WITHDRAWN: Molecular classification of colorectal cancer: Current perspectives and controversies

Journal of the Egyptian National Cancer Institute (2016) xxx, xxx–xxx

Cairo University

Journal of the Egyptian National Cancer Institute www.elsevier.com/locate/jnci www.sciencedirect.com

Review

Molecular classification of colorectal cancer: Current perspectives and controversies Amrallah A. Mohammed a,b,*, Hani El-Tanni b, Hani M. El-Khatib b, Ahmad A. Mirza c, Amr T. El-Kashif d a

Medical Oncology Department, Faculty of Medicine, Zagazig University, Egypt Oncology Center, King Abdullah Medical City-Holy Capital, Saudi Arabia c Faculty of Medicine, Umm Al-Qura University, Makkah, Saudi Arabia d Clinical Oncology Department, Faculty of Medicine, Cairo University, Egypt b

Received 19 October 2015; revised 25 November 2015; accepted 28 November 2015

KEYWORDS Colorectal cancer; Molecular classification; Colon cancer subtypes

Abstract Objective: The aim of this review is to summarize the molecular classification of colorectal cancer (CRC), recognizing that the review is a small fragment from the whole picture. Methods: A Medline search was conducted and published articles from American and European studies from 2000 to present were reviewed. Different types of molecular classifications are presented. Results: Integrative genomic studies revealed the complexity of molecular heterogeneity of CRC. Although several treatments exist, we do not have a good way to select the appropriate treatment; personalized treatment strategies are needed. Genomically-driven clinical trials with matched targeted therapies for KRAS, NRAS, BRAF and PIK3CA mutations are ongoing and offer promising treatment opportunities for patients with CRC. Conclusion: For CRC, a broad molecular classification is ‘‘still missing”. There is a real hope that the evolving application of molecular techniques to diagnosis, risk-stratification and management of CRC will be translated to reduce disease burden and improve the survival. Ó 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of National Cancer Institute, Cairo University. This is an open access article under the CC BY-NC-ND license (http://creativecommons. org/licenses/by-nc-nd/4.0/).

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . Evolution of molecular classification in CRC Clinical application . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . .

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* Corresponding author at: Muzdallifa Street, P.O. Box 57657, 21995 Makkah, Saudi Arabia. Tel.: +966 566979027; fax: +966 125532239. E-mail address: [email protected] (A.A. Mohammed). Peer review under responsibility of The National Cancer Institute, Cairo University. http://dx.doi.org/10.1016/j.jnci.2015.11.004 1110-0362 Ó 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of National Cancer Institute, Cairo University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Mohammed AA et al. Molecular classification of colorectal cancer: Current perspectives and controversies, J Egyptian Nat Cancer Inst (2016), http://dx.doi.org/10.1016/j.jnci.2015.11.004

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A.A. Mohammed et al. Conflict of interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Introduction Colorectal cancer (CRC) is one of the major causes of morbidity and mortality, more than 1.2 million patients are diagnosed every year, and more than 600,000 die from the disease. Incidence is higher in men than in women and strongly increases with age; median age at diagnosis is about 70 years in developed countries. In fact, the molecular heterogeneity of CRC is believed to be one of the factors responsible for the considerable variability in treatment response among patients with the same stage of CRC [1]. To date, although of significant interest, the molecular classification of CRC has not achieved widespread clinical use, and has not been validated or reproduced by multiple institutions on a large scale. Molecular classification is important because it reflects underlying mechanisms of carcinogenesis and may contribute to improve strategies for prevention, screening, diagnosis, and generate novel, more effective treatment [2]. In this review, we summarize the molecular pathways and classification of CRC and the impaction on patients. Evolution of molecular classification in CRC The transition from normal epithelium to adenoma to carcinoma is associated with acquired molecular events. At least three major distinct molecular pathways have been described, leading to CRCs: chromosomal instability (CIN), microsatellite instability (MSI), and CpG island methylator phenotype (CIMP). The first and most common (70%), the CIN pathway, defines the accumulation of numerical or structural chromosomal abnormalities, resulting in karyotypic variability from cell to cell, and is characterized by frequent loss-of-heterozygosity (LOH) at tumor suppressor gene loci. Moreover, CIN tumors are distinguished by alterations in the main oncogenes

