Reactive oxygen species (ROS) are produced as a by-product of cellular metabolism and through exposure to ultraviolet and ionizing radiation and environmental carcinogens. A major base damage produced by ROS is 7,8-dihydro-8-oxoguanine (8-oxoG). Unlike normal guanine, 8-oxoG has the propensity to mispair with adenine during DNA replication, resulting in the fixation of G:C to T:A transversion mutations. Oxidatively modified bases, such as 8-oxoG, are repaired primarily by the base excision repair pathway (BER), the first steps of which are the recognition and excision of the damaged base by a specific DNA glycosylase. The major mammalian enzyme for removing 8-oxoG from DNA is 8-oxoguanine-DNA glycosylase (OGG1). OGG1 is a bifunctional enzyme, having both 8-oxoG excision activity and a weak AP-lyase strand incision activity at abasic sites. Following excision of 8-oxoG by OGG1, the resultant abasic site is further processed in sequential steps by several enzymes to complete repair. Studies of OGG1 knockout mice and immunodepletion experiments suggest that OGG1 is the major mammalian 8-oxoguanine repair activity in non-transcribed DNA. It is widely accepted that accumulation of oxidative DNA damage over time can lead to cancer. Changes in the OGG1 coding sequence that result in amino acid substitutions that affect function, abundance, or intracellular location could be anticipated to impact genomic 8-oxoG levels, and thereby influence genomic stability and carcinogenesis. Several OGG1 polymorphisms have been reported and positively correlate with a variety of cancers;however, we have begun to explore the role of these polymorphisms in other age-related chronic disease as well. Alzheimer's disease (AD) is the most common form of dementia in adults. Approximately 5% of AD cases are early onset caused by mutations in known genes including APP, PSEN1, and PSEN2. The role of genetics in the remaining cases is unclear, but it is proposed that these cases of AD may arise from accrual of spontaneous mutations. One hypothesis to explain these mutations proposes they are produced as a consequence of oxidative stress. Brain tissues from late-stage AD patients identified three mutations in OGG1. Two of these mutations are single-nucleotide polymorphisms resulting in the amino acid substitutions A53T and A288V. The A288V polymorphism (rs3219012) is relatively rare, occurring in 1% of the general population, whereas the A53T polymorphism is currently only considered to be disease associated. Brain tissues from Alzheimer's disease (AD) patients show increased levels of oxidative DNA damage and 7,8-dihydro-8-oxoguanine (8-oxoG) accumulation. Little was known about how these polymorphisms may contribute to AD. In our work this year, we have examined the A53T and A288V polymorphic OGG1 proteins to further define the biochemical defects that could contribute to the accumulation of DNA damage and 8-oxoG lesions observed in AD patient brain tissues found by others. Our work shows a statistically significant decrease in catalytic activity for both polymorphic OGG1 proteins compared to wild type (WT). The decrease in catalytic activity may be explained by a decreased ability of the A53T protein to bind to DNA substrates and decreased lyase activity for the A288V protein. In addition, we found decreased binding of polymorphic OGG1 proteins to poly (ADP-ribose) polymerase 1 (PARP-1) and X-ray cross-complementing protein 1 (XRCC1) and a decreased ability of the polymorphisms to activate PARP-1. OGG1−/−mouse embryo fibroblasts (MEFs) expressing the polymorphisms had decreased long-term cell survival and were more sensitive to DNA-damaging agents. Together, the results presented in this article show that the A53T and A288V polymorphisms alter the protein function in a manner that may increase susceptibility to disease.
|Jacob, Kimberly D; Noren Hooten, Nicole; Tadokoro, Takashi et al. (2013) Alzheimer's disease-associated polymorphisms in human OGG1 alter catalytic activity and sensitize cells to DNA damage. Free Radic Biol Med 63:115-25|