Discovering effects on genomic instability of PI3K inhibitors. Mus musculus and Homo Sapiens

Activation-induced cytidine deaminase (AID) is a B-cell specific enzyme that targets immunoglobulin (Ig) genes to initiate class switch recombination (CSR) and somatic hypermutation (SHM). Through off-target activity, however, AID has a much broader impact on genomic instability by initiating oncogenic chromosomal translocations and mutations involved in lymphoma development and progression.The phosphatidylinositol 3-kinase (PI3K) delta pathway plays a key role in AID regulation by suppressing its expression in B cells. Novel drugs for leukemia or lymphoma therapy such as idelalisib, duvelisib or ibrutinib inhibit PI3K delta activity directly or indirectly through inhibition of the Bruton tyrosine kinase (BTK), thus possibly affecting AID expression and, consequently, genomic stability in B cells.Here we applied a genome-wide translocation technique we previously developed (High-Throughput Genomic Translocation Sequencing approach, HTGTS) to identify translocation partners from DNA double strand breaks (DSBs) introduced into the c-myc locus (Chiarle et al, Cell 2011) in Mouse and Human B cell treated with PI3K inhibitors , used Phusion High Fidelity DNA polymerase (Thermo-Scientific) to amplify selected regions from template genomic DNA for somatic hypermutation (SHM) analysis. We also performed GRO-Seq experiments in B cells treated with PI3K delta inhibitors to find how transcription influences translocation junction frequency.We show that treatment of primary mouse B cells with idelalisib or duvelisib, and to a lesser extent ibrutinib, enhanced the expression of AID and increased somatic hypermutation (SHM) and chromosomal translocation frequency to the Igh locus and to several AID off-target sites. These effects were both completely abrogated in AID deficient B cells. Consistently, PI3K delta inhibitors enhanced AID expression and translocation frequency to IgH and AID off-target sites in human chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL) cell lines. In summary, we show that PI3K delta or BTK inhibitors increase genomic instability in normal and neoplastic B cells by an AID-dependent mechanism. Since such inhibitors are administered for years to patients, their genotoxic potential should be carefully considered while planning therapeutic protocols.