Novel Bcl2 mutation leads to venetoclax resistance in progressive CLL patients

On 4 December 2018, a molecular mechanism behind venetoclax resistance in heavily pre-treated chronic lymphocytic leukemia (CLL) patients was identified and published in Cancer Discovery by Piers Blombery from the Peter McCallum Cancer Center, Melbourne, AU, and colleagues.

Venetoclax is a BCL2 inhibitor and is widely used to treat CLL patients. The aim of this study was to investigate the molecular and genetic mechanisms behind secondary venetoclax resistance, which is observed in heavily pre-treated CLL patients. The authors performed an in-depth genetic assessment with targeted-amplicon next-generation sequencing (NGS) of relapsed or refractory (R/R) CLL patients, who were treated with venetoclax on three early phase clinical trials.

Patient population

• N = 67 R/R CLL patients:
  • Patients who had experienced CLL-type progression after achieving an initial response: n = 21
    • Patients with available pre-venetoclax and progression samples for analysis: n = 15/21
  • Patients developing Richter transformation: n = 18
• Median time from venetoclax to CLL-type progression (range) = 36 (6.5−73) months

Key findings

• NGS of the entire Bcl2 locus revealed a single nucleotide variant in patient samples that had or were experiencing CLL-type progression
• The mutation led to a glycine substitution by valine at the 101 amino acid residue (Gly101Val)
• This mutation was undetectable in patients prior to venetoclax treatment
  • The mutation was not detected in the pre-venetoclax stage but only in the progression stages of the same patients
• Gly101Val was first detected at low allele frequency after 19−42 months on venetoclax
• Estimated number of CLL cells carrying the Gly101Val mutation in these patients was 1.4−3%
• In five patients, after venetoclax discontinuation and starting Bruton’s tyrosine kinase (BTK) inhibitor therapy, a good proportion of CLL cells harbouring the mutation persisted for at least 6 months
• The Gly101Val mutation was not identified in the general population, cancer patient population or venetoclax-naïve patients indicating its specificity to these heavily-pre-treated CLL patients
• In vitro grown CLL cells collected from the patients were more resistant to venetoclax killing, when compared to cell collected from the same patients at the pre-venetoclax phase
• Overexpression of a Gly101Val Bcl2 mutant in human B-lineage cell lines rendered the cells 30-fold less sensitive to venetoclax when compared to wild type Bcl2 cells
• The mutant cells however retained the wild-type pro-survival activity

Conclusions

• A novel Bcl2 mutation was discovered in venetoclax pre-treated R/R CLL patients
• This Gly101Val mutation impaired specifically the binding of venetoclax to BCL2 thus conferring cellular resistance to the drug in both patient-extracted leukemia cells and in cell lines in vitro
• The authors suggested that this novel Bcl2  mutation could be used potentially as an early biomarker of disease progression in CLL patients