Claudins are a family of transmembrane proteins that are involved in tight junction formation between epithelial cells [20
]. There are at least 24 members in the claudin family and they are expressed in an organ- and tissue-specific manner [21
]. In the kidney, expression patterns of claudins change with development and renal segments [24
]. Its deregulation has been associated with polycystic renal disease [27
Aberrant expression of various claudins has been reported in a variety of human neoplasms. Claudin-2 is overexpressed in gastrointestinal tumors [20
] and breast carcinoma [29
]. Claudin-4 is up-regulated in pancreatic adenocarcinoma and its precursor lesions [30
]. Overexpression of claudin-3 and -4 has been reported in prostate and ovarian carcinoma [32
]. While down-regulation of claudin-7 has been correlated with increased grade and metastasis of breast cancer and squamous cell carcinoma of esophagus [34
], its upregulation has been observed in the progression of cervical neoplasia [37
] and early stage of gastric adenocarcinoma [38
Renal epithelial neoplasms have a spectrum of biologic behavior, ranging from benign (oncocytoma), low-grade malignant (chromophobe RCC) to highly malignant (papillary RCC and clear cell RCC). A definitive subclassification is one of the most important factors in determining management of the patients and prognosis. Unfortunately, the different subtypes of renal epithelial neoplasms have overlapping histologic features, which makes differential diagnosis difficult in certain cases based on morphology alone. For example, eosinophilic cytoplasm can be seen in tumor cells of clear cell RCC, papillary RCC, chromophobe RCC and oncocytoma. Papillary structures can be seen in clear cell RCC as well as papillary RCC. Therefore, great effort has been made in identifying novel markers that may be useful in differential diagnosis. Recently, Schuetz et al [17
] applied oligonucleotide microarrays on 31 adult renal tumors and found overexpression of claudin-7 expression in chromophobe RCC, suggesting that claudin-7 may potentially be useful in the differential diagnosis of renal tumors.
In the current study, we applied immunohistochemistry using an anti-claudin-7 monoclonal antibody in a large cohort of 261 cases of renal tumors. Our study showed that all chromophobe RCCs express claudin-7 but less than 50% oncocytomas do. The difference, although not as dramatic as we expected from the oligonucleotide microarray study, was statistically significant. In addition, because of the arbitrary cut-off value adopted in this study, the number of oncocytomas scored as positive for claudin-7 is probably higher than it is. Chromophobe RCCs all show strong and diffuse membranous staining. If we apply this stringent criterion (strong and diffuse membranous staining), many of the oncocytomas with focal staining would have been considered negative and only less than 20% (8/47) have staining pattern similar to that of chromophobe RCCs. Therefore, claudin-7 may be useful in the differential diagnosis of the two tumors. For example, in a difficult case with overlapping features, a negative or focal scattered positive staining for claudin-7 would essentially rule out chromophobe RCC. This marker will be even more useful when it is used in a panel of markers that are expressed with different frequencies between the two tumors.
Certain chromophobe RCCs may have clear cytoplasm while some clear cell RCCs, particularly high grade ones, have granular cytoplasm. Distinction between these two entities can be very difficult in some cases. Colloidal iron has been proposed as a specific stain for chromophobe RCC but in reality, due to the difficulty in performing and interpreting this stain, it is hardly used in daily practice. Our study has demonstrated that all chromophobe RCCs are positive for claudin-7; in contrast, clear cell RCC are rarely positive for claudin-7. Therefore, an immunohistochemical study using claudin-7 may aid in the differential diagnosis of these two tumors in difficult cases.
Because the majority of papillary RCCs are positive for claudin-7 while the majority of clear cell RCC are negative, claudin-7 may be a useful marker in distinguishing clear cell RCC with papillary features from papillary RCC, particularly type 2 papillary RCC. Similarly it may be useful to distinguish papillary RCC with focally clear cytoplasm from clear cell RCC.
The results from our study may also shed light on the origin of different types of renal epithelial tumors. We have demonstrated that in benign kidney, claudin-7 is expressed in distal nephron, which includes thick ascending limb of Henle's loop, distal convoluted tubule and portion of cortical collecting tubule [39
]. This observation is consistent with previous reports [26
]. Traditionally, chromophobe RCC and oncocytoma are believed to arise from the intercalating cells of distal nephron but papillary RCC and clear cell RCC are considered to arise from the proximal nephron [43
]. Our results showed that the majority of chromophobe RCCs and papillary RCCs express claudin-7 while only rare cases of clear cell RCC express this marker. This observation suggests that, as far as the expression of claudin-7 is concerned, papillary RCC seems to be more closely related to the distal nephron, rather than the proximal nephron. Additional studies are required to determine the significance of this surprising finding.
In conclusion, claudin-7 is highly expressed in distal nephron of the kidney and can be readily detected immunohistochemically. Their differential expression among different types of renal cell neoplasms may be useful in the differential diagnosis of difficult cases.