Error-prone translesion replication of damaged DNA suppresses skin carcinogenesis by controlling inflammatory hyperplasia.

The induction of skin cancer involves both mutagenic and proliferative responses of the epidermis to ultraviolet (UV) light. It is believed that tumor initiation requires the mutagenic replication of damaged DNA by translesion synthesis (TLS) pathways. The mechanistic basis for the induction of proliferation, providing tumor promotion, is poorly understood. Here, we have investigated the role of TLS in the initiation and promotion of skin carcinogenesis, using a sensitive nucleotide excision repair-deficient mouse model that carries a hypomorphic allele of the error-prone TLS gene Rev1. Despite a defect in UV-induced mutagenesis, skin carcinogenesis was accelerated in these mice. This paradoxical phenotype was caused by the induction of inflammatory hyperplasia of the mutant skin that provides strong tumor promotion. The induction of hyperplasia was associated with mild and transient replicational stress of the UV-damaged genome, triggering DNA damage signaling and senescence. The concomitant expression of Interleukin-6 (IL-6) is in agreement with an executive role for IL-6 and possibly other cytokines in the autocrine induction of senescence and the paracrine induction of inflammatory hyperplasia. In conclusion, error-prone TLS suppresses tumor-promoting activities of UV light, thereby controlling skin carcinogenesis.

Mammalian polymerase zeta is essential for post-replication repair of UV-induced DNA lesions.

DNA polymerase zeta is believed to be an essential constituent of DNA damage tolerance, comprising several pathways that allow the replication of DNA templates containing unrepaired damage. We wanted to better define the role of polymerase zeta in DNA damage tolerance in mammalian cells. To this aim we have investigated replication of ultraviolet light-damaged DNA templates in mouse embryonic fibroblasts deficient for Rev3, the catalytic subunit of polymerase zeta. We found that Rev3 is important for a post-replication repair pathway of helix-distorting [6-4]pyrimidine-pyrimidone photoproducts and, to a lesser extent, of cyclobutane pyrimidine dimers. Unlike its partner Rev1, Rev3 appears not to be involved in an immediate translesion synthesis pathway at a stalled replication fork. The deficiency of Rev3(-/-) MEFs in post-replication repair of different photoproducts contributes to the extreme sensitivity of these cells to UV light.

Abrogation of microsatellite-instable tumors using a highly selective suicide gene/prodrug combination.

A substantial fraction of sporadic and inherited colorectal and endometrial cancers in humans is deficient in DNA mismatch repair (MMR). These cancers are characterized by length alterations in ubiquitous simple sequence repeats, a phenotype called microsatellite instability. Here we have exploited this phenotype by developing a novel approach for the highly selective gene therapy of MMR-deficient tumors. To achieve this selectivity, we mutated the VP22FCU1 suicide gene by inserting an out-of-frame microsatellite within its coding region. We show that in a significant fraction of microsatellite-instable (MSI) cells carrying the mutated suicide gene, full-length protein becomes expressed within a few cell doublings, presumably resulting from a reverting frameshift within the inserted microsatellite. Treatment of these cells with the innocuous prodrug 5-fluorocytosine (5-FC) induces strong cytotoxicity and we demonstrate that this owes to multiple bystander effects conferred by the suicide gene/prodrug combination. In a mouse model, MMR-deficient tumors that contained the out-of-frame VP22FCU1 gene displayed strong remission after treatment with 5-FC, without any obvious adverse systemic effects to the mouse. By virtue of its high selectivity and potency, this conditional enzyme/prodrug combination may hold promise for the treatment or prevention of MMR-deficient cancer in humans.