Mutational Landscape and Patterns of Clonal Evolution in Relapsed Pediatric Acute Lymphoblastic Leukemia

Relapse of acute lymphoblastic leukemia (ALL) remains a leading cause of childhood cancer-related death. Prior studies have shown clonal mutations at relapse often arise from relapse-fated subclones that exist at diagnosis. However, the genomic landscape, evolutionary trajectories, and mutational mechanisms driving relapse are incompletely understood. In an analysis of 92 cases of relapsed childhood ALL incorporating multimodal DNA and RNA sequencing, deep digital mutational tracking, and xenografting to formally define clonal structure, we identied 50 significant targets of mutation with distinct patterns of mutational acquisition or enrichment. CREBBP, NOTCH1, and RAS signaling mutations arose from diagnosis subclones, whereas variants in NCOR2, USH2A, and NT5C2 were exclusively observed at relapse. Evolutionary modeling and xenografting demonstrated that relapse-fated clones were minor (50%), major (27%), or multiclonal (18%) at diagnosis. Putative second leukemias, including those with lineage shift, were shown to most commonly represent relapse from an ancestral clone rather than a truly independent second primary leukemia. A subset of leukemias prone to repeated relapse exhibited hypermutation driven by at least three distinct mutational processes, resulting in heightened neoepitope burden and potential vulnerability to immunotherapy. Finally, relapse-driving sequence mutations were detected prior to relapse using droplet digital PCR at levels comparable with orthogonal approaches to monitor levels of measurable residual disease. These results provide a genomic framework to anticipate and circumvent relapse by earlier detection and targeting of relapse-fated clones. SIGNIFICANCE: This study defines the landscape of mutations that preexist and arise after commencement of ALL therapy and shows that relapse may be propagated from ancestral, major, or minor clones at initial diagnosis. A subset of cases exhibit hypermutation that results in expression of neoepitopes that may be substrates for immunotherapeutic intervention.