Dysregulation of PRMT5 in chronic lymphocytic leukemia promotes progression with high risk of Richter's transformation

Richter's Transformation (RT) is a poorly understood and fatal progression of chronic lymphocytic leukemia (CLL) manifesting histologically as diffuse large B-cell lymphoma. Protein arginine methyltransferase 5 (PRMT5) is implicated in lymphomagenesis, but its role in CLL or RT progression is unknown. We demonstrate herein that tumors uniformly overexpress PRMT5 in patients with progression to RT. Furthermore, mice with B-specific overexpression of hPRMT5 develop a B-lymphoid expansion with increased risk of death, and Eµ-PRMT5/TCL1 double transgenic mice develop a highly aggressive disease with transformation that histologically resembles RT; where large-scale transcriptional profiling identifies oncogenic pathways mediating PRMT5-driven disease progression. Lastly, we report the development of a SAM-competitive PRMT5 inhibitor, PRT382, with exclusive selectivity and optimal in vitro and in vivo activity compared to available PRMT5 inhibitors. Taken together, the discovery that PRMT5 drives oncogenic pathways promoting RT provides a compelling rationale for clinical investigation of PRMT5 inhibitors such as PRT382 in aggressive CLL/RT cases.

Targeting OXPHOS de novo purine synthesis as the nexus of FLT3 inhibitor-mediated synergistic antileukemic actions

Using a genome-wide CRISPR screen, we identified CDK9, DHODH, and PRMT5 as synthetic lethal partners with gilteritinib treatment in fms-like tyrosine kinase 3 (FLT3)-internal tandem duplication (ITD) acute myeloid leukemia (AML) and genetically and pharmacologically validated their roles in gilteritinib sensitivity. The presence of FLT3-ITD is associated with an increase in anaerobic glycolysis, rendering leukemia cells highly sensitive to inhibition of glycolysis. Supportive of this, our data show the enrichment of single guide RNAs targeting 28 glycolysis-related genes upon gilteritinib treatment, suggesting that switching from glycolysis to oxidative phosphorylation (OXPHOS) may represent a metabolic adaption of AML in gilteritinib resistance. CDK9i/FLT3i, DHODHi/FLT3i, and PRMT5i/FLT3i pairs mechanistically converge on OXPHOS and purine biosynthesis blockade, implying that targeting the metabolic functions of these three genes and/or proteins may represent attractive strategies to sensitize AML to gilteritinib treatment. Our findings provide the basis for maximizing therapeutic impact of FLT3-ITD inhibitors and a rationale for a clinical trial of these novel combinations.

Selinexor Combined with Ibrutinib Demonstrates Tolerability and Safety in Advanced B-Cell Malignancies: A Phase I Study

PURPOSE

Dual blockade of Bruton's tyrosine kinase with ibrutinib and selinexor has potential to deepen responses for patients with chronic lymphocytic leukemia (CLL) and non-Hodgkin lymphoma (NHL).

PATIENTS AND METHODS

In this phase I study (clinicaltrials.gov: NCT02303392), adult patients with CLL/NHL, relapsed/refractory to ≥1 prior therapy were enrolled. Patients received weekly oral selinexor and daily oral ibrutinib in 28-day cycles until progression or intolerance. Primary objective was to determine MTD.

RESULTS

Included patients had CLL (n = 16) or NHL (n = 18; 9 Richter transformation, 6 diffuse large B-cell lymphoma, and 3 mantle cell lymphoma). Median prior therapies were 4 (range = 1-14) and 59% previously received ibrutinib. The established MTD was 40 mg of selinexor (days 1, 8, 15) and 420 mg daily ibrutinib. Common nonhematologic adverse events were fatigue (56%), nausea (53%), anorexia (41%), and diarrhea (41%) and were mostly low grade. Overall response rate was 32%. An additional 47% achieved stable disease (SD), some prolonged (up to 36 months). Median progression-free survival for patients with CLL and NHL was 8.9 [95% confidence interval (CI), 3.9-16.1] and 2.7 (95% CI, 0.7-5.4) months, respectively. For patients with CLL who did not receive prior ibrutinib, only 20% (1/5) progressed. Estimated 2-year overall survival was 73.7% (95% CI, 44.1-89.2) and 27.8% (95% CI, 10.1-48.9) for patients with CLL and NHL, respectively.

CONCLUSIONS

The selinexor and ibrutinib combination has demonstrated tolerability in patients with relapsed/refractory CLL/NHL. Responses were durable. Notable responses were seen in patients with CLL with minimal prior therapy. Future study of this combination will focus on efforts to deepen remissions in patients with CLL receiving ibrutinib therapy.

