Synthetic lethalities to cell cycle disruption in glioma

Summary
Despite decades of research into targeted therapeutics against gliomas, the most successful treatments
remain DNA damaging agents: radiation and the alkylating agents temozolomide and lomustine. DNA damage
generates particular obstacles for rapidly dividing cells; as cells undergoing such damage progress through the
cell cycle, they can undergo genotoxic or mitotic catastrophe. Multiple compounds have recently been developed
that interfere with cell cycle regulation, with the aim of generating mitotic catastrophe in cancer cells. These
include compounds targeting regulators of the G2/M checkpoint, including CHK1 and CHK2; WEE1; and others.
Some of these are being applied to gliomas in clinical trials, including a trial of the WEE1 inhibitor AZD1775 in
patients with glioblastoma. However, a detailed understanding of which gliomas are most likely to require a
functional G2/M checkpoint, and under what conditions, is not available. Therefore, despite this pathway being
highly relevant to the most successful existing therapeutics, we do not know when or how to use modulators of
the pathway in patients with glioma. The objective of this proposal is to determine whether and in what instances
inhibitors of the G2/M checkpoint, and particularly CHK1/2, can lead to improved outcomes in gliomas. We
evaluated the effects of 400 biologically active small molecules on 78 glioma cell lines with comprehensive
genomic characterization, including conventional and neurosphere lines. One of the most prominent outcomes
was that inactivation of TP53 was associated with worse response to almost all compounds, but combined loss
of TP53 and CDKN2A/B rendered cells more sensitive to G2/M checkpoint inhibitors, especially inhibitors of
CHK1/2 (CHK1/2i). We hypothesize that combined loss of TP53 and other G1/S cell cycle regulators leads to a
reliance on the CHK1/2-controlled G2/M checkpoint to avoid uncontrolled cell cycling in the context of genotoxic
or replicative stress. By understanding the mechanisms underlying G2/M inhibitor sensitivity, we will have
potential for a major near-term impact on treatment through optimized therapeutic strategies using these
inhibitors, which are already under development, that can lead to immediate incorporation into new clinical trials
strategies. We will achieve this with the following specific aims: Aim 1: Test the hypothesis that combined loss
of TP53 and G1/S checkpoint control generates sensitivity to G2/M checkpoint inhibitors. Aim 2: Test the
hypothesis that cell differentiation state determines sensitivity to G2/M checkpoint inhibition. Aim 3: Test the
hypothesis that MDM2 inhibitors can increase the therapeutic window of CHK1/2i in the context of DNA damaging
agents. In summary, the proposal described should lead to better diagnostics and treatments for those afflicted
by gliomas and offer new avenues for clinical trial design and implementation in patient studies.

Grant Number: 5R01CA262462-04
NIH Institute/Center: NIH
Principal Investigator: RAMEEN BEROUKHIM