Transcriptional Regulation Of Human DNA Polymerase Kappa

It is now widely accepted that cancer results from the accumulation of mutations in the genes that directly control cell birth or cell death. Genetic instability is required for the generation of the multiple mutations that underlie cancer. The mutator phenotype hypothesis of cancer suggests that an early event in tumorigenesis is a mutation and inactivates a gene that normally functions to maintain genome stability, resulting in an elevated mutation rate. This elevated mutation rate in turn results in additional mutations in other genes associated with multistage carcinogenesis, which then confer selective advantages that allow mutated cells to expand and achieve clonal dominance. The efficiency and fidelity of DNA replication or repair can be key to origins of genetic instability. It is logical to consider whether cancer is causally associated with mutations in DNA polymerase genes that reduce efficiency of repair or the fidelity of DNA synthesis. For example, mice carrying a point mutation that inactivates the proofreading function of DNA polymerase 8 develop a recessive cancer phenotype characterized by reduced life span and an increased cancer incidence. This implies that DNA polymerase errors that are not proofread contribute to carcinogenesis. Investigating relationships between cancer and polymerase defects has become a much greater challenge due to the discovery of a large number of DNA polymerases that were unkown only a few years ago.Cellular DNA is continually attacked by a plethora of endogenous and exogenous genotoxins such as reactive oxygen species, UV light and environmental polycyclic aromatic hydrocarbon (PAH). The resulting lesions are usually repaired by one of multiple repair pathways in both prokaryotic and eukaryotic cells. However, because DNA repair processes are not perfect, the persisting lesions can block or slowdown thereplicative machinery. Translesion synthesis (TLS) has evolved to minimize cell death resulting from replication blockage. Recently, a number of previously unrecognized DNA polymerases have been identified to specially carry out TLS, and these enzymes are refered as specialized DNA polymerases, one of which is human DNA polymerse kappa (pol K).Human pol K is a homologue of the E.coli DinB protein, a SOS protein involved in untargeted UV-induced mutagenesis of bacteriophage X. In 1999, Geralch et al firstly isolated human pol K gene was by screening a Hela cell cDNA library, the gene was mapped to chomosome 5q13.1. The deduced 870-amino acid pol K protein contains an N-terminal conserved nucleotidyl transferase domain, 2 tandem helix-hairpin-helix domains implicated in DNA binding, and a nonconserved C terminus containing 2 zinc clusters. Northern blot analysis revealed ubiquitous expression of a 5.0-kb transcript at low but varying levels in all tissues tested. In vitro, pol K can bypass certain DNA lesions including 1 ,N6-ethenodeoxyadenosine or acetylaminofluorene-derived DNA adducts by generating base substitutions and deletions, 8-oxo-7,8-dihydrodeoxy -guanosine by incorporating dAMP opposite the lesion. Pol K is unable to perform TLS opposite either a cisplatin adduct or a TT dimer, although it is a promiscuous extender of mispaired primer termini. In contrast, pol K is able to error-freely bypass the four different stereoisomers of dG-N2-BPDE adducts resulting from B(a)P induced DNA damage. However, in most cases the TLS mediated by pol K result in mutagensis. Upregulation of pol K has been found in lung cancers and implies its role in carcinogenesis. The mouse pol K gene is developmentally regulated in testis and its transcription is partly under control of AhR. The mechanism regulating pol K differential expression between cancer and adjacent normal tissue is poorly understood. To better define the role of pol K in tumor development, we carry out the study on its transcriptional regulation.1. Human pol K is downregulated in lung, gastric and colorectal cancersAlthough previous study revealed elevated pol K expression in a high percentage...