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Quick Facts
- Select molecular markers have clinical applicability for select patient populations
- Scientists are studying specific markers for their role in both prognosis and treatment response
- Mutations in K-ras can predict lack of response to EGFR inhibitor agents
- Some markers can help predict toxicity, including dihydropyrimidine dehydrogenase (DPD) in patients receiving fluorouracil and the UGT1A1*28 allele in candidates for irinotecan therapy
Genomics in CRC 1/2: The Role of Molecular Markers
Genetic and genomic competencies are essential for all nurses, including oncology nurses. Understanding of the role of genetics and genomics in the care of patients with cancer is critical. 
  • Genetics is the study of individual genes and their impact on relatively rare single-gene disorders
  • Genomics is the study of all the genes in the human genome together, including their interactions with each other, the environment, and other psychosocial and cultural factors2
  • Nursing competencies have 2 domains1:
    • Professional responsibilities
    • Professional practice (including assessment, identification, referral, and provision of education, care, and support
Clinically, oncology nurses can apply genetic and genomic competencies to  
  • Identify patients who may benefit from specific genetic and genomic information
  • Provide clients with credible, accurate, and current information, resources, services, and technologies that facilitate decision making
  • Use genetic- and genomic-based interventions and information to improve patient outcomes1
The field of genomics has expanded exponentially for the disease of cancer and is helping to improve or refine specific therapies for many different diseases, including colorectal cancer (CRC). Nurses can incorporate genetic and genomic information to facilitate drug selection or dosage in treating adults with CRC based on molecular markers.3  

Predictive Markers
Although their use is not the standard of care in most clinical practice settings, predictive markers can help clinicians identify which patients would benefit from adjuvant chemotherapy. Some of the markers under study include the following:
  • Microsatellite instability (MSI) tumors are common across different types of cancer and are present in two-thirds of sporadic colon cancers. The MSI is caused by mismatched repair genes, and there seems to be a link between the MSI and transforming growth factor mutation. Researchers have shown that CRC patients with MSI tumors have better recurrence-free intervals than their counterparts and, interestingly, do not benefit from adjuvant chemotherapy.4 These high-frequency MSI (MSI-H) tumors may be resistant to fluorouracil (5-FU), with changes in the DNA mismatch repair system that decrease the cell death common with 5-FU.5 This genomic change may affect the prognosis of stage II and III CRC patients, since MSI-H patients may not seem to respond to 5-FU therapy.6
  •  K-ras is a gene essential to the growth and differentiation of extracellular signaling. In CRC, approximately 40% of patients have a mutation in the K-ras gene linked to increased risk of nodal metastasis and possibly more aggressive tumor behavior for this subset of patients.4 Patients with this mutation do not respond to treatment with epidermal growth factor receptor inhibitors (EGFRIs) and therefore should not be treated with those agents. The National Comprehensive Cancer Network (NCCN) has recently changed its guidelines to reflect this information and guide clinicians in formulating treatment plans for metastatic mCRC. The current recommendation is to treat only patients with wild-type K-ras, since this is the only patient population that will benefit from therapy.7-8  Additionally, the labels for both cetuximab and panitumumab have been changed as of July, 2009, to state that retrospective subset analyses of trials in patients with CRC who had mutations in K-ras in codon 12 or 13 showed a lack of benefit with the agents, and the use of these drugs is not recommended with those patients.9
  • B-type RAF kinase (BRAF) is a protein downstream of K-ras.  Approximately < 10% of patients with CRC have a mutation in BRAF, which can promote activity of the mitogen-activated protein kinase pathway. A recent retrospective analysis reported that 8.7% of 516 tumors contained mutations in BRAF, and that those patients with BRAF-mutated tumors had significantly shorter median progression-free survival (PFS) and median overall survival (OS) than patients with wild-type BRAF. Additionally, the effect (notably different from K-ras mutations) is not restricted to the outcome of cetuximab treatment.10
  • TP53 is a tumor suppressor gene that is mutated in 40% to 60% of CRC patients and may have a role both in the patient’s prognosis and as a predictor of response to chemotherapy used in CRC.4 Patients with the TP53 mutation had a poorer prognosis for stage II and III disease when treated with surgery alone; however, studies report conflicting data regarding the role of TP53 in predicting response to adjuvant 5-FU therapy (1 study showed that patients with the TP53 mutation did not benefit clinically).
  • Circulating tumor cells (CTC) have shown to be an independent predictor of PFS and OS in patients with metastatic CRC and can also provide prognostic information in conjunction with imaging studies.11 Circulating tumor cells are found in blood, and in one prospective multicenter study, CTC were measured in patients with mCRC. The patients were stratified into unfavorable and favorable prognostic groups based on CTC levels of 3 or more or less than 3 CTC/7.5 mL, respectively. The patients with unfavorable levels compared with favorable baseline CTCs had a shorter median PFS and OS.11

