Case report: Gastric carcinoma with SMARCA4 deficient: two cases report and literature review

SMARCA4-deficient gastric carcinoma has been reported sporadically since 2016. Only 29 patients have been reported; nevertheless, it is aggressive and highly malignant with poor outcomes. It has an immunohistochemical phenotype showing loss of SMARCA4 expression and can be accompanied by codeletion of other switch/sucrose non-fermentable chromatin-remodeling complex subunits. Microscopically, it displays high-grade undifferentiated histological morphology with rhabdoid cell differentiation. Rarely does the tumor contain a purely or partly adenocarcinoma component. Here, we report two cases to demonstrate these unusual morphologies analyzed using morphological and immunohistochemical techniques. In addition, there is a lack of research on the classification of these morphologies. Therefore, our report will aid the diagnosis and classification of SMARCA4-deficient gastric carcinoma.


Introduction
Undifferentiated gastric carcinoma is a primary tumor without specific cytological or architectural types of differentiation (1).Expression of switch/sucrose non-fermentable (SWI/SNF) chromatin-remodeling complex subunits is reportedly deficient in some cases; these subunits include SMARCA4, SMARCA2, SMARCB1, and ARID1A.SMARCA4deficient undifferentiated carcinomas (SD-UCs) are rare and were described first in 2016 by Agaimy et al. (2) The morphological features are solid, diffuse sheets of polygonal cells with pleomorphic giant cells.These cells have vesicular nuclei and a high degree of mitosis, which are poorly cohesive.A rhabdoid cell component is common and may be the predominant pattern.Because rhabdoid cells are a diagnostic clue, the terminology undifferentiated/rhabdoid carcinoma was proposed by Chang et al. (3) A few cases of SMARCA4-deficient undifferentiated gastric carcinoma were reported demonstrating glandular differentiation or encompassing adenocarcinoma.According to the fifth World Health Organization (WHO) classification of the digestive system, undifferentiated gastric carcinoma is a malignant epithelial tumor composed of anaplastic cells with no specific cytologic or architectural differentiation, including glandular, squamous, neuroendocrine, and sarcomatoid differentiation (1).In this context, "dedifferentiated carcinoma" was recommended for cases with adenocarcinoma components (3).There are also SMARCA4deficient adenocarcinomas (SD-ADs), as we and Huang described (4).SMARCA4-deficient gastric carcinoma can be classified as SD-UC, where all components are undifferentiated, except the adenocarcinoma portion, according to the WHO.SMARCA4deficient dedifferentiated carcinoma (SD-DC) is the term used for tumors with adenocarcinoma components.SD-AD is used for cases comprising purely adenocarcinoma components.These tumors are then categorized as well, moderately, or poorly differentiated based on the percentage of glandular components.Although, the latter two are not included in WHO classification.
The three subtypes have different immunophenotypes; panCK, SMARCA2, and E-cadherin are positive in adenocarcinoma areas; the opposite is true in undifferentiated areas.Attention should be paid to discriminating among the three subtypes to classify these unusual tumors.
3 Case presentation

Case 1
A 65-year-old man presented to the Department of Gastroenterology, Zhongshan Hospital of Xiamen University, with a one-month history of indigestion after meals accompanied by pain in the right upper abdomen without any known inducement.Gastroduodenoscopy revealed a cauliflower-like mass with an ulcer in the cardia of the stomach (Figure 1A), and a biopsy was performed.
Computed tomography revealed liver and lymphatic metastases.After careful consideration, the patient and his family chose not to undergo further treatment.Clinical follow-up was available, and the patient was still alive more than 3 months from the date of diagnosis.

Case 2
A 75-year-old man presented with persistent hematochezia and melena associated with peripheral neuropathy and abdominal pain for over a month.Gastroduodenoscopy revealed a large mass occupying 50% of the antrum.Computed tomography revealed no lymphatic metastases.He was diagnosed with a malignant gastric tumor at another hospital.He was transferred to Zhongshan Hospital of Xiamen University and underwent gastrectomy.
The pathological findings revealed an elevated solid mass measuring 6.3 cm × 5.1 cm × 1.2 cm (Figure 2A) in the antrum next to the pylorus.Microscopically, the tumor invaded the muscularis propria of the gastric wall.It abruptly transitioned from the normal epithelium (Figure 2B).Two components were differentiated.One was a poorly differentiated adenocarcinoma with gland formation (Figure 2C).The other was an undifferentiated carcinoma with diffuse sheets of polygonal and pleomorphic giant cells.Cells were discohesive.Rhabdoid cells were common; these cells had conspicuous vesicular nucleoli (Figure 2D).Both carcinomas lost expression of SMARCA4 (Figure 2E) but were positive for SMARCB1, P53, and SALL4.They were negative for CgA, CD56, and CD34.Synaptophysin was weakly positive.The poorly differentiated adenocarcinoma expressed SMARCA2 (Figure 2F), panCK (Figure 2G), and Ecadherin (Figure 2H) but lost vimentin (Figure 2I).The undifferentiated carcinoma expressed vimentin but lost E-cadherin and showed reduced expression of panCK and SMARCA2.The tumor was finally diagnosed as SD-DC (T2N0M0).
He did not receive chemotherapy, immune checkpoint inhibitors, or targeted therapy because of his poor physical condition.Follow-up was performed, and the patient was alive after over five months.

