Stacey Shiovitz, MD
Assistant Professor, Division of Oncology
University of Washington School of Medicine
Assistant Member, Clinical Research Division
Fred Hutchinson Cancer Research Center
Colorectal cancer (CRC) is increasingly being recognized as a heterogeneous disease, with molecular features defining emerging subtypes. Understanding which tests to order in which cases can better direct care for our patients. Increasingly, panel testing has advantages over individual genetic testing.
Approximately 15% of CRCs will have microsatellite instability (MSI). Tumors with MSI have a better prognosis overall. In addition, MSI can influence treatment decisions. Retrospective data show that individuals with stage II (T3-4 N0 M0) tumors with MSI have no benefit (or even detriment) from adjuvant 5-fluorouracil chemotherapy. On the other hand, microsatellite stable (MSS) patients have a trend toward a survival benefit. For high-risk stage II patients, the recommendation is to consider adjuvant chemotherapy in MSS cases only.
One-third of MSI cases are due to a germline genetic change in the mismatch repair proteins; the remaining cases are due to sporadic changes in the tumor. The hereditary cases are referred to as Lynch syndrome (LS; previously referred to has Hereditary Non-polyposis Colon Cancer).
Patients with LS have up to a 90% lifetime risk of CRC, in addition to increased risk of endometrial, genitourinary, pancreatic, and other cancers. Screening for LS was previously recommended based on clinical criteria (the Amsterdam and/or Bethesda criteria).
Now, however, universal screening for LS is the recommendation of the Evaluation of Genomic Applications in Practice and Prevention (EGAPP) working group and other groups. Furthermore, it is the recommendation that patients with CRC undergo both polymerase chain reaction (PCR)-based MSI testing and immunohistochemistry (IHC) screening tests. The PCR-based test evaluates changes in areas of repeated DNA sequences, while IHC utilizes tumor tissue staining to evaluate for loss of expression of the mismatch repair genes (MLH1, MSH2, MSH6, and PMS2). A universal screening strategy has been found to be cost-effective because recognizing undiagnosed LS patients may prevent future cancer occurrences in the individual and their family members. This strategy has been adopted by Seattle Cancer Care Alliance in the multidisciplinary Colorectal Cancer Specialty Clinic.
In summary, MSI and IHC are useful screening tools for hereditary cancer and MSI is recognized as both a prognostic and predictive marker for CRC treatment.
Expanded RAS Testing
In stage IV CRC, KRAS serves as a biomarker for determining benefit from cetuximab and panitumumab, therapies that target EGFR. The manner and extent of mutation testing is key to interpreting results. Standard methods evaluate codons 12 and 13 of exon 2. However, there is evidence from the PRIME (N Engl J Med. 2013;369:1023-1034) and FIRE-3 (Lancet Oncol. 2014;15:1065-1075) trials of clinical benefit from also testing exons 3 and 4 of KRAS, so as not to miss rare mutations that can also predict lack of benefit from anti-EGFR therapy. These retrospective analyses note that NRAS mutations in exons 2 to 4 additionally are predictive. BRAF, which is also in the EGFR pathway, appears to be a prognostic feature, but is not useful for anti-EGFR treatment prediction. Thus, the newer strategy that is advocated is “expanded RAS” testing, including sequencing of exons 2, 3, and 4 of both KRAS and NRAS. Given the expense of individual genetic tests, panel testing is a better streamlined approach to mutation analysis.
Cheaper, Faster Screening With Panel Testing
With the advent of better testing technology, next-generation sequencing has allowed for cheaper and faster genetic screening of multiple genes simultaneously. This can be used both on tumor tissue (to screen for somatic changes) or germline DNA (to screen for hereditary gene changes and/or genetic changes that may influence treatment response). Developed at the University of Washington (UW), the UW-OncoPlex panel is a 194-gene panel of genes with known or emerging relevance to cancer development. UW-OncoPlex is indicated for tumors of any type where driver genes are of interest.