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Expert Opinion

Quick Facts

CRC, like numerous other solid tumors, is a heterogeneous disease in which different subtypes may be distinguished by their specific clinical and/or molecular features.


The majority of sporadic CRCs (~85%) exhibit chromosomal instability (CIN), with changes in chromosome number and structure. These changes include gains or losses of chromosomal segments, chromosomal rearrangements, and loss of heterozygosity (LOH), which results in gene copy number variations (CNVs). These alterations affect the expression of tumor-associated genes, and/or genes that regulate cell proliferation or cell cycle checkpoints, which, in turn, may activate pathways essential for CRC initiation and progression.


The remaining sporadic cases (~15%) have high-frequency microsatellite instability (MSI) pheno-types. However, hereditary CRC has two well-described forms: Familial adenomatous polyposis (FAP) (<1%) patients inherit a mutated copy of the adenomatous polyposis (APC) gene, whereas hereditary non-polyposis colorectal cancer (HNPPC, or Lynch syndrome) (1-3%) is characterized by MSI, a consequence of a defective DNA mismatch repair (MMR) system. The other forms of hereditary CRC include a rare syndrome called hamartomatous polyposis syndrome (<1%) and the common inherited cases caused by less penetrant inherited mutations (32%) (3).


NCCN Clinical Practice Guidelines in Oncology, Colon cancer


Molecular Diagnostics in Colorectal Carcinoma

Advances and Applications for 2018

Which hereditary syndromes are associated with colorectal cancer, and when is genetic testing for these syndromes recommended?

Approximately one-fifth of colorectal cancers (CRCs) are related to a hereditary syndrome.

Lynch syndrome (LS), or hereditary nonpolyposis colorectal cancer (HNPCC), accounts for 2-4% of colorectal cancer cases in the United States.

Given the frequency of LS, multiple institutions have endorsed immunohistochemistry (IHC) and, in some cases, microsatellite instability (MSI) testing in all colorectal and endometrial cancers to determine which patients should receive germline genetic testing.

 Other less common syndromes associated with CRC include familial adenomatous polyposis (FAP), MUTYH (MYH)-associated polyposis (MAP), Peutz-Jeghers syndrome (PJS), juvenile polyposis syndrome (JPS), hereditary diffuse gastric cancer, serrated polyposis syndrome, Cowden syndrome, and Li-Fraumeni syndrome.

If a familial syndrome is suspected, germline genetic testing, such as a hereditary cancer multigene panel, single gene testing, or familial mutation testing, may be appropriate.


Should vitamin D testing be performed in colorectal cancer?

Multiple studies have suggested that a lack of vitamin D is associated with poorer outcomes in colorectal cancer (CRC); however, other studies have indicated no cancer-specific benefit from vitamin D supplementation. Studies are ongoing, but the National Comprehensive Cancer Network (NCCN) does not currently recommend routine vitamin D testing in patients with CRC.


Which Test to Use?

Regarding their performance as a screening tool for Lynch syndrome, both IHC and molecular MSI testing are considered quite sensitive and produce concordant results.

Although the sensitivity of molecular MSI testing has been reported to be as low as 58%, partially owing to the ineffectiveness of detecting MSH6 mutation carriers, more recent studies using mononucleotide markers show the high sensitivities, reaching up to 100%, with increased detection of MSH6 mutation carriers.

Although very sensitive, both IHC and molecular MSI testing have their limitations; therefore, if questionable results are obtained by either method, they should be confirmed with alternative tests.



Molecular Testing

Tissue from a primary, recurrent, or metastatic colorectal tumor is acceptable for somatic molecular testing because results are similar for these specimen types.

Formalin-fixed, paraffin-embedded tissue should be used.
Germline genetic testing for hereditary CRC syndromes should be based on clinical presentation and family history, and should be performed in conjunction with genetic consultation.
A hereditary cancer multigene panel, single gene testing, or familial mutation testing may be appropriate if a hereditary cancer is suspected.

Microsatellite Instability and Mismatch Repair Somatic Testing

Testing for MSI via polymerase chain reaction (PCR), or for mismatch repair (MMR) protein status via immunohistochemistry (IHC), is recommended in all patients with a history of CRC to evaluate for LS risk. MSI (either low or high) or MMR deficiency identifies patients at high risk.


MSI testing is also useful to determine whether to use adjuvant chemotherapy in stage II disease, and both MSI and MMR testing are useful in treatment selection in stage IV disease.

High MSI and MMR deficiency are both associated with a more favorable prognosis in stage II disease and decreased likelihood of metastases.

Extended RAS Somatic Testing

Extended RAS gene testing, specifically KRAS (codons 12 and 13 of exon 2; codons 59 and 61 of exon 3; and codons 117 and 146 of exon 4) and NRAS (codons 12 and 13 of exon 2; codons 59 and 61 of exon 3; and codons 117 and 146 of exon 4), is recommended in all patients with CRC at the diagnosis of stage IV disease  or being considered for anti-EGFR therapy. Patients with any known KRAS variant in exons 2, 3, or 4 or NRAS variant in exons 2, 3, or 4 should not receive cetuximab or panitumumab.

No specific methodology is recommended for RAS testing.
Wild-type KRAS is associated with improved prognosis and increased lymph node retrieval.

BRAF Somatic Testing

BRAF testing is recommended in all patients with metastatic CRC at the diagnosis of stage IV disease and should be considered in patients with wild-type KRAS/NRAS metastatic colon cancer.

Wild-type BRAF is associated with improved prognosis and increased lymph node retrieval.
Presence of the BRAF V600E variant in patients with MMR-deficient CRC tumors with loss of MLH1 suggests sporadic CRC, although it does not rule out Lynch syndrome. Presence of the BRAF V600E variant also decreases the probability that treatment with panitumumab or cetuximab therapy will be effective without anti-BRAF therapy (eg, vemurafenib); combination therapy is recommended.


Fluorouracil Drugs

MSI-low (MSI-L) or microsatellite stable (MSS) tumors are associated with improved outcomes with 5-fluorouracil (5-FU) adjuvant therapy.

Patients with low-risk stage II MSI-high (MSI-H) tumors should not be given 5-FU adjuvant therapy.


Decreased UGT1A1 gene expression may lead to drug toxicity, including development of severe neutropenia, from irinotecan.

The UGT1A1*28 allele is associated with an increased risk of toxicity.
Patients who are heterozygous or homozygous for the *28 allele should receive a reduced starting dose of irinotecan   and be treated with caution.Pretreatment testing for UGT1A1*28 should be considered, although guidelines have yet to be established.


MSI-H status and MMR deficiency are associated with a decreased benefit from fluoropyrimidine adjuvant therapy in stage II disease.

Certain variants in the dihydropyrimidine dehydrogenase gene (DYPD) are associated with life-threatening toxicity from fluoropyrimidine.
These variants are thought to occur in 1-2% of the population; however, universal testing for these variants before fluoropyrimidine treatment is not currently recommended.

Anti-EGFR Therapy

As mentioned above, patients with any known KRAS variant in exon 2, 3, or 4 or NRAS variant in exon 2, 3, or 4 should not receive cetuximab or panitumumab, and presence of the BRAF V600E variant also decreases the probability that treatment with these drugs will be effective without anti-BRAF therapy.

HER2 overexpression may also predict resistance to anti-EGFR treatment. However, HER2 testing is not currently recommended for treatment planning.