Prominent African-Americans aberrations were chromosome X amplification in men and deletions in chromosomes 4, 8, and 18. In African-Americans several CAN genes were altered at high frequencies. EXOC4, EPHB6, GNAS, MLL3, and TBX22 were the most frequently deleted genes and HAPLN1, ADAM29, SMAD2, and SMAD4 underwent the most amplifications. One of the most deleted genes was EPHB6 that is known to slow breast cancer cell lines invasiveness. Chromosome X amplification in men with CRC should be monitored. The observed CIN may have a key role in CRC in this population (Ashktorab et al. 2010; Brim et al. 2012). Among the ethnic groups, Ashktorab and colleagues found that African -Americans possessed the highest CRC the age-standardized incidence and mortality rates compared with other ethnic groups. Most CRCs originate from pre-existing adenoma. 30 % of the US adult population has adenomas (Nouraie et al. 2010). In African-Americans, CRC is more advanced and right-sided. African-Americans continue to have higher CRC death rates despite the reduced mortality rates as a result of screening (Nouraie et al. 2010; Lipkin and Higgins 1988; Kinzler and Vogelstein 1996; Jemal et al. 2007, 2014).

In a study, 1753 CRC cases were diagnosed from 1959 to 2006 in Howard University Hospital in America. The rate of in situ tumor reached a peak of 8.5 % in the 1990s (p = 0.0001). Ashktorab and co-workers observed a decade-to-decade increasing rate of right-sided tumors, beginning with 36 % during 1959–1970 and reaching a peak of 60 % during 2001–2006 (p = 0.0001). The recent increased rate of advanced and right-sided tumor were demonstrated which is consistent with SEER data and is highly important in developing strategies for CRC prevention and treatment for African-Americans (Nouraie et al. 2010). On the other hand, 5,013 colorectal polyps were diagnosed during 1959–2006, with tubular adenoma being the most frequent pathology (73 %). Right-sided polyps were mostly seen in the 1990s. Left-sided polyps were younger (p < 0.0001), more hyperplasic (23 vs. 5 %; p < 0.0001), and more frequent in women (56 vs. 52 %; p = 0.02) compared with right-sided polyps. The frequency of right-sided adenoma significantly increased from 18 % in the 1960s to 51 % during 2001–2006 (p < 0.0001). A higher neoplastic to hyperplastic polyps ratio (8:1) than what had been reported in Caucasians (7:1) was observed. This implies a recent shift in polyps from the left side to the right side of the colon, which is in accordance with the increased colon cancer incidence in African-Americans. This, the researchers further emphasized on the key role of screening for reducing the incidence of colon cancer in African-Americans (Nouraie et al. 2010).

African-Americans microsatellite instability-high (MSI-H) cancers was high compare to microsatellite stable (MSS) and microsatellite instability-low (MSH-L) cancers. Moreover, most MSI-H tumors were well differentiated, proximal, and highly mucinous. MSI-H colorectal tumor was 2–3-folds more prevalent, while the defect in the percentage expression of mismatch repair (MMR) genes (hMLH1 and hMSH2) was similar in African-American patients compared with American Caucasians. Here, the role of environmental or genetic factors that are more common in the development of CRC in African-Americans should not be underestimated (Ashktorab et al. 2005).

In this way, Brim et al showed the different DNA aberrations processes which can cause colorectal cancer (CRC). They conducted a comprehensive molecular characterization of 27 CRCs from Iranian patients. Array CGH was performed. The MSI phenotype and the methylation status of 15 genes were established using MSP. The CGH data was compared to two established lists of 41 and 68 cancer genes, respectively, and to CGH data from African Americans. A maximum parsimony cladogram based on global aberrations was established. The numbers of aberrations seem to depend on the MSI status. MSI-H tumors displayed the lowest number of aberrations. MSP revealed that most markers were methylated, except RNF182 gene. P16 and MLH1 genes were primarily methylated in MSI-H tumors. Seven markers with moderate to high frequency of methylation (SYNE1, MMP2, CD109, EVL, RET, LGR and PTPRD) had very low levels of chromosomal aberrations. All chromosomes were targeted by aberrations with deletions more frequent than amplifications. The most amplified markers were CD248, ERCC6, ERGIC3, GNAS, MMP2, NF1, P2RX7, SFRS6, SLC29A1 and TBX22. Most deletions were noted for ADAM29, CHL1, CSMD3, FBXW7, GALNS, MMP2, NF1, PRKD1, SMAD4 and TP53. Aberrations targeting chromosome X were primarily amplifications in male patients and deletions in female patients. A finding similar to what were reported for African American CRC patients. Therefore, this first comprehensive analysis of CRC Iranian tumors reveals a high MSI rate. The MSI tumors displayed the lowest level of chromosomal aberrations but high frequency of methylation (Brim et al. 2014). The MSI-L was predominantly targeted with chromosomal instability in a way similar to the MSS tumors. Chromosome 20 consistently in 20q13 region and its corresponding genes were amplified as shown in Fig. 13.2. The global chromosomal aberration profiles showed many similarities with other populations but also differences that might allow a better understanding of CRC’s clinico-pathological specifics in this population.

