Nonsense mutations in the rhodopsin gene that give rise to mild phenotypes trigger mRNA degradation in human cells by nonsense-mediated decay

Abstract

Eight different nonsense mutations in the human rhodopsin gene cause retinitis pigmentosa (RP), an inherited degenerative disease of the retina that can lead to complete blindness. Although all these nonsense mutations lead to premature termination codons (PTCs) in rhodopsin mRNA, some display dominant inheritance, while others are recessive. Because nonsense-mediated decay (NMD) can degrade mRNAs containing PTCs and modulate the inheritance patterns of genetic diseases, we asked whether any of the nonsense mutations in the rhodopsin gene generated mRNAs that were susceptible to degradation by NMD. We hypothesized that nonsense mutations that caused mild RP phenotypes would trigger NMD, whereas those that did not engage NMD would cause more severe RP phenotypes—presumably due to the toxicity of the truncated protein. To test our hypothesis, we transfected human rhodopsin nonsense mutants into HEK293 and HT1080 human cells and measured transcript levels by qRT-PCR. In both cell lines, rhodopsin mutations Q64X and Q344X, which cause severe phenotypes that are dominantly inherited, yielded the same levels of rhodopsin mRNA as wild type. By contrast, rhodopsin mutations W161X and E249X, which cause recessive RP, showed decreased rhodopsin mRNA levels, consistent with NMD. Rhodopsin mutant Y136X, a dominant mutation that causes late-onset RP with a very mild pathology, also gave lower mRNA levels. Treatment of cells with Wortmannin, an inhibitor of NMD, eliminated the degradation of Y136X, W161X, and E249X rhodopsin mRNAs. These results suggest that NMD modulates the severity of RP in patients with nonsense mutations in the rhodopsin gene.