(P 68) MULTI-OMIC INTRA- AND INTER-TUMOR HETEROGENEITY OF BENIGN SMOOTH MUSCLE TUMORS

Objective: Uterine leiomyomas (LM) are benign tumors originating from the myometrium, commonly affecting women of reproductive age. Approximately half of patients present with multiple tumors, termed multifocal LM. LM are thought to be monoclonal proliferations, but it is unknown whether multifocal LM exhibit inter-tumor molecular heterogeneity, which may contribute to their variable response to hormone therapy. Anecdotal reports suggest that LM may occasionally progress into malignant metastasizing leiomyosarcoma (LMS). We hypothesized that better understanding of inter- and intra-tumor heterogeneity of LM may identify molecular processes that may contribute to oncogenic transformation.

Methods: We performed multi-omic profiling of 62 specimens from 21 LM patients (median of 3 tumors per patient, range: 2-8). We analyzed multiple regions of a single tumor, and multiple concurrent LM from patients with multifocal disease. We profiled DNA methylation (EPIC microarrays), point mutations (whole exome sequencing) and gene expression (RNA-seq) in these specimens.

Results: Multi-omic profiling showed a remarkable intra- and inter-tumor heterogeneity of genomic, epigenomic and transcriptomic patterns in LM. Through reconstruction of phylogenetic trees based on single nucleotide variants, we identified for the first time different MED12 mutations in co-existing nodules, and novel clonal and subclonal somatic mutations in LM. We also observed differential enrichment of oncogenic pathways between distinct regions within the same nodule and among co-existing nodules.

Conclusion: Molecular heterogeneity has been well-documented in malignant tumors, and we report it here for the first time in histologically bland, non-metastasizing LM. Our study reveals that in multifocal LM, distinct tumors can acquire unique molecular alterations, meaning that single-tumor analysis may not capture the full spectrum of molecular changes. This highlights the importance of considering molecular heterogeneity in diagnosis and evaluation of response to treatment in LM. Our results warrant further investigation into clonal evolution and localized activation of oncogenic pathways that may contribute to the rare transformation of benign LM into malignant LMS.