Introduction to Colorectal Cancer


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Source: Colon Cancer Alliance, et al. 2014

Colorectal cancer is the third most common cancer in the entire world (Mayo, et al. 2014). Behind Lung Cancer, colorectal cancer makes up the second most cancer-related death in the United States (CDC, et al. 2014). 72% of colorectal cancer is colon caner and 28% is rectal (CDC). Like all cancers, colorectal cancer occurance is strongly correlated to environmental stimuli and as a result of increasing age. It is recommended that screening for colorectal cancer start at the age of 50, and continue till the patient is 75 (CDC). Normally, colorectal cancer is initiated in the lining of the bowel. As the cancer becomes invasive it moves to the muscle layers underneath, and eventually through the bowel wall. If the cancer remains in the bowel wall, it is often curable through surgery.
Rectal Bleeding and anemia are the common signs of a colorectal tumor, especially in those over 50.

Other symptoms include constipation, bloody stool, loss of appetite and weight, nausea, and vomiting (CDC). Diagnostic tests are used for patients that have high-risk symptoms. Regular colorectal cancer screening typically begins at 50 (Here is the CDC page for colorectal cancer screening). Routine Colonoscopies are also used to scan the bowels for tumors and other gastrointestinal tract diseases. Unfortunately, detection of a tumor can be very late, and it may already be beyond stage 1 or 2. Once metastatic a patient is often given chemotherapy intervention to increase their life span. The 5-year survival rate of patients with stage 3 colorectal cancer is around 50%, however stage 4 patient's is almost zero (Seattle Cancer Care Alliance, et al. 2014). Although surgery is the main and most successful form of treatment, once the cancer is metastatic anti-EGRF antibodies are a main line of treatment (Alberts).


Anti-EGFR Treatment


The epidermal growth factor receptor (EGFR) is often over expressed or mutated in uncontrolled cell proliferation. Growth factors bind to their cell surface receptors that signal for cell division. Some mutated receptors, common in cancers, signal cell proliferation even when receiving no signal from the growth factor leading to uncontrollable cell replication. EGFR, sends its signal down the MAP Kinase pathway, which includes the KRAS protein. KRAS is a major player in the resulting EGFR pathway in lung cancer also.

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Source-- Bardelli, Alberto and Pasi Janne. "The Road to Resistance: EGFR Mutation and Cetuximab."

Anti-EGFR monoclonal antibodies bind to the EGFR receptor to block the signal down the pathway. As seen in Figure 1 b, the anti-EGFR drug Cetuximab prevents ligand binding to the EGFR receptor, thus blocking the RAS protein that leads to cell proliferation. Anti-EGFR also can block the phosphoinositide 3-kinase (PI3K) pathway, leading to cell apoptosis instead of survival. These anti-EGFR antibodies can then be used to kill malignant tumor cells. However, mutations in RAS gene can signal both pathways to continue even withut ligand binding to the receptor. In figure 1 d, despite cetuximab binding to the receptor instead of the ligand, there is still downstream signaling. There are a variety of mutations that affect the KRAS gene, on exon 2, 3 and that continue the downstream signaling while ignoring the EGFR receptor.
There is another mutation that was found after prolonged cetuximab exposure. The tumor cells generated a EGFR mutation (S492R) that created cetuximab-resistant cells. The mutation changes the extracellular domain of the receptor such that cetuximab is blocked but ligands can still bind, as illustrated in figure 1 c. In patients with cetuximab resistance, 2 out of 10 had the S492R mutation. The S492R mutation occurs in patients with wildtype KRAS exon 2 (Bardelli and Janne 2012).

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Source: Alberto Bardelli, Salvatore Siena, "Molecular Mechanisms of Resistance to Cetuximab and Panitumumab in Colorectal Cancer" American Society of Clinical Oncology
As expected, those with KRAS mutations faired far worse then their KRAS wildtype counterparts in trials for anti-EGFR drugs, particularly cetuximab and panitumumab. Panitumumab has the same mechanism of action as cetuximab, only being effective with wildtype KRAS patients [why?]. Cetuximab and panitumumab alter the same EGFR pathway. The oncogenic pathways that are being changed by the monoclonal antibodies are colored red. Through the RAS or PI3K pathway the MAPK pathway leading to cell proliferation is stopped. However, as previously discussed, KRAS mutations hinder anti-EGFR. Interestingly, cetuximab-resistant tumors still retain panitumumab sensitivity. The opposite also holds true, panitumumab resistance does not also correlate to cetuximab resistance. Yet they work along the same pathways. The conclusion then, is that the two anti-EGFR therapies must bind to different EGFR epitopes. That is, before they inhibit the pathways they are binding to different parts of the antigen molecule. The antibodies are then binding to different sites, causing the difference in cetuximab and panitumumab treatment. With this information a cocktail of the drugs may be the best option for patients with KRAS mutations, but eventually resistance against all anti-EGFR antibodies is acquired.






