<scp> <i>PLAG1</i> </scp> ‐rearranged uterine mesenchymal tumours: A study of 11 cases showing distinct histological features beyond myxoid morphology

AIMS: PLAG1 gene fusions have been identified in a subset of uterine sarcomas and were historically associated with myxoid morphology. However, recent evidence shows that not all cases demonstrate myxoid features or conventional smooth muscle immunophenotype. Herein, we present 11 cases of PLAG1-rearranged uterine mesenchymal tumours to further characterize their clinicopathologic, immunohistochemical and molecular profiles. METHODS: Eleven cases of PLAG1-rearranged uterine mesenchymal tumours were included from Fudan University Shanghai Cancer Center, comprising nine consultation referrals and two primary diagnoses. Clinicopathological features were reviewed, and all cases underwent targeted RNA sequencing with a 631-gene panel. RESULTS: The median patient age was 43 years (range: 31-58). Tumours occurred in the uterine corpus (n = 10) or broad ligament (n = 1), with a median size of 10 cm (range: 5.8-19 cm). Most presented at FIGO stage I. During follow-up, 27.3% (3/11) of patients developed recurrence or metastasis. Histologic evaluation revealed significant heterogeneity: myxoid morphology in 54.5% (6/11), epithelioid morphology in 27.3% (3/11) and spindle cell morphology in 18.2% (2/11). Immunohistochemically, 81.8% (9/11) expressed at least one smooth muscle marker (desmin, h-caldesmon or SMA). RNA sequencing confirmed PLAG1 rearrangements with multiple partners, including RAB2A, TRPS1, PBX1, PRLR, BOC, CSDE1, TMEM68, PHLPP1, UBC, ESPL1 and NCOR2. The most frequent fusions were TRPS1::PLAG1 (27.3%, n = 3) and CSDE1::PLAG1 (18.2%, n = 2). CONCLUSION: PLAG1-rearranged uterine mesenchymal tumours demonstrate notable morphologic heterogeneity, predominantly presenting as myxoid leiomyosarcoma. We recommend using the integrated diagnostic term 'PLAG1-rearranged uterine mesenchymal tumour with features of [specific morphology]' to capture both the genetic alteration and histologic spectrum, accompanied by a comment regarding the limited experience and potential for recurrence.