Abstract PR003: DNA methylation reveals metastatic colonization routes in ovarian cancer

Abstract High-grade serous carcinoma (HGSC) remains the deadliest gynecological cancer, mostly due to the widespread metastases in abdominal cavities already at diagnosis. To improve therapy success, understanding metastasis routes and their molecular characteristics is crucial. Herein, we used >400 samples subjected to whole-genome bisulfite (WGBS) and RNA sequencing from 125 patients with HGSC, enrolled to prospective, longitudinal, multi-region, observational DECIDER trial (NCT04846933). These real-world epigenetics data allowed, for the first time, to delineate metastatic routes in HGSC. Our results show that cancer cells released from the site-of-origin sites seed to both mid-abdomen and upper abdomen in parallel. The dominant mechanism responsible for almost half of the spread was direct surface-to-surface release of ovarian tumor cells into the peritoneal cavity. The major source of cancer cells in the pelvic area was ovary, from which cancer disseminated to all intra-abdominal organs, including omentum. Tumor cells from pelvic bowel displayed strong tropism for virtually all metastatic deposits, including such distant metastases, as liver, vagina, and lungs, which suggested hematogenous spread. Ascites play a prominent role in fueling colonization, accounting for additional 20% of direct spread. The seeding via ascitic fluid remained localized within the abdominal area, however, ascites colonized primarily para-aortic and inguinal lymph nodes. Lymph node metastases were noted in one-third of possible spreading modes. HGSC dissemination via inguinal lymph nodes represented a rare, but aggressive trajectory, as these tumors, originated directly from tube were prone to invade all distant locations. Comparison of the paired samples before and after chemotherapy revealed that most sensitive to neoadjuvant chemotherapy (NACT) tumors are adnexal and omental tumors. Peritoneum and mesentery showed prominent resistance to treatment, as well as lymph nodes. Thus, patients with metastases in inguinal lymph nodes and bowel are at the highest risk of developing widespread intra-abdominal and distant metastases. Furthermore, the standard-of-care platinum/taxane therapy, along with maintenance relapse therapies are ineffective for eradication of these tumors. Ascites displayed an intriguing case: being virtually non-responding to NACT, epigenetic profiles at relapse exhibited a large change. This implies that epigenetic processes in ascites evolve along with the cancer progression, driving chemoresistance. Taken together, using DNA methylation changes from >400 samples we revealed metastasis routes in HGSC, as well as tissue-specific chemotherapy treatment response. Dissection of the key regulatory pathways of metastatic routes helps identify patients at the highest risk of distant metastases and suggests new targets for therapies. Citation Format: Alexandra Lahtinen, Giovanni Marchi, Kari Lavikka, Daria Afenteva, Veli-Matti Isoviita, Susanna Holmström, Yilin Li, Taru A Muranen, Jaana Oikkonen, Antti Häkkinen, Anni Virtanen, Johanna Hynninen, Sampsa Hautaniemi. DNA methylation reveals metastatic colonization routes in ovarian cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Methylation, Clonal Hematopoiesis, and Cancer; 2025 Feb 1-4; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2025;85(3 Suppl):Abstract nr PR003.