In spite of the characteristics of CRC ethnicity, differences in clinical presentation and surgical management of right- and left-sided large bowel cancer are well known (Fig. 13.2). Tumors that are on the right side usually present at a more advanced stage and are accompanied by symptoms such as weight loss and anemia. On the contrary, left-sided tumors often present with rectal bleeding, change in bowel habit, and tenesmus. However, the molecular pathology of carcinomas might differ with respect to the side they are one in the large bowel. These variations and differences will become more significant with advancements in systemic treatments (Richman and Adlard 2002; Fig. 13.3).

In a study conducted by Ellidokuz and colleagues, polyps and cancers were more prevalent in the left colon. Right-sided SCRCs are more fatal than left-colon cancers (Ellidokuz et al. 2003). Most SCRCs are located proximally in women and distally in men (Potter et al. 1993). Researchers have found that age is an important factor in the distribution of colorectal polyps and cancers, and a proximal trend is observed with increasing age (Samowitz et al. 2007).

A different embryological origin has been assumed for proximal and distal colons suggesting that tumors originating at any of these sites could develop along different pathways. The midgut eventually develops into distal duodenum, jejunum, ileum, caecum, appendix, ascending colon, and proximal two thirds of the transverse colon. The hindgut develops into the distal third of the transverse colon, the sigmoid colon, rectum, and upper two thirds of the anal canal. The different origins of the proximal and distal colons also lead to different vascular supplies. The proximal colon is served by the superior mesenteric artery and the distal colon by the inferior mesenteric artery. Moreover, proximal and distal colons differ in expression of several antigens, metabolism of glucose, polyamines and butyric acid, as well as in bile acid consumption, and composition and density of the bacterial population (Bufill 1990; Pocard et al. 1995; Distler and Holt 1997). Bufilland co-workers found different gene expressions in 87 genes in the ascending and descending fetal colon, indicating the occurrence of additional changes in gene expression in postnatal development (Bufill 1990). However, significant anatomic differences have already been found in the embryonic colon (Glebov et al. 2003). Gene expression was studied in fetal (17–24 weeks gestation) proximal and distal colon. More than 1000 genes were expressed differentially in adult ascending versus descending colon, with 165 genes showing > 2-fold and 49 genes showing > 3-fold differences in expression (Table 13.1).

Therefore, developmental and biologic differences in proximal and distal colon may reflect differing susceptibilities to neoplastic transformation. Insights on the mechanisms of CRC can be provided by comparing patterns of gene or protein expression in different populations or in proximal and distal tumors. One of the most important clinical applications is related to the better selection of patients needing chemotherapy. Chemoresponsive patients will be identified more accurately through molecular profiling. For example: Patients with MSI-H tumors had a modestly better prognosis than those with microsatellite-stable (MSS) or MSI-low (MSI-L) cancers, yet also did not seem to benefit from adjuvant fluorouracil (FU)-based chemotherapy. This resistance to FU is presumably due to incorporation of FU metabolites into DNA rather than inhibition of its effective target, thymidylate synthase (Kimmie and Schrag 2010).

In one study, sporadic CRCs were analyzed for microsatellite instability, expression status of mismatched repair genes (hMLH1, hMSH2), and presence of the BRAF (V600E) in Omani, Iranian, African-Americans patients to analyze the difference between molecular genetics of CRC in different populations. Tumors with BRAF mutations were on the left side in Omanis while 88 % of such mutations were found on the right side of the colon in African-American patients. The highest and lowest rates of microsatellite instability tumors at two or more markers (MSI-H) were seen in African-Americans and Omanis (31 vs. 13 %), most of which were located in the proximal colon in African-American and Iranian patients. hMLH1 gene expression defects were seen approximately two times more in African-Americans and Iranians compared with Omanis (77 vs 38 %). Tumors in all patients had BRAF mutations and mains had the most mutations (19 %). These findings suggest that Omani and Iranian patients experience CRC at a younger age. The incidence of MSI-H was lower in these groups compared with the African-American patients. This emphasizes on the key role of hMLH1 expression and BRAF mutation in MSI-H CRC in these populations compared with other populations (Brim et al. 2008).

