Dual role for mammalian DNA polymerase ζ in maintaining genome stability and proliferative responses

Abstract

DNA polymerase ζ (polζ) is critical for bypass of DNA damage and the associated mutagenesis, but also has unique functions in mammals. It is required for embryonic development and for viability of hematopoietic cells, but, paradoxically, skin epithelia appear to survive polζ deletion. We wished to determine whether polζ functions in a tissue-specific manner and how polζ status influences skin tumorigenesis. Mice were produced in which Rev3L (the catalytic subunit of polζ) was deleted in tissues expressing keratin 5. Efficient epidermal deletion of Rev3L was tolerated but led to skin and hair abnormalities, accompanied by evidence of DNA breaks. Unchallenged mice developed tumors in keratin 5-expressing tissues with age, consistent with the chromosomal instability accompanying a polζ defect. Unexpectedly, mice with the Rev3L deletion were much more sensitive to UVB radiation than mice defective in other DNA repair genes. Following irradiation, polζ-defective mice failed to mount skin-regenerative responses and responded to stress by mobilizing melanocytes to the epidermis. However, they did not develop skin tumors after chronic UVB irradiation. To determine the proliferative potential of polζ-deficient skin epithelia, keratinocytes were isolated and examined. These keratinocytes harbored chromosomal gaps and breaks and exhibited a striking proliferation defect. These results can be unified by a model in which slowly dividing cells accumulate replication-associated DNA breaks but otherwise survive Rev3L deletion, but functional polζ is essential for responses requiring rapid proliferation, both in cell culture and in vivo. The results reveal a biological role for mammalian polζ in tolerating DNA damage and enabling proliferative responses in vivo.DNA replication | double-strand breaks | UV radiation | carcinogenesis F ast, efficient genomic duplication requires that DNA be completely intact and in the B form, because replicative DNA polymerases cannot synthesize using a damaged DNA template (1, 2). When such a template is encountered, replication halts, and either a double-strand break (DSB) forms after replication fork collapse or the lesion is bypassed by translesion synthesis (TLS) polymerases or by template switching. After such damage tolerance, the DNA can be repaired. The relative importance of each pathway for maintaining genomic stability and preventing carcinogenesis is unknown.TLS is mediated by specialized DNA polymerases (reviewed in ref.3). DNA polymerase ζ (polζ, catalytic subunit REV3L) stands out as the most important DNA polymerase for bypass of lesions in template DNA. Polζ plays a major role in the bypass of many types of DNA damage, including pyrimidine(6-4)pyrimidone photoproducts induced by UV radiation (4, 5) as well as lesions formed by chemical damaging agents such as cisplatin and benzo[a]pyrene (4). Polζ also can be used for bypass of the frequent endogenously formed abasic sites in DNA (4). In the yeast Saccharomyces cerevisiae, spontaneous and DNA damage-induced mutagenesis is highly ...

A Missense Mutation in Rev7 Disrupts Formation of Polζ, Impairing Mouse Development and Repair of Genotoxic Agent-induced DNA Lesions

“…Rather than the proposed defect in germ cell maintenance (34), our results with Rev7 C70R mice indicate a defect in proliferation secondary to absent TLS activity. Consistent with our results, conditional targeting of Rev3 in skin epithelia (60) does not affect survival but is essential for responses requiring rapid proliferation of epithelial cells. Also notable is that the present findings indicated decreased growth ratio of the Rev7 C70R MEFs and increased apoptosis in Rev7 C70R intestinal epithelial cells, which are among the most rapidly proliferating cells.…”

DNA polymerase ζ limits chromosomal damage and promotes cell survival following aflatoxin exposure

“…Conditional deletion of Rev3L in hematopoietic, but not epithelial, tissues resulted in thymic lymphomas in a Tp53 −/− background, whereas mammary tumors containing the conditional Rev3L deletion developed independently of p53 status (23). Furthermore, mice that harbor selective deletion of Rev3L from cells expressing keratin 5 showed spontaneous epithelial tumors and were highly sensitive to UVB exposures (24). Recently, it has been demonstrated that the catalytic function of pol ζ is required for cell and embryonic viability and that deletion of pol η could not rescue the pol ζ deficiency (25,26).…”

Breakthrough for a DNA break-preventer

“…If such mice are not challenged with UV radiation but simply left to age, they developed tumors in Rev3L-deleting epithelial tissues, especially in specialized sebaceous glands. This tumor formation is consistent with the chromosomal instability that accompanies the Pol ζ defect (9,10). At the same time, Pol ζ is a potentially important target in cancer therapy, because in mouse xenograft models suppression of REV3L function can sensitize intrinsically resistant tumors to chemotherapy and reduce the frequency of acquired drug resistance of relapsed tumors (11).…”

“…In fact, loss of Pol ζ function is selectively favored for tumorigenesis in this setting. Specific deletion of Rev3L from epithelial tissues leads to very high skin sensitivity to UV radiation (10). If such mice are not challenged with UV radiation but simply left to age, they developed tumors in Rev3L-deleting epithelial tissues, especially in specialized sebaceous glands.…”

“…This might be related to POLD3's role as a subunit of Pol δ, but now the potential role of Pol ζ in preventing break formation under stress conditions should be considered. In fact, it is known that without Pol ζ, DNA double-strand breaks form in normally replicating cells, with ensuing chromosomal rearrangements (10,(17)(18)(19).…”