EWSR1-fusion-negative, SMARCB1-deficient primary pulmonary myxoid sarcoma

Introduction

Primary pulmonary myxoid sarcoma (PPMS) is a rare low-grade malignant neoplasm with distinctive clinicopathological, immunophenotypic, and molecular-genetic features. Since its first description by Nicholson et al. in 1999 as a low-grade malignant myxoid endobronchial tumour [1], no more than 20 well-documented cases appeared in the English-language literature (reviewed in Thway et al. [2]). The term PPMS was coined by Thway et al. in 2011, who reported a series of 10 cases including the two original cases and delineated the molecular-genetic profile of this exceptionally rare entity [2]. Primary pulmonary myxoid sarcoma is now included in the current World Health Organisation (WHO) classification of lung tumours as a separate entity [3]. In this report, we describe a new case of PPMS that showed prototypical histomorphological and immunophenotypic features of that entity. However, fluorescence in situ hybridisation (FISH) failed to show any evidence of EWSR1, FUS, or NR4A3 gene fusions. Instead, immunohistochemistry showed complete loss of SMARCB1 (INI1) in the neoplastic cells indicating a role for SMARCB1-deficiency as a possible alternative molecular mechanism underlying some EWSR1-negative PPMS cases.

Case history

A 48-year-old Caucasian male with a significant smoking history and chronic obstructive pulmonary disease (COPD) presented with partial respiratory insufficiency. Clinical and imaging investigations (computer tomography of the chest) showed extensive retention pneumonia involving the right and partially the left lung. A huge mass measuring > 14 cm was seen on CT occupying the right hemithorax and extending into the right main bronchus, compressing the left main bronchus and reaching the trachea (Fig. 1).Bronchoscopic biopsies were obtained from the endobronchial component. At the time of initial evaluation, there was no evidence of regional or distant metastases. The patient received several cycles of palliative chemotherapy (doxorubicin and trabectedin). Follow-up imaging showed initially stable disease but later investigations revealed signs of disease progression. The patient then received palliative radiotherapy again. CT examination showed a new focal lesion suggestive of cerebellar metastasis 13 months later. At last follow-up (23 months after initial diagnosis), he was alive with disease under palliative treatment.

Material and methods

The biopsy specimen was fixed in buffered formalin and embedded in paraffin for routine histological examination. Immunohistochemical studies were performed on 3-µm sections cut from paraffin blocks using a fully automated system (“Benchmark XT System”, Ventana Medical Systems Inc., 1910 Innovation Park Drive, Tucson, Arizona, USA) and the following antibodies: pan cytokeratin (clone AE1/AE3, 1 : 40, Zytomed, Berlin, Germany), vimentin (V9, 1 : 100, Dako, Hamburg, Germany), CK7 (clone OV-TL, 1 : 1000, Biogenex), CK18 (clone CY-90, 1 : 500, Sigma), CK5 (clone XM26, 1 : 50, Zytomed), TTF-1 (clone 8G7G3/1, dilution, 1 : 500, Zytomed), ERG (clone EPR3864, prediluted/ready to use, Ventana Medical Systems), CD31 (clone JC70A, 1 : 20, Dako), CD10 (clone 56C6, 1 : 20, Dako), p63 (clone SFI-6, 1 : 100, DCS), desmin (clone D33, 1 : 250, Dako), -smooth muscle actin (clone 1A4, 1 : 200, Dako), S100 protein (polyclonal, 1 : 2500, Dako), CD34 (clone BI-3C5, 1 : 200, Zytomed), CD30 (clone Ber-H2, 1 : 40, Zytomed), MUC4 (clone EP256, 1:500, Epitomics), TLE1 (polyclonal, 1 : 200, Santa Cruz), STAT6 (clone sc-621, 1 : 1000, Santa Cruz Biotechnology), and SMARCB1 (INI1) (MRQ-27, 1 : 50, Zytomed), according to the manufacturer instructions.

Results

The biopsy material was composed of multiple polypoid fragile gelatinous endobronchial exophytic tumour fragments measuring together 1.5 cm (Fig. 2A). Histological examination showed a low to moderate cellular neoplasm composed of bland looking medium-sized oval to rounded epithelioid cells arranged in a prominent reticular and microcystic lace-like chordoid pattern in a highly myxoid stroma (Fig. 2B, C). Mitoses were scant, with < 2 mitoses/10 HPFs. There was prominent vascularisation of the stroma (Fig. 2B). Less than 10% of the biopsies were composed of cellular solid areas with similar cellular features as the myxoid areas (Fig. 2D). The solid cellular foci blended with the myxoid component and were occasionally surrounded by old haemorrhages. No pleomorphism, necrosis, or atypical mitoses were observed. Immunohistochemistry showed diffuse expression of vimentin (Fig. 3A) and variable expression of EMA (Fig. 3B) and CD10 (Fig. 3C), while pan cytokeratin (Fig. 3D) and all other lineage-specific markers listed above were negative. In addition, the neoplastic cells showed complete loss of nuclear SMARCB1 (INI1) expression by immunohistochemistry with retained positivity in the normal cells in the background (Fig. 3E, F). The bronchial mucosa showed no evidence of surface epithelial dysplasia and there was no epithelial tumour component.
FISH analysis using EWSR1, FUS, NR4A3, and SMARCB1 double-colour probes revealed no recognisable translocation or copy number aberrations.

