Colorectal Cancer Overview

Colorectal cancer (CRC) is the second leading cause of cancer death in the United States, with projections of 52,000 deaths and 143,000 new cases diagnosed in 2012.¹ About three-quarters of CRC cases are sporadic. The remaining are familial (15% to 20%) or inherited (2% to 4%).² Familial CRC is not well understood and is characterized by increased risk of CRC and an unclear pattern of inheritance. Inherited CRC, on the other hand, exhibits a clear pattern of inheritance and includes Lynch syndrome (formerly known as hereditary nonpolyposis colorectal cancer [HNPCC]) as well as other less common syndromes. The type of CRC has important implications for screening and follow-up of patients and their family members.

The natural progression of CRC from benign adenoma to carcinoma to metastatic disease is associated with 1 of 2 distinct molecular pathways. The most common pathway, chromosomal instability, is characterized by DNA copy number changes and rearrangements. Such characteristics are associated with most sporadic CRCs. The remaining 15% to 20% are associated with microsatellite instability (MSI), defined as insertions or deletions of nucleotides within repeated DNA nucleotide sequences known as microsatellites. MSI reflects impairment of nucleotide mismatch repair (MMR) that normally occurs during DNA replication. The MMR defect pathway is exemplified by Lynch syndrome, which is caused by inheritance of 1 copy of a defective MMR allele and subsequent somatic mutation of the normal allele.

Lynch Syndrome

Lynch syndrome is characterized by a high lifetime cancer risk and early age at onset. In contrast to sporadic tumors, which are more often found on the left side of the colon, Lynch syndrome tumors are more often found on the right side. Patients with Lynch syndrome have an increased risk of developing CRC (40% to 80% by age 75)³ and an increased risk of recurrence. Lynch syndrome also carries an increased risk of tumors in the stomach, small bowel, biliary track, renal pelvis, ureter, or brain. Women with the syndrome have an increased risk for endometrial cancer (30% to 39% by age 70 years) and ovarian cancer (3% to 14% lifetime risk).^3,4

Among patients with Lynch syndrome, early detection of colorectal adenomas or tumors is critical. Surveillance allows detection of tumors at a more favorable Dukes stage and is associated with decreased CRC-specific mortality in individuals with HNPCC.⁵ Diagnosis of Lynch syndrome should trigger testing of first-degree relatives and early initiation of surveillance if an MMR gene mutation is found.

• Individuals with CRC who meet any of the Bethesda criteria (Table 1)

• Individuals who meet the Amsterdam II criteria (Table 2)

• First-degree relatives of individuals with a known MMR gene mutation

• Women diagnosed with endometrial cancer when under 50 years of age

Criteria to identify patients or their families with the disorder have been developed by the National Cancer Institute (Bethesda guidelines, Table 1)⁶ and the International Collaborative Group on HNPCC (Amsterdam criteria, Table 2).^7,8

The Evaluation of Genomic Applications in Practice and Prevention (EGAPP), which is an independent group formed by the CDC to evaluate clinical genomic testing, recommends offering screening and genetic testing for Lynch syndrome to individuals newly diagnosed with colorectal cancer regardless of age or family history.⁹ Although professional societies have yet to comment on this recommendation, literature studies support it. In an analysis of over 129,000 patients with CRC, testing all colorectal tumors resulted in detection of twice as many patients with Lynch syndrome as the commonly used Bethesda guidelines.¹⁰ Furthermore, testing for Lynch syndrome in all patients with CRC is cost effective when compared to testing CRC patients who are <50 years of age.¹¹

The tests recommended by the EGAPP and others include screening tests such as MSI; immuno-histochemistry of the 4 MMR proteins, supplemented by BRAF mutation testing; and confirmatory mutation testing of the MMR genes (Table 3).

The table is provided for informational purposes only and is not intended as medical advice. A physician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.

Individuals Suspected of Having Lynch Syndrome

Microsatellite Instability

The diagnosis of Lynch syndrome begins with consideration of MSI testing. All colorectal tumors can be tested for MSI, as recommended by EGAPP. Alternatively, testing can be performed when any of the Bethesda guidelines are met (Table 1).⁶ In a study of 1222 patients with CRC, combining Bethesda guidelines with MSI testing was more effective in determining which patients should be tested for MMR mutations than the use of either alone.¹²

Results are reported as MSI-high (MSI-H) if ≥2 of the 5 National Cancer Institute-recommended markers show instability. MSI-low (MSI-L) results are reported if 1 of 5 is positive; negative results are reported as microsatellite stable (MSS). An MSI-H result may require follow-up with MMR gene mutation testing. Although an MSI-H result is the hallmark of Lynch syndrome, it is also found in 10% to 15% of sporadic CRC cases.¹³ Since MSI results do not rule out Lynch syndrome, MMR gene mutation testing should be considered regardless of MSI status in families with a strong suspicion of Lynch syndrome.⁶

