Abstract 2590: Analysis of homologous recombination deficiency (HRD) scores in osteosarcoma

Abstract Objective: Outcomes for patients with metastatic or relapsed osteosarcoma (OS) remain poor. Many OS samples have demonstrated a high proportion of mutational signature 3 (ms3), suggestive of homologous recombination DNA repair deficiency (HRD) or a BRCAness molecular phenotype. The HRD score, which quantifies chromosomal structural rearrangements by combining loss of heterozygosity (LOH), large state transitions (LST), and telomere allelic imbalances (TAI), has been used to assess HRD status in patients with breast and ovarian cancer, with high scores found to be predictive of PARP inhibitor sensitivity. While high HRD scores have been described in OS, little is known about how HRD scores vary by stage, site, pre vs post-therapy, presence of homologous recombination pathway or TP53 alterations, and their correlation to ms3 contribution. Methods: To address this issue, we evaluated 48 OS samples with tumor purity ≥ 20% from 46 patients who had 80x whole genome sequencing (WGS) performed through our pediatric expanded genomics platform. HRD scores were then estimated using scarHRD, a computational tool, which quantifies LOH, LST, and TAI events from WGS data and has shown a good correlation to SNP-array based methods. Results: Of the 48 samples analyzed, 25 samples were from primary sites and 23 from metastatic sites of disease. The median HRD score was 45 with a minimum score of 6 and a maximum score of 79. The 25th, 75th, and 90th percentiles of values were 35.7, 57, and 66.3 respectively. There was a significant difference in HRD score in patients with localized disease (n=22) vs metastatic disease (n=26) at time of diagnosis with median HRD (mHRD) scores of 41 and 51.5 (p = 0.027). The presence of a TP53 structural variant or single nucleotide variant (mHRD=47) was associated with higher HRD scores than TP53 wild-type samples (mHRD=29.5, p=0.04). There was no statistically significant difference in HRD scores in pre vs post-treated samples or primary site vs metastatic site samples. Samples with homologous recombination pathway alterations (n=10) were not associated with increased HRD score. There was a positive correlation (Spearman’s rho=0.425, p=0.001) between the number of mutations attributed to ms3 and HRD score. Conclusions: The range and distribution of HRD scores in our OS cohort closely resemble BRCA1/2 deficient breast cancer. Patients with metastatic disease at diagnosis were found to have higher HRD scores, representing a higher degree of genomic instability and increasing potential to accumulate resistance mutations. Higher HRD scores in TP53 mutated samples may be due to chromosomal instability caused by mechanisms other than HRD. The BRCAness phenotype in OS is now being targeted in a phase II clinical trial, combining the PARP inhibitor olaparib with the ATR inhibitor ceralasertib (NCT0441706), which includes integrated correlative biology to prospectively assess biomarkers of response such as HRD score. Citation Format: Michael David Kinnaman, Julia Pena, Max Levine, Nancy Bouvier, Elli Papaemmanuil, Suzanne Forrest, Katherine Janeway, Paul Meyers, Julia Glade Bender. Analysis of homologous recombination deficiency (HRD) scores in osteosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2590.