Defects in mitochondrial DNA replication and human disease

Abstract

Human mitochondrial DNA is replicated and repaired by DNA polymerase gamma, composed of a 140kDa catalytic subunit and a 55kDa accessory subunit. The catalytic subunit contains DNA polymerase activity, 3′–5′ exonuclease activity, and a 5′dRP lyase activity while the accessory subunit is critical for highly processive DNA synthesis and imparts tighter DNA binding to the complex. Mutations in the gene for the catalytic subunit, POLG, are linked to several mitochondrial disorders including progressive external ophthalmoplegia (PEO), sensory and ataxic neuropathy, and Alpers syndrome, a fatal early childhood disease causing brain and liver failure. The carrier frequency of POLG disease mutations is ~2% in the population and is a major cause of mitochondrial disease. Over 150 disease mutations have been identified in POLG (see Human DNA Polymerase Gamma Mutation Database (http://tools.niehs.nih.gov/polg/). Using biochemical analysis, yeast genetics and mouse models, we are determining the consequences of disease mutations in the POLG protein as well as disease mutations in the accessory subunit, POLG2, and the Twinkle helicase, PEO1. These results will be discussed in terms of their role in the mechanism leading to defects in mtDNA replication.