Abstract 2260: Evaluating a rare t(X;14)(q28;q32) translocation reveals MTCP1 as a driving factor in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is the most prevalent adult leukemia in Western countries and is spelled by substantial genetic and clinical heterogeneity. During CLL transformation, loss or gain of genetic material appears to be a key determinant of disease phenotype and clinical outcome, with major chromosome aberrations observed in up to 80% of patients. Alternatively, balanced translocations, specifically those resulting in constitutive over-expression of various proto-oncogenes under the immunoglobulin heavy chain locus (IGH; 14q32), occur far less frequently. Despite their infrequence, molecular profiling of these rare rearrangements have revealed broad importance of un-recognized genes critical to the pathogenesis of CLL. Employing this strategy, we identified a young CLL patient with a previously undescribed t(X;14)(q28;q32) translocation, co-localization of the mature T cell proliferation 1 (MTCP1; Xq28) coding region with the IGH locus, triggering overexpression of MTCP1 in the CLL cells. Translocations involving MTCP1 are a driving factor in T-prolymphocytic leukemia; however, a role for MTCP1 in CLL has not been described. Inspired by this observation, we screened >1700 suspected CLL cases and evaluated gene expression data for further evidence of MCTP1 aberrations. This query identified seven additional Xq28 rearrangements, revealed MTCP1 mRNA was globally over-expressed in CLL cells compared to normal B-cells, and increased MTCP1 mRNA expression portends a poor response to chemoimmunotherapy. To establish a role for MTCP1 as an oncogene in B cell malignancies, we generated a mouse model with B cell-specific MTCP1 overexpression (Eµ-MTCP1). Longitudinal evaluation revealed a majority of Eµ-MTCP1 mice developed a lethal hematologic malignancy between 5-12 months of age, highlighted by the progressive emergence of clonally related CLL-like B lymphocytes (CD19+/CD5+ B cells) in the blood and accumulating in the spleen and lymph nodes. To support the use of the newly generated Eµ-MTCP1 mouse as a tool for pre-clinical evaluation of CLL therapeutics, we demonstrate that continuous ibrutinib administration in Eµ-MTCP1 mice was sufficient to delay the onset of the CLL-like disease and significantly prolonged survival. In summary, we report Xq28 translocations as rare genetic abnormalities in CLL, yet being one mechanism by which CLL cells amplify expression of MTCP1 compared to normal B cell subsets. Further, the Eµ-MTCP1 mouse model should be considered as an alternative tool for both biologic assessment of co-expressed genes and pre-clinical evaluation of novel CLL therapeutics. Lastly, relevant to all cancer types, successful application of a strategy pursuing the functional consequence of genes involved in rare translocations contributed to the understanding of this disease and identified a novel target for future therapeutic consideration.

Citation Format: Janek S. Walker, Zachary A. Hing, Steven Sher, James Cronin, Katie Williams, Bonnie Harrington, Jordan N. Skinner, Casey B. Cempre, Charles T. Gregory, Max Yano, Larry P. Beaver, Brandi R. Walker, Jadwiga M. Labanowska, Nyla A. Heerema, Krzysztof Mrozek, Jennifer A. Woyach, Amy S. Ruppert, Amy Lehman, Hatice Gulcin Ozer, Vincenzo Coppola, John C. Byrd, James S. Blachly, Rosa Lapalombella. Evaluating a rare t(X;14)(q28;q32) translocation reveals MTCP1 as a driving factor in chronic lymphocytic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2260.

Recurrent XPO1 mutations alter pathogenesis of chronic lymphocytic leukemia

BACKGROUND

Exportin 1 (XPO1/CRM1) is a key mediator of nuclear export with relevance to multiple cancers, including chronic lymphocytic leukemia (CLL). Whole exome sequencing has identified hot-spot somatic XPO1 point mutations which we found to disrupt highly conserved biophysical interactions in the NES-binding groove, conferring novel cargo-binding abilities and forcing cellular mis-localization of critical regulators. However, the pathogenic role played by change-in-function XPO1 mutations in CLL is not fully understood.

METHODS

We performed a large, multi-center retrospective analysis of CLL cases (N = 1286) to correlate nonsynonymous mutations in XPO1 (predominantly E571K or E571G; n = 72) with genetic and epigenetic features contributing to the overall outcomes in these patients. We then established a mouse model with over-expression of wildtype (wt) or mutant (E571K or E571G) XPO1 restricted to the B cell compartment (Eµ-XPO1). Eµ-XPO1 mice were then crossed with the Eµ-TCL1 CLL mouse model. Lastly, we determined crystal structures of XPO1 (wt or E571K) bound to several selective inhibitors of nuclear export (SINE) molecules (KPT-185, KPT-330/Selinexor, and KPT-8602/Eltanexor).

RESULTS

We report that nonsynonymous mutations in XPO1 associate with high risk genetic and epigenetic features and accelerated CLL progression. Using the newly-generated Eµ-XPO1 mouse model, we found that constitutive B-cell over-expression of wt or mutant XPO1 could affect development of a CLL-like disease in aged mice. Furthermore, concurrent B-cell expression of XPO1 with E571K or E571G mutations and TCL1 accelerated the rate of leukemogenesis relative to that of Eµ-TCL1 mice. Lastly, crystal structures of E571 or E571K-XPO1 bound to SINEs, including Selinexor, are highly similar, suggesting that the activity of this class of compounds will not be affected by XPO1 mutations at E571 in patients with CLL.

CONCLUSIONS

These findings indicate that mutations in XPO1 at E571 can drive leukemogenesis by priming the pre-neoplastic lymphocytes for acquisition of additional genetic and epigenetic abnormalities that collectively result in neoplastic transformation.