The Cooperation that Causes Cancer

This article was originally published on the Meyer Cancer Center website.

The enemy approaches. Scouts pick up the scent and send signals to the relay team. The troops are rallied and an attack is prepared, strategically aligned to the specific weaknesses of the foe.

So it is with the body’s immune system. The scouts are T-cells, which pick up the antigen ‘scent’ of foreign invaders and notify their B-cell brethren to shift forms into plasma cells and produce weapons – antibodies – to attack.

Most of the B-cells respond by rapidly producing plasma cells, but this first line of defense is often weak. Luckily, a select few form into a special forces unit: the germinal center. Germinal centers are balls of furiously dividing B-cells that are mutating in order to tailor their antibody genes to attack offending antigens. The result is a much more powerful immunological army, with high affinity antibodies that can fully get rid of the antigen.

Once the job is done, the special forces unit is disbanded, the rapid proliferation and mutations end, and the germinal center cells are reprogrammed into normal plasma cells.

Ari Melnick, M.D.

Exactly how this shape shifting process happens has been a mystery – until now. A team of researchers at Weill Cornell Medicine, led by Gebroe Family Professor of Hematology and Oncology Ari Melnick, M.D., has uncovered the mechanisms by which B-cells transform, and found that it requires a special cooperation between regulatory protein EZH2 and transcription protein BCL6.

“The transition from a resting mature B-cell to a germinal center B-cell is nothing short of astonishing, as these cells are radically different from each other and this dramatic change requires extensive epigenetic reprogramming, or loading completely different software into the B-cell hardware,” Melnick said.

The discovery is key to understanding how certain blood cancers, such as the fast-growing diffuse large B-cell lymphoma, form and evolve. And it may have an impact on a potential new therapy.

“The basic properties of germinal center B-cells is to proliferate and mutate. Hence, they are prone to becoming lymphomas if something stops them from unloading the germinal center software,” Melnick said.

Scientists have long debated which regulatory system was responsible for B-cell programming.

EZH2 is an enzyme that chemically modifies the regulatory backbone proteins that control our genomes. It drives germinal center formation by suppressing cell-cycle checkpoint genes, repressing genes involved in plasma cell differentiation, and possibly impairing DNA damage responses.

BCL6 is a master regulator protein that attaches to the genome at sites that are crucial for the development of germinal centers.

Both EZH2 and BCL6 are known to repress transcription, the first step of gene expression, in which a particular segment of DNA is copied into RNA (mRNA).

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Wendy Beguelin, Ph.D.

In a study published August 8 in Cancer Cell, Melnick and first author Wendy Beguelin, Ph.D., show that neither alone is responsible – rather, the two form separate but nearby attachment points to the genome to capture a molecular machine called “BCOR complex” which then carries out the actual work of shutting down regions of the genome.

Just as the Air Force requires two officers to simultaneously punch in personal codes and insert and turn individual keys in order to launch dangerous missiles, the immune system seems to require two biochemical keys to shapeshift a friendly B-cell into a freakazoid germinal center B-cell capable of causing lymphomas.

“We show that the ‘missile’ that causes the final effect of controlling B-cell genes is the BCOR complex, which requires the EZH2 and BCL6 key to launch,” Melnick said. Lymphomas occur when these keys get “stuck” due to mutations or other influences.

“Although it may seem a subtle point it is actually quite profound,” Melnick added. “This is the first time it has been shown that such combinatorial repressive mechanisms exist between these kinds of regulatory proteins.”

The Melnick team also discovered the key missing link between EZH2 and BCL6: the CBX8 protein. It acts as a biochemical bridge by attaching to the histone mark placed by EZH2 and the BCOR protein that interacts with BCL6.

“It is the glue that makes the whole thing work,” Melnick said.

To stop out-of-control, lymphoma-generating germinal centers, the Melnick team suggests that disarming the biochemical “keys” could be key.

“If you hit both keys, then the missile silo goes dark and the germinal center B-cells fall apart,” Melnick said. “Either one alone is only partially effective; you have to disable both arms of the PRC1-BCOR tethering mechanism.”

