XYLT1 Mutations in Desbuquois Dysplasia Type 2

Desbuquois dysplasia (DBQD) is a severe condition characterized by short stature, joint laxity, and advanced carpal ossification. Based on the presence of additional hand anomalies, we have previously distinguished DBQD type 1 and identified CANT1 (calcium activated nucleotidase 1) mutations as responsible for DBQD type 1. We report here the identification of five distinct homozygous xylosyltransferase 1 (XYLT1) mutations in seven DBQD type 2 subjects from six consanguineous families. Among the five mutations, four were expected to result in loss of function and a drastic reduction of XYLT1 cDNA level was demonstrated in two cultured individual fibroblasts. Because xylosyltransferase 1 (XT-I) catalyzes the very first step in proteoglycan (PG) biosynthesis, we further demonstrated in the two individual fibroblasts a significant reduction of cellular PG content. Our findings of XYLT1 mutations in DBQD type 2 further support a common physiological basis involving PG synthesis in the multiple dislocation group of disorders. This observation sheds light on the key role of the XT-I during the ossification process.     

Identification of a Recognizable Progressive Skeletal Dysplasia Caused by RSPRY1 Mutations

Skeletal dysplasias are highly variable Mendelian phenotypes. Molecular diagnosis of skeletal dysplasias is complicated by their extreme clinical and genetic heterogeneity. We describe a clinically recognizable autosomal-recessive disorder in four affected siblings from a consanguineous Saudi family, comprising progressive spondyloepimetaphyseal dysplasia, short stature, facial dysmorphism, short fourth metatarsals, and intellectual disability. Combined autozygome/exome analysis identified a homozygous frameshift mutation in RSPRY1 with resulting nonsense-mediated decay. Using a gene-centric “matchmaking” system, we were able to identify a Peruvian simplex case subject whose phenotype is strikingly similar to the original Saudi family and whose exome sequencing had revealed a likely pathogenic homozygous missense variant in the same gene. RSPRY1 encodes a hypothetical RING and SPRY domain-containing protein of unknown physiological function. However, we detect strong RSPRY1 protein localization in murine embryonic osteoblasts and periosteal cells during primary endochondral ossification, consistent with a role in bone development. This study highlights the role of gene-centric matchmaking tools to establish causal links to genes, especially for rare or previously undescribed clinical entities.Keyword: matchmaking, autozygome, exome, skeletal dysplasia, mucopolysaccharidosis, craniosynostosis     

Cranioectodermal Dysplasia, Sensenbrenner Syndrome, Is a Ciliopathy Caused by Mutations in the IFT122 Gene

Cranioectodermal dysplasia (CED) is a disorder characterized by craniofacial, skeletal, and ectodermal abnormalities. Most cases reported to date are sporadic, but a few familial cases support an autosomal-recessive inheritance pattern. Aiming at the elucidation of the genetic basis of CED, we collected 13 patients with CED symptoms from 12 independent families. In one family with consanguineous parents two siblings were affected, permitting linkage analysis and homozygosity mapping. This revealed a single region of homozygosity with a significant LOD score (3.57) on chromosome 3q21-3q24. By sequencing candidate genes from this interval we found a homozygous missense mutation in the IFT122 (WDR10) gene that cosegregated with the disease. Examination of IFT122 in our patient cohort revealed one additional homozygous missense change in the patient from a second consanguineous family. In addition, we found compound heterozygosity for a donor splice-site change and a missense change in one sporadic patient. All mutations were absent in 340 control chromosomes. Because IFT122 plays an important role in the assembly and maintenance of eukaryotic cilia, we investigated patient fibroblasts and found significantly reduced frequency and length of primary cilia as compared to controls. Furthermore, we transiently knocked down ift122 in zebrafish embryos and observed the typical phenotype found in other models of ciliopathies. Because not all of our patients harbored mutations in IFT122, CED seems to be genetically heterogeneous. Still, by identifying CED as a ciliary disorder, our study suggests that the causative mutations in the unresolved cases most likely affect primary cilia function too.