficiencies in the enzymes involved in the biosynthesis of the linkage region between the core of the proteoglycan protein and its glycosaminoglycan side chains are known as linkeropathies. We report on a patient from a second family with a homozygous c.830G>A (p.Arg277Gln) mutation in the B3GAT3 gene. The clinical features expand the previously reported phenotype of B3GAT3 mutations and of linkeropathies in general. This patient has short stature, facial dysmorphisms, skeletal findings, joint laxity, and cardiac manifestations similar to those previously associated with B3GAT3 mutations. However, he also has developmental delay, a refractive errors, dental defects, pectus carinatum, and skin abnormalities that have only been associated with linkeropathies caused by mutations in B4GALT6 and B4GALT7. He has bilateral inguinal hernias and atlanto-axial as well as atlanto-occipital instability that have not been previously associated with B3GAT3 mutations. We provide a detailed clinical report and a comparative overview of the phenotypic features of the linkeropathies caused by mutations in B3GAT3, B4GALT6, and B4GALT7.