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Renal medullary carcinoma is a rare, highly aggressive primary renal tumor first described by Davis et al1, typically affecting young patients with sickle-cell trait or disease. This tumor predominantly arises in the renal medulla and may exhibit a variety of growth patterns including reticular, solid, tubular, trabecular, cribriform, sarcomatoid and micropapillary.1, 2, 3, 4, 5, 6, 7 The tumor cells usually have large vesicular nuclei and prominent nucleoli with densely opaque eosinophilic cytoplasm. Eccentric nuclear displacement in some cases creates a rhabdoid appearance. The fatality approaches 100% within several weeks to months after diagnosis. An explanation for its aggressive behavior is unknown.

INI1 (hSNF5/SMARCB1/BAF47) is a highly conserved factor, from yeasts to humans, in an ATP-dependent chromatin-modifying complex-SWI/SNF.8 Loss of this factor in transgenic mice results in aggressive rhabdoid tumors or lymphomas with 100% mortality.9, 10 In humans, loss of INI1 expression has been reported in pediatric renal and extrarenal malignant rhabdoid tumors,11, 12 atypical teratoid/rhabdoid tumors of the central nervous system12, 13 and epithelioid sarcomas, most likely a primary soft tissue rhabdoid sarcoma.14, 15 INI1 loss and rhabdoid histology are independently associated with aggressive tumor behavior. The recognized histologic spectrum of renal medullary carcinoma includes but is not restricted to rhabdoid morphology. The extent to which INI1 and rhabdoid-morphology may be linked, if at all, in those tumors may help to understand the biologic behavior.

The present study investigates INI1 expression in renal medullary carcinomas and other high-grade renal tumors, including those with rhabdoid histologic features. The results show a loss of INI1 in all renal medullary carcinomas and pediatric renal rhabdoid tumors, but persistent INI1 expression in high-grade renal carcinomas including those with rhabdoid features. The results in this study demonstrate that loss of INI1 is a common feature of renal medullary carcinoma with or without rhabdoid morphology and may be linked to an underlying molecular mechanism, which may at least in part explain the extremely aggressive behavior.

Materials and methods

Five renal medullary carcinomas were recovered from the archived surgical pathology files of the University of Chicago between 1994 and 2007. Table 1 summarizes the relevant clinical and pathological information. For comparison, 10 examples of high-grade renal cell carcinoma and two high-grade urothelial carcinomas of the renal pelvis were selected from recent surgical pathology accessions. Two cases of pediatric rhabdoid tumor were available, one from the archived surgical pathology files and one from the consultation files of one of the authors (JBT). All tumors had been thoroughly sampled for routine H & E light microscopy and a representative block from each case was selected for detailed further study.

Table 1 Clinical and pathologic features of renal medullary carcinoma, rhabdoid tumors of kidney and other high-grade renal tumors
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INI1 immunohistochemistry was performed utilizing a mouse monoclonal antibody (BD Transduction Labs, San Diego, CA, USA) at 1:200 dilution. EZH2 (a factor in the polycomb chromatin-modifying complex) immunohistochemistry was performed using a polyclonal rabbit anti-human antibody (Zymed, San Francisco, CA, USA) at 1:1000 dilution. Slides were subjected to heat-induced epitope retrieval pretreatment in citrate buffer (pH 6.0) for 3 min, followed by cooling to room temperature. Sections were incubated with the primary antibodies for 2 h at room temperature. An additional panel of antibodies was used in each case: CD10 (Novocastra Laboratories Ltd, Newcastle Upon Type, United Kingdom, at 1:20 dilution), vimentin (DakoCytomation Inc., Carinteria, CA, USA at 1:100 dilution), epithelial membrane antigen (Cell Marque Corporation, Rocklin, CA, USA at 1:10 dilution), S100 (DakoCytomation Inc., at 1:100 dilution), cytokeratins AE1/AE3 (Signet Laboratories Ltd, Deham, MA, USA at 1:20), CAM5.2 (BD Biosciences, San Jose, CA, USA), CK7 and CK20 (DakoCytomation Inc., at 1:100 dilution). These immunohistochemical stains were carried out in routine fashion using a DAKO Autostainer.

