Homozygosity for Frameshift Mutations in XYLT2 Result in a Spondylo-Ocular Syndrome with Bone Fragility, Cataracts, and Hearing Defects

• Published in: American journal of human genetics Vol. 96; no. 6; pp. 971 - 978
• Main Authors: Munns, Craig F., Fahiminiya, Somayyeh, Poudel, Nabin, Munteanu, Maria Cristina, Majewski, Jacek, Sillence, David O., Metcalf, Jordan P., Biggin, Andrew, Glorieux, Francis, Fassier, François, Rauch, Frank, Hinsdale, Myron E.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 04.06.2015; Cell Press; Elsevier
• Subjects: Base Sequence; Cataract - drug therapy; Cataract - genetics; Cataract - pathology; Craniofacial Abnormalities - drug therapy; Craniofacial Abnormalities - genetics; Craniofacial Abnormalities - pathology; Diphosphonates - therapeutic use; Exome - genetics; Eye Diseases, Hereditary - drug therapy; Eye Diseases, Hereditary - genetics; Eye Diseases, Hereditary - pathology; Frameshift Mutation - genetics; Genetic disorders; Genetics; Genotype & phenotype; Hearing Disorders - genetics; Hearing Disorders - pathology; Homeostasis; Homozygote; Humans; Immunohistochemistry; Male; Molecular Sequence Data; Mutation; Osteochondrodysplasias - drug therapy; Osteochondrodysplasias - genetics; Osteochondrodysplasias - pathology; Osteoporosis - diagnostic imaging; Osteoporosis - genetics; Pamidronate; Pedigree; Pentosyltransferases - blood; Pentosyltransferases - genetics; Radiography; Real-Time Polymerase Chain Reaction; Retinal Detachment - drug therapy; Retinal Detachment - genetics; Retinal Detachment - pathology; RNA-protein interactions; Sequence Analysis, DNA; UDP Xylose-Protein Xylosyltransferase
• ISSN: 0002-9297; 1537-6605
• DOI: 10.1016/j.ajhg.2015.04.017

Heparan and chondroitin/dermatan sulfated proteoglycans have a wide range of roles in cellular and tissue homeostasis including growth factor function, morphogen gradient formation, and co-receptor activity. Proteoglycan assembly initiates with a xylose monosaccharide covalently attached by either xylosyltransferase I or II. Three individuals from two families were found that exhibited similar phenotypes. The index case subjects were two brothers, individuals 1 and 2, who presented with osteoporosis, cataracts, sensorineural hearing loss, and mild learning defects. Whole exome sequence analyses showed that both individuals had a homozygous c.692dup mutation (GenBank: NM_022167.3) in the xylosyltransferase II locus (XYLT2) (MIM: 608125), causing reduced XYLT2 mRNA and low circulating xylosyltransferase (XylT) activity. In an unrelated boy (individual 3) from the second family, we noted low serum XylT activity. Sanger sequencing of XYLT2 in this individual revealed a c.520del mutation in exon 2 that resulted in a frameshift and premature stop codon (p.Ala174Profs∗35). Fibroblasts from individuals 1 and 2 showed a range of defects including reduced XylT activity, GAG incorporation of 35SO4, and heparan sulfate proteoglycan assembly. These studies demonstrate that human XylT2 deficiency results in vertebral compression fractures, sensorineural hearing loss, eye defects, and heart defects, a phenotype that is similar to the autosomal-recessive disorder spondylo-ocular syndrome of unknown cause. This phenotype is different from what has been reported in individuals with other linker enzyme deficiencies. These studies illustrate that the cells of the lens, retina, heart muscle, inner ear, and bone are dependent on XylT2 for proteoglycan assembly in humans.