Characterization of DNA Damage Response in a Menin-Deficient Model

MEN1, the gene responsible for the cancer predisposition syndrome multiple endocrine neoplasia type I, has been implicated in DNA repair, cell cycle control, and transcriptional regulation. It is unclear to what degree these processes are integrated into a single encompassing function in normal cellular physiology and how deficiency of the MEN1-encoded protein, "menin," contributes to cancer pathogenesis. In this study, we found that loss of Men1 caused hypersensitivity to ionizing radiation and crosslinking agents, and abrogation of the G1/S and infra-S checkpoints following challenge by these mutagens. In the mammalian model, the cyclin-dependent kinase inhibitor, p21, failed to be upregulated in the mutant although upstream checkpoint signaling remained intact. Menin localized to the p21 promoter in a DNA damage-dependent manner. The MLL histone methyltransferase, a positive transcriptional regulator, bound to the same region in the presence of menin but not in Men1 -/- cells. Finally, p53 retained damage-responsive binding to the p21 promoter in the Men1 mutant. These data indicate that menin participates in the checkpoint response in a transcriptional capacity, upregulating the DNA damage-responsive target p21. Characterization of the relationship between cell cycle arrest and hypersensitivity indicated that menin has no separable role within DNA repair. No deletion construct was able to differentially rescue the two phenotypes, suggesting that menin possesses no physically separable role within crosslink repair. Expression of menin's transcriptional checkpoint target p21 within the deficient system rescues hypersensitivity to crosslinking agents in a manner dependent upon p21's ability to interact with PCNA and independent of checkpoint rescue. Upregulation of p21 within the menin-deficient model appears to be critical to both checkpoint execution and regulation of crosslink repair during the G1 phase of the cell cycle, suggesting that menin has no functionally separable role in crosslink repair. Taken together, our data indicate that menin functions within the maintenance of genome integrity as a transcriptional regulator of G1/S and intra-S damage-dependent checkpoints and of G1-specific crosslink repair.