Mutational Mechanisms in Multiply Relapsed Pediatric Acute Lymphoblastic Leukemia

Pediatric acute lymphoblastic leukemia (ALL) is marked by low mutational load at initial diagnosis, which increases at relapse. To determine which processes are active in (relapsed) ALL and how they behave during disease progression before and after therapy, we performed whole genome sequencing on tumor samples of 29 multiply relapsed ALL patients. Mutational load increased upon relapse in 28 patients and every subsequent relapse in 22 patients, with UV-like damage, APOBEC activity, reactive oxygen species, thiopurine-associated damage and an unknown therapy component driving mutagenesis. Thiopurines were the most prominent source of new mutations in relapse, affecting over half of the studied patients and causing potential relapse-driving mutations in multiple patients. Mutational processes often affected patients over longer time periods, but could also occur in isolated events, suggesting the requirement of additional triggers. Our data pose mutational processes as prominent secondary drivers with an important role in ALL development and progression.EGA study EGAS00001007900