Clonal hematopoiesis, myeloid disorders and BAX-mutated myelopoiesis in patients receiving venetoclax for CLL

Abstract

The BCL2 inhibitor venetoclax has established therapeutic roles in chronic lymphocytic leukemia (CLL) and acute myeloid leukemia. As BCL2 is an important determinant of survival of both myeloid progenitor and B cells, we investigated whether clinical and molecular abnormalities arise in the myeloid compartment during long-term continuous venetoclax treatment for CLL in 89 patients (87 with relapsed/refractory CLL). Over a median follow-up of 75 (range 21-98) months, persistent cytopenias (³ 1 of neutropenia, thrombocytopenia, anemia) lasting ³4 months and unrelated to CLL occurred in 25 patients (28%). Of these patients, 20 (80%) displayed clonal hematopoiesis, including 10 with therapy-related myeloid neoplasms (tMNs). tMNs occurred exclusively in patients previously exposed to fludarabine-alkylator combination therapy with a cumulative 5-year incidence of 10.4% after venetoclax initiation, consistent with rates reported for patients exposed to fludarabine-alkylator combination therapy without venetoclax. To determine whether the altered myelopoiesis reflected acquisition of mutations, we analyzed samples from patients with no or minimal bone marrow CLL burden (n = 41). Mutations in the apoptosis effector BAX were identified in 32% (13/41). In cellular assays, C-terminal BAX mutants abrogated outer mitochondrial membrane localization of BAX and engendered resistance to venetoclax killing. BAX-mutated clonal hematopoiesis occurred independently of prior fludarabine-alkylator combination therapy exposure and was not associated with tMNs. Single cell sequencing revealed clonal co-occurrence of mutations in BAX with DNMT3A or ASXL1. We also observed simultaneous BCL2 mutations within CLL cells and BAX mutations in the myeloid compartment of the same patients, indicating lineage-specific adaptation to venetoclax therapy.

Progress in understanding the mechanisms of resistance to BCL-2 inhibitors

“…These mutations not only lead to a change in the number of α9 helices but also remove the critical terminal amino acids [ 48 , 49 ]. As a result, BAX cannot anchor to mitochondria, thereby blocking venetoclax-induced apoptosis in vivo and in vitro [ 50 , 51 ].…”

Clinical experiences with venetoclax and other pro-apoptotic agents in lymphoid malignancies: lessons from monotherapy and chemotherapy combination

“…Interestingly, mutations of the intrinsic death pathway effector BAX were identified in the myeloid compartment in 32% of evaluable patients with a low burden of bone marrow (BM) CLL. Although these mutations conferred resistance to venetoclax, they were not clearly associated with the emergence of myeloid neoplasms [ 39 ]. These findings are of uncertain relevance to patients receiving time-limited venetoclax, but nevertheless demonstrate that chronic BH3-mimetic therapy can exert on-target survival pressures and clonal selection within the non-malignant myeloid compartment.…”

Targeting Acute Myeloid Leukemia with Venetoclax; Biomarkers for Sensitivity and Rationale for Venetoclax-Based Combination Therapies

“…These BCL-2 mutations are usually acquired when CLL patients relapse. Mutations in BAX, inhibiting its localization to the mitochondrial outer membrane and thereby inhibiting the release of cytochrome C, are also found in CLL and prevent venetoclax-induced apoptosis [ 124 ]. In AML patients who relapse after treatment with venetoclax, mutations in BCL-2 and BAX play a less prominent role in the development of resistance [ 125 ].…”