LAUSR

e15701 Background: Pseudomyxoma peritonei (PMP) is a rare malignant tumor characterized by the infiltration of the peritoneum by mucus-secreting tumor cells. The genetic landscape of these tumors and correlation with clinico-pathological tumor features is unclear to date. Methods: We performed whole-exome sequencing (WES) on 8 PMPs and matched normal. We then validated our finding using ultra-deep sequencing of hotspot mutations (~82.270 x) in 45 clinically annotated samples. In addition, bulk RNAseq was performed on 10 samples and MCP-counter was used to infer absolute abundance of eight immune and two stromal cell populations. Results: Overall, 323 somatic mutations were identified through WES with most frequent mutations involving GNAS (R186H) (50%), KRAS (G12D) (50%), AHNAK2 (25%) and ATXN1 (25%). Furthermore, ultra-deep sequencing uncovered KRAS, GNAS and IDH1/2 hotspots mutations in 16 (35.5%), 11 (24.4%) and 2 (4.4%) PMPs, respectively. Strikingly, KRAS mutations were enriched in females (79% vs 39%, p = 0.03) while GNAS in males (67% vs 28%, p = 0.04). No other significant associations were identified between mutations and other clinico-pathological features. Using MCP-counter, fibroblasts, endothelial cells and moncoytic cells were the most frequent inferred cells. Unsupervised clustering using expression of most variable cells identified two PMP clusters, namely C1 (n = 4) and C2 (n = 6). C2 cluster displayed higher T cells as compared to C1 (p < 0.0001), consistent with increased cytotoxic lymphocytes (p = 0.04). Notably, C2 was enriched for tumors with higher grade as compared to C1 (80% vs 0%; p = 0.04). Conclusions: Our study represents the largest study to data exploring genetic and immune alterations in PMPs. We uncovered puzzling associations between genetic landscape of PMPs and patients gender, which deserve further validation in an independent cohort. The association of high-grade tumors with increased tumor infiltrating lymphocytes suggests the existence of an immunogenic microenvironment; PD-1/PD-L1 blockade might represent a therapeutic option for these patients.