Discussion
SMARCA4 is located on chromosome 19p13 and encodes the transcription activator BRG1.It is an ATP-dependent catalytic subunit of SWI/SNF chromatin-remodeling complexes that regulate chromatin structure and gene expression by supplying energy (11).The SWI/SNF chromatin-remodeling complexes usually consist of 12-15 proteins, including ATPase subunits (SMARCA4 and SMARCA2), core subunits (SMARCB1, SMARCC1, and SMARCC2), and various regulatory subunits (ARID1A, ARID1B, and ARID2).The essential diagnostic criteria of SMARCA4-deficient undifferentiated tumor depend on the detection of SMARCA4 (BRG1) deficiency by immunohistochemistry but not a genetic diagnosis (12).
Sequencing can be helpful to clarify the significance of reduced expression of SMARCA4, but it is not necessary for the diagnosis, because immunohistochemistry shows complete loss in most cases and is sufficient to document SMARCA4 deficiency.In addition, the mutation may not be detectable, depending on the limitations of the methods used.Because the second hit often copy-neutral loss heterozygosity (i.e., accompanied by duplication of the mutated allele) (13).SMARCA4 loss is characteristic of thoracic sarcomas but, now, it represents primarily undifferentiated and dedifferentiated carcinomas rather than primary thoracic sarcomas (13).It has been sporadically identified in human carcinomas in a variety of regions, including endometrioid adenocarcinoma, non-small cell lung carcinoma, carcinoma of the sinonasal tract, and small cell carcinoma of the ovary-hypercalcemic type (14).SMARCA4-deficient carcinoma has an extremely low incidence.A literature search on SMARCA4deficient gastric carcinoma returned 29 cases.Huang et al. screened SMARCA4 alterations using immunohistochemistry on 1,199 surgically resected gastric carcinomas and, in only six (0.5%), SMARCA4 was completely lost (4).
We reported two cases and reviewed the literature to classify these rare tumors.The clinicopathological features of our cases and reported cases were as follows (1): The tumor often occurred in middle-aged and older patients, 30-75 years old (average age: 62.3 years).Males predominated (77% [20/26]), and the clinical stages were III or IV in 78% (18/23).There was rapid progression and poor outcomes; the median overall survival was 8 months (3-190.1 months).The effect of conventional chemotherapy was poor (2).Histomorphologically, the tumors demonstrated sheets, trabecular, solid, nest, abortive gland, tubular distribution, and large epithelioid  or rhabdoid cells with low adhesion.Anaplastic cells had vesicular nuclei, prominent nucleoli, and high mitosis indexes (3).All tumors lost SMARCA4 expression; panCK was negative, and SMARCA2 was reduced or lost in the undifferentiated carcinomas.SMARCA2 was expressed in epithelial differentiation.In our cases, differentiated adenocarcinoma expressed SMARCA2, panCK, and E-cadherin but lost vimentin.The undifferentiated carcinoma expressed vimentin and but lost E-cadherin and showed reduced panCK and SMARCA2.
Considering the histomorphology and histochemistry phenotype, we propose a new category: SMARCA4-deficient gastric carcinoma, which may be divided into three subtypes (1): SD-UC, demonstrating diffuse sheets without epithelioid differentiation.Rhabdoid cells occur frequently and may be prominent.There are discohesive cells with anaplastic features.SMRCA2, E-cadherin, and epithelioid markers are negative or reduced, but vimentin is positive (2).SD-DC, demonstrating partly adenoid differentiation in SD-UC, as Chang (3) recommended.The adenocarcinoma areas express epithelioid markers, E-cadherin, and SMARCA2 but are negative for vimentin.The opposite is seen in the undifferentiated areas (3).SD-AD, encompassing purely gland, abortive glands, or nests like conventional adenocarcinoma without rhabdoid or discohesive cells.Epithelioid markers, E-cadherin, and SMARCA2 are positive.
Vimentin and E-cadherin are markers of epithelialmesenchymal transition expressed in the undifferentiated and glandular areas, respectively, in SD-DC.In our cases, a novel observation in SD-DC was the notable loss of SMARCA2 in the transition from adenocarcinoma to undifferentiated carcinoma.In the reported literature, we also found SMARCA2 expressed in the adenocarcinoma areas in SMARCA4-deficient carcinoma.This finding suggests that SMARCA2 expression changes in the transition to SMARCA4-deficient carcinomas, as Rekhtman proposed (13).
Diagnosing this rare entity is often challenging and relies on an extensive panel of immunohistochemical stains to exclude various morphologic mimics such as neuroendocrine carcinoma, melanoma, and small cell carcinoma of the ovary-hypercalcemic type.

