The clinical project is the translational research arm of the ProgramProject. We have ssembled the largest, fully characterized cohort of families (51) harboring mtDNA mutations. Nearly 90% (44 families)carry a A3243G mutation and the fully symptomatic probands manifest the MELAS phenotype. The family members are assigned to three clinical groups (asymptomatic, oligosymptomatic and fully symptomatic) when first evaluated. Longitudinal and cross- sectional studies of these subjects have provided valuable information regarding natural history. The longitudinal study, now in its 10th year, expands our understanding of the clinical phenotypes and the frequency of medical complications. Key biological variables have been identified that predict increasing morbidity in the asymptomatic/oligosymptomatic group and mortality in the fully symptomatic group. Brain ventricular lactate is the most sensitive biomarker of brain cellular metabolism. Urine sediment DNA is the most reliable measure of mtDNA mutation. Neuropsychological studies show visual memory to be the most vulnerable brain domain. Symptomatic MELAS patients are midway through a three-year, randomized DCA/placebo double-blinded study to determine whether chronic cerebral lactic acidosis contributes to brain injury. We propose three Specific Aims. Specific Aim #1 continues our natural history study correlating genotype and phenotype. This aim tests the hypothesis that brain ventricular lactate correlates with clincal phenotype and prognosis. Specific Aim #2 continues our MELAS/DCA clinical trial. This aim tests the hypothesis that chronic cerebral lactic acidosis exacerbates the MELAS phenotype. Specific Aim #3 is a new study assessing early biomarkers of brain dysfunction using PET, fMRI and voxel-based morphometric analysis. This aim tests the hypothesis that brain ventricular lactate correlates with brain cellular dysfunction. We anticipate that these studies will allow us to determine whether some subjects with the A3243G mutation will remain asymptomatic for the duration of their expected life (low risk group) whereas others who reveal minimal early evidence of structural/functional brain disturbances will become symptomatic and are deserving of early prophylactic treatment (high risk group). The ultimate objective of this clinical project is to find a cure for clinical syndromes associated with mtDNA point mutations.