Thymine DNA Glycosylase (TDG) as a Novel Potential Target for Melanoma Therapy

Starting from the experimental observations of Prof. Bellacosa, Prof. Giordano and Prof. Larue research groups, in the following study we have tried to elucidate the relationship between TDG (Thymine-DNA Glycosylase) and melanoma disease. Originally, we detected variable endogenous levels of TDG protein within the human melanoma cell lines banked in our labs and a correlation between TDG and tumorigenicity was tentatively established, since cell lines with low to undetectable levels of TDG are known to be tumorigenic in xenotransplant experiments. Moreover, TDG knock-out mouse embryos showed a dramatic phenotype that affects neural crest cells -that are precursors of melanocytes- (see Supplementary Material) therefore, according to those scientific evidences above mentioned, we initially hypothesized and eventually demonstrated, that modulation of TDG levels may significantly affect the biology of melanoma. Melanoma is an aggressive neoplasm with increasing incidence that is classified by the National Cancer Institute (NCI) as a recalcitrant cancer, i.e., a cancer with poor prognosis, lacking progress in diagnosis and treatment. In addition to conventional therapy, melanoma treatment is currently based on targeting the BRAF/MEK/ERK signaling pathway and immune checkpoints. As drug resistance remains a major obstacle to treatment success, advanced therapeutic approaches based on novel targets are still urgently needed. We reasoned that the base excision repair enzyme TDG could be such a target for its dual role in safeguarding the genome and the epigenome, by performing the last of the multiple steps in DNA demethylation. Here, we show that TDG knockdown in melanoma cell lines causes cell cycle arrest, senescence and death by mitotic alterations, and impairs xenograft tumor formation. Importantly, untransformed melanocytes are not affected by TDG knockdown, and adult mice with conditional knockout of Tdg are viable. Candidate TDG inhibitors, identified through a high-throughput fluorescence-based screen, reduced viability and clonogenic capacity of melanoma cell lines and increased cellular levels of 5-carboxylcytosine, the last intermediate in DNA demethylation, indicating successful on-target activity. These findings suggest that TDG may provide critical functions specific to cancer cells that make it a highly suitable anti-melanoma drug target. By potentially disrupting both DNA repair and the epigenetic state, targeting TDG may therefore represent a completely new approach to melanoma therapy.