Case report: A mesenchymal chondrosarcoma with alternative HEY1::NCOA2 fusions in the sella turcica

Introduction Mesenchymal chondrosarcoma (MCS) is a rare subtype of chondrosarcoma that occurs at widespread anatomical locations, such as bone, soft tissue, and intracranial sites. The central nervous system (CNS) is one of the most common origins of extraosseous MCS. However, alternative HEY1::NCOA2 fusions have not been reported in this tumor. Case report We report a case of intracranial MCS with HEY1::NCOA2 rearrangement. A 52-year-old woman presented with a 15-mm calcified mass around the sella turcica. She initially underwent transsphenoidal surgery for tumor resection and then additional resections for five local recurrences over 5 years. Histologically, the tumor was composed of small round to spindle-shaped cells admixed with well-differentiated hyaline cartilaginous islands. A hemangiopericytoma-like vascular pattern and small sinusoid-like vessels were also observed. RNA sequencing using RNA extracted from formalin-fixed paraffin-embedded samples from the last operation revealed two alternative variants of the HEY1::NCOA2 fusion: HEY1(ex4)::NCOA2 (ex13) and HEY1(ex4)::NCOA2(ex14). Both variants were confirmed as in-frame fusions using reverse transcription-polymerase chain reaction. Discussion Cartilaginous components were often not apparent during the recurrences. In addition to the non-typical pathological finding, the correct diagnosis was hampered by the poor RNA quality of the surgical specimens and non-specific STAT6 nuclear staining. Conclusion This is the first reported case of intracranial MCS with an alternative HEY1::NCOA2 fusion.


Introduction
Mesenchymal chondrosarcoma (MCS) is a rare subtype of chondrosarcoma, accounting for only 2%-4% of all chondrosarcomas [1].MCS is distributed in the bone, soft tissue, and intracranial sites.Although the meninges are one of the most common extraskeletal origins of MCS [2], intracranial MCS is an extremely rare tumor of the central nervous system (CNS).Intracranial MCS usually occurs in adolescents and young adults and accounts for less than 1% of all intracranial tumors [3].When limited to sarcomas, MCS accounts for 11.5% of all CNS sarcomas [4].One of the most frequent sites for MCSs is the head and neck region (which contains bones in addition to the CNS), accounting for 13% of MCSs [5].In 2012, HEY1::NCOA2 fusion was identified in MCS [6].Subsequently, IRF2BP2::CDX1 fusion was detected [7].In 2020, NKX3.1 expression was reported as a useful immunohistochemical marker for MCS [8,9].By identifying such fusion genes or confirming NKX3.1 expression by immunohistochemistry (IHC), it is easier to reach an accurate diagnosis of MCS.MCS is still a very rare tumor, and it goes without saying that listing MCS in the differential diagnoses is important to perform the above diagnostic utilities.We describe a case of intracranial MCS harboring alternative variants of the HEY1::NCOA2 fusion gene in a 52year-old woman.

Clinical case
A 52-year-old Japanese woman initially noticed haze in her left eye.She was referred to our hospital because she subsequently showed gradual exacerbation of bitemporal hemianopia.She and her family had no specific medical history.Computed tomography of the head revealed a 15-mm calcified mass in the sella turcica.Magnetic resonance imaging (MRI) of the brain revealed a mass protruding over the sella turcica, with the normal pituitary gland pressed to the upper right and the optic chiasm pressed to the upper left (Figures 1A, B).

Surgery
Transsphenoidal surgery was performed, but unfortunately it was incomplete resection.The pathological diagnosis was limited to malignant tumor, with differential diagnoses of solitary fibrous tumor (SFT) and MCS.Ten months later, MRI revealed tumor recurrence.The relapsed tumor was resected via craniotomy, and the patient received stereotactic radiotherapy (SRT).Four years after SRT, she underwent four of surgical resections and one SRT for several local recurrences.

