Evaluation of the Role of AID-Induced Mutagenesis in Resistance to B-Cell Receptor Pathway Inhibitors in Chronic Lymphocytic Leukemia

: Chronic lymphocytic leukemia (CLL) is the most common leukemia in Western countries, and B-cell receptor (BCR) pathway inhibitors such as idelalisib and ibrutinib are currently established therapies for CLL. Although effective, these drugs frequently lead to resistance, but the mechanisms are still not fully understood. Activation-induced cytidine deaminase (AID) is a B-cell enzyme essential for antibody diversification. However, it can also introduce off-target mutations, leading to genomic instability. This study investigates whether treatment with BCR pathway inhibitors increases AID activity in CLL and whether this activity contributes to the development of drug resistance. Peripheral blood samples from CLL patients were collected before and after treatment with idelalisib or ibrutinib. Targeted sequencing was used to identify mutations in known AID off-target genes. Concurrently, AID-wild type (AID-WT) and AID-knockout (AID-KO) CLL cell lines were established and subsequently exposed to escalating doses of BCR pathway inhibitors to develop drug-resistant models. In patient samples, treatment with BCR pathway inhibitors was associated with an increase in AID-dependent mutations in off-target genes, including BCL2, MYC, and IRF8. The in vitro models efficiently recapitulated the patients' data, as only AID-WT CLL cells accumulated mutations in the same AID off-target genes after drug exposure. However, no mutations were detected in genes that could mediate drug resistance. We conclude that BCR pathway inhibitors enhance AID mutational activity in CLL, but this does not appear to be directly involved in driving drug resistance. AID-targeted loci may nonetheless serve as biomarkers for monitoring genomic instability during treatment and inform further study.