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 97 tumor samples of 29 multiply relapsed ALL patients. Mutational load increased upon relapse in 28 patients and upon every subsequent relapse in 22 patients. In addition to two clock-like mutational processes, we identified UV-like damage, APOBEC activity, reactive oxygen species, thiopurine-associated damage and an unknown therapy component as drivers of mutagenesis. Mutational processes often affected patients over longer time periods, but could also occur in isolated events, suggesting the requirement of additional triggers. Thiopurine exposure was the most prominent source of new mutations in relapse, affecting over half of the studied patients in first and/or later relapse and causing potential relapse-driving mutations in multiple patients. Our data demonstrate that multiple mutational processes frequently act in parallel as prominent secondary drivers with dynamic activity during ALL development and progression.

小儿急性淋巴细胞白血病 （ALL） 的特点是初诊时突变载量低，复发时突变载量增加。为了确定哪些过程在 （复发） ALL 中是活跃的，以及它们在治疗前后疾病进展期间的行为，我们对 29 名多发性复发 ALL 患者的 97 个肿瘤样本进行了全基因组测序。28 例患者复发时突变负荷增加，22 例患者随后每次复发时突变负荷增加。除了两个时钟样突变过程外，我们还确定了 UV 样损伤、APOBEC 活性、活性氧、硫嘌呤相关损伤和未知治疗成分是诱变的驱动因素。突变过程通常会在较长时间内影响患者，但也可能发生在孤立事件中，这表明需要额外的触发因素。硫嘌呤暴露是复发中新突变的最突出来源，影响了超过一半的研究患者在首次和/或晚期复发，并导致多名患者出现潜在的复发驱动突变。我们的数据表明，在 ALL 发展和进展过程中，多个突变过程经常作为动态活动的重要次要驱动因素并行发挥作用。