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FUS-ERG gene fusion in isolated myeloid sarcoma showing uncommon clinical features

FUS-ERG gene fusion has not been reported in cases of myeloid sarcoma (MS), a subtype of acute myeloid leukemia involving extramedullary anatomic sites. Here, we report a case of a 48-year-old man with primary isolated MS of the anterior mediastinum, who later developed multiple extramedullary recurrences without bone marrow infiltration throughout the course. G-banding analysis of the cells in pericardial effusion at recurrence showed complex karyotypic abnormalities including t(16;21)(p11.2;q22). FUS break-apart fluorescent in situ hybridization analysis showed split signals in biopsy sections at initial diagnosis and recurrence. Reverse transcriptase polymerase chain reaction and direct sequencing demonstrated the presence of the FUS-ERG chimeric gene transcript. The patient underwent cord blood transplantation, but died of pneumonia on day 64. To our knowledge, this is the first report of isolated MS carrying FUS-ERG gene fusion. In future study, relationship between the fusion gene and uncommon clinical features should be investigated in isolated MS.


INTRODUCTION
FUS-ERG gene fusion can be detected in patients with acute myeloid leukemia (AML) and has been reported to be associated with poor prognosis [1]. Myeloid sarcoma (MS) is a subtype of AML involving extramedullary anatomic sites. MS usually arises in patients with AML involving bone marrow (BM) both at first presentation and in the clinical course. Here, we report a patient with primary isolated MS of the anterior mediastinum, who had FUS-ERG gene fusion and later developed multiple extramedullary recurrences without BM infiltration throughout the course.

CASE REPORT
A 48-year-old man was referred to our hospital because of a supraclavicular mass. He had been well until 3 months before admission, when he had developed cough and noticed the mass. He had a history of pulmonary emphysema. Fluorodeoxyglucose-positron emission tomography (FDG-PET)/computed tomography (CT) showed increased FDG uptake in the supraclavicular lymph nodes, and a bulky anterior mediastinal tumor ( Fig. 1A-C). Fine needle biopsy from the mediastinal tumor was performed. Hematoxylin and eosin-stained mediastinal sections showed diffuse infiltration of medium-sized tumor cells ( Fig. 1F and G). Immunohistochemistry demonstrated that the tumor cells were negative for CD1a, CD3, CD8, CD20 and TdT, and positive for CD4, CD43, CD56 (Fig. 1H) and CD99 (Fig. 1I). BM examination revealed no evidence of the infiltration of tumor cells throughout the course. The patient was diagnosed as having T-lymphoblastic lymphoma of clinical stage IIA and underwent an acute lymphoblastic leukemia-type chemotherapy regimen [2]. After the induction chemotherapy, he achieved partial response, and received subsequent post-remission and central nervous system prophylaxis therapies according to the treatment schedule. He also underwent 30 gray (Gy)/15 fraction (Fr) radiation therapy (RT) for the mediastinal bulky mass according to the treatment schedule. However, marginal relapse of the irradiated field was detected during the post-remission therapy. He underwent an additional 36 Gy/18 Fr RT for the lesion that progressively increased in size. Because he noticed painless right testicular swelling 10 months after the start of treatment, right orchiectomy was performed for diagnosis (Fig. 1D). Histopathologically, slides of testicular mass sections showed diffuse stromal infiltration of medium-to large-sized atypical cells ( Fig. 1J and K). Immunohistochemical analysis revealed that the tumor cells were negative for CD3, CD20, CD68 (PGM1) and TdT, and positive for CD4, CD56, MPO (Fig. 1L) and CD34 (Fig. 1M). Initial slides of mediastinal sections were retrospectively examined and positivity for MPO was confirmed. The tumor was diagnosed as MS. Although he had no neurological symptoms, cerebrospinal fluid examination showed infiltration of myeloid blasts. We started high-dose cytarabine (HDAC) therapy. However, the patient gradually developed headache, and head magnetic resonance imaging (MRI) detected multiple intraparenchymal masses, which were considered as the invasion of MS (Fig. 1E). He underwent 30 Gy/ 10 Fr whole-brain RT. We also started intrathecal (IT) injection of cytarabine, and blasts in cerebrospinal fluid were decreased.
The patient received salvage chemotherapy and subsequently underwent cord blood transplantation in non-CR, but died of severe pneumonia on day 64. Autopsy was performed, and unexpectedly, we found a residual MS lesion only in the left testis, despite the aggressive clinical course.

DISCUSSION
Primary isolated MS is very rare, and especially that presenting mediastinal tumor was reported to account for only 5% of isolated MS [4], and has been described in only a few case reports [5,6]. The patient developed multiple extramedullary recurrences without BM infiltration, which is unique even in the context of isolated MS because patients with isolated MS often develop leukemic infiltration of BM during the course [7]. The clinical characteristics of this case, such as primary mediastinal involvement, were not reported to be associated with the distinct clinical course in AML. Thus, FUS-ERG gene fusion may be one of the reasons for the uncommon clinical features.
In terms of the transcript subtype, four types of FUS-ERG chimeric gene transcript have been reported in AML [1], and this patient had type A and B subtypes. Subtype B is the transcript produced by in-frame fusion, which is frequently found in AML with t(16;21). Subtype A is an out-of-frame transcript, which is thought to be produced by alternative splicing. We confirmed strong expression of ERG protein in the sections both at first presentation and at relapse (data not shown). Therefore, we assume that the fusion protein was expressed in tumor of this patient.
It was reported that chromosomal aberration in BM or peripheral blood blasts was detected in approximately half of patients with MS who developed leukemia, and monosomy 7, trisomy 8 and MLL were common abnormalities [8]. Recently, rare fusion genes have been reported in MS, which were related to uncommon clinical manifestations [9]. Although we report for the first time MS with t(16;21) producing FUS-ERG chimeric gene transcript, this disease entity might not be so very rare among isolated MS cases because G-banding analysis often cannot detect mitotic cells from solid tumors. In future study, isolated MS with uncommon features should probably be examined by FISH analysis with probes for detecting FUS-ERG or other gene fusions.
Optimal therapy for MS has not been determined. AML-type chemotherapy followed by RT for a localized tumor site may be performed for the treatment of patients with isolated MS [10]. In our report, RT was effective for the local control of the disease because the sites of recurrence were outside of the field of RT. Additionally, if a suitable donor is available, allogeneic hematopoietic stem cell transplantation (HSCT) may be effective for isolated mediastinal MS [6]. In fact, given that the patient underwent allogeneic HSCT in non-CR, the disease was well controlled at 2 months after HSCT.
In conclusion, we report for the first time primary isolated MS with FUS-ERG gene fusion. In future study, it should be investigated whether a causal relationship exists between the fusion gene and uncommon clinical features in isolated MS.

ETHICAL APPROVAL AND CONSENT
No ethical approval is required for case reports in our institution.