SMARCA4 -deficient Undifferentiated Uterine Sarcoma: Clinicopathological Features of an Emerging Entity

Introduction

Mesenchymal neoplasms of the uterus are classified as smooth muscle or endometrial stromal tumors based on morphological and immunohistochemical studies. In the absence of evidence of such differentiation, and with limited application of molecular techniques, undifferentiated uterine sarcoma was considered a diagnosis of exclusion. However, with the evolving genetic landscape of uterine tumors, a subset of molecularly defined neoplasms with distinct clinicopathological characteristics have emerged viz. SMARCA4-deficient undifferentiated uterine sarcoma. ¹ SMARCA4 encodes the BRG1 protein, a member of Switch/ Sucrose Non-Fermenting (SWI/SNF) chromatin remodeling complex, and functions as a tumor suppressor gene. Inactivating germline and/or somatic mutations in the SMARCA4 gene were recently established as the key driver molecular alteration in small cell carcinoma of ovary, hypercalcemic type (SCCOHT) and thoracic undifferentiated carcinomas, both of which demonstrate rhabdoid features akin to SMARCB1 (INI1)- deficient malignant rhabdoid tumors at other sites.^2,3 Mutations in the various subunits of SWI/SNF complex ie SMARCA4, SMARCB1, ARID1A, and ARID1B genes have also been described in other gynecologic cancers, as an early or late event in carcinogenesis. ⁴ Since their documentation as a distinct entity, 28 cases of SMARCA4-deficient undifferentiated uterine sarcoma have been described in three studies, where SMARCA4 mutation was the sole driver mutation in majority of cases.^5-7 Loss of SMARCA4 by immunohistochemistry (IHC) corroborates well with SMARCA4 mutation status and thus serves as a valid diagnostic biomarker in routine practice.^5,8 Interestingly, secondary loss of SMARCA4 has also been described in undifferentiated or dedifferentiated endometrial carcinomas, thus raising implications for differential diagnosis. ⁴ Herein, we describe a case of SMARCA4-deficient undifferentiated uterine sarcoma to expand the current knowledge regarding this novel entity.

Case Report

A 52-year-old lady, with no significant family history, presented to our hospital with complaints of postmenopausal bleeding for one year. She underwent hysteroscopic-guided polypectomy which was diagnosed as a poorly differentiated malignant tumor, likely carcinosarcoma on histopathological examination. Ultrasonography revealed a 75 × 70 mm tortuous cystic lesion in the endometrium with partial septate and homogenous internal echoes. Subsequently, she underwent hysterectomy with bilateral salpingo-oophorectomy and pelvic lymph node dissection. Grossly, an exophytic growth of 5 × 3 × 1.5 cm was identified in the fundus of the uterus infiltrating more than half the myometrium and extending through to the serosa. The attached parametrial tissue was not involved by the tumor. Microscopically, the tumor was composed of sheets of undifferentiated epithelioid cells with moderate amount of cytoplasm and vesicular nuclei with prominent nucleoli (Figure 1A–C). A proportion of tumor cells demonstrated eccentric nuclei and bright eosinophilic cytoplasm imparting a rhabdoid appearance (Figure 1B–D). Mitotic activity was brisk. Lymphovascular tumor emboli (Figure 1E) at both intratumoral and extratumoral locations were present. Focally, the tumor cells were surrounded by hyalinized stroma. Towards the endometrial surface, many entrapped atrophic endometrial glands were noted (Figure 1F). Phyllodes-like architecture, periglandular stromal condensation, glandular architecture, small cell and spindle cell areas were not seen. Reticulin stain highlighted vague nested architecture of tumor cells. Immunohistochemically, the neoplastic cells were diffusely positive for vimentin and CD99; epithelial membrane antigen (EMA) and synaptophysin staining was present in occasional cells, while pancytokeratin, estrogen receptor, smooth muscle actin (SMA), desmin, FLI1, chromogranin, cyclin D1, leukocyte common antigen, WT1, p16, SALL4, CD10, KRT7 and PAX8 were negative (Figure 2). Wild type positivity was noted for p53. Mismatch repair (MMR) proteins (MLH1, MSH2, MSH6 and PMS2) showed intact nuclear expression in the tumor cells. Β-catenin displayed membranous positivity in tumor cells. INI1 expression was retained. The tumor cells showed loss of SMARCA4/ BRG1 expression (Figure 3). Bilateral ovaries and fallopian tubes were uninvolved by the tumor. Based on these findings, the diagnosis of SMARCA4- deficient undifferentiated uterine sarcoma was suggested. One of 22 pelvic lymph nodes was infiltrated by metastatic tumor. The initial post-operative period was uneventful; however, the patient developed cardiorespiratory arrest on the 15th post-operative day and expired.

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Figure 1.

Histopathology of SMARCA4-deficient undifferentiated uterine sarcoma. The tumor is composed of relatively monomorphic epithelioid cells arranged in solid sheets (A, H&E), with moderate amount of cytoplasm, and vesicular nuclei with prominent nucleoli. Mitotic figures are present (B, C; H&E). Few cells show rhabdoid morphology with eosinophilic paranuclear inclusions (arrow) highlighted on PAS stain (D). Lymphovascular tumor embolus is present (E; H&E). Atrophic endometrial glands are seen entrapped within the tumor (F; H&E). [PAS, periodic acid Schiff].

