Phenotypic diversity associated with the MT-TV gene m.1644G>A mutation, a matter of quantity.

We describe four patients from three independent families with the m.1644G>A in the MT-TV gene, previously reported without demonstration of its deleterious impact. Very high mutation proportion co-segregated with cytochrome oxidase defect in single muscle fibers and respiratory defect in cybrids as shown by spectrophotometric assays and polarography. The mutation appeared to have a very steep threshold effect with asymptomatic life up to 70% mutation proportion, progressive encephalopathy above 80% and severe Leigh-like syndrome above 95% mutation. One patient did not fit within that frame but presented with characteristics suggesting the presence of an additional disease.