(P 12) DNA REPLICATION STRESS AND DEFICIENT HOMOLOGOUS RECOMBINATION IN OSTEOSARCOMA RISK AND THERAPY

Objective: For 40 years, osteosarcoma (OST) treatments have not improved children's survival, mainly due to the lack of patient stratification. OST is marked by high genomic instability linked to DNA double-strand breaks (DSBs). Two sources of DSB accumulation were explored: replication stress (RS) and homologous recombination deficiency (HRD). A cohort of 100 patients was used to assess their potential for new stratification strategies and targeted therapies.

Methods: We used complementary approaches to study replication stress (RS) and homologous recombination deficiency (HRD) in osteosarcoma (OST): The Illumina TruSight™ Oncology 500 HRD assay to assess genomic instability scores and detect mutations and amplifications in 523 genes, the Shallow whole genome sequencing (sWGS) to measure large-scale genomic alterations, RNA sequencing to analyze gene expression patterns related to RS and HRD, and Digital pathology on FFPE tissues to evaluate expression and activation of key pathway components, supporting biomarker development.

Results: We identified two distinct subgroups of OST patients based on genomic instability: one showing signs of replication stress (RS), including oncogene amplifications and chromothripsis-like profiles, and another resembling HRD ovarian cancer patients, marked by high genomic instability without chromothripsis. We then developed innovative biomarkers linked to each subgroup’s survival mechanisms: ATR, which controls the replication checkpoint in RS-OST tumors, and DNA polymerase θ (Polθ), a backup DSB repair factor in HRD-OST tumors. These biomarkers were validated by immunohistochemical (IHC) staining in OST samples.

Conclusion: These results define potential novel molecular subtypes of osteosarcoma based on replication stress and DNA repair deficiency signatures. This stratification will open new therapeutic possibilities with already existing drugs, which target specifically compensatory mechanisms that limit excessive DSB and genomic instability: ATR inhibitors for RS OST and Polθ inhibitors for HRD OST.