We are investigating specific studies from a paper we found in the journal Nature. The studies examine the relationship between KRAS mutations and anti-EGFR therapy, specifically cetuximab, and the authors claim that KRAS mutations drive secondary acquired resistance to cetuximab (Misale, et al. 2012). We are looking at how the data could induce a causal conclusion.


Cellular Models


The study was conducted with two colorectal cancer cellular models. One is called DiFi, the other Lim1215. Both lines were generated to be similar on a molecular level to the colorectal cancer patients who most likely would respond to cetuximab. DiFi cells overexpress EGFR due to the amplification of the EGFR gene. On the other hand, Lim1215 cells express normal levels of EGFR, while still being similarly sensitive to cetuximab as DiFi cells. From these two models, the researchers produced the cetuximab-resistant variants (DiFi-R, Lim1215-R), which are extremely sensitive to EGFR inhibition. One concern is that the methods section of the paper did not delve much into the specifics of how the cells were generated, so we are unsure how the cells function and how they are equally resistant to cetuximab.

DiFi cells



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Source-- Misale, Sandra et al. "Emergence of KRAS Mutations and Acquired Resistance to Anti EGFR Therapy in Colorectal Cancer."


Graph (a) depicts the resistance of DiFi R1 and DiFi R2 to increasing concentrations of cetuximab over the course of a week. The significant difference between the wild type DiFi and the R lines shows that there is a resistance. The figure does not distinguish the difference between R1 and R2, but it is later clarified that DiFi R1 was generated by exposure to a constant dose of cetuximab for a year, while R2 was exposed to stepwise increasing cetuximab dosage over a year. In addition, graph (a) shows that the cell viability of R1 and R2 are extremely similar and almost equally resistant. The inclusion of these two different drug treatment protocols reduces possibility of the variable that the method of treatment could affect resistance. Parts (b) and (c) display the amplification of the KRAS gene in DiFi R. Part (b) compares the exome gene copy number between the control DiFi and resistant DiFi R, clearly showing the difference and transition from EGFR expression to KRAS expression. Part (c) provides the reader with a visual representation confirming the amplification of KRAS in DiFi R. However, the study consisted of two R lines, so which one is actually shown?

Western blot analysis of the different cells in parts (d) and (e) gives us a better look at the protein levels. The parental DiFi line clearly has a lot of EGFR present and very little KRAS. DiFi R1 and R2 are similar in that both have low levels of EGFR but high levels of KRAS. However, a slight difference between R1 and R2 after exposed to cetuximab is that R2 has more pMEK and pERK, possibly as a result of method of treatment. Despite these minor differences, the resistant lines clearly depict the decrease of EGFR but increase of KRAS from the control, continuing to support the hypothesis. Part (e) was obtained by infecting DiFi cells with a KRAS lentivirus, which is used for gene manipulation because it specializes in infecting non-diving cells. Unfortunately, the function of the lentivirus was not expanded upon neither in the article nor the methods section, but we assumed that it silenced the KRAS gene according to outside sources. The similar presence of KRAS in “KRAS over” (which we assumed to mean “overexpressed”), R1, and R2 connects the resistant lines to having an overexpression of KRAS as well. The details of this Western blot analysis are lacking, which could undermine its significance as a part of the study.