Predicting Response to Chemotherapy and /or Targeted Therapy

Studies have examined the role of genomic markers in predicting response to chemotherapy, including unexpected toxicities. Although testing for these markers is not yet routine, if patients have signs of toxicity that are unexpected for their treatment protocols, such testing should be considered.   

  • 5-FU is the oldest drug in CRC treatment, approved for both adjuvant and metastatic disease; it kills cancer cells by inhibiting the nucleotide synthetic enzyme thymidylate synthase (TS) by its active metabolite fluorodeoxyuridine monophosphate (FdUMP), resulting in the depletion of thymidylate. Some studies have shown that TS can be both a predictive and a prognostic marker in patients treated with 5-FU and that overexpression of TS may lead to resistance to the drug.12 Low levels of thymidylate phosphorylase (TP) expression are also associated with response and survival in this disease; expression of TP may indicate more aggressive tumors that are less responsive to treatment.4,12 Patients with dihydropyrimidine dehydrogenase (DPD) deficiency may experience extreme toxicity with 5-FU therapy; DPD is essential to the drug’s metabolism.12 This deficiency can be fatal for patients, as it leads to a much higher systemic level of drug.4 Studies have shown that patients who had low DPD expression experienced longer survival and disease-free recurrence, although some researchers have found the opposite to be true in patients treated with surgery alone.4 Further research in this area will continue to define the role of particular molecular markers in this disease.
  • Oxaliplatin is the standard of care for patients with node-positive CRC and is given in both the adjuvant and metastatic settings. Researchers have examined the role of ERCC1 (a protein involved in the NER pathway) and discovered that ERCC1 gene expression correlates with OS in patients receiving 5-FU and oxaliplatin chemotherapy for CRC in the refractory setting.4 ERCC1 mRNA and TS mRNA expression levels were also found to be independent predictive markers of survival for both 5-FU and oxaliplatin in a study of patients with progressive stage IV disease after unsuccessful 5-FU and irinotecan chemotherapy.13
  • Irinotecan is approved in the metastatic setting and functions as a DNA topoisomerase I inhibitor. The relationship between the DNA topoisomerase I activity and the sensitivity of cells to irinotecan is under study.4 Pharmacogenetic research has identified specific patient populations at risk for increased toxicity with irinotecan by their ability to metabolize SN-38, the active metabolite of the drug. Patients homozygous for the UGT1A1* 28 allele are at increased risk for neutropenia following administration of the drug and should be considered for a dose reduction.14
  • Capecitabine is an oral agent that functions as a 5-FU prodrug and mimics infusion of 5-FU; it is approved in both the adjuvant and metastatic settings. TP expression was found to be associated with response to capecitabine plus irinotecan for patients with metastatic CRC in one recently reported study, with time to tumor progression superior in patients with tumors that expressed TP and longer overall survival for positive expression as well.15
  • Cetuximab and panitumumab are monoclonal antibodies that target epidermal growth factor (EGF) by inhibiting the EGF receptor (EGFR). EGF may be expressed in 60% to 75% of CRC patients and overexpressed in approximately 50%; it is therefore an excellent target for therapy, and both agents are approved in the metastatic setting.4 However, recent data have shown that the EGFR gene copy number may predict clinical outcome of patients treated with panitumumab and cetuximab.16 In one trial, the panitumumab-treated patients who had a mean EGFR gene copy number of less than 2.5/nucleus or less than 40% of tumor cells displaying chromosome 7 polysomy within the tumor had a shorter PFS and OS. The researchers concluded that these patients have a reduced likelihood of response to the agent.17
  • Mutations in K-ras have been noted in approximately 40% of patients with CRC (Figure 1). EGFR inhibitor agents block ligand from the receptors in the extracellular region of the cell, preventing subsequent dimerization and activation of phosphorylation, which in turn causes signal transduction and activation of wild-type K-ras protein. Mutated K-ras is continually active, however, and does not respond to inhibition of the EGF receptor (cetuximab [Erbitux] and panitumumab [Vectibix] prescribing information).18,19 Patients who have the mutation have been virtually nonresponsive to treatment with cetuximab and panitumumab. Lievre and colleagues20 demonstrated that the K-ras mutations had an association with resistance to cetuximab in 89 patients (27% of whom had a documented mutation). There were 0% responders versus 40% in the 24 mutated and 65 nonmutated patients (P < .001), and a poorer survival was also noted in the patients with K-ras mutation.20 In a trial of 463 patients, K-ras mutations were found in 43% of those enrolled to receive panitumumab or best supportive care (BSC), with the treatment effect on PFS in the wild-type K-ras group significantly greater (P < .0001) than in the mutated group.21 The median PFS in the wild-type group was 12.3 weeks versus 7.3 weeks for BSC patients, and wild-type patients had longer overall survival as well.21 The researchers concluded that panitumumab monotherapy efficacy is limited to patients who have wild-type status K-ras tumors, with the recommendation to consider testing when selecting patients with metastatic CRC for therapy with panitumumab.21
  • Bevacizumab is a monoclonal antibody that targets vascular endothelial growth factor (VEGF), the ligand for the VEGF receptor. Because increased VEGF expression seems to correlate with a poorer prognosis and a higher recurrence rate,4 targeting CRC with an inhibitor of VEGF is an important clinical strategy. Bevacizumab is approved in the metastatic setting.

