Non-Small Cell Lung Cancer (NSCLC)
The American Cancer Society estimates that over 220,000 new cases of lung cancer will be diagnosed in 2015, with over 85% of those cancers being classified as non-small cell lung cancer. Lung cancer represents approximately 13% of all new cancer diagnoses, and approximately 27% of cancer deaths. The estimated 5yr survival rate for all lung cancer patients is 17% and is only 4% for patients with metastatic disease.
Most lung cancers are epithelial in origin, with squamous cell carcinomas, adenocarcinomas, and small cell carcinomas being the predominant histologic types. The first two, squamous and adenocarcinomas, have been traditionally grouped as non-small cell lung cancer (NSCLC). Surgery remains the cornerstone of treatment for early stage NSCLC of either type, however treatment of advanced stage disease is based primarily on drugs. Distinctive response patterns to specific therapeutic drugs have been demonstrated over the past 12 years, necessitating the distinction between squamous cell and adenocarcinoma morphology. Consequently, the most recent WHO guidelines advocate sub-classification of all NSCLC in to a more specific subtype whenever possible. This is typically accomplished by histologic evaluation with support from specific immunohistochemical studies, which are particularly useful in the evaluation of small biopsies.
Adenocarcinomas account for approximately 40% of all lung cancers, and are the most common lung cancer in never- or light smokers. Adenocarcinomas are characterized by glandular differentiation, mucin production, or pneumocyte marker expression. Certain genomic alterations are more commonly found in lung adenocarcinomas (when compared to squamous or small cell carcinomas) and clinical laboratory testing to identify these alterations is important in two respects: First, some mutations are now recognized as “driver mutations,” which are essential for tumor cell survival. Inhibition of these mutated proteins results in tumor cell death, making them attractive therapeutic targets. While not all driver mutations have specific therapies at this time, an important corollary of this concept is that with rare exception, such driver mutations are mutually exclusive, i.e. the identification of one driver mutation in a tumor effectively makes the likelihood that another driver mutation is present extremely unlikely.
The second reason clinical laboratory testing for specific driver mutations in lung adenocarcinomas is important is the association of specific genomic alterations with response to specific drugs. Different driver mutations respond to different targeted therapies, and only genetic testing can clearly identify which mutation is present and, therefore, which treatment should be administered. While some of the genetic alterations that affect response to targeted inhibitors are found in a higher proportion of adenocarcinomas from patients with certain clinical risk factors (i.e., low to no history of tobacco exposure, female gender, young age, Asian ethnicity) , these clinical associations are not sufficiently predictive of mutation status to appropriately determine therapy without genetic testing. Accordingly, professional practice guidelines from CAP-IASLC-AMP, WHO, and NCCN all advise against using smoking history, or other clinical risk factors, to exclude testing patients for specific genomic alterations.
NCCN Guidelines (v 7.2015) for Non-Small Cell Lung Cancer recommend testing all non-squamous NSCLCs (i.e. adenocarcinomas, large cell carcinomas, and NSCLC not otherwise specified) for specific alterations in EGFR and ALK and recommend consideration of such testing in tumors with mixed squamous and adeno histology, and in the rare squamous cell carcinomas in never smokers. The NCCN NSCLC Panel “strongly endorses broader molecular profiling to identify rare driver mutations using multiplex/NGS to ensure that patients receive the most appropriate treatment”. In addition to testing for alterations in EGFR and ALK, NCCN Guidelines explicitly recognize the prognostic and predictive value of KRAS mutations as well as alterations in the BRAF, MET, and ROS1 genes to select a therapeutic agent, the use of which may be “off-label” but which also meets Medicare coverage requirements for off-label cancer drugs (CMS Publication Pub 100-02, Medicare Benefit Policy Manual, Chapter 15, §50.4.5).
Genomic alterations contribute to the development of non-small cell lung carcinoma. Two of the best studied alterations are EGFR mutations and ALK (e.g., EML4-ALK) gene fusions. EGFR mutations are permissive for the use of oral EGFR inhibitors, such as erlotinib. Similarly, activating fusions of ALK permit treatment with oral ALK inhibitors, such as crizotinib. Gene alterations for which a targeted therapeutic agent is available and the use of which meets Medicare coverage requirements (outside of a clinical trial) are listed in Table 1.
