Alternative, Noninvasive Tissues for Quantitative Screening of Mutant Mitochondrial DNA

Since the molecular basis of the first mitochondrial DNA (mtDNA) disorder, Leber hereditary optic neuropathy (LHON), was established (1), mtDNA mutations have become increasingly more recognized as an important cause of genetic disease. The most common mutations identified are A3243G, A8344G, T8993G/C, G11778A, and large mtDNA deletions. The A3243G mutation accounts for 80% of patients with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) (2), and the A8344G mutation accounts for 80% of patients with MERRF (myoclonic epilepsy and ragged-red fibers) (3)(4). Substitution of T8993 with G or C causes NARP (neuropathy, ataxia, retinitis pigmentosa) (5)(6), and the G11778A mutation is responsible for >50% of patients with LHON (1). MtDNA deletions and rearrangements are found in Kearns–Sayre syndrome and chronic progressive external ophthalmoplegia patients.Most pathogenic mtDNA mutations are heteroplasmic, with normal and mutant DNA coexisting in the same cell or tissue. The phenotypic expression of a pathogenic mutation depends on tissue-specific energy requirements (7). Accordingly, age of onset, tissues involved, and clinical severity vary greatly between individuals, even within a given family, depending on the proportions of mutant and normal mtDNA. Direct mtDNA mutation analysis allows for identification of asymptomatic relatives who harbor the mutant mtDNA. Prediction of future disease expression is assisted by quantitative determination of the proportion of mutant mtDNA in the relevant tissues.DNA-based molecular diagnosis of mtDNA disorders has been routinely performed on blood or biopsied skeletal muscle (8)(9). Because of variation in percentage of mutant mtDNA among different tissues, mutant mtDNA may not be detectable in blood. Nevertheless, it is difficult to justify obtaining muscle biopsies from multiple asymptomatic at-risk family members. In this setting, the examination of alternative tissues such as cheek cells or hair follicles is especially relevant. Studies of MELAS mutant heteroplasmy …