Colorectal cancers have been widely studied due to their massive impact on Americans. In 2019, estimates from the American Cancer Society expect slightly over 100,000 new cases of colon cancer and 44,000 new cases of rectal cancer to be diagnosed. Additionally, colorectal cancer is expected to cause around 51,000 deaths in the U.S. in 2019 alone. Although these statistics are quite powerful, the number of cases and deaths have been decreasing as further scientific understanding of the mechanisms of these cancers is elucidated.
This summer, I am working in the Kalady Lab at the Lerner Research Institute of the Cleveland Clinic. My lab specializes in providing insight into the genetic underpinnings of colorectal cancers and applying this knowledge to the clinical realm in order to treat patients. I will be reporting on my project further along in the summer, but I first wanted to start with a blog post explaining how colon cancer manifests and how the worst aspects of this illness are often preventable.
Understanding tumor formation requires understanding the cellular mechanisms that tumor cells hijack. Tumors cells are abnormal cells that have somehow beaten the system and have acquired the capacity to uncontrollably divide. The methods in which tumor cells diversify or transform often confer a selective advantage in comparison to regular cells, indicating why tumors can grow at such a fast and uncontrollable pace. For colorectal cancers, there have been two major pathways established for how tumors can develop. These separate pathways include modifications to genetic, epigenetic, and DNA mismatch-repair (MMR) systems that are associated with cell growth, differentiation, motility, and survival. For the purpose of this blog post, I am going to define the genetic and epigenetic roots of tumor formation in addition to a shorter explanation of MMR.
The underlying genetics involved in cancer have been studied for decades, leading to an understanding that certain mutations that can lead to benign and malignant tumors. The Chromosomal Instability Pathway, described by Bert Vogelstein in 1988-1990, establishes that a certain number of genetic mutations in specific genes are correlated to different stages of tumor development.
As seen in the figure, three genes are often referred to within the Vogelstein pathway as genes or processes that must be mutated or upset in order to undergo tumor formation. All of the mutations in the Vogelstein Pathway are additive; there isn’t a necessary order to the mutation process. Because tumor formation requires all of these mutations but not in any order, tumors are much more likely to win out as random mutation is a byproduct of the imperfection of nature.
The APC gene, or adenomatous polyposis coli gene, codes for a regulatory protein in the Wnt pathway. When a mutation occurs in APC, its protein’s ability to interact with and bind to β-catenin is ceased. β-catenin is a signaling molecule that can call for the upregulation of genes associated with proliferation. Therefore, if APC is mutated and loses functionality, proliferative genes can be more highly expressed, leading to one of the hallmarks of cancer. The next gene mentioned in this figure is Kirsten-ras or KRAS, which codes for a cell-signaling protein that works within the RAS/ERK pathway of signaling. When RAS proteins are phosphorylated, they can pass their phosphate to the next protein in the pathway: BRAF. Eventually, the pathway involves a protein called ERK that has the potential to upregulate more genes associated with proliferation and survival. Mutations in either KRAS or BRAF have been clinically observed in early adenoma formation, a benign growth that needs only one more mutation to become cancerous. The final gene that provides a barrier against tumor formation is the p53 gene. p53 is a transcription factor that assists in the control of the cell cycle. This transcription factor will bind to DNA and can downregulate genes associated with survival and proliferation; however, if p53 is mutated, this function is lost. p53 is somewhat of a last resort in tumor prevention, although the exact reasoning as to why has not yet been fully elucidated.
The second pathway to colon cancer development was established in a study by Toyota et al. in 1999. This pathway arises from the CpG island methylator phenotype (CIMP), which is an observed phenotype of epigenetic silencing of certain DNA repair and cell maintenance genes through promoter methylation. The physical process that occurs includes the sequestering of a protein complex to certain promoter locations of DNA. At the promoter, methyl groups are added by the complex to cytosines that share phosphodiester bonds to guanine nucleotides. When CpG’s form in groups, this clustering of methylation is deemed an island and can lead to the inability of other transcription factors successfully interacting with the promoter. Ultimately, methylation leads to downregulation of transcription of the following gene, thus cutting off any cellular outcome attached to the target gene.
The identification of CIMP has been made only in relation to genes associated with regulatory processes within the cell, which, when hijacked, can lead to tumor formation. Most of the genome is actually methylated at any given time; proteins must often demethylate DNA via nucleotide excision repair or mismatch repair in order to express the following gene. Therefore, when studying CIMP, it is critical to have a thorough and accurate process in how to delineate between CIMP+, CIMP-, and CIMP0. This delineation has been established differently between studies, but the most common identification technique of CIMP-status is a five-panel marker established by Weisenberger et al. in 2006. This panel includes genes that are all somehow associated with CIMP and that when methylated, resulting tumors show symptoms correlated to CIMP+.
The distinction of why CIMP-status becomes important is in the type of tumor formed by CIMP+ versus CIMP- status. CIMP+ tumors are often less differentiated and are more aggressive tumors. Prognostically, data differ between whether CIMP+ or CIMP- possess more clinically favorable outcomes, though. These differences arise from the multiple panels used to analyze CIMP status. Regardless of which panel is used, a certain gene correlated to MMR is always addressed: MLH1.
MLH1, mutL homolog 1, is a protein that assists in fixing errors in DNA replication prior to cell division. MLH1 complexes with PMS2 to cut out erroneous nucleotides and properly replace the necessary nucleotide as replication occurs. MLH1 mutation causes the gene to lose its ability to regulate DNA replication through mismatch repair. Without MLH1 functionality, mutation rates drastically increase causing DNA hypermutability, designated as microsatellite instable, or MSI-H. The outcome of microsatellite instability is that if a tumor is formed, the tumor genome is unstable as MLH1 cannot spell-check its replication. Conversely, without an MLH1 mutation, a tumor is designated as microsatellite stable, or MSS. This distinction provides a more controlled environment for tumor growth and is disadvantageous prognostically.
Throughout this background on the formation of cancer, it is still crucial to acknowledge that although it may be interesting to study the causes of this disease and where the body is prone to failure, individual people still suffer from the outcomes of these failures. The only reason that we have access to such a great deal of information about colorectal cancers is because thousands of patients have been willing to undergo additional testing or provide samples of their tumors during surgery. Ideally, more effective screening should reduce the number of patients who have to experience the physical and mental ramifications of colorectal cancer. Currently, the American Cancer Society recommends that people with average risk for colorectal cancer should begin screening at age 45.
Major issues exist in making this screening accessible to all peoples, but if the option exists to receive the screen, there is no reason to not immediately schedule testing. Colorectal cancer is one of the only cancers that can be controlled or prevented simply by adequate screening, and for most, there is no reason why this illness should be a risk. Therefore, please advocate for screening or sign up for your own screening soon! This can easily be a life-changing or even life-saving decision.