CMG Helicase Assembly and Activation: Regulation by c-Myc through Chromatin Decondensation and Novel Therapeutic Avenues for Cancer Treatment

The CMG (Cdc45, MCM, GINS) helicase is required for cellular proliferation and functions to unwind double-stranded DNA to allow the replication machinery to duplicate the genome. Cancer cells mismanage helicase activation through a variety of mechanisms, leading to the potential for the development of novel anti-cancer treatments. Mammalian cells load an excess of MCM complexes that act as reserves for new replication origins to be created when replication forks stall due to stress conditions, such as drug treatment. Targeting the helicase through inhibition of the MCM complex has sensitized cancer cells to drugs that inhibit DNA replication, such as aphidicolin and hydroxyurea. However, these drugs are not used in the clinical management of cancer. We hypothesized that the effectiveness of the clinically relevant drugs gemcitabine and 5-FU against pancreatic cancer cells, and oxaliplatin and etoposide against colorectal cells, could be increased through co-suppression of the MCM complex. The oncogene c-Myc also leads to the mismanagement of CMG helicases in part due to a non-transcriptional role in overactivating replication origins and causing DNA damage. We sought to elucidate the mechanism by which Myc causes overactivation of CMG helicases. Herein we demonstrate that co-suppression of reserve MCM complexes in pancreatic or colorectal cancer cell lines treated with clinically applicable chemotherapeutic compounds causes significant loss of proliferative capacity compared with cells containing the full complement of reserve MCMs. This is in part due to an inability to recover DNA replication following drug exposure, leading to an increase in apoptosis. Targeting of Myc to genomic sites induced large-scale decondensation of higher order chromatin that was required for CMG helicase assembly and activation at reserve MCM complexes. The physiological mediators of Myc, GCN5 and Tip60, are required for the chromatin unfolding and Cdc45 recruitment. We conclude that depletion of the reserve MCM complexes causes chemosensitization of multiple human tumor cell types to several chemotherapeutic drugs used in the clinical management of human cancer. This argues for the development and use of anti-MCM drugs in combination with chemotherapeutic compounds, which has the potential to increase the therapeutic index of existing clinical compounds. We have also identified a previously unknown role for Myc in normal cell cycle progression whereby DNA replication initiation is regulated through the assembly and activation of CMG helicases on Myc-mediated open chromatin regions. Our results also provide new mechanistic insight into Myc oncogenic transformation in which overstimulation of DNA replication could result in genomic instability and provide an explanation for Myc driven oncogenic transformation