Histone H3 modifying genes may serve as a predictive marker for metastasis in synovial sarcoma.

Abstract

11058 Background: Synovial sarcomas (SS) are rare, mesenchymal tumors that exhibit heterogeneous clinical behavior. SS is characterized molecularly by a fusion of the SS18 and one of three SSX genes. While fusion type has been shown to play a role in prognosis, little is known regarding the role of secondary genomic variants in SS. We hypothesized that frequently mutated secondary mutations corresponded to altered molecular pathways associated with accelerated disease progression. Here we present further analysis of the largest study of secondary SS genomic alterations and their potential clinical implications particularly in regard to Histone H3 modifying genes. Methods: Comprehensive Genomic Profiling (CGP) from 203 SS18-SSX fusion positive SS subjects were obtained from Foundation Medicine (FMI). Correlation between gene expression and survival endpoints were assessed in two independent datasets (GSE40018, n = 34; GSE40021, n = 58). A histone H3 score (HH3Score) was calculated through summation of expression values of MLL2, MLL3, SETD2. Altered expression was assessed for genes mutated in > 5% of SS in the FMI dataset. Datasets were also combined with appropriate adjustments for multivariate analysis. Cox proportional hazard, and Chi-squared test were used as appropriate. Results: In CGP profiling of SS, only 4 genes were seen in > 5% of subjects ( MLL3 9%, SETD2 7%, MYO18A 6%, MLL2 6%). 3 of these variants are known to regulate histone H3 activity and are collectively altered in 22% (44 of 203) of the FMI SS subjects. Notably, these H3 targeted gene alterations were found to be mutually exclusive of one another (p < 0.001). In 2 independent SS datasets, subjects with high histone H3 modifying gene RNA expression levels (HH3Score) demonstrated worsened outcomes with earlier tumor metastasis (GSE40018: HR = 0.41, 95%CI 0.15-1.07, p = 0.07; GSE40021: HR = 0.22, 95%CI 0.09-0.52, p < 0.001). Multivariate analysis demonstrated that HH3Score remained significantly correlated to metastasis-free survival (HR = 0.41, 95%CI 0.22-0.77, p < 0.01) when subjects’ age (p < 0.001) and fusion type (p = 0.60) were taken into consideration. Conclusions: Altered expression of histone H3 modifying genes may serve as a key driver for SS progression. Further prospective research is necessary to confirm its prognostic importance.