FIGURE 1 - Blocking EGFR and Tyrosine Kinase
From Harari and Huang. Clin Can Res. 2004;10:428-432, with permission.

Molecular Markers
Molecular markers have played a significant role in the treatment of breast cancer with the routine testing of hormone and HER2 receptors. Specific markers can help predict response to therapy in CRC, guiding clinicians in deciding the stage at which cancer patients would derive benefit from potentially toxic chemotherapy, or those who might benefit from targeted therapies. Treatment therefore becomes individualized, with the goal of sparing patients who might not respond to potentially toxic chemotherapy.22 Studies have been conflicting in this area, and the recently published trial assessing TS and DPD expression in patients receiving 5-FU/LV did not show higher response rates among those with higher expression; more research is needed.23 Two very recent papers have also indicated that the correlation between MSI-H tumors and chemotherapy response is still not completely understood, and research in this area has shown inconsistent results.24 Perhaps the benefit is more pronounced for its role as a marker for prognosis and response in families with Lynch syndrome (an hereditary nonpolyposis CRC syndrome) versus sporadic colon cancers.25

Testing for K-ras mutation should be considered in order to determine which patients may benefit from EGFR monoclonal antibody therapy, and the current NCCN recommendations call for testing prior to considering therapy with these agents, since virtually no mutated K-ras patients will respond to therapy.7

Although no FDA-approved test is yet available, a number of labs conduct the test (Table 1).

A commercial test is available (Invader UGT1A1 Molecular Assay; Third Wave Technologies, Inc) to test for the UGT1A1*28 allele, and some clinicians believe that all new patients should be tested prior to beginning therapy with irinotecan.14 This is an exciting time to be participating in the care of patients with CRC, but it is incumbent on all oncology nurses to stay abreast of new developments in the treatment of this patient population. Although the use of molecular markers is just beginning, these specialized examinations may benefit some patients.

