Acute myeloid leukemia (AML) is one of the deadliest hematological malignancies and typically has a poor prognosis characterized by differentiation blockade, which enables its continual self-renewal and proliferation. Moreover, epigenetic dysregulation is a hallmark characteristic of AML and helps enforce an oncogenic state of differentiation arrest.
Accordingly, epigenetic regulators are of critical importance for sustaining the transcriptional programs that drive AML differentiation arrest and represent more attractive therapeutic targets than their associated transcription factors. Moreover, while chemotherapy aims to eliminate AML cells by directly inducing cell death or cell cycle arrest, differentiation therapy provides an alternative in which self-renewal is depleted by inducing myeloid maturation programs, thereby extinguishing proliferation
The KAT6A (MOZ) translocation MOZ-TIF2 has been long known as a rare driver event in AML, but a function for wild-type KAT6A in leukemia differentiation has not been previously reported. Now, a team led by M. Andres Blanco, BA, PhD, Assistant Professor of Biomedical Sciences at the University of Pennsylvania School of Veterinary Medicine, has identified a new approach to triggering differentiation in AML—one with potential to treat a much wider array of AML patients. Their study was published in the journal Cancer Discovery (2022; https://doi.org/10.1158/2159-8290.CD-20-1459).
Specifically, Blanco and colleagues wanted to understand how cells commit to the differentiation program and what molecular players are involved in the epigenetic regulation of cell differentiation in AML more generally. To identify key epigenetic regulators whose inhibition induces therapeutic differentiation on AML cells, the team designed and performed a positive selection gain-of-differentiation CRISPR/Cas9 system screen. This screen identified the H3K9 acetyltransferase KAT6A/MYST3/MOZ as a novel regulator of myeloid differentiation that drives critical leukemogenic gene-expression programs.
Next, Blanco and his team aimed to identify the steps it took, and the other molecules KAT6A interacted with, to support AML growth by blocking differentiation. Mechanistically, the team demonstrated that KAT6A is the key H3K9 acetyltransferase that cooperates with the acetyl-lysine reader ENL in a “writer-reader” epigenetic transcriptional control module, which in turn cooperates with a network of chromatin factors to induce transcriptional elongation in AML.
Inhibition of KAT6A has strong anti-AML phenotypes in vitro and in vivo. When KAT6A was eliminated from a human AML cell line, the cells grew more slowly. When the researchers manipulated the cells to have lower levels of KAT6A, a marker of differentiation increased, signaling that the enzyme was somehow setting up an obstacle in the way of cellular differentiation.
To see how KAT6A might behave in vivo, KAT6A was first blocked in an AML cell line, which was then transferred to immunodeficient mice. The investigators discovered that mice that received cells with KAT6A knocked out had a slower growing disease and lived longer than mice that received AML cells containing the protein.
Oncology Times caught up with Blanco, senior author of the research article, for additional insights into their novel pathway that activates promoters of AML oncogenes. His lab's focus in on elucidating the mechanisms by which epigenetic information is encoded, interpreted, and propagated in cell identity programs.
Oncology Times: What are some of the major challenges when it comes to treating patients with AML?
Blanco: “There are a lot of challenges, unfortunately. One is that AMLs are very heterogenous. There are many different types of genetic mutations that can drive AML—and only a few are particularly common. So, each patient is different.
“A second major challenge is that many AML patients are of more advanced age—median age at diagnosis is about 65 years. Patients at and above this age can have significant difficulty tolerating the toxicity associated with treatments like chemotherapy.
“A third challenge is one that is a tremendous difficulty for almost all cancers: drug resistance. Cancer is sometimes described as being a ‘Darwinian’ process of natural selection. Random mutations occur all the time, and the rare ones that happen to confer resistance to a given treatment will come to dominate the cell population, making it all the harder to treat.”
Oncology Times: Why are epigenetic regulators of such critical importance? What led to your interest in epigenetic regulation of cell differentiation in AML?
Blanco: “One reason epigenetic regulators are of great importance in AML is because they are powerful regulators of normal tissue gene expression programs—the same programs that become altered in AML. For example, all AMLs are characterized by a differentiation block. To become oncogenic, they must disable normal myeloid cell differentiation programs. A good way for cells to do this is to epigenetically silence the gene expression programs that drive myeloid differentiation. But, unlike genetic mutations, epigenetic alterations are in principle reversible.
“This greatly interests me because I'm very enthusiastic about a therapeutic approach called differentiation therapy. This refers to treating AMLs not by directly killing them with toxic agents, such as chemotherapies, but by reactivating latent normal myeloid differentiation programs, which entail cell cycle exit followed by apoptosis. This powerful approach is curative in one subtype of AML. But many researchers, including myself, would like to figure out how to achieve differentiation therapy in the other AML subtypes. I believe epigenetics may hold the key for differentiation therapy. Normal tissue differentiation programs of AML cells are epigenetically silenced, so it may be possible to epigenetically reactivate them.”
Oncology Times: Would you please elaborate on the significance of the KAT6A-ENL pathway and the potential clinical implications of the study?
Blanco: “Our findings suggest that KAT6A and ENL cooperate to help maintain aberrantly high levels of transcription of many critical AML oncogenes—in particular, those that oppose normal tissue differentiation programs and allow AML cells to keep proliferating. Certainly, many other genes also help keep oncogene expression high in AML cells. But KAT6A and ENL are particularly attractive because the proteins they encode appear, in principle, quite possible to inhibit with small molecules. KAT6A is an enzyme, so it's catalytic domain can be targeted. ENL is a chromatin ‘reader’ that binds to post-translational modification with a small, well-defined, pocket that can also be targeted. If KAT6A and ENL-targeting small molecule inhibitors were to be made, they could be considered for testing in AML clinical trials.”
Oncology Times: In terms of potential future studies, what questions still remain to be answered to create a new class of differentiation therapies?
Blanco: “There are several questions to pursue, but a big one is how to identify and utilize drug combinations to induce complete, ‘terminal,’ cell differentiation. Many inhibitors induce partial differentiation when used as single agents. But that's not good enough. Do I think that KAT6A and ENL inhibitors could induce terminal differentiation in mono-agent therapy? Probably not.
“Many cellular programs are dysregulated in cancer cells; there are very few cases—in any cancer—of an inhibitor that is curative when used alone. But drug combinations are powerful. So, I think a critical area for continued KAT6A-ENL studies would be to identify genes that, when inactivated in combination with KAT6A or ENL inactivation, induce terminal differentiation of AML cells. Studies in these directions could open up a new type of treatment that is not really on the map yet—combination differentiation therapy.”
Dibash Kumar Das is a contributing writer.