PLCG2 exon-skipped variants: insights into their potential role in chronic lymphocytic leukemia.

Bruton tyrosine kinase inhibitors (BTKi) are successful in the treatment of chronic lymphocytic leukemia (CLL), yet acquired resistance remains a challenge. In our study, REC-1 cells that acquired resistance to the BTKi tirabrutinib by long-term treatment gained a splice site mutation (SSM) c.2236-1G>T in phospholipase C gamma 2 (PLCG2). Notably, a deletion spanning c.2236-23_2238 at the same splice site was identified in CLL from a patient who had disease progression during ibrutinib therapy. These alterations resulted in the exon-skipped variant Δ21, which exhibited enhanced phospholipase activity. PLCG2 SSM REC-1 exhibited increased anti-immunoglobulin M-mediated calcium flux and resistance to BTK inhibition but retained sensitivity to PLCγ inhibitors. By analyzing additional primary CLL samples, we identified widespread expression of 2 additional exon-skipped variants, Δ22 and Δ20-22, independent of PLCG2 genetic alterations. The active Δ20-22, which is also described in PLCG2-associated immune dysregulation, was predominantly present in CLL compared to healthy donor samples. Expression analysis in CLL cells isolated from patients before and after targeted treatments revealed a decrease in the expression of Δ20-22 after venetoclax treatment (P = .02), whereas no change in expression was observed after ibrutinib treatment, indicating that these variants may persist in the presence of ibrutinib but might be lost in the absence of drug selection pressure. Our study reveals novel PLCG2 splice site alterations and exon-skipped variants beyond the classical BTK or PLCG2 mutations to have a potential role in BTKi resistance and inform treatment selection.