Identification of ATP-Competitive Human CMG Helicase Inhibitors for Cancer Intervention that Disrupt CMG-Replisome Function.

The human CMG helicase (Cdc45-MCM-GINS) is a novel target for anti-cancer therapy. Tumor-specific weaknesses in the CMG are caused by oncogene-driven changes that adversely affect CMG function, and a requirement for CMG activity during recovery from replicative stresses such as chemotherapy. Here, we developed an orthogonal biochemical screening approach and identified CMG inhibitors (CMGi) that inhibit ATPase and helicase activities in an ATP-competitive manner at low micromolar concentrations. Structure-activity information, in silico docking, and testing with synthetic chemical compounds indicate that CMGi require specific chemical elements and occupy ATP binding sites and channels within MCM subunits leading to the ATP clefts, which are likely used for ATP/ADP ingress or egress. CMGi are therefore also MCM complex inhibitors (MCMi). Biological testing shows that CMGi/MCMi inhibit cell growth and DNA replication using multiple molecular mechanisms distinct from other chemotherapy agents. CMGi/MCMi block helicase assembly steps that require ATP binding/hydrolysis by the MCM complex, specifically MCM ring assembly on DNA and GINS recruitment to DNA-loaded MCM hexamers. During S-phase, inhibition of MCM ATP binding/hydrolysis by CMGi/MCMi causes a 'reverse allosteric' dissociation of Cdc45/GINS from the CMG that destabilizes replisome components Ctf4, Mcm10, and DNA polymerase-a, -d, -e, resulting in DNA damage. CMGi/MCMi display selective toxicity toward multiple solid tumor cell types with K-Ras mutations, targeting the CMG and inducing DNA damage, Parp cleavage, and loss of viability. This new class of CMGi/MCMi provides a basis for small chemical development of CMG helicase-targeted anti-cancer compounds with distinct mechanisms of action.