A Genetic Perspective on Colorectal Cancer Progression

2.1 Chromosomal instability

The most frequent model of phenotype group in colorectal cancer are allelic losses in the short arm of chromosome 17 and 8 and in the long arm of chromosomes 5, 18 and 22, being approximately 80% of the sporadic form and are related to mutations in tumor suppressor genes of TP53 genes, APC, SMAD2, and SMAD4.

The first major accepted model for cancer development in colon was described by Fearon and Vogelstein in 1990. The majority of the sporadic CCR tumors originates from premalignant precursor lesions known as polyps, which over time progress to clinically relevant tumors. In this model, the sequence of events leading from the adenoma (polyps) to carcinoma was based on mutation on APC and TP53 genes [13, 14].

The proto-oncogene K-ras (Kirsten-ras) tumor suppressor genes, APC protein (adenomatous polyposis coli), DCC protein (deleted in colorectal cancer) and TP53; and DNA repair or mismatch repair genes (MSH2, MLH1, PMS1, PMS2 and MSH6) are fundamental in development of CCR. Repetitive nucleotide sequences form approximately 25–40% of the DNA molecule being observed several times across the genome as dispersed replicates and tandem or satellite replicates [15]. These replicates can be classified according to the extension of the repetitive sequence in: satellite, minisatellite and microsatellite, depending on the number of nucleotides [16].

Most of the cases of CCR originate from polyps, but it was evidenced that about 45% of the tumors located in the proximal colon originated from epithelium without preexisting polyps, being considered new cancer [17]. In this model of carcinogenesis, which affects 10–15% of cases of sporadic CCR, instead of chromosomal instability there would be genomic instability due to mutations of DNA repair proteins, a phenomenon known as microsatellite instability (IMS) [18]. This pathway of colorectal carcinogenesis due to chromosomal instability is more frequently observed in tumors in the proximal colon, presenting characteristic histological features, being diploid, exophytic growth, worse histological grade, greater tendency to mucus production and lower mutation index in the TP53 gene, and paradoxically, they are associated with a better prognosis [18, 19].

2.2 Microsatellite instability

DNA is a molecule that often undergoes changes through loss of segments, mutations that occur during the process of cell division. To correct these changes, it has proteins with the function of performing the repairs necessary to maintain its integrity. These proteins are produced from some genes known as repair (mismatch repair genes—MMR) and this function is carried out continuously, preserving the cellular tissues [20, 21, 22]. The hMLH1 gene is located on chromosome 3p21-23 [23, 24], hMSH2 on chromosome 2p21 [25, 26, 27] and hMSH6 is on 2p16 [28].

In hereditary CCR there are two genetic pathways of carcinogenesis: a chromosomal instability pathway, which occurs in PAF (familial adenomatous polyposis), where the patient inherits a mutation of the APC (adenomatous colonic polyposis) tumor suppressor gene, and DNA hypermutability pathway, which occurs in HNPCC (inherited non-polyposis colorectal cancer) in which the inherited genetic change is the inactivation of one of the alleles of genes involved in DNA repair (hMSH2 and hMLH1 genes). However, other genes are involved in colorectal carcinogenesis, such as K-ras gene, DCC gene, Tp53 gene, etc. [29].

DNA is a molecule that often undergoes changes through loss of segments, mutations that occur during the process of cell division. To correct these changes, it has proteins with the function of performing the repairs necessary to maintain its integrity. These proteins are produced from some genes known as repair (mismatch repair genes—MMR) and this function is carried out continuously, preserving the cellular tissues [20, 21, 22]. The hMLH1 gene is located on chromosome 3p21-23 [23, 24], hMSH2 on chromosome 2p21 [25, 26, 27] and hMSH6 is on 2p16 [28].

MMRs have the function of recognizing the occurrence of the mutation and blocking cell division in order to prevent the emergence of a defective cell line, which is done by inducing cell death (apoptosis) or performing DNA repair. For the latter, these proteins remove a segment of DNA containing the change and insert a new segment containing the right sequence, based on the “template” of the complementary DNA [30].

The failure of these proteins to function will cause a great instability in the genome, that is, defects in the sequence of base pairs occurring at random in DNA replication cannot be adequately repaired, generating an accumulation of genetic abnormalities that favor the emergence of cancer [30].

A genetic instability that appears in 12–15% of CCR cases, named MSI (microsatellite instability) as a result of a mismatch repair (MMR), what leading to the accumulation of mutations in genes controlling cell cycle and apoptosis (TGFBRII, BAX or CASPASE5) [31].

Cells with changes in MMRs are not able to correctly repair errors during DNA replication. Because of their repetitive structure, microsatellite regions in DNA are particularly prone to these repair errors [32, 33]. The DNA of cells of certain tumors presents differences in the number of repetitive units in one or more microsatellites, when compared to the same microsatellites in the DNA of normal cells, a fact called microsatellite instability (MSI). as positive for replication errors, that is, RER (+) [34].

