Frequent FOS Gene Rearrangements in Epithelioid Hemangioma: A Molecular Study of 58 Cases With Morphologic Reappraisal

Abstract

Epithelioid hemangioma (EH) is a unique benign vasoformative tumor composed of epithelioid endothelial cells. Although a small subset of EHs with atypical features harbor ZFP36-FOSB fusions, no additional genetic abnormalities have been found to date in the remaining cases. On the basis of a novel FOS-LMNA gene fusion identified by RNA sequencing in an index case of a skeletal EH with typical morphology, we sought to investigate the prevalence of FOS rearrangement in a large cohort of EHs. Thus 57 additional EH cases lacking FOSB rearrangements were studied for FOS gene abnormalities by fluorescence in situ hybridization, and results were correlated with morphologic appearance and clinical presentation. The EHs were subclassified as typical (n=25), cellular (n=21), and angiolymphoid hyperplasia with eosinophilia (ALHE) (n=12) variants. The ALHE was defined as an EH with a vascular "blow-out" pattern associated with a variable degree of inflammation. There were 17 (29%) cases bearing FOS gene rearrangements among 58 cases tested, including 12 male and 5 female patients, with a mean age of 42 years. Most FOS-rearranged EHs occurred in the bone (10) and soft tissue (6), whereas only 1 case was cutaneous. The predominant anatomic site was the extremity (12), followed by trunk (3), head and neck (1), and penis (1). The incidence of FOS rearrangement was significantly higher in bone (59%, P=0.006) and lower in head and neck (5%, P=0.009). Twelve of the FOS-rearranged cases were cellular EH (P=0.001) associated with moderate mitotic activity (2 to 5/10 HPF) and milder inflammatory background. All 12 ALHE cases lacked FOS gene abnormalities, suggesting different pathogenesis. In conclusion, FOS rearrangement was present in a third of EHs across different locations and histologic variants; however, it was more prevalent in cellular EH and intraosseous lesions, compared with those in skin, soft tissue, and head and neck. This genetic abnormality can be useful in challenging cases, to distinguish cellular EHs from malignant epithelioid vascular tumors. These results also suggest that dysregulation of the FOS family of transcription factors through chromosomal translocation is as a key event in the tumorigenesis of EH except for the ALHE variant.

Figure 1. Pathologic findings of the FOS-rearranged EH cases

(A) The index case (case #1), positive for FOS-LMNA fusion, showed a typical morphology with epithelioid cells surrounding vascular lumina. (B) Lesional cells showed glassy eosinophilic cytoplasm with scattered vacuoles, vesicular nuclei, and prominent nucleoli (case #1). (C) One of the cellular EH (case #14) was confined to a dilated vascular channel, which at higher power (D) was composed of solid sheets of tumor cells. Cellular EH showed (E) variably prominent vascular spaces (case #12), (F) distinctive epithelioid cell features (case #11), and (G) surrounding lymphoid and eosinophilic infiltrates (case #13). (H) One typical EH was characterized by a prominent inflammatory background obscuring the proliferating vascular channels (H, case #15). Eosinophils were usually seen. (I) The penile EH (case #4) was composed of typical vasoformative structures but showed a more infiltrating growth pattern.

Figure 2. RNA sequencing and experimental validation of a novel FOS-LMNA gene fusion in a typical EH of bone (case#1)

(A) Schematic representation of RNA sequencing indicating the LMNA locus joined with the FOS gene, resulting in a t(1;14)(q22;q24.3) translocation. (B) RT-PCR confirmed both the FOS-LMNA and the reciprocal LMNA-FOS chimeric transcript. (C) FISH assay validated break-apart signals in both FOS and LMNA genes. (D) Normalized RNA sequencing read per kilobase per million mapped reads (RPKM) showed significant FOS mRNA upregulation in the index case (FOS-positive EH1), FOSB-rearranged cases (FOSB-positive EHa & EHb), and a fusion-negative EH (EHc), compared to epithelioid hemangioendothelioma (EHE), angiosarcoma (AS), and glomus tumor (G).

Figure 3. Histologic features of cellular EHs harboring FOS-VIM fusion

(A) The intraosseous cellular EH (case #2) extended to the skin and showed a distinctive lobulated growth. (B) Higher magnification demonstrated solid sheets with scattered vascular channels. (C) Transition between central solid growth and peripheral maturation into well-formed vascular structures was identified (case #3). (D) VIM break-apart signals by FISH assay (case#2).