Leiomyoma with bizarre nuclei (LBN) is an uncommon variant of uterine smooth muscle neoplasm. Involvement of fumarate hydratase (FH) has been suggested in the pathogenesis of a subset of LBN. The goal of our study is to assess the clinicopathological, morphological, immunohistochemical and molecular findings focusing on FH in LBNs (n = 108) and compare it with the findings in usual leiomyomas (UL; n = 50) and leiomyosarcomas (LMS; n = 42). Immunohistochemically, loss of FH expression was found in 67/108 of LBN, 1/50 of UL and in no LMS. Class 4/5 FH mutations were detected in 15/53 LBN with sufficient DNA quality for molecular analysis. Pathogenic variants of the FH gene were detected in neither UL nor LMS. Local recurrence after surgery was present in 18/92 of LBN patients, 7 of which were histologically verified and 2 of which were found to be LBN. Our results confirmed that LBN behave in a benign fashion, although they may relapse. FH gene mutations were a common finding only in LBN, but not in UL and LMS. Immunohistochemistry with an antibody against FH seems to have a good sensitivity (87%) and moderate specificity (58%) with regard to predicting FH gene mutations and could be used as a screening method in tumors with features suggestive of FH alterations to identify patients who are at risk for the FH aberrations.

Department of Obstetrics and Gynecology 1st Faculty of Medicine Charles University and General University Hospital Prague Prague Czech Republic

Institute of Pathology 1st Faculty of Medicine Charles University and General University Hospital Prague Studnickova 2 12800 Prague 2 Czech Republic

The Fingerland Department of Pathology Faculty of Medicine in Hradec Králové and University Hospital in Hradec Králové Charles University Prague Czech Republic

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