Selection of Labs for K-ras Testing (non-FDA approved)




Genzyme Genetics


Polymerase chain reaction (PCR) based allele-specific primer extension

8-10 days

Laboratory Corporation of America


DxS K-ras gene mutation test kit


Quest Diagnostics



Click on Diagnostic Services

Insert K-ras Testing in Search feature

PCR and sequencing


Testing for predicting toxicity from irinotecan therapy



DxS K-ras gene mutation test kit

5 business days

Transgenomic Labs


Denaturing high-performance liquid chromatography (DHPLC) mutation scanning and DNA sequencing

Up to 7 days

TrimGen Genetic Diagnostics  www.trimgen.com

Mutector II multiplex mutation detection platform

3-5 business days

Response Genetics


ResponseDX specimen collection and transportation kit

5-7 business days


  1. Calzone K, Macdonald D, Masny A.  The essential nursing competencies for genetics and genomics: impact on oncology nursing practice. Instructional session, Oncology Nursing Society Congress; May 15-18, 2008, Philadelphia, Pa. 
  2. Guttmacher AE, Collins FS. Genomic medicine—a primer. N Engl. J Med. 2002;347:1512-1520.
  3. American Nurses Association. Essential nursing competencies and crucial guidelines for genetics and genomics.  American Nurses Association, Silver Spring, Md. http://www.genome.gov/Pages/Careers/HealthProfessionalEducation/geneticscompetency.pdf. Accessed December 14, 2010.
  4. Allen WL, Johnston PG. Role of genomic markers in colorectal cancer treatment. J Clin Oncol. 2005;23:4545-4552.
  5. Jo WS, Carethers JM. Chemotherapeutic implications in microsatellite unstable colorectal cancer. Cancer Biomark. 2006;2:51-60.
  6. Benatti P, Gafa R, Barana D, et al. Microsatellite instability and colorectal cancer prognosis. Clin Cancer Res. 2005;11:8332-8340.
  7. NCCN Clinical Practice Guidelines in Oncology: Colon Cancer—v.2.2011.  http://www.nccn.org/professionals/physician_gls/PDF/colon.pdf. Accessed December 14, 2010.
  8. Artale S, Sartore-Bianchi A, Veronese SM, et al. Mutations in KRAS and BRAF in primary and matched metastatic sites of colorectal cancer. J Clin Oncol. 2008;26:4217-4219.
  9. Food and Drug Administration.  http://www.fda.gov/AboutFDA/CentersOffices/CDER/ucm172905.htm  Accessed December 14, 2010.
  10. Tol J, Nagtegaal ID, Punt CJA. BRAF mutation in metastatic colorectal cancer. N Engl J Med. 2009;361:98-99.
  11. Cohen SJ, Punt CJ, Iannotti N, et al. Relationship of circulating tumor cells to tumor response, progression-free survival, and overall survival in patients with metastatic colorectal cancer. J Clin Oncol. 2008;26:3213-3221.
  12. Igbal S, Lenz HJ. Determinants of prognosis and response to therapy in  colorectal cancer. Curr Oncol Rep. 2001;3:102-108.
  13. Shirota Y, Stoehlmacher J, Brabender J, et al. ERCC1 and thymidylate synthase mRNA levels predict survival for colorectal cancer patients receiving combination oxaliplatin and fluorouracil chemotherapy. J Clin Oncol. 2001;19:4298-4304.
  14. O’Dwyer PJ, Catalano RB. Uridine diphosphate glucuronosyltransferase (UGT)1A1 and irinotecan: practical pharmacogenomics arrives in cancer therapy. J Clin Oncol. 2006;24:4534-4538.
  15. Meropol NJ, Gold PJ, Diasio RB, et al. Thymidine phosphorylase expression associated with response to capecitabine plus irinotecan with metastatic colorectal cancer. J Clin Oncol. 2006;24:4069-4077.
  16. Moroni M, Veronese S, Benvenuti S, et al. Gene copy number for epidermal growth factor receptor (EGFR) and clinical response to anti-EGFR treatment in colorectal cancer: a cohort study. J Clin Oncol. 2005;6:279-286.
  17. Sartore-Bianchi A, Moroni M, Veronese S. Epidermal growth factor receptor gene copy number and clinical outcome of metastatic colorectal cancer treated with panitumumab. J Clin Oncol. 2008;25:3238-3245.
  18. Vectibix™ prescribing information. Amgen.
    Accessed December 14, 2010. 
  19. Erbitux™ prescribing information. Bristol-Myers Squibb. http://packageinserts.bms.com/pi/pi_erbitux.pdf. Accessed December 14, 2010.
  20. Lievre A, Bachet JB, Boige V, et al. KRAS mutations as an independent prognostic factor in patients with advanced colorectal cancer treated with cetuximab. J Clin Oncol. 2008;26:374-379.
  21. Amado RG, Wolf M, Peeter M, et al. Wild-type Kras is required for panitumumab efficacy in patients with metastatic colorectal cancer. J Clin Oncol. 2008;26:1626-1634.
  22. Allen WL, Coyle VM, Johnston PG. Predicting the outcome of chemotherapy for colorectal cancer. Curr Opin Pharmacol. 2006;6:332-336.
  23. Smorenburg CH, Peters GJ, van Groeningen CJ, et al. Phase II study of tailored chemotherapy for advanced colorectal cancer with either 5-fluorouracil and leucovorin or oxaliplatin and irinotecan based on the expression of thymidylate synthase and dihydropyrimidine dehydrogenase. Ann Oncol. 2006;17:35-42.
  24. Kim GP, Colangelo, LH, Wieand S, et al. Prognostic and predictive roles of high-degree microsatellite instability in colon cancer: a National Cancer Institute–National Surgical Adjuvant Breast and Bowel Project Collaborative Study. J Clin Oncol. 2007;25:767-772.
  25. Boland CR. Clinical uses of microsatellite instability testing in colorectal cancer: an ongoing challenge. J Clin Oncol. 2007;25:754-756.

