CALRET

Calretinin (calbindin-2) is an intracellular calcium-binding protein encoded by the CALB2 gene on chromosome 16q22–q23. Its principal function is the buffering of intracellular calcium and the modulation of calcium-dependent signaling pathways, particularly within specific neuronal populations where it contributes to synaptic plasticity. Beyond the nervous system, calretinin is also expressed in mesothelial and leydig cells. In calretinin positive cells, the protein is generally found in both the cytoplasm and nuclei. 

In pathology, calretinin is widely used as an immunohistochemical marker to assist in the differential diagnosis of various tumors, especially to identify those of mesothelial or sex cord-stromal origin. 

• Malignant mesothelioma (pleural/peritoneal) Calretinin is highly sensitive for mesothelial cells and is routinely incorporated into IHC panels, often in combination with other mesothelioma positive markers like WT1, CK5 and Podoplanin/D2-40, to distinguish malignant mesothelioma from e.g. lung adenocarcinoma. Lung adenocarcinomas are typically negative for calretinin but express markers such as TTF-1, Napsin A and Claudin-4. However, rare calretinin-positive cases have been reported, underlining the importance of correlating with a comprehensive IHC panel. Calretinin reaction is seen in 80–100% of epithelioid malignant mesotheliomas and is also positive in most biphasic mesotheliomas, but its expression decreases in sarcomatoid mesotheliomas, where it can be weak, partial or absent.
• Ovarian sex-cord stromal tumors (SCST)  In SCST (granulosa cell tumor, Sertoli–Leydig cell tumor, etc.)  calretinin is a highly sensitive marker, showing positivity in nearly all cases. Its sensitivity generally exceeds that of inhibin and steroidogenic factor 1 (SF-1) but has lower specificity. Accordingly, it is best interpreted as part of an IHC-panel to support SCST lineage and distinguish these neoplasms from mimics. 
• Adrenal tumors  Calretinin is expressed in adrenal cortex and adrenocortical neoplasms (adenoma/carcinoma), whereas pheochromocytomas are typically negative. Interpreted within an immunohistochemical panel, calretinin together with inhibin and SF-1 reliably separates adrenocortical tumors (calretinin/ inhibin / SF-1 positive) from pheochromocytoma/paraganglioma (calretinin/ inhibin/ SF-1 negative and positive for SYP, CGA, INSM1) and from metastatic tumors. 
• Hirschsprung disease (HD) evaluation In normally innervated bowel, calretinin highlights mucosal and submucosal nerve fibers and ganglion cells. Loss of these calretinin-positive fibers and ganglion cells supports aganglionosis and serves as a sensitive, specific addition to HE. 

Adrenal gland and appendix/colon are recommendable positive and negative tissue controls for calretinin. The adrenal gland serves as a low-level expressor (LE) positive tissue control, in which at least weak to moderate, distinct cytoplasmic and nuclear staining reaction in the majority of cortical epithelial cells must be seen. Appendix/colon serves both as a negative and a high-level expressor (HE) positive tissue control. Columnar epithelial cells and smooth muscle cells should be negative, while strong, distinct cytoplasmic and nuclear staining reaction of peripheral nerves (ganglion cells and axons) and macrophages should be seen. Furthermore, fat cells in the submucosa of the appendix/colon can serve as an additional LE positive tissue control and will typically show a weak to moderate cytoplasmic and nuclear staining reaction. If only a cytoplasmic staining reaction for calretinin is seen this indicates a false positive result e.g. caused by a poorly calibrated protocol.