Abstract 4069: Transcriptome-wide splicing analysis reveals changes in transcript usage of <i>MDM2</i> and <i>ESR1</i> in uterine leiomyomas

Abstract Uterine leiomyomas (ULs) are smooth muscle tumors of the uterus. ULs are the most common tumors in women affecting up to 70% by the age of 50. ULs are often divided into molecular subtypes based on genetic driver alterations that are usually mutually exclusive. Transcriptome-wide aberrations in messenger-RNA (mRNA) splicing have been reported in many cancer types due to disturbed regulation of splicing. In addition, splice site mutations of individual driver genes, especially tumor suppressor genes, are common in cancer. Splice mutations of a few driver genes have been identified also in ULs. However, to our knowledge transcriptome-wide changes in splicing have not been studied in ULs. Our aim was to study the potential role of mRNA-splicing alterations in UL genesis. We generated RNA-sequencing (RNAseq) data from 410 ULs and 151 corresponding normal myometrium samples and used LeafCutter for running splicing analyses. The preliminary results suggest that splicing patterns differ between UL subtypes. Unsupervised hierarchical clustering based on splicing separates normals and molecular UL subtypes to their respective clusters. The different UL subtypes shared many splicing alterations compared to normal myometrium. mRNAs for components of extracellular matrix and contractile muscle fibers were enriched among differentially spliced transcripts in tumors compared to normals. The only differentially spliced gene shared by all molecular UL subtypes was MDM2, a well-known oncogene that plays a role in the regulation of p53. Interestingly, ZMAT3, which is the third most overexpressed gene in ULs, is known to regulate MDM2 splicing. Earlier GWAS studies have revealed a UL predisposition locus close to MDM2, affecting MDM2 expression levels in ULs but not normal myometrium. The UL subtypes with driver alterations in MED12 or SRCAP complex genes showed differential usage of estrogen receptor 1 (ESR1) transcript isoforms. It is well known that excessive estrogen promotes UL genesis. The differential expression of ESR1 isoforms may play a role in estrogen sensitivity of ULs. ULs almost never progress to leiomyosarcoma. However, ULs share common driver genes and dysregulated signaling pathways with many cancer types. Better understanding of UL genesis broadens our knowledge on the common and distinguishing characteristics of benign and malignant tumors, perhaps shedding light on mechanisms preventing malignant degeneration in a benign neoplastic lesion. Citation Format: Vilja Jokinen, Niko Välimäki, Kristiina Rajamäki, Auli Karhu, Emma Siili, Ralf Bützow, Oskari Heikinheimo, Annukka Pasanen, Lauri A. Aaltonen. Transcriptome-wide splicing analysis reveals changes in transcript usage of MDM2 and ESR1 in uterine leiomyomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4069.