Determination of HER2 status has now become of major clinical importance with the advent of anti-HER2 therapy, the recombinant humanised anti-p185Her-2/neu antibody trastuzumab (Herceptin®) (Pegram et al, 1998; Cobleigh et al, 1999; Slamon et al, 2001).
Immunohistochemistry (IHC) is expected to be the best method for the determination of HER2 status, as IHC assesses the level of HER2 overexpression, which is the target of Herceptin® therapy. Moreover, the patient's selection for Herceptin® therapy is mainly based on IHC because previous studies demonstrated a good correlation between IHC results and gene status, as determined by fluorescence in situ hybridisation (FISH) (Pegram et al, 1998; Jacobs et al, 1999,2000; Couturier et al, 2000; Jimenez et al, 2000; Lebeau et al, 2001; Lehr et al, 2001). However, the HER2-IHC detection was criticised because of a lack of interlaboratory reproducibility and, furthermore, Herceptest®, a standardised IHC method, was shown to be a method with excessive sensitivity when compared to FISH (Persons et al, 1997; Bartlett et al, 2001; Tubbs et al, 2001). Even though HER2 overexpression without gene amplification was reported in 2.9–8.3% of cases (Kallioniemi et al, 1992; Persons et al, 1997; Couturier et al, 2000; Jimenez et al, 2000; Pauletti et al, 2000), discordant results between IHC and FISH were mainly observed for tumours that were scored 2+ by IHC (Persons et al, 1997; Bartlett et al, 2001; Tubbs et al, 2001). For this reason, and particularly in Europe, a confirmation of HER2 gene amplification by FISH became mandatory for a patient's inclusion in a clinical trial using Herceptin®, when the corresponding tumour is scored 2+ by IHC (Hoang et al, 2000; Ridolfi et al, 2000; Diaz, 2001; Tubbs et al, 2001; Vogel et al, 2002).
Some authors found that HER2 status determined by FISH was more reproducible (Press et al, 1994; Persons et al, 1997; Bartlett et al, 2001; Tubbs et al, 2001). Thus, these authors thought that FISH had to be proposed as the only method to select patients for Herceptin®. However, FISH is a long and expensive procedure that requires trained personnel and fluorescence microscopy.
Chromogenic in situ hybridisation (CISH) is a recently introduced technique in which the DNA probe is detected using an immunoperoxidase reaction (Tanner et al, 2000). This method is very close to FISH but does not require the use of fluorescence microscopy. Moreover, FISH signals fade within a few weeks and the FISH results have to be recorded with expensive digital systems. This is not the case for CISH staining. Owing to the similarity with IHC staining, CISH is also easier to interpret for pathologists who are not trained with fluorescence. In one study (Tanner et al, 2000), CISH was demonstrated to be well correlated with FISH.
The aims of our study were to: (a) confirm the good correlation between FISH and CISH in a nonhomogeneous series of breast tumours coming from eight different laboratories using different fixation procedures, (b) analyse this correlation according to the expression of HER2 protein analysed by IHC (c) focus on IHC 2+ cases and analyse in this situation if CISH gives the same information as FISH for the treatment of patients.