Immunohistochemical (IHC) Testing of MMR Proteins
MMR gene mutation usually leads to a loss of protein expression. Thus, the absence of IHC staining serves as a surrogate marker of MMR gene mutation that can be followed up with directed genetic testing. The overall clinical sensitivity of IHC testing for MMR proteins (correctly identifying a tumor positive for an MMR mutation) is 77% (95% CI, 69% to 84%).⁸

In patients fulfilling the Bethesda criteria, MSI and Lynch syndrome IHC screening tests were both found to be effective in selecting patients for MMR mutation testing.^2,12,14 However, using either MSI or IHC testing alone resulted in false-negative results.¹⁵ Thus, sensitivity can be maximized by using both MSI and IHC testing^2,4; this approach is incorporated into a testing algorithm for Lynch syndrome (Figure 1).

MMR Gene Mutation Testing
MMR gene mutation testing is based on MSI-H pattern and/or loss of MMR protein immunostaining (Figure 2). Mutation testing should include analysis of various types of alterations. Small DNA sequence changes, such as point mutations and alterations involving just a few nucleotides, are the most frequent. However, roughly 27% of families with Lynch syndrome have larger rearrangements, which may include deletions and duplications involving parts of, whole, or multiple gene exons.² Approximately 32% of the mutations occur in MLH1, 38% in MSH2, 14% in MSH6, and 15% in PMS2.⁸ Testing for mutations in more than 1 MMR gene may be appropriate if the gene implicated by IHC testing does not harbor a mutation. Detection of a deleterious MMR gene mutation in a patient with CRC is diagnostic for Lynch syndrome, but failure to identify a mutation does not rule out the diagnosis.

The EPCAM gene immediately precedes the MSH2 gene in the genome. Mutation of the 3′ end of EPCAM, which is closest to the MSH2 gene, can influence MSH2 gene expression. When present, a 3′-deletion in EPCAM inactivates the MSH2 gene.¹⁶ Thus, loss of MSH2 staining can be explained either by MSH2 mutation or by 3′-deletion in EPCAM (Figure 2). The presence of either the mutation or the deletion is diagnostic of Lynch syndrome.

Germline mutation testing can supplement tumor testing and is especially useful for Lynch syndrome risk assessment in family members (Table 3).

BRAF Mutation Testing
Loss of staining for MLH1 by IHC should be followed up with BRAF mutation testing (Figure 2). BRAF gene mutation is predominantly associated with non-Lynch syndrome CRC; tumors positive for BRAF mutation V600E essentially rule-out Lynch syndrome.¹⁰ When both MLH1 staining and BRAF mutation testing are negative, MLH1 gene mutation testing is suggested (Figure 2).

Risk Assessment of Family Members At-risk for Lynch Syndrome

Carriers of an MMR mutation have an increased colon cancer risk: by age 70, 45% for men and 35% for women.⁸ The risk for other Lynch syndrome-associated cancers by age 70 is 22% for men and 34% for women, including endometrial cancer.⁸ Surveillance colonoscopy of at-risk family members reduces the rate of CRC by 62% and overall mortality by 65%.¹⁷ Because surveillance should be initiated at 20 to 25 years of age or 2 to 5 years earlier than the youngest age when CRC was diagnosed in the family (whichever is earlier), gene testing should precede these timeframes to rule in or out the need for such surveillance.² If mutation testing rules out the family mutation, screening can be performed as for the average-risk population.

Germline MMR gene testing is guided by whether the MMR gene mutation is known or unknown. First-degree relatives of a patient with diagnosed Lynch syndrome and known MMR gene mutation can be tested for the known mutation using a point mutation sequencing assay or a deletion/duplication gene assay (Figure 3). Genetic counselors are key to guide proper gene testing when the familial mutation is known. For first-degree relatives of Lynch syndrome patients who meet the Amsterdam criteria but do not know the family mutation, the Lynch Syndrome Panel offers comprehensive gene testing (Figure 3). Because not all Lynch syndrome families meet the Amsterdam criteria, MMR mutation testing should also be considered when there is a strong suspicion of Lynch syndrome based on family history.² Identification of a deleterious mutation in an MMR gene confirms the clinical diagnosis of Lynch syndrome and thereby identifies individuals who should undergo routine surveillance for associated cancers. Failure to identify a deleterious mutation does not rule out the diagnosis and patient management should be based on family history.