DNA Damage Response-Independent Role for MDC1 in Maintaining Genomic Stability

MDC1 is a central player in checkpoint activation and subsequent DNA repair following DNA damage. Although MDC1 has been studied extensively, many of its known functions, to date, pertain to the DNA damage response (DDR) pathway. Herein we report a novel function of phosphorylated MDC1 that is independent of ATM and DNA damage and is required for proper mitotic progression and maintenance of genomic stability. We demonstrate that MDC1 is an in vivo target of Plk1 and that phosphorylated MDC1 is dynamically localized to nuclear envelopes, centrosomes, kinetochores, and midbodies. Knockdown of MDC1 or abrogation of Plk1 phosphorylation of MDC1 causes a delay of the prometaphase-metaphase transition. It is significant that mice with reduced levels of MDC1 showed an elevated level of spontaneous tumors in aged animals. Our results demonstrate that MDC1 also plays a fundamentally significant role in maintenance of genomic stability through a DDR-independent pathway. KEYWORDS MDC1, genomic instability, mitosis, Plk1, phosphorylation I t has been well established that the DNA damage response (DDR) pathway is essential for the maintenance of genome stability and that dysfunction of the DDR results in tumorigenesis (1). Mediator of DNA damage checkpoint 1 (MDC1) is a large protein that contains both forkhead-associated (FHA) and BRCA1 C-terminal (BRCT) domains, both of which are frequently found in proteins involved in the DDR (2). In light of these features, several groups independently discovered the critical role of MDC1 in the DDR (3-6). Indeed, dysfunction of MDC1 has been reported to cause multiple disorders (6, 7). Genetic variants of MDC1 have been associated with Epstein-Barr virus antibody titers and radiosensitivity, both of which are associated with high risk for human cancers (8, 9). MDC1 / mice recapitulate many phenotypes of H2AX knockout mice, including growth retardation, male infertility, immune defects, chromosome instability, DNA repair defects, and radiation sensitivity (6, 10). Interestingly, mice heterozygous for MDC1 did not show any DNA repair defects or radiation sensitivity (6). Therefore, MDC1 is haplosufficient for the DDR pathway. However, moderately reduced expression of the MDC1 protein was found for lung cancer, breast carcinoma, gastric carcinoma, and glioma (11-13). Furthermore, single nucleotide polymorphisms (SNPs) of MDC1 that affect the MDC1 expression level have been associated with an increased risk of lung cancer (14). All of this emerging evidence is suggestive of other MDC1 functions that are unrelated to DDR signaling. In support of this, MDC1 was shown to interact with anaphase-promoting complex/cyclosome (APC/C), thus regulating mitotic progression Therefore, it is crucial to discern new MDC1 functions in order to fully understand how MDC1 suppresses tumorigenesis and how reduced expression of MDC1 predisposes cells to tumorigenicity.