Renal epithelioid angiomyolipoma: Histopathologic review, immunohistochemical evaluation and prognostic significance

Renal angiomyolipoma (AML) is a benign mesenchymal neoplasm composed of variable proportions of triphasic histology, abnormal thick-walled blood vessels, spindle and epithelioid smooth muscle cells and adipose tissue.1 AML accounts for approximately 1% of surgically resected renal tumors, and the association with tuberous sclerosis complex (TSC) is well known.2 There is growing evidence that AML is in the perivascular epithelioid cell tumor (PEComa) family.3-5 Immunohistochemically, AML is characterized by co-expression of melanocytic markers (HMB-45 and Mart-1/Melan A) and myoid markers (SMA and muscle-specific actin).3, 5, 6 Though benign mesenchymal tumor, AML may extend into the renal venous system and may show vascular invasion and lymph node involvement.1

In contrast to classic AML, epithelioid AML (EAML) is considered as a potentially malignant mesenchymal neoplasm with possible lymph node metastasis and distant metastasis.7-14 EAML was first described as a variant of AML by Martignoni G et al. and Mai KT et al. in 1995 and 1996, respectively.15, 16 By definition, EAML consists of at least 80% epithelioid cells.7 EAML shows a higher association with TSC than does classic AML.8 Large-scale EAML studies and meta-analytic studies revealed the following adverse prognostic factors8-11, 14: large tumor size (>7 cm),9, 11, 14 severe cytologic or nuclear atypia,9, 10 extent of nuclear atypia (≥70% atypical epithelioid cells),10, 14 (extensive) tumor necrosis,9-11, 14 large epithelioid cell component,10, 14 mitotic count (≥2 mitosis per 10 high power fields (HPFs)),10 atypical mitotic figures,10 lymphovascular invasion,10 associated TSC or concurrent AML,11 extrarenal extension and/or renal vein involvement,11 and carcinoma-like growth patterns.11

Activation of the mammalian target of rapamycin (mTOR) signaling in AML has been identified.13, 17 As TSC1/TSC2 complex inhibit mTOR pathway, loss of TSC1/TSC2 complex can induce over-actvation of mTOR signaling and cause disease progression in AML. Moreover, association with mTOR activation and reduction in membranous E-cadherin, thus, induction of epithelial-mesenchymal transition (EMT), has been elucidated.18 Treatment of AML patients with mTOR inhibitor has shown tumor regression and favorable outcome.19 Also, some studies have identified that TP53 mutation or p53 immunoreactivity is associated with malignant transformation of AML.20-22

In the present study, we aimed to evaluate the clinicopathologic features of a large series of AML patients in single institution by a comprehensive review with an emphasis of EAML cases. Additionally, we evaluated the prognostic significance of EAML with suggested adverse prognostic factors.

Materials and Methods

Results

A total of 78 patients who underwent surgical resection and were diagnosed with AML or EAML were analyzed. The clinicopathological characteristics of the patients are summarized in Tables 1, 2, and 3. There were 73 AML patients and 5 EAML patients. The EAML cases occupied 6.4% of total AMLs. Among the EAML patients, there were three men and two women. The mean age was 41.4 years old (range 20–64), and mean tumor size was 12.7 cm (range 1.6–22.0). TSC was found in two cases (40.0%), and the patients were diagnosed based on clinical diagnostic criteria of TSC (angiofibromas (≥3), tubers in bilateral frontal lobe and temporal lobe, subependymal nodule and AML in both kidneys and liver for case E4 and angiofibromas (≥3) and AML in kidney for case E5).23 The TSC1/2 gene mutation study was not performed. Recurrence of EAML and distant metastasis was observed in one case (20.0%). Multiple AMLs were found in two cases (40.0%). PEComas in other site were identified in one case (20.0%), and the patient diagnosed AML in liver and lymphangioleiomyomatosis in lung. Extrarenal involvement was found in four cases (80.0%). Compared to classic AML, EAML showed higher association with TSC and more frequent hemorrhage, cytologic atypia and extrarenal involvement (Table 1).

