Impact of ATRX loss on survival and immune microenvironment in multiple sarcoma subtypes.

11511 Background: Sarcomas are heterogeneous mesenchymal malignancies often with few treatment options and poor outcomes. ATRX is commonly altered in sarcoma, with loss-of-function (LOF) mutations observed in up to 35% of cases. ATRX plays roles in DNA damage response, telomere maintenance, and epigenetic regulation. However, the impact of ATRX loss on sarcoma biology and the potential of ATRX mutation to serve as a prognostic biomarker in sarcoma are under investigation. Methods: The predominant alteration seen in ATRX in sarcoma is LOF mutation, thus tumor specimens were scored for presence or absence of ATRX staining by immunohistochemistry (IHC) in clinically-annotated tissue microarrays (TMAs) including 95 patients with uterine leiomyosarcoma (ULMS), 127 with soft tissue leiomyosarcoma (STLMS), 82 with undifferentiated pleomorphic sarcoma (UPS), and 84 with well-differentiated/dedifferentiated liposarcoma (WD/DDLPS). We also performed IHC staining using a panel of immune cell markers and checkpoints on the ULMS TMA, including CD8, CD20, CD68, PDL1, CTLA4, CD47, and CD163, among others. Kaplan-Meier analysis with log rank tests were used to assess the impact of ATRX loss on overall survival (OS), disease-specific survival (DSS), and progression free survival (PFS). Cox proportional hazards modeling was used to determine whether ATRX loss is an independent predictor of survival. T tests, ANOVA, and Chi squared tests were used to test for association between ATRX loss and immune markers in the ULMS TMA. Results: ATRX loss was seen in 14.1% of STLMS (n = 18), 52.6% of ULMS (n = 50), 36.6% of UPS (n = 30), and 8.3% of WD/DDLPS (n = 7). In univariate analysis, ATRX loss was associated with significantly worse OS (HR 0.50, 95% CI 0.31-0.80, p = 0.004) and PFS (HR 0.51, 95% CI 0.33-0.81, p = 0.004) in ULMS, worse DSS in UPS (HR 0.46, 95% CI 0.23-0.95, p = 0.03), and worse PFS in WD/DDLPS (HR 0.13, 95% CI 0.02-0.73, p = 0.02). In multivariate analyses, ATRX loss was also associated with worse OS and PFS in ULMS and worse DSS in UPS. In ULMS, ATRX loss was associated with increased density of CD8+ T cells (p = 0.01), PDL1 expression (p = 0.03), CTLA4 expression (p = 0.049), IDO-positive tumor cells (p = 0.01), and CD163-positive cells (p = 0.003). Conclusions: ATRX loss is associated with worse outcomes in ULMS, UPS, and WD/DDLPS. ATRX loss is also associated with a distinct immune microenvironment in ULMS characterized by T cell exhaustion and immunosuppressive M2 macrophages. Future work will focus on understanding the molecular underpinnings of these associations and potential ways to exploit these findings therapeutically.