Results

A total of 19 cases were studied: five renal medullary carcinomas, two rhabdoid tumors of kidney, 10 renal cell carcinomas and two urothelial carcinomas. The salient clinical, histologic and immunohistochemical features are summarized in Table 1. All renal medullary carcinomas were young adults (three female, two male) with tumor sizes ranging from 5.5–10 cm. All patients were Black, all had sickle-cell trait and presented with extensive extrarenal metastases at the time of diagnosis. One of those patients died 5 months after diagnosis. In the comparison group of high-grade renal carcinomas, the sizes ranged from 4 to 11 cm. Five of the ten renal cell carcinomas had rhabdoid features histologically. Two had extrarenal metastases at the time of diagnosis. The two rhabdoid tumors of kidney were 11 and 8 cm.

The renal medullary carcinomas exhibited a broad spectrum of histologic features with rhabdoid features ranging from <5–50% of tumor cells in four cases. One had no rhabdoid features, although this tissue sample was a needle biopsy. Renal medullary carcinoma no. 1 (Figure 1a) showed a reticular pattern with rhabdoid cells: eccentrically placed vesicular nuclei, prominent nucleoli and dense, dark eosinophilic cytoplasm. Renal medullary carcinoma no. 2 had a diffuse pattern with less prominent rhabdoid cells (Figure 1c). The immunohistochemical studies are summarized in Table 2. INI1 immunohistochemical staining in these cases (Figure 1b and d) was totally negative. Focal microcystic, trabecular and sarcomatoid patterns, in addition to rhabdoid areas, were all INI1 negative. Thus, there was a complete loss of INI1 in all five cases of renal medullary carcinoma, irrespective of the histopathologic features. Since INI1 is ubiquitously expressed in non-neoplastic nuclei, endothelial cells and lymphocytes within the tumor served as internal controls. Renal medullary carcinomas were also positive for EZH2, a factor in the polycomb chromatin-modifying complex; vimentin, epithelial membrane antigen, cytokeratin AE1/AE3 and CAM5.2 were also positive as shown in Table 2.

Figure 1
figure 1

Morphological features and INI1 immunohistochemistry of renal medullary carcinomas. (a) Hematoxylin and Eosin stain of Case 1 demonstrating a reticular histologic pattern with rhabdoid type cells. (b) INI1 immunohistochemical stain of Case 1 showing absent nuclear staining. (c) Hematoxylin and Eosin stain of Case 2 with a solid growth pattern. (d) INI1 immunohistochemical stain of Case 2; nuclear staining absent. Positive nuclear staining is seen in background lymphocytes.

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Table 2 Immunohistochemical summary
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Histologic examination of the high-grade renal cell carcinomas and urothelial carcinomas of the renal pelvis demonstrated rhabdoid morphology in five cases of renal cell carcinoma. In Case 8 (Figure 2a), the tumor cells having predominant rhabdoid features (Figure 2a) showed strong nuclear reactivity to INI1 (Figure 2b). The sarcomatoid features of Case 12 (Figure 2c) also demonstrated strong INI1 reactivity (Figure 2d). Reactions for the remaining immunohistochemical markers were similar to the renal medullary carcinoma cases.

Figure 2
figure 2

Morphological features and INI1 immunohistochemistry of renal cell carcinomas with rhabdoid cytologic features. (a) Hematoxylin and Eosin stain of Case 8 with prominent rhabdoid features. (b) INI1 immunohistochemical stain of Case 8 showing positive nuclear reactions. (c) Hematoxylin and Eosin stain of Case 12 with solid growth and focal rhabdoid features. (d) INI1 immunohistochemical stain of Case 12 with strongly positive nuclear reactions.

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Two primary rhabdoid tumors of the kidney showed typical histologic features (Figure 3). There was a complete loss of INI1 staining in both cases. The remaining antibody panel was positive as found in the renal cell carcinoma and renal medullary carcinoma groups.