This is possible by combining BCL6 and EZH2 inhibitors. EZH2 inhibitors are in clinical trials for patients with lymphomas at Weill Cornell and elsewhere, and BCL6 drugs have been developed by Dr. Melnick and colleagues and are not yet in human trials. The Melnick team showed that the EZH2-BCL6 drug combination synergistically suppressed human diffuse large B-cell lymphomas in pre-clinical studies.

“Although both EZH2 and BCL6 inhibitors inhibited tumor growth alone, the combination more potently and significantly suppressed lymphoma growth,” Melnick said. “In addition, the combination was not toxic to normal cells.”

The work was supported in part by grants from the National Institutes of Health, the National Cancer Institute, the American Society of Hematology, and the Leukemia and Lymphoma Society.

Dr. Wendy Beguelin Receives Basic Research Fellows Award from the American Society of Hematology

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Dr. Wendy Beguelin

Last week the American Society of Hematology announced recipients for its 2015 awards. Dr. Wendy Beguelin a postdoctoral scientist at WCMC, under the supervision and mentorship of Dr. Ari Melnick received a Basic Research Fellows award for her work in lymphoma epigenetics. Recipients receive $100,000 over a two to three year period to support basic translational and clinical research that advances the hematology field.

In her work Dr. Beguelin identified novel epigenetic and transcriptional mechanisms that contribute to B-cell differentiation and lymphomagenesis. She revealed the role of the Polycomb protein EZH2 in humoral immunity and the mechanism through which somatic gain of function EZH2 mutations occurring in human lymphoma patients reprograms the epigenome to mediate malignant transformation. Her group introduced the novel concept of “bivalent gene targeted therapy”, by showing that mutant EZH2 freezes bivalent domains (genes featuring overlapping repressive and activation histone marks) in a locked silenced configuration that can be reversed to great therapeutic effect using specific EZH2 inhibitors. This led to the identification of biomarkers indicating the subset of human lymphoma patients amenable to EZH2 inhibitor therapy. This new information was used to design the first in vivo clinical trial of the first GSK EZH2 specific inhibitor that is currently underway at our institution.

AACR 2014: How deregulation of histone methyltransferases drive malignant transformation of B-cells

wendybeguelinBy Wendy Béguelin, PhD

DLBCLs are a heterogeneous group of diseases initiating from germinal center (GC) B cells. GC B cells are uniquely specialized to tolerate rapid proliferation, and physiological genomic instability, thus generating a diverse set of clones of cells encoding high affinity antibodies. The GC phenotype poses a significant risk in the malignant transformation to B cells, with epigenetic regulatory complexes playing a critical role in lymphomagenesis. During a symposium session at the recent American Association for Cancer Research, the Melnick Lab, reported how the deregulation of histone methyltransferases causes the malignant transformation of B-cells.

EZH2, which epigenetically silences genes through histone 3 lysine 27 methylation is upregulated in normal and malignant GC B cells. EZH2 is often affected by gain of function mutations in lymphomas that alter its enzymatic specificity. EZH2 mediates GC formation by transiently suppressing checkpoint genes and terminal differentiation genes through formation of bivalent chromatin domains. EZH2 somatic mutations induce germinal center hyperplasia and malignant transformation, and cooperate with other oncogenes such as BCL2. EZH2 specific inhibitors can suppress the growth of GC derived lymphoma cells in vitro and in vivo, and are currently being evaluated in early phase clinical trials. DNA methyltransferase 1 (DNMT1) is required for B cells to form GC, and GC B cells display cytosine methylation redistribution as compared to resting or naïve B cells. DLBCL in turn exhibit prominent and heterogeneous disruption of cytosine methylation distribution, with specific and distinct DNA methylation profiles occurring in different lymphoma subtypes.

Epigenetic heterogeneity is associated with unfavorable outcomes in B-cell lymphoma. This suggests that epigenetic diversity may provide a survival advantage to lymphoma cell populations. DNA methyltransferase inhibitors can reprogram lymphoma cells to develop a form of incomplete senescence that allows for a more complete response to chemotherapy treatment. These DNA methyltransferase inhibitors can be safely combined with standard lymphoma therapies for first line treatment of patients with DLBCL. However, further research will be required to confirm this targeted therapy approach for clinical use in patients.

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