(P 326) MOLECULAR LANDSCAPE OF ENDOMETRIAL STROMAL SARCOMA IN A LARGE REAL-WORLD PATIENT COHORT

Objective: Endometrial stromal sarcomas (ESS) comprise 10-25% of uterine sarcomas. ESS can be categorized into two types: low-grade (LG-ESS) and high-grade (HG-ESS), each with distinct genomic changes and clinical characteristics. Gene fusions are commonly found in both LG-ESS and HG-ESS, with JAZF1-SUZ12 being prevalent in LG-ESS, while YWHAE-NUTM2 fusion and BCOR alterations, such as ZC3H7B:BCOR fusions or BCOR internal tandem duplications (ITD), are common hallmarks in HG-ESS. However, there is limited knowledge about additional molecular features. Our aim was to perform a comprehensive molecular profiling of the largest ESS cohort so far to uncover novel biological events in these tumors.

Methods: ESS patient samples underwent next-generation sequencing (NGS) of DNA (592-gene, N = 20; whole exome, N = 165) and RNA (whole transcriptome, N = 178) at Caris Life Sciences. All samples were centrally reviewed by an expert sarcoma pathologist. Expression of estrogen (ER) and progesterone (PR) receptors was evaluated by immunohistochemistry (IHC). Proliferation was assessed by KI67 transcriptional expression. Tumor mutational burden (TMB) was calculated by the number of non-synonymous somatic mutations (single nucleotide variants and small insertion/deletions) per mega-base (Mb); a high TMB was defined by a value ≥ 10 mutations/Mb. Tumor microenvironment (TME) was analyzed by RNA deconvolution (quanTIseq). Statistical significance was tested by χ2, Fisher’s exact, or Mann-Whitney U when appropriate.

Results: 185 ESS samples (110 LG-ESS, 75 HG-ESS) were analyzed with NGS. Median age was 58 years (range 21-84). Most cases (56% LG and 68% HG) were from metastatic disease. HG-ESS showed higher proliferation (mKi67 RNA expression 21.9 vs 6.7, p < 0.001), and more common nuclear pleomorphism and nuclear irregularities. IHC expression of ER and PR was higher among LG-ESS v. HG-ESS samples (73.5% vs 23.2% for ER, p < 0.001; 65.0% vs 24.6% for PR, p < 0.001).
JAZF1:SUZ12 was the most common translocation in LG-ESS (21%), which was also found in 4% of HG-ESS (4/5 were ER/PR positive). JAZF1:PHF1 fusion was found in 3 LG-ESS, while other JAZF1 fusions occurred in 3 LG-ESS and 2 HG-ESS. ZC3H7B:BCOR fusions were detected in both LG-ESS (N=9, 8%) and in HG-ESS (N=4,6%). Finally, YWHAE:NUTM2B fusion was found in 5 LG-ESS (5%) and 3 HG-ESS (4%); other YWHAE rearrangements were found in 2 LG-ESS and 2 HG-ESS.
Mutational events were uncommon irrespective of ESS grade. Compared with LG-ESS, HG-ESS showed more mutations in TP53 (28.0% v. 3.7%, p < 0.001) and MED12 (20.7% v. 2.9%, p = 0.04). Mutations in ATRX were found exclusively in HG-ESS (10.1% vs 0.0%, p = 0.001). BCOR mutations were detected in 0.9% LG-ESS case v. 6.7% HG-ESS.
Copy-number alterations detected in > 5% of the cases were uncommon and mainly affected MDM2 (7.3%), CDK4 (6.2%), LRIG3 (5.9%) and HMGA2 (5.7%). TMB was globally low (median 1.0 and 3.0 mut/Mb for LG-ESS and HG-ESS, respectively). dMMR/MSI-H was observed only in 1.6% cases.
TME analysis revealed low T-cell infiltration in both LG-ESS and HG-ESS ( < 1% for all T-cell subsets); LG-ESS were enriched in neutrophils (2.0-fold, p = 0.01) and M2 macrophages (1.5-fold, p < 0.001) while the most differentially represented cell fractions in HG-ESS were dendritic cells (5.2-fold, p < 0.001).

Conclusion: This series represents the largest cohort of multi-omic characterization of ESS. Our data confirmed the characteristic presence of JAZF1:SUZ12 fusions in LG-ESS. However, this fusion was also found in HG-ESS cases with retention of ER and PR expression. In addition, both HG-ESS associated with ZC3H7B:BCOR or YWHAE:NUTM2 fusions were also detected in LG-ESS samples. These findings may suggest that some HG-ESS cases might evolve from low-grade tumors. The ER/PR retention in ~ 25% of HG-ESS could provide valuable treatment options.
Compared to LG-ESS, HG-ESS presented more frequently mutations in known tumor-suppressor genes such as TP53, MED12 and ATRX. However, the TMB was low across the whole cohort.
Our works also provides the first description of TME in ESS, indicating globally a low T-cell infiltration for both LG-ESS and HG-ESS, while the relative abundance of neutrophil and M2 macrophage in LG-ESS is of uncertain significance and warrants further investigation.