On the other hand, 36 (23.8 %) tumors out of 151 were MSI+ in Iranians, especially in those with proximal tumors (OR = 10.4; 95 %CI = 3.9–27.8) and in smokers (OR = 2.9; 95 %CI = 1.3–6.7). MTHFR 677CT+ TT genotype was strongly associated with MSI (OR = 2.6; 95 %CI = 1.3–5.3) and a positive relationship was found between the hypermethylation of mismatch repair genes and the incidence of MSI (p = 0.001) Therefore, the researchers suggested the MTHFR 677CT+ TT variant genotype as a risk factor for MSI+ cancer (Naghibalhossaini et al. 2010). A study on the Iranian population provided insights into the mechanisms of colorectal tumor development. K-ras, p53 mutations were significantly more common in left-sided than in right-sided tumors, indicating differences in the carcinogenesis pathways in these tissues. This finding is consistent with previous reports (Russo et al. 2005; Calistri et al. 2005; Toribara and Sleisenger 1995). This could be explained by the fact that the left-sided bowel lumen exposed to ingested carcinogens and mutagens more than the right-sided bowel lumen. Moreover, simultaneous p53 and K-ras mutations were rarely seen in the same tumor. In the mentioned study, the researchers evaluated the hot spot genetic changes leading to the development of CRC in three key genes (APC, K-ras, and p53) in a CRC series from southern Iran for the first time. Mutations in exons 5 and 7 of p53, and exons 1 or 2 of K-ras genes were investigated in 151 sporadic CRC tumors by PCR-SSCP, and K-ras results were confirmed by pyrosequencing. p53 was the most frequently mutated gene, with a frequency of 62/151 (41.1 %). This finding is consistent with previous studies on CRC, with reported frequencies ranging from 40 to 50 % of p53 alterations in CRC (Soong et al. 2000; Soussi et al. 2000; Leroy et al. 2014). K-ras gene mutations were identified in 46 (30.5 %) of 151 cases which is in line with the reported frequency of 27.4 % (Smith et al. 2002). Mutations of K-ras result in specific amino acid substitutions that lead to permanent activation of the encoded p21 ras protein (Barbacid, 1987). It is well known that the activating K-ras mutations cluster at codons 12/13 (GTP-binding domain). Similar with previous reports, most samples of K-ras mutations were found in codon 12, with a smaller number of nucleotide substitutions in codon 13. No K-ras mutations were detected in codon 61 in Iranian patients. This confirms that codons 12 and 13 are also the hot spots of mutations in the Iranian population. K-ras gene mutations were localized more in distal (41.8 %) than proximal (13.3 %) tumors (p = 0.001). Most K-ras mutations were base pair transitions which mostly occurred at the second bases of codons 12 and 13. All mutations in codon 12 and 13 were G→A transitions (data not published). Ras proteins are key components of signal transduction pathways leading from cell surface receptors to the control of cell proliferation, differentiation or death. Active Ras as well as mutated Ras stimulates the RAS-RAF-MEK-ERK-MAP kinase signaling pathway through tyrosine kinase receptors (RTKs) and EGF ligand. Therefore, anti-EGFR therapies via monoclonal antibody or EGFR-tyrosine kinase inhibitors are able to block cell proliferation, survival, invasion and metastasis. When the EGFR is blocked wild type K-ras doesnot signal and the tumor cells do not proliferate. However, mutated K-ras is permanently turned on, tumor cells proliferate.

On the other hand, Mokarram and colleagues found that one of the critical factors for cancer progression is the hypermethylation of the specific locus near the MGMT start codon. In patients with CRC, the assessment of MGMT-B which is associated with K-ras mutation has high prognostic value (Mokarram et al. 2012). Moreover, 41.1 % of Iranian cases carried a pathogenic p53 mutation, which is similar to those reported for CRC in other populations (Mcdermott et al. 2002; Iacopetta 2003; Russo et al. 2005). The type of p53 mutations was not determined in the aforementioned study. However, in other populations (Russo et al. 2005; Béroud and Soussi 2003; Olivier et al. 2003; Soussi and Beroud 2003), most detected p53 mutations were G→A transitions at CpGs (Pfeifer 2006). This type of mutation could be the result of spontaneous deamination or preferential adduct for mutation at methylated CpGs (Soussi and Beroud 2003; Pfeifer 2006; Greenblatt et al. 1994), followed by G→A transitions at non-CpGs that might be associated with hypemethylation of MGMT in our population (Mokarram et al. 2012). However, since simultaneous mutations in K-ras and p53 were very rare (p = 0.007) and the frequency of p53 mutations in tumors differed in the left and right colon, it seems that MGMT does not follow the same mechanism in two genes.

