Introduction
Osteoid osteoma and osteoblastoma are common bone-forming tumors and typically present during the second decade of life. They have no malignant potential, but osteoblastoma can behave locally aggressive [
1,
2]. Both lesions are more or less histologically indistinguishable, and distinction is predominantly based on size (diameter below or above 2 cm, respectively) [
3]. In addition, osteoid osteomas are usually located in the long bones and present with nocturnal pain relieved by nonsteroidal anti-inflammatory drugs (NSAIDs), while osteoblastomas have a preference for the posterior column of the spine. The most essential feature in osteoid osteoma is the radiographic presence of a central lucent area (nidus), which is surrounded by dense sclerotic bone tissue. In the nidus, regular trabeculae of woven bone are present. These trabeculae are lined by active osteoblasts with vascularized stroma in between. In osteoblastoma, the distribution of woven bone can be slightly less organized, as compared to the nidus of an osteoid osteoma. In the past years, deep sequencing has rapidly advanced the field, as it has provided increased knowledge on the molecular background of bone tumors. Based on these findings, molecular testing as well as specific immunohistochemistry has found its way in routine bone tumor diagnostics that historically heavily relied on morphology and has improved diagnostic accuracy [
4,
5]. Recently, recurrent translocations in
FOS (87%) and
FOSB (3%) were found in osteoblastoma and osteoid osteoma [
6]. Both FOS and FOSB are part of the FOS family of transcription factors and were shown to play a role in diverse biological processes, including osteoblast differentiation and proliferation [
7]. Also, similar rearrangements are found in vascular tumors [
8‐
11]. The aim of this study was to evaluate whether the recently found
FOS and
FOSB rearrangements can be used as an auxiliary diagnostic tool in routine bone tumor diagnosis. We compared immunohistochemistry of FOS and FOSB between osteoid osteoma and osteoblastoma and other lesions with bone deposition. We evaluated the influence of decalcification and, in addition, correlated the immunohistochemical findings to the underlying genetic alteration using interphase fluorescence in situ hybridization (FISH).
Discussion
In this study, we evaluated the utility of the use of immunohistochemistry of FOS to diagnose osteoid osteoma and osteoblastoma. So far this has only been tested in small series, with divergent results, as positivity ranged from 0 (
n=11) to 100% (
n=3) [
6,
19]. Strong overexpression of FOS at immunohistochemistry correlated strongly with the underlying
FOS rearrangement. While in a previous study in a minority of cases,
FOSB rearrangements were present, instead of
FOS rearrangements [
6], we did not find any, rendering FOSB immunohistochemistry diagnostically not relevant. Our study indicates that there are two important caveats that pathologists should be aware of when applying immunohistochemistry for FOS to diagnose osteoid osteoma and osteoblastoma.
First, we showed that after > 3 days of acid-based decalcification, immunoreactivity for FOS disappeared. Though decalcification in EDTA preserves DNA and immunogenicity, acid-based solutions are still commonly used and may affect antigen preservation, leading to loss of sensitivity of immunohistochemistry [
20]. In this study, a striking difference between osteoid osteoma and osteoblastoma samples for FOS expression was noticed, as all osteoid osteoma, but only 57% of osteoblastomas showed positivity. In general, osteoid osteoma samples were all small and were decalcified for a short period of only 4 h in most cases, as opposed to osteoblastoma samples. The additional decalcified placental series confirmed diminished nuclear staining after a longer period of decalcification for FOS, while FOSB remained intact. Thus, long decalcification times specifically affect FOS immunohistochemistry, and immunohistochemistry should not be used on resection specimens after prolonged acid-based decalcification.
Second, we scored FOS overexpression as strong and diffuse (> 50% of tumor cells) nuclear expression that we found in all 22 osteoid osteomas and in 12 of 21 osteoblastomas. As could be expected based on their role in normal osteoblast maturation and differentiation [
7], we noticed moderate to strong nuclear positivity for FOS and FOSB in the areas of bone deposition in several reactive and proliferative bone-forming lesions. Of the neoplasms, only 1 of 6 aneurysmal bone cysts showed moderate staining in > 50% of the tumor cells, while this was absent in other tumors. This can be a pitfall when using immunohistochemistry, necessitating confirmation by FISH under these circumstances. Partial weak staining was noticed in the majority of other samples and should be considered as not representative of translocation-induced overexpression. Moreover, consistent with previous findings in which copy number gains were noticed in FOS immunopositive osteosarcoma [
6], we also observed FOS positivity in two osteosarcoma samples (osteoblastic and sclerosing subtype). In one case, FISH was possible, which showed gains of
FOS and
FOSB, potentially resulting in overexpression at immunohistochemistry.
The FOS transcription factor family includes FOS, FOSB, FOSL1, and FOSL2 and encodes leucine zipper proteins that can dimerize with proteins of the JUN family, thereby forming the transcription factor complex AP-1. This way, the FOS proteins regulate a diverse array of biological processes, including cell proliferation, differentiation, and survival. Functional studies have shown that FOS and FOSB, together with other family members of FOS family, are highly expressed during normal osteoblast maturation [
21]. Retroviral
FOS oncogene can cause osteosarcoma in mouse model systems, when fused with a highly active promoter and the v-
fos 3’ untranslated region [
22].
Similar rearrangements of
FOS and
FOSB were previously found in vascular tumors [
8‐
11]. Identical to
FOS-rearranged epithelioid hemangioma, the translocations involve various genes or intergenic regions and lead to a premature stop codon, at or early after the break points that always involve exon 4 of
FOS [
6,
8]. This causes loss of the C-terminal end of the protein, rendering the protein resistant to degradation causing high expression in tumor cells [
23]. The
FOSB fusions described in atypical epithelioid hemangioma and pseudomyogenic hemangioendothelioma occur at the N-terminal part of the protein and are most likely induced by promoter swap events, causing upregulation of
FOSB [
10,
11,
24].
For bone tumor pathologists, a challenging diagnostic problem is to discriminate epithelioid osteoblastoma from high-grade osteoblastic osteosarcoma. Epithelioid osteoblastomas can be composed of large, plump osteoblasts, surrounded by abundant eosinophilic cytoplasm. Additional degenerative nuclear atypia can be present, accompanied by mitotic figures. Similarly, osteoblastoma-like osteosarcoma can mimic osteoblastoma. Distinction is of crucial importance, as prognosis and treatment differ significantly. While infiltration of host bone and lack of differentiation towards the periphery seem to be the most discriminating features between (epithelioid) osteoblastoma and (osteoblastoma-like) osteosarcoma, this is not often assessable in biopsy and curettage specimens [
25,
26]. Although numbers are small, our present results indicate that immunohistochemistry and/or FISH for FOS can be of help in distinguishing (epithelioid) osteoblastoma from osteosarcoma, especially since there are no specific antibodies or molecular tests for osteosarcoma.
To summarize, FOS immunohistochemistry can be used as an auxiliary tool for osteoid osteoma and osteoblastoma in short decalcified tissue, while FOSB immunohistochemistry is diagnostically not useful. However, FOS immunohistochemistry should not be used after long decalcification, and the low-level focal expression found in other lesions and tissues, especially reactive bone, might be confusing. Under these circumstances, the use of FISH for FOS could be diagnostically useful, for cases where it is difficult to distinguish osteoid osteoma and osteoblastoma from their histologic mimics.
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