Positive/retained SDHB immunostaining in renal cell carcinomas associated to germline SDHB-deficiency: case report

The 2016 WHO classification of tumors of the urinary system and male genital organs recognizes succinate dehydrogenase–deficient renal cell carcinoma (SDH-deficient RCC) as a distinct entity [1, 2]. We now report a case of two SDHB-positive RCC associated to a germline SDHB mutation. Interestingly, SDHB immunohistochemistry showed a diffuse granular cytoplasmic positivity (mitochondrial pattern) in tumor cells, thus not fulfilling the 2016 WHO diagnostic criteria for SDH-deficient RCC, despite the characteristic morphological features and the association with a germline pathogenic SDHB mutation.

According to the 2016 WHO classification, SDH-deficient RCC is composed of uniform cells characteristically displaying eosinophilic granular material intermixed with fewer cells exhibiting clear intracytoplasmic inclusions and bland centered nuclei [1, 2]. SDH-deficient renal carcinomas commonly entrap benign tubules and have cystic changes, as observed in the tumors in this patient [1, 2, 6, 7]. They lack the distinct granularity of oncocytes for which they may be mistaken. Immunohistochemistry is useful for diagnosis revealing a loss of expression of SDHB. The novel term SDH-deficient RCC should allow the correct classification of tumors with a specific pathogenesis, usually occurring in the context of hereditary conditions, and characteristic histological features, that, for the lack of a common term, had been previously assigned very different names in routine clinical practice and in the literature, as reflected in the 2009 description of the association of renal oncocytoma with SDHB mutations [8]. Prior to coining the term, most cases may have been misdiagnosed as clear cell RCC, chromophobe RCC, oncocytomas or hybrid tumors (Table 1). In fact, these authors concluded that “SDHB mutations should also be considered in the context of genetic testing when renal tumors, regardless of histopathology, present in families with other tumors consistent hereditary paraganglioma syndrome”. While we are not aware of bona fide cases of SDH-mutated RCC that weakly retained expression of SDHB, the phenomenon has been described in paragangliomas [9].

SDH-deficient renal carcinoma is rare, less than 0.5% of all renal carcinomas [1, 2, 6, 7]. The added risk of RCC in patients with SDHB mutations was first described in 2008, when familial RCC was reported, even in the absence of a personal or family history of pheochromocytoma or head and neck paraganglioma, the most characteristics tumors associated to SDHB mutations, that cause pheochromocytoma/paraganglioma syndrome type 4 (PGL4) [10]. However, it was not until 2011 that the specific morphological features were reported and both the morphology and negative immunohistochemistry for SDHB in tumor cells proposed to allow the identification of kindreds with germline SDHB mutations [6]. Although the WHO classification of SDH-deficient RCC just refers to negative SDHB immunostaining [1, 2], a more detailed explanation of the criteria to consider SDHB immunostaining negative is provided by Gill et al. [1, 2, 6, 7]: A negative SDHB immunohistochemistry requires the entire tumor to demonstrate absent granular cytoplasmic (that is, mitochondrial) staining and the presence of readily identifiable internal positive controls in non-neoplastic cells. The present results are significant for the presence of clearly SDHB positive tumor cells displaying the characteristic staining pattern of SDHB positivity, in two tumors from the same patient with a bona fide SDHB genetic defect and concordant individual and familial phenotype, which otherwise displayed the morphological characteristics and clinical features of SDHB mutation-associated RCC. This illustrates the need to develop further markers of this type of RCC. Whether it also questions the need for homozygous SDHB deficiency for the pathogenesis of associated RCC may be discussed, since the molecular explanation for SDHB positivity in the tumor may theoretically be an acquired inactivating mutation in the wild-type SDHB allele that causes dysfunction but allows the complex to form (Fig. 3). As an alternative interpretation, a mutation may have resulted in expression of a dysfunctional SDHB subunit protein somehow rendered more stable as a monomer, rather than participating in assembly of an intact complex. Since we did not sequence the tumor, we cannot exclude this possibility.

The SDH complex is a key aerobic respiratory enzyme composed of four subunits: SDHA, SDHB, SDHC and SDHD, succinate dehydrogenase assembly factor 2 (SDHAF2) which form the mitochondrial complex 2, also known as succinate dehydrogenase/succinate–ubiquinone oxireductase or SDH. The complex is destabilized by the loss of any component, releasing the SDHB subunit into the cytoplasm where it degrades rapidly. The loss of SDHB observed by immunohistochemistry implies a double-hit inactivation, usually in the presence of a prior germline mutation, that prevents formation of the complex [1, 2, 6, 7]. It has been suggested that negative immunohistochemical staining for SDHB in tumors can be used as a surrogate marker for syndromic disease related to germline mutation of any of the components of the complex and it has been proposed that the term SDH-deficient can therefore be applied to all tumors which show loss of expression of SDHB, and SDH deficiency can be considered prima facie evidence of syndromic disease, usually dependent on a germline mutation [1, 2, 6, 7]. In this regard, rare cases of SDH-deficient RCC have also been associated with SDHA mutations (also with loss of SDHA expression by immunohistochemistry) and SDHC mutations have been reported.

The case provides lessons for other tumors. Thus, GIST, paraganglioma and pheochoromocytoma and the literature suggesting SDHD mutation in paraganglioma may be prone to difficult to interpret staining (diffuse blush negative may appear positive, like case 2 in this report). The reader should also be aware that tumors with very SDH-deficient RCC-like morphology, show retained expression of SDHB, but are in fact paradoxically FH-deficient [11, 12]. The implication of “missing” such a FH-mutated individual with HLRCC, which might present an identical morphology and SDH-retained phenotype as that described would be potentially even more serious than that of missing a SDH-deficient diagnosis.

False positive blush cytoplasmic staining may be challenging to differentiate from true mitochondrial positivity especially when non-neoplastic adjacent tubules are not available to be used as an internal control. Different results may be observed using different immunostaining platforms at the same dilution.

In individuals with SDHB deficiency, asynchronic SDHB-positive RCCs may occur, questioning the sensitivity of SDHB negative staining to detect individuals with germline SDH deficiency-associated multiple RCC. Since increasingly somatic tumor mutation profiling is performed, which might even obviate the need for an immunostaining, the WHO may reconsider its definition of SDHB-deficient tumor by adding a direct mutation profiling criterion.