Graph (f) looks again at cell viability in relation to cetuximab concentration but this time in DiFi cells overexpressing KRAS, wildtype cells, and empty vectors. The significant difference between the cell viability of DiFi “KRAS over” and DiFi wildtype and DiFi empty show a resistance in DiFi “KRAS over”. However, as higher concentrations of cetuximab, the cell viability of DiFi “KRAS over” is very close to DiFi wildtype and DiFi empty. In addition, the cell viability of DiFi wildtype in graph (f) is extremely different from graph (a), where graph (a) seemed to show that cetuximab was more effective, while graph (f) showed more resistance in the wildtype. The authors use graph (f) to connect overexpression of KRAS to cetuximab resistance, but with these two possible points, the conclusion may have been weakened.

From this data, the authors conclude that in DiFi cells, KRAS amplification mediates the acquired resistance to cetuximab.

Lim1215 cells



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Source-- Misale, Sandra et al. "Emergence of KRAS Mutations and Acquired Resistance to Anti EGFR Therapy in Colorectal Cancer."

Graph (a) depicts the relationship between cetuximab concentration and cell viability, ultimately exhibiting the successful resistance created in the R lines. The overlap of error bars of the R lines show that the resistance is similar between the two, despite method of treatment. Just like the DiFi R lines, the Lim1215 R1 line was exposed to a constant concentration of cetuximab, while R2 was exposed to a concentration that increased stepwise over time for a year. However, there is a striking difference between the Lim1215 R lines and the DiFi R lines. The Lim1215 R lines were exposed to an overall concentration of 1400 nM over a course of 3 months, while DiFi R lines were exposed to an overall concentration of 350 nM over the course of a year. The higher concentration was used for Lim1215 to possibly induce mutations instead of amplifications that would be affecting the resistance.

According to the Sanger sequencing results in part (b), which is just a method of DNA sequencing, the two resistant lines show inconsistencies compared to the control, as pointed out by the arrows. The other peaks not pointed out do not match perfectly with the control results, but the peaks seem to be relatively proportional, so they are deemed as insignificant differences. In Lim1215 R1, the difference turned out to be a G12R mutation, which means that at position 12 in KRAS, there is an amino acid substitution from a glycine to an arginine. On the other hand, the difference in Lim1215 R2 is a G13D mutation, where an aspartic substituted a glycine at position 13 in KRAS. Because the mutation differed in R1 and R2, we would assume that the type of cetuximab treatment could possibly affect the type of mutation, but the researchers do not expound further on this topic.

The Western blot analysis in part (c) shows that regardless of cetuximab exposure, active GTP-KRAS is present in the resistant lines but not the parent Lim1215 line. All the other protein expression levels seem similar, emphasizing the significance of the active GTP-KRAS. The evidence of active GTP-KRAS paired with the KRAS mutations magnifies the possibility that there is a correlation between KRAS mutation and acquired cetuximab resistance.

The methods to back up the data in parts (d) and (e) are not focused on much in the paper. Part (d) is a schematic representation of how the G12R and G13D mutations were “knocked-in” to the genome of Lim1215 parental cells. The paper does not explain this procedure or its significance further, but its relationship with the graph in part (e) can help one understand how the authors come to their conclusions. One can assume that the purpose of knocking-in the mutations into the genome of the parent cells is to test if the mutations are the reason why there is resistance, or if they are just side products. Illustrated in graph (e), the control parent line has significantly lower cell viability as the cetuximab concentration increases, consistent with the control in graph (a) as well. The Lim KI G12R, which corresponds to the mutation in Lim1215 R1, has a similar resistance when compared to graph (a). Lim KI G13D, which corresponds to the mutation in Lim1215 R2, is still resistant compared to the control, but does not show the same cell viability compared to R2 in graph (a). Therefore, another variable may play a role in affecting the resistance in R2. However, the mutation in Lim1215 R1 seems to be a very strong candidate.

From the results of these two cellular models, the authors then needed to determine if KRAS mutation and amplification are clinically relevant.



Clinical Trials


Through two different models, the authors concluded that KRAS amplification and/or mutation had an effect on resistance to cetuximab. To determine if these conclusions are clinically relevant, the researchers examined tumor biopsies from colorectal cancer patients. The results are presented in this figure.


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Source-- Misale, Sandra et al. "Emergence of KRAS Mutations and Acquired Resistance to Anti EGFR Therapy in Colorectal Cancer."


