Collagen 2A1 Isoforms Delineate Stickler/Wagner Syndrome Phenotypes

Abstract

Purpose: : Stickler syndrome is an arthro-ophthalmopathy with phenotypic overlap with Wagner syndrome, which clinically has only ocular vitreo-retinopathy manifestations. The common Type I Stickler syndrome (STL1) is inherited as an autosomal dominant trait, with causal mutations in collagen type II alpha 1 (COL2A1) onchromosome 12q13.11-q13.2. Wagner syndrome maps to chromosome 5q13-q14, and is associated with mutations in CSPG2 encoding for the proteoglycan versican. We ascertained a three-generation Caucasian family variably diagnosed with either syndrome, and screened for sequence variants in the COL2A1 and CSPG2 genes.

Methods: : Genomic DNA samples derived from saliva were collected from all family members (5 affected and 6 unaffected individuals). Complete sequencing of COL2A1 and CSPG2 was performed on two affected individuals. Sequencing primers were designed to cover all exon and intron-exon boundaries. Direct sequencing of remaining family members was conducted if discovered variants followed segregation. Allelic frequencies of variants of interest were determined by genotyping 1100 ethnically matched controls.

Results: : No detectable CSPG2 mutations were noted. A base-pair substitution (c.258C->A) in exon 2 of COL2A1 co-segregated with familial disease status. This known mutation occurs in a highly conserved site that causes a premature stop codon (p.C86X). The mutation was not seen in 1142 ethnically matched control DNA samples.

Conclusions: : Wagner syndrome due to CSPG2 mutations was ruled out in this pedigree. In COL2A1, pre-mRNA undergoes tissue-dependent alternative splicing leading to two COL2A1 isoforms- type IIA including exon 2, and type IIB excluding exon 2. While type IIB is mainly expressed by adult differentiated chondrocytes, type IIA is expressed in embryonic chondroprogenitor cells and in the vitreous. Thus, premature stop codons in COL2A1 exon 2 lead to a STL1 ocular-only phenotype with few or no systemic manifestations. Mutation screening of COL2A1 in autosomal dominant vitreo-retinopathy families is important for accurate clinical diagnosis. In vivo functional modeling using zebrafish is underway.

Keywords: gene screening • genetics • vitreous