Other study showed when neoplasia gains K-ras mutations, the neoplasia would transform into tubular adenoma with villous architecture and accumulate dense methylation in the SFRP2/MGMT promoter, consequently becoming non-MSI cancer (Takeda 2011; Fig. 13.4).

21/62 (33.8 %) mutations in left-sided tumors were in exon 5, and 12/62 (19.4 %) were in exon 7. In the right colon, only 6/62 (9.6 %) mutations occurred in exon 5, while 12/62 (19.4 %) of mutations occurred in exon 7. 11/62 (17.7 %) of the simultaneous mutations in both exon 5, 7 were localized in the left colon. Mokarram and co-workers did not find alterations in p53 (exons 5 or 7) or K-ras genes in 35.7 % of Iranian patients with cancer. Therefore, these mutations possibly lie on alternate pathways in the development of CRC which depends on epigenetic mechanisms (data not published).

The researchers found the MSI-H phenotype in 23.8 % of the patients which is higher than reports from Western countries but not in African-Americans (Gryfe and Gallinger 2001; Peltomaki 2001). Their finding was consistent with the results reported from the North of Iran (Bishehsari et al. 2006). The high prevalence of MSI-H could be related to the relatively high number of right-sided tumors found in the studied group (43.3 %). Most MSI-H tumors in the mentioned study were right sided (72.2 %) which is consistent with results from developed countries indicating a relatively high rate of MSI-H in proximal colon cancer (Ashktorab et al. 2005; Urso et al. 2008). A higher percentage of K-ras gene mutations were detected in non-MSI-H tumors (40/46, 87 %) compared with MSI-H tumors (6/46, 13 %, p = 0.04). Similar results were obtained for total mutations (individuals with mutation in any of two genes, p53 or K-ras) and MSI status (p = 0.04). This finding is consistent with another report showing that K-ras mutation is less frequent in patients with sporadic MSI-H with hMLH1 hypermethylation compared with non-MSI-H CRC without hMLH1 methylation and MSS CRC (Oliveira et al. 2004). There is an inverse relationship between microsatellite instability and K-ras, p53 mutations regardless of the type of microsatellite. However, the existence of such a relationship is controversial among different studies (Fujiwara et al. 1998).

A previous report from Iran showed that sporadic MSS and sporadic MSI-H CRCs shared similar K-ras mutation frequencies. However, the type of K-ras mutations differed between MSI-H and MSS subgroups, suggesting a link between this specific type of mutation and MMR defect in MSI-H CRCs. Defects in other DNA repair systems, such as O6-methylguanine-DNA methyltransferase (MGMT) activity might be related to inability to protect from G→A transition in K-ras induced by alkylating agents (Esteller et al. 2000; Lees et al. 2004; Qi et al. 2005), which was shown in a recent study (Mokarram et al. 2012).

The relatively low frequency of tumors with mutations in both genes (K-ras and p53) as well as the fact that 57 % of tumors had mutations in only one gene (p53, or K-ras), implies that the progressive accumulation of multiple mutations in these genes is not a prerequisite for tumor development and does not represent a synergistic evolutionary pathway. Therefore, the simultaneous alterations in K-ras and p53 are not frequent, suggesting that the sequential K-ras-p53 module is not obligatory in the progression of CRC in Iranian population. While some studies have shown that K-ras mutations may be lost through selection in the progression of tumors from adenoma to carcinoma, (Pretlow et al. 1993; Ines et al. 2014) we found no statistically significant difference in K-ras mutations between stage I and II. However, a significant difference was found in the frequency of p53 mutations between stage I and II, suggesting that p53 inactivation may be an important determinant of tumor progression. In contrast to other reports suggesting that MSS and MSI-L tumors have a common molecular background (Bouzourene et al. 2000; Laiho et al. 2002). Mokarram and co-workers observed more p53 mutations in the MSI-L tumors in lower stages (I, II). It seems that MSI-L CRCs were distinct from both MSI-H and MSS cancers in our study population. Correlations between p53 mutations and tumor stage correspond with the notion that p53 mutations accumulate during CRC progression (Bouzourene et al. 2000; Akkiprik et al. 2007). An important aspect of this study is that two major gene mutations were analyzed in Iranian patients with CRC in relation to prognosis for the first time. Although we and other researchers showed that there are genetic variations between population and proximal and distal CRCs, the issue of whether proximal and distal CRC should be considered as a single entity or two distinct entities is still debated. Further studies addressing the heterogeneity of the pathogenetic pathways leading to sporadic CRC in different populations as well identifying biological and/or molecular differences between proximal and distal CRCs should be done in this regard. With respect to the obtained data, future therapies must be targeted on individual patients, based on the detailed understanding of their genetic background and the nature of the mutation present in individual tumors.