Molecular characterization and biomarker identification in pediatric B-cell acute lymphoblastic leukemia.

6524 Background: B-cell acute lymphoblastic leukemia (B-ALL) is the most prevalent hematologic malignancy in children and a leading cause of mortality. Despite high cure rates, the management of B-ALL remains challenging due to its high heterogeneity and propensity for relapse. This study aimed to delineate molecular features of pediatric B-ALL and explore the clinical utility of circulating tumor DNA (ctDNA) as a biomarker in B-ALL. Methods: The study analyzed 146 primary pediatric B-ALL patients who were diagnosed from August 2020 to April 2023 and received systemic chemotherapy in Wuhan Children's Hospital. Baseline bone marrow (BM) and plasma samples were collected for next-generation sequencing (NGS) to profile the mutational landscape. BM samples collected on day 19 of the treatment were utilized for minimal residual disease (MRD) testing to assess treatment efficacy. Transcriptomic profiling of baseline BM samples was performed via bulk RNA sequencing. Fisher's exact test and the Wilcoxon rank sum test were utilized to compare categorical and continuous variables between groups, respectively. A two‐tailed P-value<0.05 was considered statistically significant unless indicated otherwise. Results: Transcriptomic analysis revealed that 86.3% of patients (126 out of 146) could be categorized into 13 distinct molecular subtypes, with hyperdiploidy emerging as the predominant subtype. Baseline BM NGS identified gene fusions in 61% of patients, including 37 novel fusions previously unreported in B-ALL. Specifically, the KMT2A-TRIM29 novel fusion was detected and validated in a male child diagnosed with the KMT2A-rearranged subtype. Despite initially responding well to therapy, the patient experienced disease progression after a year, indicating a poor prognosis. We also found that elevated mutant counts and maximum-variant-allele-frequency (maxVAF) in baseline BM were associated with significantly poorer response to chemotherapy ( P=0.0012 and 0.028, respectively). Additionally, MRD-negative patients exhibited upregulated expression of immune-related pathways ( P<0.01) and increased CD8+ T cell infiltration ( P=0.047), indicating a more active immune microenvironment in better responding tumors. Baseline plasma ctDNA, which exhibited high mutational concordance with paired baseline BM samples, also demonstrated significant associations with chemotherapy efficacy, highlighting its potential as a non-invasive tool for disease monitoring and treatment outcome prediction. Conclusions: This study elucidated novel gene fusions and potential biomarkers for treatment response in pediatric B-ALL. Importantly, both baseline plasma ctDNA and BM samples offer promising prognostic insights into chemotherapy responses, paving the way for non-invasive monitoring strategies in the management of pediatric B-ALL.