HER2 evaluation using the novel rabbit monoclonal antibody SP3 and CISH in tissue microarrays of invasive breast carcinomas

Background: Laboratory methods for HER2 assessment currently include immunohistochemical (IHC) methods ( measuring protein overexpression) and fluorescence in situ hybridisation ( FISH) ( measuring gene amplification). The measure of HER2 protein by IHC is usually assessed by the mouse monoclonal antibody CB11, and polyclonal antibodies ( Herceptest) directed against the internal portion of the receptor. Recently, chromogenic in situ hybridisation ( CISH), in which HER2 is detected by a peroxidase reaction and the gene amplification can be determined by regular bright-field microscopy, has emerged as an alternative to FISH. Aims: To evaluate the status of HER2 in tissue microarrays ( TMAs) of invasive breast cancer using the novel rabbit monoclonal antibody SP3 directed against the external portion of HER2, and correlate the results with CB11 and CISH. Methods: IHC was performed with two antibodies (CB11 and SP3) and CISH for HER2 in 10 TMA blocks with 190 formalin-fixed paraffin-embedded cases of invasive breast carcinomas. Results: The correlation between SP3 and CB11 was significant (p < 0.001) with an agreement rate of 86.9%. When the staining pattern of the two antibodies was compared, the majority of SP3 immunostainings were assessed more easily, with a strong complete membrane staining pattern without non-specific cytoplasmic staining. There was a good correlation between SP3 and CISH ( p < 0.001). 23/ 24 SP3 3+ cases showed gene amplification, 97.3% of the cases without gene amplification were SP3 negative, and 6/ 7 SP3 2+ were amplified. Conclusion: The high level of agreement between SP3, a monoclonal antibody that recognises the extracellular domain of the HER2 receptor, and CB11 and CISH, shows that this novel antibody is a reliable candidate to evaluate the expression of HER2 in breast cancer.