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Figure 2.

Immunohistochemistry shows diffuse positivity for vimentin (A; IHC), and focal positivity for EMA (B; IHC) and synaptophysin (C; IHC). p53 (D; IHC), cyclin D1 (E; IHC) and SMA (F; IHC) are negative [EMA, epithelial membrane antigen; IHC, immunohistochemistry; SMA, smooth muscle actin].

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Figure 3.

Immunohistochemistry shows intact nuclear expression of mismatch repair proteins (A, MLH1; B, MSH2; C, MSH6; D, PMS2) and INI1 (E) in the tumor cells. The tumor cells showed loss of SMARCA4 expression (F).

Discussion

SMARCA4-deficient undifferentiated uterine sarcoma is a recently proposed distinct entity characterized by truncating mutations and deletions in the SMARCA4 gene. ⁵ SMARCA4 and SMARCB1 are components of the catalytic ATPase subunit of the SWI/SNF complex; thus, SMARCA4-deficient undifferentiated uterine sarcoma adds to the rapidly growing family of SW1/SNF-deficient tumors. The clinicopathological and molecular features of SMARCA4- deficient undifferentiated uterine sarcoma were first described in five patients by Kolin et al, ⁵ followed by an expanded cohort of cases by the same authors, ⁷ and another study by Lin and colleagues. ⁶ Being a novel entity with only 28 cases documented till date,^6,7 our case adds to the spectrum of clinical, morphological and immunohistochemical features of this neoplasm. The patient is a 52-year-old female; the age of presentation is similar to that observed by Lin et al, ⁶ and in contrast to the younger age group observed by Kolin et al.^5,7 Patients present with non-specific symptoms including vaginal bleeding or a utero-cervical mass.^5,6

Morphologically, SMARCA4- deficient undifferentiated uterine sarcoma is characterized by diffuse tumor growth composed of uniformly atypical epithelioid tumor cells with a variable proportion of cells displaying rhabdoid morphology.^1,5 Vague nested pattern highlighted by reticulin stain, phyllodiform architecture, spindle cell and small cell morphology have been described. Increased mitotic count, necrosis, myometrial invasion and lymphovascular invasion are consistent findings. Focal myxoid change or stromal hyalinization may also be identified.^1,6,7 Infiltration or entrapment of benign endometrial glands suggests that the endometrial stroma is the cell of origin of this neoplasm.^1,6 Immunohistochemically, these tumors are diffusely positive for vimentin, and focally positive for EMA, CD10, and synaptophysin. They are mostly negative for a wide spectrum of cytokeratins, smooth muscle markers, S100, chromogranin, KIT, claudin-4, and estrogen/ progesterone receptors.^6,7 All reported SMARCA4- deficient undifferentiated uterine sarcoma cases have shown loss of SMARCA4 expression with retained SMARCB1 staining, and are microsatellite stable, either by IHC or genetic testing.

As SMARCA4-deficient undifferentiated uterine sarcoma appears to be a distinct clinicopathological entity, this diagnosis may be considered after excluding potential mimics ie other malignancies with high grade undifferentiated morphology. This includes undifferentiated endometrial carcinoma, epithelioid leiomyosarcoma, malignant perivascular epithelioid cell tumor (PEComa), high grade endometrial stromal sarcoma, and adenosarcoma. Undifferentiated endometrial carcinoma, the principal differential diagnosis, usually presents in older patients, with a median age of 55 to 61 years, ⁷ while the median age of all SMARCA4- deficient undifferentiated uterine sarcoma patients reported till date is 43.5 years.^5-7 SMARCA4- deficient undifferentiated uterine sarcoma and undifferentiated endometrial carcinoma can have overlapping morphology and immunophenotypes, with rhabdoid cells having been observed in 25% of undifferentiated endometrial carcinoma, and loss of SMARCA4 expression being documented in 20%.^7,9 However, a key distinguishing feature is the striking nuclear pleomorphism of tumor cells in undifferentiated endometrial carcinoma. ⁷ Also, undifferentiated endometrial carcinoma is often accompanied by the presence of a differentiated endometrioid component.^8-10 Interestingly, loss of SMARCA4expression in undifferentiated endometrial carcinoma is limited to the undifferentiated carcinoma and is intact in the differentiated endometrioid carcinoma component, indicating its oncogenic role in tumor dedifferentiation and progression.^8-10 Loss of staining for MMR proteins is frequent in undifferentiated endometrial carcinoma, ranging from 44% to 73%.^8,11 Although tumor cells in undifferentiated endometrial carcinoma express most epithelial markers only focally, significant positivity for claudin-4 and cyclin D1 has been found to be helpful in making this diagnosis. Molecular testing, when available, would further aid in distinguishing the two entities, as SMARCA4 mutation is the only pathogenic driver alteration in SMARCA4- deficient undifferentiated uterine sarcoma, while SMARCA4-deficient undifferentiated endometrial carcinomas additionally show mutations in genes frequently altered in endometrial carcinoma such as PTEN, KRAS, CTNNB1, PIK3CA, and commonly exhibit microsatellite instability. Thus, a uterine undifferentiated epithelioid neoplasm with the absence of a morphologically differentiated, microsatellite unstable endometrioid carcinoma component, and lacking PTEN, KRAS, CTNNB1 and PIK3CA alterations should raise suspicion for SMARCA4- deficient undifferentiated uterine sarcoma.^5-7

