[Fibrodysplasia ossificans progressive: a clinicopathological and molecular genetic analysis of three cases].

Objective: To investigate the clinicopathological and molecular genetic characteristics of fibrodysplasia ossificans progressiva (FOP). Methods: Three cases of FOP were collected in the First Affiliated Hospital of Zhengzhou University from January 2018 to December 2024. The clinicopathological characteristics were reviewed. Immunohistochemistry and DNA-based NGS were performed. Results: The cohort included two males and one female, with ages of 1, 11, and 11 years, respectively. All patients presented with multiple soft tissue masses in the trunk and hallux valgus deformity. Due to clinical suspicion of malignancy, biopsies were performed in all three cases, and one patient subsequently underwent excision. Histologically, biopsy specimens revealed spindle cell proliferation within a myxoid and collagenous background, infiltrating skeletal muscle fibers, accompanied by thin-walled blood vessels and minimal inflammatory infiltrates. Multifocal ossification was observed in the excised specimen. Immunohistochemically, the spindle cells were positive for SMA (3/3), and negative for desmin (3/3), S-100 (3/3), CD34 (3/3), and MUC4 (1/1). AB-PAS staining revealed extensive mucin deposition in the stroma (2/2). Next-generation sequencing (NGS) identified the ACVR1 R206H mutation in all three cases. Conclusions: Early-stage FOP exhibits distinct histological features, with congenital hallux deformity serving as a crucial diagnostic clue. Molecular testing for ACVR1 mutations facilitates early disease diagnosis.