Novel Mechanism of Resistance to Venetoclax Identified in CLL, Lymphoma

Findings from a new study show a probable driver behind resistance to venetoclax in patients with aggressive lymphoma or high-risk chronic lymphocytic leukemia (CLL).

The researchers, who used whole exome sequencing, methylated DNA immunoprecipitation sequencing, and genome-wide CRISPR/Cas9 screening, found that resistance to the BCL2 inhibitor was driven by methylation and epigenetic silencing of PUMA (p53 upregulated mediator of apoptosis). Their findings were recently published in Blood.

From 10 cell lines of different leukemia and lymphoma entities with resistance to venetoclax, the researchers observed MCL1, BCL2-associated protein (BAX), and PUMA protein levels were repeatedly affected, in 7 of 10, 6 of 10, and 8 of 10 cell lines, respectively.

“While venetoclax is increasingly used for the therapy of hematologic malignancies, the frequency of resistance to venetoclax is also increasing,” explained the researchers. “The underlying mechanisms for venetoclax resistance are not completely understood and strategies to overcome this resistance are needed.”

Notably, the group found that PUMA regulation was specific to venetoclax, not seen in resistance to MCL1 or chemotherapy, and that expression of PUMA and its sensitivity toward venetoclax can be restored by inhibiting methyltransferases. The researchers highlighted that their findings suggest that immunotherapy may be a potential approach to overcome venetoclax resistance, also citing previous research, which showed that CLL resistant to venetoclax and ibrutinib responded successfully to chimeric antigen receptor T cells.

Based on their initial findings, the researchers also studied samples from 6 high-risk patients with CLL treated with venetoclax, noting that BAX plays an important role in apoptosis. Three patients exhibited regular CLL morphology and 2 showed signs of Richter’s transformation. Notably, half of the patients did not show measurable BAX expression, indicating that resistance to venetoclax often comes with loss of BAX in cell lines.

“Since we found downregulation of BAX in venetoclax-resistant CLL patients with Richter’s transformation or more aggressive course of diseases, we introduced a CLL mouse model with B-cell–specific BAX loss,” wrote the researchers. “Intriguingly, our data suggest that BAX-associated apoptosis resistance is a major cause for therapy resistance but not for disease progression or transformation, which stands in contrast to the roles of Tp53 or constitutive active Akt signaling in TCL1 mice. Since we did not find an expansion of B cells in BAX knockout (TCL1 wildtype) mice until the age of 83 weeks, oncogenic stimulation or genomic instability seem to be necessary for development of CLL and other B-cell lymphoma.”

From their research, the group also found that the loss of PUMA led to metabolic reprogramming of lymphoma cells. Both respiration and glucose metabolism were increases in the PUMA-depleted cell lines. According to the researchers, the observed increase in basal and maximal respiration level and oxidative phosphorylation–dependent adenosine triphosphate production was similar between cell lines with PUMA knockout and those with acquired venetoclax resistance.

Reference

Thomalla D, Beckmann L, Grimm C, et al. Deregulation and epigenetic modification of BCL2-family genes cause resistance to venetoclax in hematologic malignancies. Blood. Published online June 15, 2022. doi:10.1182/blood.2021014304