12S-rRNA and COX1 carriers require comprehensive individual and family investigations and follow-up

In a recent article, Ding et al. reported about the diagnostic-work up of non-syndromic hearing loss in a Han Chinese family revealing two different mtDNA mutations [1]. The two variants were made responsible for the phenotype but there are some concerns with regard to the reliability of the presented data. The main shortcoming of the study is that no heteroplasmy rates of the m.1494C>T and m.7444G>A variant were provided. Phenotypic expression of an mtDNA variant may not only depend on the type and location of a mutation, on the penetrance, haplotypes, and polymorphisms, but also on heteroplasmy rates. Thus, we should be informed about heteroplasmy rates in hair follicles, fibroblasts, buccal mucosa cells, lymphocytes, muscle cells, or urine bladder epithelial cells and if heteroplasmy rates differed between these cell types. It should be also provided if heteroplasmy rates correlated with the severity of the phenotype.