Investigating Molecular Mechanisms Underlying Mild Phenotype in Friedreich Ataxia Patients with G130V Missense Mutation

Friedreich’s Ataxia (FRDA) is an incurable neurodegenerative disease caused by mutations in the frataxin (FXN) gene, resulting in decreased expression of the mitochondrial protein FXN. 2-3% of FRDA patients carry a GAA expansion on one FXN allele, and a missense mutation on the other. The mechanism behind the disease?causing features remains elusive. The phenotype associated with patients carrying point mutations cannot be predicted with certainty; these patients can have a mild or severe clinical outcome, creating a unique platform to understand clinical heterogeneity. FXN is important for proper mitochondrial function, and is involved in Fe-S cluster biogenesis, metabolism, and ATP production. Defining how missense mutations influence FXN’s processing and role in energy production and cellular metabolism will help identify pathways that are affected during disease progression, begin to explain the varying phenotypes, and establish a biochemical genotype-phenotype correlation. Of all disease-associated mutations, patients carrying the G130V missense mutation are of most interest because they have less than 5 % of control mature FXN levels but evolve to a milder phenotype with slower disease progression and significantly lower occurrence of cardiomyopathy, scoliosis, and diabetes. In this thesis, I identified impaired protein processing from FXN42-210 to FXN81-210 as the mechanism by which FXN missense mutations result in lower mature FXN81-210 levels in mutation-selective ways by overexpression studies and subcellular fractionation. This was also true for a novel FXN W168R missense mutation associated with severely low FXN levels and phenotype. Multiple features of mitochondrial dysfunction associated with severe phenotype in typical FRDA, and compared them to G130V patients were also assessed in order to understand the molecular mechanisms underlying the milder phenotype. Fibroblasts from G130V patients have increased mitochondrial ferritin immunoreactivity by immunocytochemistry, increased mitochondrial aconitase activity measured by enzymatic conversion of citrate to isocitrate, and increased Krebs cycle metabolic activity measured by LC-MS isotopologue tracer studies, compared to typical FRDA fibroblasts. Overall, fibroblasts from G130V patients appear to have improved mitochondrial function compared to typical FRDA patients, thus providing a rationale linking G130V functional capacity with milder phenotype