Follow up
After several times of surgery, the patient experienced blindness and hydrocephalus due to invasion of the recurrent tumors.Regarding the additional therapy to this patient, chemotherapy was not performed, because resistance to chemotherapy and radiation therapy has been reported in conventional chondrosarcoma and it is still controversial in mesenchymal chondrosarcoma [10][11][12][13].

Discussion
At the initial resection, characteristic pathological findings of small round cells admixed with welldifferentiated hyaline cartilaginous islands suggested MCS as a differential diagnosis.Tumor-specific HEY1::NCOA2 fusion was not detected at that time, probably because of poor sample quality.In this case, we could not confirm the simultaneous presence of cartilaginous components and small round cell areas in several recurrent surgical specimens.In addition, undifferentiated small round cells frequently showed a spindle-shaped morphology with a hemangiopericytomatous vascular pattern.Furthermore, repeated STAT6 IHC revealed no nuclear staining which ruled out the possibility of an SFT.
Clinicopathologic characteristics of MCS in head and neck regions including brain origin was well described in a recent study [11].It occurs in relatively younger age, and the median age at diagnosis was 19 years (range: 6-54 years) [11].The patient in this case was oldest among 4 MCS of brain origin in that study [11].Absence of cartilagenous area was observed in 4 of 13 cases [11] as seen in the recurrent tumor of our case.
Histologically, the tumor was composed of small round to spindle-shaped cells with a hemangiopericytomatous vasculature.Staghorn/hemangiopericytomatous vessels were also described as a frequent histological feature as seen in this case [11].The differential diagnoses were Ewing's sarcoma, synovial sarcoma, and SFT.It is difficult to correctly diagnose MCS, especially when only a small round cell area is collected, as was the case for the second and subsequent surgical specimens in this case.Immunostaining for NKX3.1 [8] and SOX9, a master regulator of chondrogenesis, has been reported to be useful [14].IHC for NKX3.1 and SOX9 was also performed in this case.Tumor cells in both small round/spindle cell and cartilage areas showed positive staining for SOX9, while almost only small round/spindle cell area showed positive staining for NKX3.1.Thus, IHC using these antibodies may be able to distinguish tissues composed of only small round cells without cartilage components.
Therapeutic strategies that are currently believed to reduce the risk of recurrence include radical resection with radiation and chemotherapy.Xu et al. reported that two patients with MCS who received neoadjuvant chemotherapy did not show any therapeutic response [11], and Huvos et al. also demonstrated that four patients with MCS did not show response to preoperative high dose methotrexate-based chemotherapy [12].However, Tsuda et al. found stable disease in three MCS patients and partial response in one MCS patient treated with neoadjuvant chemotherapy [13].Thus, chemotherapeutic effect on MCS is still controversial, and new therapeutic option is expected.
Regarding tumorigenesis, Qi et al. reported that plateletderived growth factor receptor alpha, which belongs to a family of receptor tyrosine kinases, was upregulated by HEY1::NCOA2 fusion in a study using transduced induced pluripotent stem cell MSCs with inducible expression of the HEY1::NCOA2 fusion protein [16].In addition, a recent report showed that imatinib, a tyrosine kinase inhibitor (TKI), significantly reduced tumor growth in the HEY1:: NCOA2 fusion-driven cellular model as well as in MCSpatient derived xenograft models [17].Therefore, although further research is required, TKI may be effective in treating MCS.

Conclusion
We encountered a case of intracranial MCS harboring two alternative forms of the HEY1::NCOA2 fusion transcripts.

Data availability statement
The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

FIGURE 1
FIGURE 1 Magnetic resonance imaging (MRI) with contrast enhancement.Preoperative MRI showed a protruding mass above the sella turcica [(A) coronal view, (B) sagittal view].(C-F) Representative pathological findings.Hematoxylin-eosin staining showed a tumor composed of small round cells and islands of well-differentiated hyaline cartilage with atypical chondroid cells floating in lacunar spaces (original magnification, ×200) (C).A neoplastic cartilage cells do not show higher nuclear atypia, but they have constricted nuclei (original magnification, ×200) (D).Areas of dense proliferation (Continued )