Table 1
| Gene |
NCCN Category 1 or 2A Recommended Therapeutic Option |
| ALK |
crizotinib, ceritinib, and alectinib |
| EGFR |
afatinib, erlotinib hydrochloride, and gefitinib |
| ROS 1 |
crizotinib |
| KRAS |
Avoid TKI |
| BRAF |
dabrafenib and vemurafib |
| MET |
crizotinib |
Metastatic Colorectal Cancer (mCRC)
Despite improvements in mortality secondary to advances in screening and treatment over the last several decades, CRC is the second leading cause of cancer deaths in the US, with about 50,000 deaths per year, 8% of all cancer deaths (1,2). Approximately 50-60% of patients with CRC develop metastases (usually metachronous, but synchronous in 20-34%), with 80-90% unresectable metastatic liver disease. Twenty percent present with metastatic disease. Today, the median overall survival (OS) for patients with mCRC is approximately 30 months, more than double that of 20 years ago (3).
The level of understanding of the molecular events underlying CRC is far greater than for other common solid tumors. Sporadic disease, in which there is no family history, accounts for approximately 70% of all CRCs (4). Specific germline mutations are responsible for the inherited CRC syndromes, while a stepwise accumulation of somatic mutations is thought to underlie most sporadic cases.
RAS and BRAF Mutations
The activation of the epidermal growth factor receptor (EGFR) signaling cascade is a well-described pathway leading to colon tumorigenesis. The receptor for EGFR is overexpressed in 49-82% of CRC. Cetuximab and panitumumab are monoclonal antibodies directed against EGFR that inhibit downstream signaling pathways, but are only effective in about 10-20% of CRC. Mutations within the RAS and BRAF oncogenes located downstream of EGFR within this pathway lead to its constitutive activation, even if the EGFR is blocked.
KRAS exon 2–4 and NRAS exon 2–4 mutations (RAS mutations) are found in about 50% of mCRC and exclude affected patients from EGFR-directed therapy (5,6). There is good concordance between the primary and synchronous distant metastases (but not lymph node metastases) (7). Besides their negative predictive value, RAS mutations have been independently associated with a worse prognosis in most studies (5,8). Some studies suggest that EGFR inhibition may even be detrimental in patients with RAS-mutant mCRC (9).
Activating BRAF mutations (mostly V600E), mutually exclusive from KRAS mutations, are found in approximately 5-10% of mCRCs, and are associated with poor overall prognosis (10). Evidence increasingly suggests that response to EGFR-targeted agents is unlikely in BRAF V600E patients, even if RAS wild type (11,12).
The most recent TNM staging classification considers RAS and BRAF V600E mutations to have both prognostic (level 1 evidence) and predictive (level 1 evidence) significance (2). In a Provisional Clinical Opinion (PLO), the American Society of Clinical Oncology (ASCO) recommended that all patients who are candidates for anti-EGFR therapy should have their tumor tested for mutations in both KRAS and NRAS exons 2 (codons 12 and 13), 3 (codons 59 and 61), and 4 (codons 117 and 146) (13). Because anti-EGFR agents have no role in the management of stage I-III disease, this genotyping is reserved for metastatic disease.
Mismatch Repair Deficiency (dMMR)
MMR genes are responsible for correcting the ubiquitous nucleotide base mispairings and small insertions or deletions that occur during DNA replication. Cells that are MMR-deficient accumulate DNA errors throughout the genome (14). The dMMR genetic signature is a high number of DNA replication errors (RER+) and high levels of DNA microsatellite instability (MSI-H), defined as expansion or contraction in ≥30 percent of microsatellite loci; microsatellites are short sequences of nucleotide bases that are repeated dozens to hundreds of times within the genome.
The gold standard for assessment of MSI has been concurrent analysis of patient tumor and normal DNA for five mononucleotide microsatellite loci with polymerase chain reaction (PCR) (15). The standard for detecting MMR protein expression status has been immunohistochemistry (IHC) for MLH1, MSH2, PMS2, and MSH6 expression. MMR can also be measured by total mutational burden (TMB) via next generation sequencing (NGS). In sporadic tumors, epigenetic changes (acquired hypermethylation of the promoters of both alleles of MLH1 gene), rather than gene mutations, usually account for defective MMR gene expression (20,21).
MSI-H tumors are associated with longer survival in both Lynch syndrome and sporadic CRC, for unclear reasons (4). However, the prognostic influence of MSI is less clear in patients with metastatic CRC, a population in which the prevalence of MSI-H disease is only 3.5% (16). Emerging data have shown MMR status to predict the clinical benefit of immune checkpoint blockade with pembrolizumab in patients with mCRC (17, 22, 23).