Five cases of EAML have shown variable histologic features (Table 3). All five cases showed an epithelioid component of over 60%, with the average proportion of the epithelioid component being 77.0% (range 60–95). The mean atypical epithelioid component with higher than moderate degree was 61.0% (range 0–100). Among the five EAML cases, we found necrosis in two cases (40.0%), with case E5 especially representing geographic necrosis. One case (20.0%) revealed severe cytologic atypia, and that case had four mitoses per 10 HPFs with atypical form. There were no sarcomatous components in any of the five cases. Renal capsule invasion and lymphovascular invasion was observed in one E5 case (20.0%). In an evaluation of the growth pattern, we categorized two cases as carcinoma-like growth pattern (40.0%), epithelioid and plump spindle cells in a diffuse growth pattern in one case (20.0%), and a mixed pattern in two cases (40.0%).

Among five cases of EAML, case E5 presented malignant outcome (Fig. 1 and Supporting Information Fig. S1). On macroscopic examination, the tumor was relatively well demarcated solid mass and showed variegated cut surface and red-brown and yellowish color. There was focal hemorrhage and necrosis. On microscopic view, our malignant E5 case had a carcinoma-like growth pattern, geographic necrosis, severe cytologic atypia, four mitoses per 10 HPFs with atypical mitosis, renal capsule invasion and lymphovascular invasion. Additionally, the malignant case revealed positive for melanocytic markers and negative for CK (Fig. 1). On follow-up, case E5 developed local recurrence and distant metastasis to scapula, liver, pelvic bone and peritoneal seeding and the disease-free survival times were 12 months. Interestingly, the malignant E5 case revealed decreased membranous E-cadherin expression and diffuse strong p53 immunoreactivity compared to other EAML cases (Fig. 2 and Table 4).

Discussion

EAML is considered a potentially malignant neoplasm, and lymph node metastases, local recurrence or distant metastases have been reported.7-14 The disease progression of EAML has been reported as a variable percentage from 0% to 51.5%.8-11, 21 These variable results are thought to be due to different samples and inclusion criteria of EAML. Though the definition of EAML was written as ‘at least 80% of epithelioid cells’ in the 2016 WHO classification of tumors of the kidney,7 the precise diagnostic criteria of EAML have not been established. Different diagnostic criteria for EAML may lead to collecting heterogeneous samples, potentially resulting in imprecise histopathologic features. Moreover, those lead to confusion of prognosis of EAML and would be an obstacle for studying pathogenesis or genetic alterations. Meta-analytic study, including six EAML cases with diagnostic criteria of epithelioid cells occupying one medium power field of 5.5 mm in diameter, revealed that 38% (16 out of 42 cases) of EAML had unfavorable outcomes.9 Also, pure EAMLs having an epithelioid histology of >95% showed a high disease progression rate (51.5%).11 However, one large study with diagnostic criteria of a minimum of 10% of epithelioid component showed no malignant EAML cases (0 out of 15 cases).8 Also, EAMLs with >80% epithelioid histology exhibited a low disease progression rate (5%).24

The percentage of atypical epithelioid component would be more important than the epithelioid proportion. Study of EAML with atypia included EAMLs with ≥5% epithelioid portion and showed malignant behavior in 26% (9 out of 34 cases).10 A recent consensus meeting resulted in an agreement that AML with epithelioid morphology should be divided into “EAML with atypia” and “EAML without atypia”, though it may be somewhat subjective.12 Therefore, we suggest that it will be useful and helpful to describe the proportion of epithelioid component and degree of atypia in epithelioid cells in the diagnosis of EAML.

In our study, we determined the diagnostic criteria as having an epithelioid portion of more than 10% of the tumor, rather than 80%, arbitrarily. The reason is that an epithelioid component of more than 10% would be able to cause different clinical behavior from the classic AML. In previous studies, EAML with epithelioid cells occupying one medium power field of 5.5 mm in diameter and EAML with atypia with ≥5% epithelioid portion revealed unfavorable behavior.9, 10 In this study, all of 73 AMLs were ≤5% of epithelioid portion, and only two cases had an epithelioid portion of about 5%. The epithelioid component of the five EAMLs ranged 60–95%. Our results revealed that there were five cases of EAML (6.4%) and one case (20.0%) showed malignant behavior with local recurrence and distant metastasis to scapula, liver, pelvic bone and peritoneal seeding. Though, AML showed multicentric PEComa lesions, the malignant EAML revealed lymphovascular invasion, the specimen from scapula was similar to renal EAML cancer cells, and involvement of scapula and other sites as a multicentric lesions is extremely rare. So, we concluded that lesions of scapula and other sites were distant metastatic lesion rather than multicentric PEComas in proper clinical context. The malignant case (case E5) showed a tumor size >7 cm (13.0 cm), a higher percentage epithelioid component (95%), ≥70% atypical epithelioid cells (95%), ≥2 mitosis per 10 HPFs (4/10 HPFs), atypical mitotic figures, necrosis (20%, geographic necrosis), extrarenal extension (capsule invasion), lymphovascular invasion, and carcinoma-like growth pattern.

