The protein methyltransferases (PMTs) methylate specific locations on histones using a universal methyl donating co-factor, S-adenosyl methionine (SAM). At least for one disease, MLL rearranged leukemia, a change of function of a PMT enzyme is recognized as a causative element of this malignancy.
“In MLL, recruitment of a specific PMT called DOT1L results in overexpression of leukemogenic genes,” says Margaret Porter Scott, director of biochemistry and molecular pharmacology, Epizyme.
“Finding small molecule inhibitors of this and other PMTs may lead to development of a new class of targeted cancer therapies as personalized therapeutics for genetically defined cancers.”
Epizyme has established an integrated discovery platform for PMT inhibitors. The new molecules from each PMT project are continuously aggregated and subjected to highly parallel pipeline analysis, including inhibition assays in a panel of many disease-relevant PMTs. As a result, Epizyme has now created a large and highly enriched proprietary library of novel PMT inhibitors.
“Different PMTs require particular substrates for maximum activity,” continues Dr. Porter Scott. “Some of them only work on full-length nucleosomes. When we started designing our screening schema, there were no published methods for assaying inhibitor libraries against a full-length nucleosome. We had to develop proprietary methods of PMT screening."
Epizyme uses a battery of substrates ranging from native and recombinant histones to synthetic peptides in which methylation sites are systematically modified. Once the correct substrate is identified, screening moves into a high-throughput format.
Two advanced therapeutic leads generated by this process target DOT1L and EZH2, an enzyme implicated in non-Hodgkin lymphoma and several solid tumors; both are in preclinical development.
“As we learn more about the family of PMT enzymes, we start seeing analogies with the family of protein kinases. Just from one kinase co-factor, ATP, many different kinase inhibitors were discovered and are in clinical use today,” says Dr. Porter Scott.
“Similarly, our strategy is to target the PMTs as a family of SAM-utilizing enzymes, making full use of lessons learned from kinases and exploiting technological platforms that allow parallel processing of multiple enzymes of similar mechanism.
“Using this strategy, we have discovered and are developing small molecules with multiple modes of inhibition including competitive, noncompetitive, and uncompetitive antagonists of PMTs,” concludes Dr. Porter Scott.