Large cell neuroendocrine carcinoma
Tumors are solid with nests or pseudoglandular epithelioid monoclonal and adhesive cells.It expresses epithelioid markers and at least two neuroendocrine markers.SD-UC may express synaptophysin, mainly focal or weakly positive.

Melanoma
Immunohistochemical detection of HMB45, Melan-A, and S-100 is helpful.

SMARCA4-deficient malignant rhabdoid tumors
These tumors show substantial overlap in histomorphology and immunohistochemistry. Malignant rhabdoid tumors predominantly occur in children under 3 years old.

Metastatic small cell carcinoma of the ovary-hypercalcemic type
This tumor displays unique immune features, including the expression of Wilms' tumor suppressor gene 1, EMA, vimentin, cytokeratin, and neuroendocrine markers.SMARCA4-deficient gastric carcinoma is aggressive and resistant to traditional chemotherapy.Based on the antagonism of SWI/SNF and polycomb repressive complex2 (PRC-2), SWI/SNF deletion leads to loss inhibition of enhancer of zeste homolog 2 (EZH2) methyltransferase, which accelerates PRC2-mediated tumorigenesis (15).Urgent, efficient therapy is required.Tazemetostat is a small molecule enhancer of the EZH2 inhibitor approved by the U.S. Food and Drug Administration in 2020 for treating INI1-negative or SMARCA4-negative tumors.Tazemetostat reduces the trimethylation of H3K27 and induces durable tumor responses.Data from a phase I clinical trial of EZH2 inhibitors showed clinical activity consisting of objective responses (complete responses and partial responses) or prolonged stable disease (6.4 to > 20 months), which exceeded 2 years in 5 (38%) of 13 patients with INI1-negative or SMARCA4-negative solid tumors (16).
In conclusion, SMARCA4-deficient gastric carcinoma should be divided into three subtypes: SD-UC, SD-DC, and SD-AD, depending on histomorphology and immunophenotype.Even though they have no significant clinical characteristics, they have different histomorphology and immunohistochemistry.We must recognize these subtypes and collect more cases to characterize SMARCA4-deficient gastric carcinoma.

No Gross
Patterns Cytology

1
FIGURE 1 Gastroduodenoscopy showed (A) a cauliflower-like tumor with an ulcer in the cardia.(B) Hematoxylin and eosin staining showed glandular adenocarcinoma gradually transferred from the normal epithelium.High-grade intraepithelial neoplastic was seen with differentiated, moderately differentiated, and poorly differentiated adenocarcinoma.(C) Cells had large round to oval nuclei and coarse chromatin.The degree of mitosis was high (arrow).(D) Immunohistochemical staining revealed that SMARCA4 was lost in tumors while normal epithelium was retained.(E) PanCK, (F) E-cadherin, and (G) SMARCA2 were both positive.

2
FIGURE 2 Gross appearance showed (A) an elevated solid mass, sized 6.3 cm × 5.1 cm × 1.2 cm in the antrum.(B) Hematoxylin and eosin staining showed the tumor abruptly transferred from the normal epithelium.(C) The tumor encompassed two components with a clear-cut surface.Poorly differentiated adenocarcinoma with gland formation and undifferentiated carcinoma with diffuse sheets without epithelial differentiation.(D) Undifferentiated carcinoma of polygonal cells and pleomorphic giant cells.Rhobdoid cells were common with vesicular nuclei and conspicuous nucleoli.(E) Immunohistochemical staining revealed both areas lost SMARCA4.(F) SMARCA2 was positive in poorly differentiated adenocarcinoma and was reduced in undifferentiated carcinoma.(G) PanCK was positive in poorly differentiated adenocarcinoma and was reduced in undifferentiated carcinoma.(H) E-cadherin was positive in poorly differentiated adenocarcinoma and lost in undifferentiated carcinoma.(I) Vimentin was positive in undifferentiated carcinoma and lost in poorly differentiated adenocarcinoma.

TABLE 1
Clinical characteristics reported in SMARCA4-deficient gastric carcinoma.

TABLE 2 Continued
NA, not available.