Pathogenic Mitochondrial tRNA Point Mutations: Nine Novel Mutations Affirm Their Importance as a Cause of Mitochondrial Disease

ABSTRACT Mutations in the mitochondrial genome, and in particular the mt‐tRNAs, are an important cause of human disease. Accurate classification of the pathogenicity of novel variants is vital to allow accurate genetic counseling for patients and their families. The use of weighted criteria based on functional studies—outlined in a validated pathogenicity scoring system—is therefore invaluable in determining whether novel or rare mt‐tRNA variants are pathogenic. Here, we describe the identification of nine novel mt‐tRNA variants in nine families, in which the probands presented with a diverse range of clinical phenotypes including mitochondrial encephalomyopathy, lactic acidosis, and stroke‐like episodes, isolated progressive external ophthalmoplegia, epilepsy, deafness and diabetes. Each of the variants identified (m.4289T>C, MT‐TI; m.5541C>T, MT‐TW; m.5690A>G, MT‐TN; m.7451A>T, MT‐TS1; m.7554G>A, MT‐TD; m.8304G>A, MT‐TK; m.12206C>T, MT‐TH; m.12317T>C, MT‐TL2; m.16023G>A, MT‐TP) was present in a different tRNA, with evidence in support of pathogenicity, and where possible, details of mutation transmission documented. Through the application of the pathogenicity scoring system, we have classified six of these variants as “definitely pathogenic” mutations (m.5541C>T, m.5690A>G, m.7451A>T, m.12206C>T, m.12317T>C, and m.16023G>A), whereas the remaining three currently lack sufficient evidence and are therefore classed as ‘possibly pathogenic’ (m.4289T>C, m.7554G>A, and m.8304G>A). We describe novel mitochondrial tRNA point mutations in nine families associated with a diverse range of clinical presentations, providing functional evidence to classify six of these variants as definitely pathogenic, with the remaining three classed as possibly pathogenic. Our report adds to the large number of mitochondrial tRNA variants associated with disease and confirms mitochondrial tRNA mutations as an important subgroup of human genetic disorders.