Rational design of anti-cancer therapeutics harnessing the synthetic lethality of methionine metabolism and arginine methyltransferases

Proposal Abstract
Methionine adenosyltransferase 2 alpha (MAT2A) and protein arginine methyltransferase 5 (PRMT5) are
cancer targets that are synthetically lethal with MTAP deletions and have several drug candidates in clinical
trials targeting MTAP-/- cancers. MTAP is deleted in ~15% of human cancers and encodes the metabolic
enzyme 5’-methylthioadenosine phosphorylase, the sole enzyme in humans responsible for recycling of
methylthioadenosine (MTA) to methionine. MAT2A synthesizes S-adenosyl methionine (SAM), the methyl
donor substrate for methyltransferase reactions. PRMT5 utilizes SAM as a substrate and is inhibited by MTA,
and MTAP-/- cells in culture demonstrate elevated MTA levels. In vivo observations of glioblastoma tumors
suggest however, that MTAP-/- does not always lead to increased tumoral MTA levels due to MTA efflux into
matrix MTAP-competent cells. Additionally, MTAP deletions are a rare (~2%) occurrence in colorectal cancers
(CRCs), precluding MAT2A and PRMT5 inhibitors’ use for most CRCs. The Schramm laboratory has
previously solved the transition state (TS) structure of MTAP and synthesized a potent small molecule inhibitor
methylthio-DADMe-immucillin-A (MTDIA) that recapitulates the in vitro effects of MTA accumulation within
tissues. MTDIA has been shown to inhibit tumor growth in several cancer models, including CRC, and is linked
to a decrease in PRMT5 activity through elevation of MTA levels. We propose that MTDIA be used in
combination with MAT2A inhibitor AG-270, currently in Phase I clinical trials, to harness their synthetic lethality
by targeting PRMT5. We will test the safety, target engagement, and anti-cancer efficacy of MTDIA in
combination with AG-270 in ApcMin/+ and CRC patient-derived xenograft (PDX) mice. To determine
mechanisms of anti-cancer effects, we will probe the upstream and downstream effects related to PRMT5
activity. We will perform tumor metabolomic quantification of relevant metabolites and histone and protein-
arginine methylation characterization using immunohistochemistry and proteomic techniques. We will also
profile the gene expression changes using single-cell RNA sequencing to determine how combination therapy
alters tumor architecture and growth. Finally, we will solve the transition state structure of PRMT5 with the goal
of laying the foundations for development of novel transition state analogue inhibitors. This work will expand
upon the use of MAT2A and PRMT5 inhibitors beyond the ~15% of MTAP-deleted cancers and provide
avenues for MTDIA to be used in clinical trials.

Grant Number: 5F30CA275213-04
NIH Institute/Center: NIH
Principal Investigator: Gabriel Bedard