This study compares the KRAS mutational statuses of colorectal cancer patients who had chemotherapy resistant tumors and those with anti-EGFR resistant tumors. The patients with anti-EGFR resistant tumors have either been treated with cetuximab or panitumumab. The other group had been treated by cytotoxic chemotherapy and had not been previously exposed to anti-EGFR therapies. Through multiple methods of sequencing, KRAS mutations in the patients’ tumors were identified. Some patients who had anti-EGFR therapies, shown in table (b), acquired KRAS mutations, while the chemotherapy patients, table (a), were all wild type for KRAS.

The sample size for the chemotherapy group is sufficient because they all are wild type, but a larger sample size for the anti-EGFR group would be rewarding and valuable to this study. This could show possibly a variety of mutations as well as the frequency of these mutations in colorectal cancer patients. Also, three anti-EGFR patients are KRAS wild type, leading to the question of what caused their acquired resistance? Would it be similar to the chemotherapy patients?

The molecular experiments focused solely on the drug cetuximab, but this study allows patients who have also used panitumumab. How similar are these drugs to assume that panitumumab could also lead to the same mutations? The authors of this paper made their conclusions in terms of cetuximab, so incorporating panitumumab data may weaken the strength of their claim. The type of sequencing also differed for some patients, which could lead to inconsistencies in the data.

The number of mutated reads from sequencing quantifies the total number of KRAS mutations for the two groups of patients. Graph (c) illustrates the significant difference between the two groups, including a small p-value, supporting the significance. However, this study focused on sequencing KRAS in patients that have already had a form of treatment, but without the knowledge of their KRAS status before the treatment, one cannot say that anti-EGFR therapy causes KRAS mutations, leading to resistance. We only know from this data that there is a correlation, but in combination with the molecular data, other assumptions can be made. This study concludes that there is a relationship between anti-EGFR treatments and KRAS mutations in patients with resistant tumors. This information allows for the initiation of further clinical trials and the decrease of anti-EGFR treatments used for patients with KRAS mutations.

Another Trial


We wanted to investigate the relationship of KRAS mutations and anti-EGFR therapies in more than one type of study, so the molecular data and clinical trial data work well together. However, these three data sets are from the same research paper, meaning that they could have been exposed to the same biases. We decided to look at another clinical trial and compare the results.


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Source-- Liever, Astrid et al. "KRAS Mutation Status Is Predictive of Response to Cetuximab Therapy in Colorectal Cancer."


In this study from the journal Cancer Research, 30 metastatic colorectal cancer patients were given different types of treatment. One received cetuximab monotherapy, 25 received cetuximab with irinotecan, and four received cetuximab with two other drugs.[How can the other therapies be generally described?] After treatment, DNA extraction and mutation analysis were performed in multiple locations, but we will only focus on the KRAS analysis.

Unlike the first clinical study we looked at, this one used other drugs in combination with cetuximab as well, possibly weakening the study’s conclusions, which only focuses on cetuximab. In addition, this study looked at the effectiveness of cetuximab (and other drugs) with KRAS status, while the other compared treatment types. This study may give us abetter understanding when it comes to the relationship of cetuximab resistance and KRAS mutations.

The patients diseases were monitored to follow the effects of cetuximab. Eleven out of the 30 patients responded to cetuximab, and no KRAS mutation was found in their tumors. However, a KRAS mutation was found in the tumor of 13 patients, and they all were not positively affected by cetuximab; the patients’ diseases were either stable or progressive. This indicates a relationship between cetuximab effectiveness, possibly resistance, and KRAS mutations. The study found the presence of KRAS mutation to be significantly associated with the absence of response to cetuximab. Patients nonresponsive to cetuximab have a 52.5% KRAS mutation frequency, while responders are at 9.5%. In the presence of a KRAS mutation, the frequency of no response to cetuximab is 91.3%, leading to conclude that patients with wild type KRAS status would be more responsive to cetuximab and have a longer overall survival.

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Source-- Liever, Astrid et al. "KRAS Mutation Status Is Predictive of Response to Cetuximab Therapy in Colorectal Cancer."

This Kaplan-Meier plot portrays the overall survival curves of patients with a KRAS-mutated or nonmutated tumor.
The median survival for those with mutated KRAS is around 6.5 months, while those with nonmutated KRAS have a median survial rate of around 14 months, basically double the amount of time. It clearly shows that patients with tumors with mutated KRAS have a much lower overall survival. However, the use of other drugs in combination with cetuximab may have affected the survival of certain patients. How were the therapies decided? Were the best ones chosen for each individual?

