A molecular test for a treatable disease
Stickler et al. (1965) described a new dominant entity consisting of progressive myopia beginning in the first decade of life and resulting in retinal detachment and blindness. Affected persons also exhibited premature degenerative changes in various joints with abnormal epiphyseal development and slight hypermobility in some.
Freddi et al. (2000) described a novel strategy for screening families with type I Stickler syndrome due to nonsense mutations in the COL2A1 gene, using a modified RNA-based protein truncation test. To overcome the problem of the unavailability of collagen II-producing cartilage cells, they performed RT-PCR on the illegitimate transcripts of accessible cells (lymphoblasts and fibroblasts), which were preincubated with cycloheximide to prevent nonsense mutation-induced mRNA decay. The 5 overlapping RT-PCR fragments covering the COL2A1 coding region were then transcribed and translated in vitro to identify smaller truncated protein products resulting from a premature stop codon. Using this method, Freddi et al. (2000) screened a 4-generation family with Stickler syndrome and identified a protein-truncating mutation that was present in all affected individuals. Targeted sequencing identified the mutation as a transition at the 5-prime splice donor site of intron 25 (120140.0032), resulting in a translational frameshift that introduced a premature stop codon. Mutant mRNA was undetectable without cycloheximide protection, demonstrating that the mutant mRNA was subjected to nonsense-mediated mRNA decay. As well as providing further evidence that type I Stickler syndrome results from premature stop codon mutations of the COL2A1 gene, this study suggested that mutant mRNA instability leading to haploinsufficiency may also be an important but previously unrecognized molecular basis of Stickler syndrome. The authors concluded that this rapid test for COL2A1 nonsense mutations is of particular clinical importance to families with Stickler syndrome, where the identification of individuals who are at risk for this potentially preventable form of blindness will allow them to undergo regular ophthalmologic surveillance and preventive or early ameliorative treatment.