A New Target for Poor-Prognosis Non-Hodgkin Lymphoma

As scientists improve their understanding of the signaling pathways that cancer cells depend on to grow and survive, new potential strategies for treatments can emerge. Sometimes an unexpected discovery leads researchers in the direction of a completely new target. Such was the case for researchers in the laboratory of Peter Lucas, MD, PhD, and Linda M. McAllister-Lucas, MD, PhD, at UPMC Children’s Hospital of Pittsburgh, who have identified a potential new treatment target for a subtype of non-Hodgkin lymphoma (NHL) that currently has a poor prognosis.

Survival for most patients with NHL has increased greatly over the last few decades, following the introduction of the targeted therapy rituximab. But for patients with a subtype of NHL called Activated B-cell diffuse large B-cell lymphoma (ABC–DLBCL), three-year progression-free survival has remained at about 40%, compared with 75% for patients with other NHL subtypes.

Most ABC–DLBCL lymphomas are dependent on a series of cellular events known as the NF-κB signaling pathway for growth and survival. Much recent research has focused on a protein that plays a key role in regulating this NF-κB signaling pathway called MALT1.

“We know that MALT1 plays a critically important role in normal lymphocyte function but dysregulation of MALT1 contributes to the pathogenesis of lymphoma,” says Jing Cheng, MD, PhD, a research assistant professor in the Lucas-McAllister Laboratory, who led the new work.

Multiple laboratory studies have shown that blocking MALT1 function can kill or slow the growth of ABC–DLBCL cells. Because MALT1 has also more recently been implicated in driving the growth of other NHL subtypes, blocking MALT1 may kill or slow growth in other lymphomas as well.

“MALT1 is required for the growth and survival of multiple lymphoma subtypes in addition to ABC–DLBCL, including mantle-cell lymphoma and cutaneous T-cell lymphoma,” says Dr. Cheng. “So MALT1 has emerged as a promising pharmaceutical target.”

Dr. McAllister-Lucas and her team have been working for years to tease out the many functions of MALT1 in lymphoma cells. Several years ago, they were contacted by a research group at Michigan State University who studies a family of proteins called the “G protein-coupled receptor kinases”. The Michigan State team had just discovered that one such protein, G protein-coupled receptor kinase 2 (GRK2), can bind to MALT1.

“The Michigan State team were experts in GRK2, but they had no experience in studying MALT1,” says Dr. Cheng. “So, they contacted us.”

That call led to a years-long collaboration that also drew in participants from the University of California, San Francisco, as well as KU Leuven University in Belgium and University of Lausanne in Switzerland. The results from the work, published in February 2020 in the Journal of Clinical Investigation, suggest that manipulating GRK2 may be a new way to shut down MALT1 activity in the lymphomas that depend on it.

In the new research, Dr. Cheng and her colleagues confirmed that GRK2 binds to MALT1 and determined the exact region of MALT1 where this interaction takes place. When bound to MALT1, GRK2 inhibited the NF-κB signaling pathway. The team found that GRK2 binding inhibits two different functions of MALT1. Normally, MALT1 can serve as both a scaffold and a protease. As a scaffold, it physically brings other proteins in the NF-κB pathway together so they can interact. As a protease, it cleaves other proteins that would otherwise interfere with the NF-κB pathway signaling.
The fact that GRK2 can inhibit both MALT1 functions is exciting, says Dr. Cheng. “All the MALT1 inhibitors developed so far specifically target the protease activity of MALT1, and do not impact the scaffold activity,” she says. “If we could develop an inhibitor that targets both these functions of MALT1, it might be more effective in treating MALT1-dependent lymphoma.”

The researchers also examined survival in people with ABC–DLBCL in relation to GRK2 levels in their tumors. They found that both progression-free and overall survival were significantly worse for patients with the lowest GRK2 levels (bottom 25%) compared with those whose tumors had the highest levels of the protein (top 25%). GRK2 levels did not affect survival in patients with non-MALT1-dependent subtypes of lymphoma.

In follow-up studies, knocking down GRK2 in ABC-DLBCL cells led to more aggressive tumor growth in mice implanted with those cells. “We know that ABC–DLBCL requires active MALT1 for its growth and survival,” says Dr. McAllister-Lucas, chief of the Division of Pediatric Hematology/Oncology at UPMC Children’s and a professor of pediatrics at the University of Pittsburgh School of Medicine. “The results of this study identify GRK2 as a binding partner and negative modulator of MALT1.”

“To our knowledge, GRK2 is the first protein that has been shown to inhibit the scaffolding and protease activities of MALT1,” she says. “Our findings could suggest that GRK2 has a tumor suppressor role in ABC–DLBCL.”

Next Steps Toward Treatment

The research team considered the possibility that mutations or deletions in the GRK2 gene may account for the low levels of the protein in a subset of diffuse large B-cell lymphomas. However, they found only one example of such a mutation in six published data sets, comprising more than 300 patients.

“We are now exploring whether other molecular mechanisms could be responsible for downregulation of GRK2 in a subset of diffuse large B-cell lymphoma — for example, epigenetic alterations or microRNA regulation,” says Dr. Cheng.

The Lucas-McAllister Laboratory also is collaborating on structural studies of the GRK2–MALT1 interaction. In addition, they are working with the Small Molecule Therapeutic Center of the University of Pittsburgh, led by Beibei Bill Chen, PhD, to identify small molecules that could potentially mimic GRK2 in its role as a MALT1 inhibitor.

“Developing MALT1 inhibitors is an area of great interest in our field, and combining these types of inhibitors with other anticancer therapies might greatly benefit patients with ABC–DLBCL and other tumor types that depend on this protein,” says Dr. Cheng.

Reference

Cheng J, Klei LR, Hubel NE, Zhang M, Schairer R, Maurer LM, Klei HB, Kang H, Concel VJ, Delekta PC, Dang EV, Mintz MA, Baens M, Cyster JG, Parameswaran N, Thome M, Lucas PC, McAllister-Lucas LM. GRK2 Suppresses Lymphomagenesis by Inhibiting the MALT1 Proto-oncoprotein. J Clin Invest. 2020 Feb 3; 130(2): 1036-1051.