Pathogenesis of renal AML or EAML has been studied. In one study, the pathogenic role of the mTOR pathway activation in sporadic AML was identified.17 The authors revealed that mTOR signaling pathway is up-regulated and upstream molecule, phospho-AKT, is down-regulated, suggesting TSC1/2 complex disruption. As mTOR activation is associated with cell growth, proliferation, survival, and thus, tumorigenesis, mTOR inhibitor can be considered as a therapeutic target.25 Treatment of renal AML or EAML with mTOR inhibitor, such as sirolimus, has shown tumor regression and favorable outcome.19

The mTOR signaling pathway activation can affect membranous E-cadherin expression.18 Barnes EA et al. showed that tuberin, product of TSC2 gene, regulates membranous expression of E-cadherin through mTOR pathway. Loss of tuberin leads mTOR activation, and further, decrease in membranous E-cadherin expression. E-cadherin is associated with EMT in many cancers.26 Decreased membranous E-cadherin expression can affect EMT and tumor progression. Previous study of renal EAML with malignant features revealed that all three cases showed weakly positivity for E-cadherin.13 These findings were consistent with our results that membranous E-cadherin immunoreactivity was decreased in malignant EAML, however, it was revealed moderate to strong positivity in other four EAML cases.

Some reports revealed that malignant EAML showed p53 immunoreactivity.20-22, 27 Kawaguchi K et al. presented malignant transformation of renal AML and showed that atypical epithelioid cells were p53 positive, whereas, typical AML foci were negative.20 Also, the malignant epithelioid cells showed p53 mutations. Sato K et al. reported malignant EAML case and the immunohistochemical study revealed diffuse p53 positivity, though p53 mutation was not detected by polymerase chain reaction-single-strand conformation polymorphism analysis.21 Report from Li J et al. showed malignant EAML with pulmonary metastases.22 Both renal EAML and pulmonary tumor revealed positivity for p53. Moreover, the p53 gene mutation was identified in renal tumor. Another study analyzed five renal EAML, and revealed that p53 staining was much stronger in EAML cases than that in classic AML.27 Among five renal EAML cases, two cases revealed p53 gene mutations. Similarly, our immunohistochemical results showed diffuse strong p53 immunoreactivity in malignant EAML and negative or focal weak positivity (no overexpression) in other EAML cases. Based on previous studies and our results, we suggest that decreased membranous E-cadherin expression and strong p53 positivity may be the late stage alterations and responsible for malignant transformation of EAML. Furthermore, immunohistochemical analysis of E-cadherin and p53 may serve as an ancillary test for risk stratification of EAML patients.

In conclusion, we identified 5 EAML cases (6.4%) among 78 cases of AML. All five EAMLs had an epithelioid component of over 60%. Among the five EAMLs, one case (20.0%) showed malignant behavior with local recurrence and distant metastasis to multiple sites. This malignant EAML revealed larger tumor size, higher percentage of epithelioid component and atypical epithelioid cells, ≥2 mitosis per 10 HPFs with atypical mitotic figures, necrosis, extrarenal extension (capsule invasion), lymphovascular invasion, and carcinoma-like growth pattern. Also, immunohistochemical results of decreased membranous E-cadherin expression and diffuse strong p53 immunoreactivity were identified. For precise prognosis estimation of EAML, pathologists should aware aforementioned adverse prognostic factors and those factors should be evaluated for further risk stratification of EAML (Fig. 3).

Disclosure Statement

None declared.