Concerted action of two novel tRNA mtDNA point mutations in chronic progressive external ophthalmoplegia

Cornelia Kornblum (1) , Gábor Zsurka , Rudolf J. Wiesner , Rolf Schröder , Wolfram S. Kunz

1 Neurology

Chronic progressive external ophthalmoplegia (CPEO)1 is a common mitochondrial disease phenotype in adults which is due to mitochondrial DNA (mtDNA) point mutations in a subset of patients. Attributing pathogenicity to novel tRNA mtDNA mutations still poses a challenge particularly when several mtDNA sequence variants are present. Here we report on a CPEO patient in whom sequencing of the mitochondrial genome revealed three novel tRNA mtDNA mutations: G5835A, del4315, T1658C in tRNATyr, tRNAIle, and tRNAVal genes. In skeletal muscle, tRNAVal and tRNAIle mutations were homoplasmic, while the tRNATyr mutation was heteroplasmic. To address the pathogenic relevance, we performed two types of functional tests: (i) single skeletal muscle fiber analysis comparing G5835A mutation loads and biochemical phenotypes of corresponding fibers, (ii) Northern blot analyses of mitochondrial tRNATyr, tRNAIle, tRNAVal. We demonstrated that both the G5835A tRNAMTyr and del4315A tRNAIle mutation have serious functional consequences. Single fiber analyses displayed a high threshold of the tRNATyr mutation load for biochemical phenotypic expression on the single cell level indicating a rather mild pathogenic effect. In contrast, skeletal muscle tissue showed a severe decrease in respiratory chain activities, a reduced overall cytochrome c oxidase (COX) staining intensity, and abundant COX-negative fibers. Northern blot analyses showed a dramatic reduction of tRNATyr and tRNAIle levels in muscle with impaired charging of tRNAIle, whereas tRNAVal levels were only slightly decreased with aminoacylation unaffected. Our findings suggest that the heteroplasmic tRNATyr and homoplasmic tRNAIle mutation act together resulting in a concerted effect on the biochemical and histological phenotype. Thus, homoplasmic mutations may influence the functional consequences of pathogenic heteroplasmic mtDNA mutations.

Mots clés: Life Sciences

Cornelia Kornblum, Gábor Zsurka, Rudolf J. Wiesner, Rolf Schröder, Wolfram S. Kunz. Concerted action of two novel tRNA mtDNA point mutations in chronic progressive external ophthalmoplegia. Bioscience Reports, 2008, 28 (2), pp.89-96. ⟨10.1042/BSR20080004⟩. ⟨hal-00479296⟩