Uterine sarcoma with KAT6B/A::KANSL1 fusion: a molecular and clinicopathological study on 9 cases.

Uterine sarcomas with KAT6B/A::KANSL1 fusion represent a new entity characterized by bland morphology, commonly with hybrid features of low-grade endometrial stromal sarcoma (LG-ESS) and tumors with smooth muscle differentiation. In our study, we performed a detailed morphological, immunohistochemical, and molecular analysis of 9 cases of these tumors. Six of those had been originally diagnosed as LG-ESS, one as leiomyoma, one as leiomyosarcoma, and the remaining case as sarcoma with the KAT6B/A::KANSL1 fusion. Seven cases showed overlapping features between endometrial stromal and smooth muscle tumors, one case resembled cellular leiomyoma, and one case resembled high-grade endometrial stromal sarcoma. Immunohistochemically, the tumors showed a common expression of smooth muscle markers and endometrial stromal markers. Molecular findings showed the KAT6B/A::KANSL1 fusion in all cases (by NGS and FISH). In addition, mutations affecting genes such as TP53, PDGFRB, NF1, RB1, PTEN, ATM, RB1, FANCD2, and TSC1 were present in all 5 cases with aggressive behavior. One patient with no evidence of disease showed no additional mutations, while another harbored a mutation of a single gene (ERCC3). Of the 8 patients with available follow-up, two died of disease, 3 are currently alive with disease, and 3 have no evidence of disease. The correct recognition of tumors with the KAT6B/A::KANSL1 fusion is essential because despite the bland morphological features of most cases, these tumors have a propensity for aggressive behavior.