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The majority of patients with MELAS (mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes) have the A3243G point mutation. The much rarer T3271C mutation has been reported predominantly in Japanese subjects. Our objective was to better define the clinical phenotype and mutation load in patients with MELAS and the T3271C mutation in mitochondrial DNA. We present clinical and molecular genetic data in two pedigrees with the T3271C mutation. The age at onset was 8 years in one proband and 14 years in the other. Both patients had migrainelike headache, seizures, and strokelike episodes. Mutation loads were quantified in multiple tissues from the patients and from family members by polymerase chain reaction-restriction fragment length polymorphism analysis. The symptoms in both probands were typical of MELAS, and, contrary to previous reports, onset was early. Hearing loss was less common than in typical MELAS, and ragged red fibers were absent. The proportion of mutant genomes was consistently and markedly greater in DNA from urinary sediment than from blood. In the mother of one proband, mutant genomes were detected only in DNA from hair follicles and cheek mucosa The phenotype of patients with the T3271C mutation might not be as distinct as that of the A3243G mutation, as previously described. Our data also suggest that urine is a better source of DNA than blood for diagnosis and that multiple tissues should be studied in maternal relatives, especially when the mutation cannot be detected in blood.

Testing for common mutations in mitochondrial DNA (mtDNA), including the A3243G MELAS (mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes) mutation, is routinely done in DNA isolated from blood. Since the blood level of the A3243G mutation may be low in probands and even lower in asymptomatic or oligosymptomatic maternal relatives, we assessed the proportion of A3243G mutant genomes in other accessible tissues. We studied five tissues (leukocytes, skin fibroblasts, hair roots, urinary sediment, and cheek mucosa) in 61 individuals from 22 families harboring the A3243G mutation. Although mutational loads varied widely among these tissues, as a rule DNA from urinary sediment had the highest and blood the lowest proportion of mutant genomes; individual hair roots differed markedly from one another; fibroblasts and cheek mucosa tended to have higher mutation loads than blood but lower than urinary sediment. In all individuals in whom the mutation was detectable in blood, it was also detected in other tissues. Conversely, in some individuals the mutation was undetectable in blood but clearly present in other tissues. We conclude that urinary sediment and cheek mucosa are tissues of choice for the diagnosis of mtDNA mutations, as they are easy to obtain and the mutation load is almost always greater than in blood.

Background: The mendelian forms of progressive external ophthalmoplegia (PEO) associated with multiple mitochondrial DNA deletions are clinically heterogeneous disorders transmitted as dominant or recessive traits. Autosomal dominant PEO is caused by mutations in at least 3 genes: adenine nucleotide translocator-1 (ANT1), encoding the muscle-specific adenine nucleotide translocator; chromosome 10 open reading frame 2 (C10orf2), encoding Twinkle helicase; and polymerase gamma (POLG), encoding the alpha subunit of polymerase gamma. Mutations in POLG can also cause autosomal recessive PEO, which is often associated with multisystemic disorders., Objective and Methods: To further investigate the frequency and genotype-phenotype correlations of mutations in the POLG gene, we used single-stranded conformational polymorphism analysis and direct sequencing to screen 30 patients with familial or sporadic PEO and multiple mitochondrial DNA deletions in muscle but without mutations in ANT1 and C10orf2., Results: Four unrelated patients had novel POLG mutations. A woman with PEO and mental retardation had a heterozygous Gly1076Val mutation. Two patients, one with PEO, exercise intolerance, and gastrointestinal dysmotility and the other with PEO, neuropathy, deafness, and hypogonadism, both had a Pro587Leu change. The fourth patient, who was compound heterozygous for Ala889Thr and Arg579Trp mutations, had PEO, gastrointestinal dysmotility, and neuropathy. These mutations were not detected in 120 healthy control alleles., Conclusions: Our results demonstrate that POLG mutations account for a substantial proportion of patients (13%) with PEO and multiple mitochondrial DNA deletions and cause both clinically and genetically heterogeneous disorders.