Wild-type and missense mutants of retinoschisin co-assemble resulting in either intracellular retention or incorrect assembly of the functionally active octamer.

The X-linked disease retinoschisis is caused by mutations in the RS1 gene encoding retinoschisin, most commonly missense mutations leading to a lack of secretion of functional protein. One potential approach to treat this disease would be the introduction of the wild-type protein by gene therapy in affected individuals. Retinoschisin normally forms homo-octamers, so co-expression of the wild-type protein with the mutant could result in their co-assembly. In the present study, we show that retinoschisin assembles into an octamer before transport from the endoplasmic reticulum and that co-assembly of wild-type and mutant protein can occur when they are co-expressed in the same cell. This co-assembly results in the retention of some, but not all, expressed wild-type retinoschisin. Moreover, when the wild-type protein is expressed with a missense mutant that is normally secreted, co-assembly occurs resulting in the secretion of a heterogeneous mixture of oligomers. Missense mutations of retinoschisin which cause intracellular retention also lead to an unfolded protein response. However, this is not sufficient to decrease cell viability suggesting that the pathology of the disease is not likely to be linked to programmed cell death.

Identification of a mutation in the ubiquitin-fold modifier 1-specific peptidase 2 gene, UFSP2, in an extended South African family with Beukes hip dysplasia.

BACKGROUND: Beukes hip dysplasia (BHD) is an autosomal dominant disorder of variable penetrance that was originally identified in a large South African family of European origin. BHD is characterised by bilateral dysmorphism of the proximal femur, which results in severe degenerative osteoarthropathy. Previous studies mapped the disorder to a 3.34 Mb region on chromosome 4q35. OBJECTIVE: To fine-map the BHD locus and identify the disease-causing mutation by direct sequencing. RESULTS: The linked BHD allele was refined to 1.33 Mb, reducing the number of candidate genes from 25 to 16. Analysis of protein coding and invariant splice-site sequences in three distantly related individuals identified a single-candidate disease-causing variant c.868T>C within exon 8 of the ubiquitin-fold modifier 1 (Ufm1)-specific peptidase 2 gene, UFSP2. The presence of this unique mutation was confirmed in all 17 affected members of the BHD family who were genotyped. The mutation segregated with the BHD phenotype in the extended family with a two-point (single marker) LOD score of 10.4 (θ=0.0 and 80% penetrance). The mutation predicts the substitution of a highly conserved amino acid, p.Tyr290His, in the encoded protein. In vitro functional assays performed using purified recombinant wild-type and mutant UFSP2 protein demonstrated that the BHD mutation abolishes UFSP2-mediated C-terminal cleavage of its substrate, Ufm1. CONCLUSION: We report a unique UFSP2 mutation that segregates with the BHD phenotype. The predicted amino acid substitution inactivates UFSP2 proteolytic function, thus implicating the ubiquitin-fold modifier 1 cascade in this form of severe hip osteoarthropathy. The facile polymerase chain reaction-based assay we describe could be used to confirm the diagnosis of BHD, or for presymptomatic testing of members of the extended BHD family.

A mutation in the variable repeat region of the aggrecan gene (AGC1) causes a form of spondyloepiphyseal dysplasia associated with severe, premature osteoarthritis.

Spondyloepiphyseal dysplasia (SED) encompasses a heterogeneous group of disorders characterized by shortening of the trunk and limbs. The autosomal dominant SED type Kimberley (SEDK) is associated with premature degenerative arthropathy and has been previously mapped in a multigenerational family to a novel locus on 15q26.1. This locus contains the gene AGC1, which encodes aggrecan, the core protein of the most abundant proteoglycan of cartilage. We screened AGC1 for mutations and identified a single-base-pair insertion, within the variable repeat region of exon 12 in affected individuals from the family with SEDK, that introduces a frameshift of 212 amino acids, including 22 cysteine residues, followed by a premature stop codon. This is the first identification of an AGC1 mutation causing a human disorder. This finding extends the spectrum of mutated genes that may cause SED and thus will aid in the molecular delineation of this complex group of conditions.