Genomic characterization and histologic analysis of uterine leiomyosarcoma arising from leiomyoma with bizarre nuclei

Leiomyoma with bizarre nuclei (LM-BN) is a rare variant of leiomyoma with a benign clinical course. In contrast, leiomyosarcoma (LMS) is a high-grade, malignant neoplasm characterized by high recurrence rates and poor survival. While LM-BN and LMS show distinct morphologies, they share similar immunoprofiles and molecular alterations, with both considered ‘genomically unstable’. Rare cases of LM-BN associated with LMS have been reported; however, the histogenesis and molecular relationship between these two tumors remains unclear. In this study, we assessed 11 cases of LMS arising in conjunction with LM-BN confirmed by histology and immunohistochemistry (IHC), further analyzed by clinical, histologic, and molecular characteristics of these distinct components. LM-BN and LMS had similar p16 and p53 IHC patterns, but LMS had a higher Ki-67 index and lower estrogen and progesterone eceptor expression. Digital image analysis based on cytologic features revealed spatial relationships between LMS and LM-BN. Genomic copy number alterations (CNAs) demonstrated the same clonal origin of LMS arising from existing LM-BN through conserved CNAs. LMS harbored highly complex CNAs and more frequent losses of the TP53, RB1, and PTEN genomic regions than LM-BN (p = 0.0031), with CDKN2A/B deletion identified in LMS only. Mutational profiling revealed many shared oncogenic alterations in both LM-BN and LMS; however, additional mutations were present within LMS, indicative of tumor progression through progressive genomic instability. Analysis of spatial transcriptomes defined uniquely expressed gene signatures that matched the geographic distribution of LM-BN, LMS, and other cell types. Our findings indicate for the first time that a subset of LMS arises from an existing LM-BN, and highly complex genomic alterations could be potential high risks associated with disease progression in LM-BN. © 2024 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.