Introduction
The routine use of IHC has never been greater or more widespread. The overall market for IHC antibodies and reagents continues to grow, outpacing almost all pathology disciplines. Sadly, this correlates to the increase in the rates of cancer globally. At Leica, our business growth is fuelled by testing volumes, and the continuous release of clinically validated antibodies providing better diagnosis and therapy prediction, not to mention an increase in their application in clinical research.
The possibility remains that one day molecular applications, including many based on the Polymerase Chain Reaction (PCR), may replace some routine IHC tests. Gene profiling of tumours offers the potential to completely characterise the patient sample to more thoroughly classify cancers and provide both prognostic and predictive data for better patient management.
PCR based tests can provide prognostic data
Some PCR based tests provide complex prognostic data, particularly in breast cancer. While the data is useful, there is debate amongst pathologists whether this data is truly better than that already provided by existing methods such as the Nottingham Prognostic Index, derived from routinely assessed pathological factors on H&E stained sections. One factor that all can agree on is that the latter is significantly more economical.
There are also PCR based tests entering routine histopathology laboratories for the detection of somatic tumour mutations such as K-Ras, in the setting of colon and lung cancer. While theranostic in nature, its utility has driven the uptake of PCR methodology in laboratories that previously never provided PCR testing as a clinical service. There are a number of commercial tests available for this assessment and most of them have proven comparable to sequencing. A key issue with all such tests, especially when offered by laboratories with no track record in molecular pathology, is a definite requirement for thorough quality assurance.1
In-situ hybridisation can provide data within the context of tissue morphology
In-situ hybridisation (ISH) as a molecular application is already widely adopted in histopathology laboratories. With ISH, one can appreciate the targets directly in relation to histology and morphology, at least with chromogenic formats, which provides greater context regarding the target and the associated pathology. This therefore lends itself to histopathology far more easily than expression profiling or PCR. Even fluorescent in-situ hybridisation, while not necessarily being able to view the target directly in relation to the tumour and surrounding tissue, still provides the “gold standard” in many assessments such as Her2.
PCR has advantages where antibodies have been difficult to develop
This desire to see abnormalities in context has led to many requests for a monoclonal antibody to K-Ras mutations. However, point mutations within the variable codons resulting in ever so subtle amino acid substitutions make it difficult to develop antibodies, regardless of species, that are sufficiently sensitive. Here applications such as PCR have a distinct advantage. One of the few ways to circumvent this is to detect surrogate markers that are consistently detectable in the presence of mutations, alas this is not always achievable.
Cases where immunohistochemistry has allowed PCR to be supplanted
But let’s consider previous PCR-based evaluations such as that of mismatch repair markers, MLH1, MSH2, MSH6 and PMS2. These were, up until approximately ten years ago, largely restricted to PCR-based microsatellite instability (MSI) analysis, until reliable and robust monoclonal antibodies were made available. Leica have just released PMS2 to complete our range of antibodies for the analysis of tumours possibly related to Lynch syndrome (Hereditary Non-Polyposis Colorectal Cancer, HNPCC). Pathologists were quick to switch to IHC for investigation of possible Lynch syndrome because it allows one to look at these abnormalities in context and in relation to the tumour and surrounding tissue, in positive cases identifying the likely causative gene and providing vital information for subsequent germline mutation screening. Most importantly, the test employed a technique already familiar to pathology laboratories, IHC.
IHC and PCR are complementary approaches rather than replacements
Given the current technologies and utilities available, we can confidently predict a long life for IHC. We shouldn’t forget that most diagnoses are still made on H&E examination alone. However, IHC comes into its own when confirming and/or refining a cancer diagnosis, with respect to prognostic or predictive information, more accurately classifying, or suggesting a likely primary tumour site. Selected molecular techniques have the capability of complimenting routine applications including IHC in cases where perhaps interpretation is difficult or that the targets are highly mutated. Examining either tumour DNA or RNA is not guaranteed to reflect protein expression and therefore phenotype. As most, if not all, drugs target these proteins it is sometimes important to be looking at protein not only for diagnosis but also for therapeutic decisions. The pathologist’s overriding need to observe the morphology of the tumour and protein expression is likely to remain.