Figure 3
figure 3

Morphological features and INI1 immunohistochemistry of rhabdoid tumors of kidney. (a) Hematoxylin and Eosin stain of Case 6. (b) INI1 immunohistochemical stain of Case 6 demonstrating a diffuse negative nuclear reaction. (c) Hematoxylin and Eosin stain of Case 7. (d) INI1 negative immunohistochemical stain of Case 7. Positive nuclear staining is seen in background lymphocytes.

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Discussion

The rhabdoid phenotype in tumors of the kidney is evidence of potentially aggressive behavior. This includes tumors in the pediatric group and less commonly in adults.1, 2, 3, 4, 5, 6, 7, 16, 17 Prominent rhabdoid features have also been identified and are characteristic of epithelioid sarcoma and atypical/teratoid tumors of the central nervous system. The rare renal medullary carcinoma seems restricted to patients with sickle-cell disease or trait,1, 2, 3, 4, 5, 6, 7 and despite varied histologic growth patterns also manifests variably prominent rhabdoid cytology. The possibility of a genetic connection between a rhabdoid cytologic phenotype and the aggressive biologic behavior of tumors, which exhibit this feature has not yet been elucidated.

INI1 protein is a member of the SWI/SNF multiprotein complex, which is involved in chromatin modification in an ATP-dependent manner.8 Loss of this factor in mice results in aggressive rhabdoid tumors or lymphomas with 100% mortality.9, 10 The function of this protein is incompletely understood. Previous studies of pediatric rhabdoid tumors have shown uniform absence of immunoreactivity to INI1 protein while previous studies of rhabdoid elements in renal cell carcinomas have shown positive reactions.11, 12, 13 It is possible that the absence of INI1 staining in primary rhabdoid tumors of the kidney and renal medullary carcinomas indicates a common biallelic inactivation of the INI1/hSNF5 tumor suppressor gene of chromosome 22q11.2. Recently, Sigauke et al12 also reported the absence of INI1 immunoreactivity in one renal medullary carcinoma case of a series of extrarenal and renal rhabdoid tumors.

In the present series, in addition to the loss of INI1 expression in two pediatric rhabdoid tumors of kidney, there was a complete absence of INI1 in all five renal medullary carcinoma cases exhibiting varying histologic patterns, including rhabdoid features. All high-grade renal cell carcinomas and urothelial carcinomas of renal pelvis in the present study, even those with rhabdoid features, retained strong INI1 immunohistochemical reactivity. This study did not pursue further identifying INI1/hSNF5 alterations using FISH, Reverse Transcriptase-PCR, DNA sequencing or other molecular methods. Previous studies have shown that up to 20–30% of renal and extra-renal rhabdoid tumors with loss of immunoreactivity to INI1 protein do not show alterations at the DNA level.11, 12, 18, 19 If INI1 has a diagnostic significance, it is, therefore, at the level of protein expression by INI1 immunohistochemistry in support of the morphologic diagnosis of malignant rhabdoid tumor. In the present series, the relation between INI1 immunoreactivity and potential upstream molecular alterations, while interesting, does not add diagnostic relevance and is beyond the scope of this study.

The results of this study suggest that rhabdoid cytologic features, while indicative of biologic aggressiveness in the neoplasms in which it has been described,11, 12, 13, 14, 15, 16 may represent a phenotypic change or series of changes for which loss of INI1 expression may be one factor. A recent report showed that INI1-deficient tumors and rhabdoid tumors are convergent, but not fully overlapping entities.20 Loss of INI1 is not restricted to pediatric renal rhabdoid tumors, as the present study has shown, since INI1 expression is lost in renal medullary carcinomas as well. Renal medullary carcinoma, an admittedly rare primary renal tumor, seems to represent a specific clinicopathologic entity, occurring thus far only in patients with sickle-cell trait or disease. However, INI1-loss tumors have been identified in a spectrum of organ systems.11, 12, 13, 14, 15 While those tumors are histologically unrelated, the loss of this protein most likely identifies an underlying molecular aberration accounting for the aggressive clinical behavior.