Overall, this study seems to show the relationship between cetuximab effectiveness and KRAS mutation well, while the previous study linked anti-EGFR treatments to KRAS mutations.


Conclusions


Although the molecular mechanism for anti-EGFR therapy resistance in unknown, the studies we have focused on helped us come to a strong conclusion that KRAS mutations are correlated with acquired resistance to cetuximab. Cellular models showed us KRAS amplification and KRAS mutations as a result of cetuximab treatment, with the latter being more influential. The clinical trial in the same paper illustrated a general correlation between anti-EGFR treatment and KRAS mutations linked to resistant tumors, but a look at another trial shower a greater connection between the drug's efficacy and KRAS mutations. Overall, KRAS mutations have been concluded to be a predictor of restistance to cetuximab, and those with wild type KRAS have a better response and longer overall survival.

Estimates are that around 40% of patients with metastatic colorectal cancer have KRAS mutations. For most patients with KRAS mutations, this will mean they forgo the use of anti-EGFR drugs like cetuximab and panitumumab to avoid the high cost and adverse side effects. In 2009 the FDA updated the labels of cetuximab (Erbitux) and panitumumab (Vectibix) to include information about the effects of KRAS mutations, then licensed the drugs for use for patients with wildtype KRAS only (Pazdur 2013). Although the licensing remains (in the USA and Europe), the National Comprehensive Cancer Network revised their guidelines to recommend anti-EGFR therapy to all patients, even those with any KRAS mutation. Clearly, more specific anti-EGFR drugs need to be researched to provide more personalized care. A drug that blocks the ligands and signals from KRAS mutations can help inhibit the MAPK pathway. Otherwise patients with a KRAS mutation and metastatic colorectal cancer will find themselves looking at less effective interventions then their wildtype counterparts taking anti-EGFR monoclonal antibodies.


Sources


Alberts, Steven, Deborah Citrin, and Miguel Rodriguez-Bigas. "Colon, Rectal, and Anal Cancers." Oncology and Hematology News and Journal Articles. Cancer Management Handbook, n.d. Web. 02 June 2014.http://www.cancernetwork.com/cancer-management/colon-rectal-and-anal-cancers

Bardelli, Alberto and Pasi Janne. "The Road to Resistance: EGFR Mutation and Cetuximab." Nature Medicine. Nature Medicine. Nature (2012). Web. http://www.nature.com/nm/journal/v18/n2/full/nm.2646.html

Bardelli, Alberto and Salvatore Siena. "Molecular Mechanisms of Resistance to Cetuximab and Panitumumab in Colorectal Cancer." Journal of Clinical Oncology, n.d. Web. 02 June 2014. http://jco.ascopubs.org/content/28/7/1254.long

Liever, Astrid et al. "KRAS Mutation Status Is Predictive of Response to Cetuximab Therapy in Colorectal Cancer." KRAS Mutation Status Is Predictive of Response to Cetuximab Therapy in Colorectal Cancer. The Journal of Cancer Research, n.d. Web. 02 June 2014. http://cancerres.aacrjournals.org/content/66/8/3992.long

Misale, Sandra et al. "Emergence of KRAS Mutations and Acquired Resistance to Anti EGFR Therapy in Colorectal Cancer." NIH Public Access. NCBI, n.d. Nature (2012) 486:7404, Web. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3927413/?report=classic#SD1

Pazdur, Richard. "FDA Approval for Cetuximab." National Cancer Institute. National Cancer Institute, 07 July 2013. Web. 02 June 2014. http://www.cancer.gov/cancertopics/druginfo/fda-cetuximab

"Colon Cancer." Definition. Mayo Clinic, n.d. Web. 02 June 2014. http://www.mayoclinic.org/diseases-conditions/colon-cancer/in-depth/CON-20031877

"Basic Information About Colorectal Cancer." Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 26 Feb. 2014. Web. 01 June 2014. http://www.cdc.gov/cancer/colorectal/basic_info/index.htm

"Winning the Battle Against Colon Cancer." Colon Cancer Survival Rates. Seattle Cancer Care Alliance, n.d. Web. 04 June 2014. http://www.seattlecca.org/diseases/colon-cancer-survival-rates.cfm