A critical observation in sporadic cancers is that not all individuals are equally likely to develop cancers with a given exposure to an environmental carcinogen. The difference in the timing of cancer disease onset is most likely attributable to genetic variations, such as polymorphisms in the human genome. Molecular epidemiological studies employing statistical analysis of case-control genotyping and biochemical and molecular biological function analyses have now provided convincing evidence for such an association, while in other cases, the results and conclusions are contradictive. We hypothesize that various functional alterations in genes involved in the base excision repair pathway (BER), in the form of polymorphisms, result in differences in genetic susceptibilities to environmental stresses in the human population, which will subsequently leading to individual differences in the timing of disease initiation and progression. A powerful approach to test this hypothesis is to incorporate the human prevalent polymorphisms into the mouse genome and observe if the model organisms will develop cancer due to the genetic alteration and derived molecular events. The BER pathway is responsible for repair DNA base damage. Defects in this pathway could have two distinct impacts on the genome: 1) accumulation of mutagenic abasic sites, which can result in high mutation frequency, and 2) accumulation of DNA damage such as ssDNA nicks or gaps and dsDNA breaks due to improper processing of the intermediates. DNA damage may cause damaged cells to generate signals to arrest the cell cycle progression. Prolonged cell cycle arrest is known to cause tetraploidy and aneuploidy, a hallmark of cancer. In this exploratory grant application, we choose two distinct polymorphisms on APE1 and Pol 2 ?both are two key BER components. Our preliminary data indicate that APE1 P311S is deficient in endonuclease activity, leading to failure to further process abasic sites, and subsequently inducing mutations in the next round of DNA replication. The DNA damage-induced mutator phenotype causes cancer. On the other hand, we found that Pol 2 R137Q significantly reduced BER capacity in terms of decreased polymerase activity as well as PCNA interaction capacity. We predict that, in this case, although the initial steps for the removal of the damaged base is not affected, the resulting gap will not be able to be filled. This will lead to un-ligated gaps with subsequent DNA breaks in the genome that, in turn, will disturb cell division, inducing tetraploidy and aneuploidy. In the current application, we will use the representative APE1 and Pol 2 mutant mouse models to 1) determine if the APE1 P311S or Pol 2 R137Q polymorphism will predispose a high incidence of cancer in knock-in mouse models and 2) elucidate the molecular mechanisms underlying cancer initiation and development, including overall BER capacity, as evaluated by biochemical assays, the status of genome stability, mutation frequency, formation of tetraploidy and aneuploidy, cellular transformation of mutant cells, and the susceptibility of mouse variant cells to DNA damage agents and chemical and radiation carcinogens.
The current application aims to establish a comprehensive relationship among genetic alterations, functional deficiency at molecular and cellular levels, and cancer pathological consequences, using human BER gene polymorphisms as an example and employing a combined approach of molecular biological and mouse genetic analyses. Successful completion of the proposed studies will reveal a vast majority of information of functional mechanisms of individual polymorphisms during transformation from a normal cell to a cancer cell and provide a reference in developing new therapeutic regimens and personalized medicine.
|Dey, Sanjib; Maiti, Amit K; Hegde, Muralidhar L et al. (2012) Increased risk of lung cancer associated with a functionally impaired polymorphic variant of the human DNA glycosylase NEIL2. DNA Repair (Amst) 11:570-8|