Mechanism of mammalian DNA repair

DNA repair prevents cancer and early aging and is essential for the immune system. Our objective is to obtain a detailed understanding of the mechanism of base excision repair (BER) and direct base damage repair. BER is a template-dependent multistep proc ess initiated by a DNA glycosylase. The UNG-gene encodes uracil-DNA glycosylases UNG1 located in mitochondria and UNG2 in nuclei. UNG-mutations are associated with immune deficiency and lymphoma. UNG2 engages in multiprotein complexes that carry out compl ete BER. We will determine the composition and function of different complexes. They seem to vary both depending on type of damage cell cycle status. We will also continue our studies on the significance of UNG2 for antibody production in the immune syste m. Repair of mtDNA has been little studied, and UNG1 is among the few distinct mitochondrial repair proteins identified. Mitochondria do not have capacity for nucleotide excision repair, and probably not mismatch repair, leaving BER as a major repair mech anism in this organelle. Furthermore, mutation frequencies are several-fold higher in mtDNA than in nuclear DNA. This could be due to less efficient or less accurate repair. We will examine the capacity and accuracy of BER in mitochondria, as compared wit h nuclei. We will also explore the mechanism of BER in mitochondria. Direct DNA-base repair by oxidative dealkylases is a template- independent, one-step mechanism carried out by homologs of the E. coli protein AlkB. Eight mammalian AlkB homologs (ABH1-8) have been identified, but only two significantly explored. At least one homolog also repairs RNA. We will examine the function of the mammalian AlkB homologs in repair of DNA, RNA and possibly methylated proteins, and their biomedical significance using both in vitro systems and mouse models.