Discussion

Primary pulmonary myxoid sarcoma (PPMS) is exceedingly rare with no more than 20 well-documented cases in the English-language literature (Table I) [1, 2, 4, 5, 6, 7, 8]. The size of reported cases varied from 1.5 cm to 13 cm (median, 4 cm). All 16 tumours with detailed gross description presented as predominantly endobronchial masses leading to significant retention pneumonia. The disease affected females and males with equal frequency (11 females and 10 males) in the age range 26 to 68 years (median, 48 years). There was no difference in the disease distribution in the lower (n = 10) vs. upper (n = 8) lung lobes or in the left (n = 10) vs. right (n = 11) lung. Metastatic disease occurred in 4 patients a few to 13 months after diagnoses and was localised in the brain, kidney, and contralateral lung. Nine patients were alive without evidence of disease at last follow-up (12-180 months; median 69).
Variable immunohistochemical stains have been performed on individual cases. These revealed consistent expression of vimentin (18/18) and focal staining for EMA (9/17) and S100 protein (2/21). Desmin was expressed in a single case (1/19). None of the cases tested showed expression of pan cytokeratin (0/20), p63 (0/11), or smooth muscle actin (0/7).
Although available data is still limited, the molecular profile of PPMS seems to impact prognosis. Thus, all of the eight patients who remained disease-free at last follow-up had detectable EWSR1 gene fusions, with seven of them having EWSR1-CREB1 translocations and one case showing positive EWSR1 status with unknown fusion partner. On the other hand, of the four patients who developed metastatic disease (one died of his disease a few months after diagnosis), three had wildtype EWSR1 status (including the current report).
Primary pulmonary myxoid sarcoma shows histological and molecular similarity to extraskeletal myxoid chondrosarcoma [9], myxoid angiomatoid fibrous histiocytoma [10, 11], and myxoid myoepithelial carcinoma of soft tissue [12]. Although, the unique growth pattern of this enigmatic entity with a predominantly endobronchial growth might suggest a salivary-analogue neoplasm [13], the consistent morphology lacking any salivary-like myoepithelial or mixed areas, absence of specific myoepithelial markers by immunohistochemistry, and the molecular features of this distinctive entity are more in line with true mesenchymal (soft tissue) origin than with a myoepithelial neoplastic entity. In terms of reproducible morphology and other distinctive clinicopathological features, PPMS represents a unique rare histologically low-grade sarcoma that needs to be distinguished from more aggressive primary and metastatic soft tissue tumours such as extraskeletal myxoid chondrosarcoma, chordoma, myxoid epithelioid sarcoma, and myoepithelial carcinoma as well as from lung sarcomatoid carcinoma with prominent myxoid changes [9, 13, 14].
Most difficult and even arbitrary is the distinction of PPMS from rare pulmonary cases of myxoid angiomatoid fibrous histiocytoma [10, 11] and extraskeletal myxoid chondrosarcoma [14]. Indeed, it is still unclear if some of the former represented unusual variants of PPMS. However, the most relevant differential diagnostic distinction when encountering a putative case of PPMS is to exclude pulmonary metastasis from occult or previous soft tissue primary because this may herald a more aggressive behaviour with significant prognostic implications.
The available literature (Table I) suggests genotypic heterogeneity in the group of PPMS. Absence of ESWR1 gene fusions in a subset of cases (mostly with more aggressive course) suggests different molecular subsets of lesions unified by the phenotype of PPMS. The current report is in line with the existence of different molecular subtypes in the spectrum of primary pulmonary myxoid sarcoma and indicates SMARCB1 loss as a possible alternative molecular driver in some PPMS. In this context, it should be emphasised that SMARCB1 loss seems to be exceptionally rare in pulmonary neoplasms because none of 316 non-small cell lung cancer specimens analysed in a recent study showed loss of SMARCB1 [15]. This is in sharp contrast to the reported SMARCB1 loss in variable subsets of mesenchymal, myoepithelial, and epithelial neoplasms in different organs [16]. Thus, it would be of relevance to investigate future PPMS cases for SMARCB1 status to see if this is mutually exclusive with the EWSR1 fusion (then as a primary driver event) or if it merely represents a secondary epigenetic event.

The authors declare no conflict of interest.

References

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Address for correspondence

Abbas Agaimy, MD
Pathologisches Institut
Universitätsklinikum Erlangen
Krankenhausstraße 8-10
91054 Erlangen
Germany
tel. +49-9131-85-22288
fax: +49-9131-85-22613
E-MAIL: ABBAS.AGAIMY@UK-ERLANGEN.DE