Tumor Suppression By Regulation Of Mini-Chromosome Maintenance Expression In Response To Dna Replication Stress

Abstract

DNA replication is an essential process during cell proliferation, when genomic information is completely and precisely duplicated. DNA replication machinery (replisome) is responsible for the generation and maintenance of replication structure (replication fork), where genomic DNA is replicated semi-conservatively. Conditions that can inhibit replisome progression are recognized as DNA replication stress (RS). RS originates from many endogenous sources, thus proliferating cells inevitably experience RS during DNA replication throughout their replicative lifespan. DNA damage response (DDR) is responsible for the detection and removal of DNA lesions associated with aberrant DNA replication. However, some of these DNA damages may escape the detection from DDR and accumulate in proliferating cell lineages, which pose great threat to genome integrity maintenance. Furthermore, genome alterations of oncogenic natures may contribute to malignant transformation and eventual neoplasia. Little is known about how proliferating cells record the overall experience of accumulative RS during their prolonged replicative lifespan to prevent cancerous transformation. Minichromosome Maintenance 2-7 (MCM2-7) factors are essential for replication origin licensing and which also constitute catalytic core of replicative helicase. MCM2-7 proteins also partner with DDR, especially in the presence of RS, to faithfully replicate the genome. One of such actions is through sufficient origin licensing. MCM2-7 proteins are excessively expressed in proliferating cells to sufficiently license origins, including dormant origins, which are only activated in response to RS to rescue the nearby stalled replisomes. In contrast to the idea that cells requires high MCM2-7 expression to ensure proper response to RS, I found that chronic RS actually suppresses MCM2-7 expression. This regulation is mediated by the DDR mechanism, and depends on the central tumor suppressor Trp53 in mice, which is partly accomplished through microRNA mediated gene silencing. Moderate MCM2-7 downregulation does not affect normal DNA replication and cell proliferation, however reduced MCM expression sensitized cells to additional RS by inducing terminal cell cycle arrest such as senescence to prevent cellular transformation. Thus, MCM2-7 expression level serves as a molecular memory of accumulative RS experienced by the proliferating cells, when normal high expression level ensures sufficient RS tolerance during their replicative lifespan.