BAX-Mutated Clonal Hematopoiesis in Patients on Long-Term Venetoclax for Relapsed/Refractory Chronic Lymphocytic Leukemia

Venetoclax (Ven) is an effective element of treatments for chronic lymphocytic leukemia (CLL) with high response rates observed in the upfront and relapsed/refractory (R/R) settings. In addition to inducing apoptosis in CLL cells, Ven also induces apoptosis within normal and malignant myeloid lineage populations (accounting for its efficacy in the treatment of acute myeloid leukemia). We investigated the effects of Ven outside the target tumor compartment in patients (pts) with CLL receiving long-term continuous Ven and make the novel observation of the development of BAX-mutated clonal hematopoiesis in this heavily pre-treated patient group.

92 pts with CLL receiving continuous non time-limited Ven have been treated at our institutions on clinical trials. Of these, 41 had sufficient (>6 mo) follow up (median 70; range 14-95 mo) and suitable samples available for further analysis. 38/41 (93%) pts had received previous treatment with alkylators and/or fludarabine. In order to assess the non-CLL compartment in these 41 pts we identified those with peripheral blood or bone marrow aspirate samples taken during deep response to Ven demonstrating either minimal (<5%) or no CLL involvement by flow cytometry (sensitivity 10^-4). We initially performed unique molecular index (UMI)-based targeted next generation sequencing of apoptosis pathway genes as well a panel of 60 genes recurrently mutated in lymphoid and myeloid malignancy. From these 41 pts we identified mutations in the apoptosis effector BAX in samples from 12 (29%). 20 different BAX mutations were observed across these 12 pts at variant allele frequencies (VAF) consistent with their occurrence in the non-CLL compartment. Mutations included frameshift, nonsense, canonical splice site and missense mutations occurring in key structural elements of BAX consistent with a loss-of-function mechanism (Fig 1A). Interestingly, an enrichment of missense and truncating mutations predicted to escape nonsense mediated decay were observed at the C-terminus of the BAX protein affecting the critical α9 helix. Mutations in this region have previously been shown in cell lines to cause aberrant intracellular BAX localization and abrogation of normal BAX function in apoptosis (Fresquet Blood 2014; Kuwana J Biol Chem 2020).

For comparison, NGS targeted sequencing for BAX mutations was performed on samples from cohorts of pts with (i) myeloid or lymphoid malignancy (n=80) or (ii) R/R CLL treated with BTK inhibitors (n=15) after a similar extent of preceding chemotherapy. Neither of these cohorts had previous exposure to Ven. BAX mutations were not detected in any samples from these pts.

Longitudinal sampling from pts on Ven harboring BAX mutations in the non-CLL compartment was performed to further understand compartment dynamics over time (in 9 pts over 21-93 months of follow up). Multiple pts demonstrated a progressive increase in VAF of single BAX mutations over time to become clonally dominant within the non-CLL compartment and with observed VAFs consistent with their presence in the myeloid compartment. Mutations in other genes implicated in clonal hematopoiesis and myeloid malignancy including ASXL1, DNMT3A, TET2, U2AF1 and ZRSR2 were also detected in these pts samples. Targeted amplicon single cell sequencing (Mission Bio) demonstrated the co-occurrence of clonally progressive BAX mutations within the same clones as mutations in DNMT3A and ASXL1 as well as the existence of further BAX mutations at low VAF outside these dominant clones which remained non-progressive over time (Fig 1B). In addition, fluctuations in the presence and VAF of myeloid-disease associated mutations was noted with Ven exposure. In aggregate these data are consistent with the existence of a selective pressure within the myeloid compartment of these pts and an interplay of BAX with other mutations in determining survival and enrichment of these clones over time with ongoing Ven therapy.

In summary, we have observed the development of BAX-mutated clonal hematopoiesis specifically in pts with CLL treated with long-term Ven. These data are consistent with a multi-lineage pharmacological effect of Ven leading to a survival advantage for clones harboring BAX mutations within the myeloid compartment during chronic Ven exposure. Finally, our data support the further investigation of BAX mutations as a potential resistance mechanism in myeloid malignancies treated with Ven.

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