More than 90% of HNPCC patients present RER (+), while about 15% of sporadic CCRs present this genetic trait [30].

Repair proteins in their normal state form heterodimers [35, 36, 37, 38]. MSH2 dimerizes with MSH6 forming the MutSα [38] functional complex, and MLH1 dimerizes with PMS2 to form MutLα [37, 39]. It has been shown that MSH2 and MLH1 proteins are obligatory parts of their respective heterodimers [40, 41, 42]. Their abnormalities may result in a proteolytic degradation of their dimers and consequently loss of both mandatory and secondarily associated proteins, the exception includes only MLH1 mutations, when the mutations result in the antigenically activation of the mutated MLH1 protein, which may be the loss of PMS2 only. The reverse, however, is not true, when mutation occurs in the genes of secondary proteins, for example, in MSH6 and PMS2, loss of MSH2 and MLH1 proteins that may not occur, inasmuch as other proteins compensate the function of the secondary proteins, such as MSH3, MLH3 and PMS1. In effect, mutations of MLH1 or MSH2 routinely cause loss of MLH1/PMS2 or MSH2/MHS6, respectively, while mutations of PMS2 or MSH6 cause isolated loss of PMS2 or MSH2 only [43].

Figure 4 shows the model of DNA repair proteins in patients with colorectal cancer [44].

Studies performed in CCR demonstrated a positivity index for greater microsatellite instability in young patients or located in proximal segments of the colon. A study restricted to rectum tumors, the incidence of repair errors was only in 2% of cases, confirming the relationship between microsatellite instability and tumors located in the right or transverse colon [45]. The CCR associated with repair errors tend to present the same location and biological behavior independent of their sporadic or hereditary nature (HNPCC) [45].

2.3 Serrated pathway

The serrated pathway was first described by Longacre and Fenoglio-Preiser in 1990 and differs from the classical adenoma-carcinoma sequence one because in this way there is the participation of other genetic alterations other than chromosomal instability and KRAS mutations like BRAF mutations and gene promoter hypermethylation. But in the serrated pathway, microsatellite instability can also be detected.

The serrated polyps are characterized by glandular serration in which the colonic epithelial crypts show luminal “saw-toothed” pattern. The nomenclature is not well established but the World Health Organization in 2010 classified them into three main groups: hyperplastic polyps, sessile serrated adenomas/polyps and traditional serrated adenomas [46].

Genetically, carcinoma arising from serrated polyps are MSI-H and shows epithelial serrations, clear, eosinophilic and abundant cytoplasm, vesicular nuclei, absence of necrosis, mucin production and presence of cell balls and rods. The finding of serrated lesion in the peripheral of an invasive carcinoma also leads to the diagnostic of this pathway.

Serrated adenocarcinoma is found in about 10% of sporadic colorectal cancers and is originated in the serrated polyp-carcinoma pathway. In this way, hyperplastic polyps now are recognized as neoplastic lesions because they may predispose to cancer in a sequence in which they progress to serrated adenomas and then to colorectal cancer in at about 7 years. It is not clear why only a few groups of hyperplastic polyps, mainly the ones located in the right colon, will progress to carcinoma and the answer is probably dependent on the genetic findings not elucidated until now. A great number of studies have shown that the right colon is not the same organ as the left colon and the right-sided cancer tends to be more aggressive and this difference is caused by the difference of genetic standard between the sides.

The clinical management of hyperplastic polyps and serrated polyps is essential for avoiding the carcinoma transformation. The most important procedure for prophylaxis is the complete removal of these polyps in colonoscopy and the subsequent surveillance but it is not well established how to follow up. Just for comparison, in the classical adenomatous polyps, the size, number and histological variants (if tubular, villous or tubulovillous) are taken in account to determine the interval of surveillance and this knowledge is more than a decade old. For serrated polyps, as the understanding of this pattern of via is recent, the follow up is not clear hence studies have not shown yet which features are important to determine the risk of progression of these lesions. Moreover, the majority of serrated polyps will not progress to carcinoma and studies answering why are not yet available. A few studies have demonstrated that sessile serrated lesions larger than 10 mm are at high risk for carcinoma progress. But despite the lack of information and in order to prevent cancer arising via serrated pathway, the complete removal of serrated polyps is the goal in these cases.

If there are significant differences regarding the genetic markers and pathological findings in serrated pathway, it is expected to have differences in the presentation of the disease and response to therapy. And some evidences have showed these: carcinomas arising from serrated pathway tends to have lesser 5-year survival but again the causes of this comportment are not available, and the answer may be found as the genetic alterations becomes evident [47].

For now, it is clear that serrated pathway is a well-established pattern that explains the behavior of some hyperplastic and serrated polyps that could not have been explained in the classical adenoma-carcinoma sequence using CIN and MSI models. Even with lacks in knowledge for profound understanding, the pathological and molecular characterization of these polyps are constantly progressing and studies in the next few years will probably show the best way to manage these cases in clinical practice [47].