Key Definitions
dihydropyrimidine dehydrogenase (DPD)—the principal enzyme involved in the degradation of fluorouracil (5-FU); thymidylate phosphorylase—the first enzyme in the metabolic activation pathway of 5-FU; nucleotide synthetic enzyme thymidylate synthase (TS)—the target enzyme of the antimetabolite 5-FU. These 3 proteins assessed together have been indicated as possible predictive markers of 5-FU–based treatment

ERCC1 mRNA—a key enzyme of the repair pathway associated with response and survival in 5-FU–refractory patients with metastatic colorectal tumors undergoing 5-FU/oxaliplatin combination chemotherapy

fluorodeoxyuridine monophosphate (FdUMP)—the end metabolic process of 5-FU

genomics—the study of an organism’s genome that includes the DNA sequence, structure, and regulatory mechanisms. The goal is to find each gene to develop medicines and predict susceptibility of diseases. Proteomics is the study of the proteins expressed by the genetic material

mismatched repair genes—genes that fix problems arising during DNA replication when bases are incorrectly paired. Patients with a variant of a DNA repair gene, known as MLH1, may have an increased risk of a subtype of CRC

molecular marker (biomarker, genetic marker)—fragment of a DNA sequence that indicates the presence of a disease state. DNA sequences in most patients with CRC are either short or long and fluctuate. These sequences are called microsatellites. Their fluctuation is termed microsatellite instability (MSI)

tumor suppressor gene—any of a class of genes (as TP53) that act in normal cells to inhibit unrestrained cell division and through mutation place the cell at increased risk for malignant proliferation; also called antioncogene

UGT1A1*28 allele—a genetic marker that can be screened for determining the proper dosage of irinotecan for persons with CRC or other types of cancer as well as susceptibility to neutropenia

wild-type K-ras—wild-type denotes the phenotype characteristic of the majority of individuals of a species under natural conditions. Wild-type K-ras is the most frequent proto-oncogene mutation for the epidermal growth factor receptor (EGFR). Mutations in EGFR and K-ras genes appear to be mutually exclusive

Article Created On : 8/14/2009 9:12:59 AM             Article Updated On : 12/14/2010 3:51:53 PM