Other morphological differential diagnoses of uterine malignant epithelioid tumors include epithelioid leiomyosarcoma and malignant PEComa. Both these tumors show mild to marked nuclear pleomorphism, accompanied by immunoreactivity with smooth muscle markers. PEComas, in addition, are immunopositive for melanocytic markers (HMB-45 and Melan-A). Contrarily, SMARCA4- deficient undifferentiated uterine sarcoma is negative for both smooth muscle and melanocytic markers. Adenosarcomas show classical leaf-like architecture with predominantly spindled morphology; epithelioid foci are rare, but can occur focally with smooth muscle differentiation. ⁵ When present in SMARCA4- deficient undifferentiated uterine sarcoma, this phyllodiform pattern is focal and poorly developed. IHC lacks utility, as adenosarcomas, like SMARCA4- deficient undifferentiated uterine sarcoma, are microsatellite stable and do not express epithelial markers. However, overt sarcomatous features are usually absent in adenosarcomas, helping to distinguishing them from SMARCA4- deficient undifferentiated uterine sarcoma. ⁷ High grade endometrial stromal sarcomas are highly infiltrative tumors composed of fascicles of spindle cells in a myxoid stroma, or high grade small round cells, corresponding to genetic aberrations involving the BCOR gene. These tumors demonstrate variable CD10, but strong and diffuse cyclin D1 positivity, along with diffuse BCOR positivity in a subset of cases. ¹ Karnezis et al ² analyzed the immunoexpression of SMARCA4 in various gynecological tumors, and while none of the leiomyosarcomas and adenosarcomas included showed loss of SMARCA4 expression, 8% of endometrial stromal sarcomas included showed loss; one of these cases was high grade and had rhabdoid features, which, if reconsidered, may possibly fall into the diagnostic category of SMARCA4- deficient undifferentiated uterine sarcoma. Also, high grade endometrial stromal sarcomas can be characterized by translocations involving YWHAE. Thus, in view of overlapping morphological and immunohistochemical features, a high index of suspicion is necessary for recognizing SMARCA4- deficient undifferentiated uterine sarcoma from among its morphological mimics.

Comprehensive genomic profiling has revealed inactivating mutations of the SMARCA4 gene in SMARCA4- deficient undifferentiated uterine sarcoma.^5,6 While Kolin et al identified other somatic mutations in their cohort, Lin et al found that SMARCA4 mutations were the only pathogenic driver genetic alteration in 81% of SMARCA4- deficient undifferentiated uterine sarcoma, with no or few co-occurring oncogenic alterations in TP53, RB1 and CTNNB1 genes that are commonly associated with endometrial carcinomas.^5,6 Lin et al also detected a germline SMARCA4 gene mutation in a 55-year-old woman with a uterine sarcoma, who also had positive family history of a 32-year-old daughter who died of SCCOHT and carried the same SMARCA4 mutation. ⁶ Heterozygous germline mutations in SMARCA4 gene cause Rhabdoid tumor predisposition syndrome 2 (RTPS2), while rhabdoid tumor predisposition syndrome 1 is caused by SMARCB1 alterations. ⁶ Based on this, it is postulated that SMARCA4- deficient undifferentiated uterine sarcoma is a component of RTPS2, suggesting that SMARCA4- deficient undifferentiated uterine sarcoma patients should undergo thorough clinical evaluation for other SWI/SNF-deficient tumors including malignant rhabdoid tumors and SCCOHT, along with screening of family members.^5,6

Like other known SW1/SNF-deficient neoplasms, SMARCA4- deficient undifferentiated uterine sarcoma has an aggressive clinical course, where patients present with high clinical stage, extrauterine spread, and metastases.^5,6 The clinical data regarding this entity is scarce, and the limited number of reported cases have shown a median survival of 7 to 9 months.^5,7 Recognition of this entity may have possible therapeutic implications in future, as these patients could be benefited by enrollment in ongoing clinical trials for targeted therapies including PD-L1, EZH2, and CDK4/6 inhibitors for SCCOHT which shares genetic features with SMARCA4- deficient undifferentiated uterine sarcoma. ¹

Conclusion

A newly recognized entity is described in a postmenopausal woman with a uterine mass, and is differentiated from its histologic mimics. Apart from prognostication, diagnosing SMARCA4- deficient undifferentiated uterine sarcoma is imperative as the patient may be considered for inclusion in clinical trials for targeted therapies, and for screening the patient and family members for germline SMARCA4 mutations and associated neoplasms. Due to its rarity, documentation of clinicopathological features and clinical outcome of this rare neoplasm are imperative to gain further insight into its biological behavior.