Peposertib suppresses generation of FLT3-internal tandem duplication formed by contralateral double nicks

Fms-like tyrosine kinase 3-internal tandem duplication (FLT3-ITD) is the most frequent gene mutation in acute myeloid leukemia. The consequences of FLT3-ITD have been analyzed in detail; however, the molecular mechanisms underlying the generation of FLT3-ITD remain to be elucidated. We analyzed FLT3-ITDs in clinical samples using deep sequencing and identified not only oligoclonal ITDs but also rare deletion clones clustered at the palindrome-like sequence at FLT3 exon 14. We hypothesized that FLT3 exon 14 is genetically unstable due to the palindrome-like sequence at the region and that genomic damage at the site initiates FLT3-ITD formation. We used clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 to induce DNA damage for creating artificial FLT3-ITDs in human and mouse cell lines. We found that double nicks on the adjacent contralateral strand most efficiently generate ITDs. The artificial ITDs resembled clinical ITDs in the length distribution and characteristics at the joint. We further compared the inhibitory effects of olaparib and peposertib, specific inhibitors of single-strand break (SSB) and double-strand break (DSB) repair, respectively. Peposertib remarkably reduced ITD formation, but olaparib did not affect the mutation pattern. The findings indicated that nonhomologous end joining has a crucial role in the generation of ITDs. Our data shed light to the new role of peposertib, which potentially suppresses the generation of de novo FLT3-ITDs caused by mis-repair events of the DNA damages in a clinical course.