Abstract
In the past 25 years, treatment of metastatic colorectal cancer (mCRC) has undergone profound changes. The approval of newer chemotherapeutics such as irinotecan and oxaliplatin was followed in 2005 by the first targeted therapies, for example, monoclonal antibodies directed against the epidermal growth factor receptor (EGFR), as cetuximab and panitumumab, or the angiogenesis inhibitors bevacizumab, ramucirumab, and aflibercept. With the rapidly progressing molecular characterization of mCRC in the last 10 years and the classification of the disease in four consensus subtypes, further changes are emerging, which will promote, among other things, the introduction of protein-kinase inhibitors developed for specific molecular aberrations as well as immune checkpoint inhibitors into the treatment algorithm.Thorough molecular pathologic testing is indispensable today for guideline-compliant treatment of mCRC patients. In addition to RAS testing as a precondition for the therapy decision with regard to cetuximab and panitumumab, BRAF testing is of considerable relevance to allow decision making with regard to the newly approved chemotherapy-free combination of the BRAF inhibitor encorafenib and cetuximab in cases where a BRAF-V600E mutation is detected. Additional diagnostic tests should also include genome instability (microsatellite instability). Overall, more and more molecular alterations need to be investigated simultaneously, so that the use of focused next-generation sequencing is increasingly recommended.This overview describes the prognostic relevance of BRAF testing in the context of molecular pathologic diagnostics of mCRC, presents new treatment options for BRAF-mutated mCRC patients, and explains which modern DNA analytical and immunohistochemical methods are available to detect BRAF mutations in mCRC patients.
Highlights
More than 95% of all BRAF mutations are BRAF-V600 mutations, where valine (V) is mostly substituted by glutamic acid (E) at position 1799 in codon 600 of the BRAF gene
B-Raf is a key kinase in the Ras/RAF/ MEK-mitogen-activated protein kinase (MAPK) signaling pathway, which is involved in the regulation of cell growth
KRAS mutations predominantly occur in the epithelial, “metabolic” subtype CMS3, which is characterized by metabolic dysregulation and partly by chromosomal and microsatellite instability (MSI) [2, 7]
Summary
Discrepancies observed betweenlocal and central testing showthe relevance of standardization of diagnostic procedures, especially in view of the increasing importance of targeted therapeutic approaches: clear confirmation of the locally detected BRAFV600E mutation was found in just 90.7% of central testing Of note, this discrepancy was largely due to insufficient neoplastic tissue in the sample, most likely resulting from the fact that BRAF-mutant tumors are generally associated with mucinous adenocarcinoma that contain fewer tumor cells. Since testing for KRAS, BRAF, MSI/dMMR, MLH1, and possibly other genes constitutes a prerequisite of CRC diagnostics, panel-based assays are understandably more prominent in current pathological practice—in Germany, all major university and non-university institutions are using focused NGS for this purpose. Used platforms include Illumina (MiSeqTM or NextSeqTM) and Thermo Fisher (Ion Gene Studio S5TM) [61, 62, 63]: in a multicenter validation study across Germany, a high level of consistency between different NGS platforms and gene panels was shown; apart from CRC, samples of lung and breast cancers were tested [61]
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