(P 033) REDUCED H3K27ME3 IMMUNOEXPRESSION IN PEDIATRIC OSTEOSARCOMA IS ASSOCIATED WITH POOR RESPONSE TO NEOADJUVENT THERAPY

Objective: As the most common primary bone tumour, osteosarcoma (OS) is often defined as having a non-recurrent and heterogeneous genetic profile, which differs from other pediatric bone tumours such as Ewing sarcoma. Most OS patients are treated with conventional chemotherapy and subsequent resection which allows for the evaluation of the neoadjuvant response, which may facilitate ongoing therapeutic choices to better improve the 5-year overall and event-free survival. Although 5-year overall survival rates have been static for those without metastatic disease, those with or who subsequently develop metastatic disease have markedly worse outcomes. Identifying potential biomarkers that may impact event-free and overall survival could better support future clinical trials attempting to improve the outcomes in OS. One potential area that may be explored could be the epigenetic modifications in OS, such as the methylation pattern at various histone residues. Particular methylation studies using immunohistochemistry (IHC) have found both diagnostic and prognostic significance. Tri-methylation at lysine 27 of histone H3 (H3K27me3) has been associated with gene repression, and loss of tri-methylation may allow for growth of tumours. H3K27me3 can be used in the diagnosis of malignant peripheral nerve sheath tumours and may be prognostic in these and other tumours, including meningiomas. Limited studies have investigated the role of H3K27me3 in OS. Our aim is to evaluate the role of H3K27me3 in OS by evaluating its expression in diagnostic biopsies.

Methods: We identified 58 OS cases with a primary diagnostic biopsy tissue block available for evaluation from 2002-2020. The clinical charts were reviewed to evaluate each patient’s demographics, post-therapy response, development of metastatic disease, and overall survival. Non-OS samples were also included in the TMA, ranging from normal bone and bone marrow, cartilage, and other tumours within the spectrum of OS differential diagnoses. After validating tri-methylation antibodies including H3K27me3 and H3K9me3, we evaluated the OS cores using a dichotomous scale of low (< 50%) and high (>50%) tumour nuclei expression.

Results: H3K27me3 immunoexpression was associated with post-therapy tumour necrosis response. Diagnostic biopsies that showed low H3K27me3 nuclear staining were associated with poor treatment response (< 90% necrosis) at the time of definitive excision (P< 0.05). Although interesting trends were seen, there was no significant association between pre-treatment biopsy H3K27me3 immunoexpression and overall or event-free survival. Immunoexpression of H3k9me3 was retained in nearly all tumour nuclei in all OS cores evaluated.

Conclusion: In this exploratory study, we identified H3K27me3 as a potential biomarker in OS diagnostic biopsies which may predict a poor neoadjuvant response, while H3K9me3 provided no further diagnostic or prognostic value. Although further studies with a larger patient cohort is needed to explore the role of H3K27me3 in OS, these encouraging results support the expanded evaluation for risk stratification of other histone methylation and acetylation markers.