(e.g., KRAS, NRAS, BRAF, PIK3) and loss of chromosome 18q and deletion of chromosome 17p, which contains the important tumor suppressor genes (e.g., APC, TP53, and PTEN). Key pathways include Wnt/ß-catenin, transforming growth factor beta (TGF-ß), epidermal growth factor receptor (EGFR, HER1), downstream mitogen-activated protein kinase (MAPK), and phosphoinositide 3-kinase (PI3K) signaling activation [3]. The CIN pathway and its accompanying pathological adenoma–carcinoma sequence has provided a foundation for the molecular classification of colorectal carcinogenesis and has established a reference against which one may contrast other CRC molecular profiles, but it is now clear that CRC may develop by other means. The second pathway is the MSI, occurs in 15% of CRCs and caused by inactivation of DNA mismatch repair genes (DNA MMR) leading to the accumulation of abnormalities in short sequences that are repeated up to hundreds of times within the genome (microsatellites). Basically, the MSI subtype of CRC has a clear molecular origin (MLH1, MLH3, MSH2, MSH3, MSH6, or PMS2 inactivation) arising on a hereditary or sporadic background, and a specific clinicopathological phenotype: more often located in the proximal colon, with a poorly differentiated and a mucinous or medullary histotype, and often presents intense peritumoral and intratumoral lymphocytic infiltrations. In general, the prognosis and survival of patients affected by MSI-high CRC are better and longer than those of patients with CIN positive CRC [4]. In addition to CIN and MSI, a third epigenetic instability pathway (found in approximately 15–20% of CRCs) based on methylation status in different markers had identified the tumors as CpG island methylation phenotype (CIMP).The non-cancerous cell genome contains approximately 80–85% methylated CpGs in the non-promoter region, but CpG islands located around the promoter region of genes are usually

colorectal cancer

chromosomal instability 70%

(a) Accumulation of chromosomal abnormalities. (b) Frequent LOH. (c) Alterations in the main oncogenes*and tumor suppressor genes**, Key pathways include Wnt/ßcatenin, TGF-ß, EGFR, HER1, MAPK and PI3K. (d) Leftt-sided location

e.g., *KRAS, NRAS, BRAF, PIK3 e.g., **APC, TP53, and PTEN

Figure 1

Microsatellite instability 15%

(i) (ii) (iii) (iv) (v)

Inactivation of DNA MMR. Inefficiently DNA binding. Right-sided location. Mucinous cell type. Tumor infiltrating lymphocytes.

CpG island methylator phenotype 15%

There are two general types; (a) CIMP high, are usually located in the proximal site of the colon related to BRAF mutations and MLH1 methylation. (b) CIMP low, related to KRAS mutations.

Adapted from Marzouk O and Schofield J (8)

The main features of the three genetic pathways of CRC.

Please cite this article in press as: Mohammed AA et al. Molecular classification of colorectal cancer: Current perspectives and controversies, J Egyptian Nat Cancer Inst (2016), http://dx.doi.org/10.1016/j.jnci.2015.11.004

Molecular classification of colorectal cancer Table 1

The main differences between the hypermutated and nonhypermutated.

Somatic events BRAF mutations KRAS and PIK3CA mutations a b

3

Hypermutated group

Nonhypermutated group

➢ ➢ ➢ ➢

➢ ➢ ➢ ➢

Mismatch-repair genesa DNA repair geneb About 50% of cases Very uncommon

TP53 mutations, which characterize CIN tumors. More gene copy number alterations Less than 5% of cases Common

(MSI/CIMP-H). (such as POLE) Polymerase, DNA, Epsilon.

Another useful way to subcategorize CRC is by mutation rate. The Cancer Genome Atlas (TCGA) study showed that over 15% of all CRC had distinctly elevated gene mutation rates compared to the remainder of the CRC. The hypermutated CRC had a high fraction of tumors with MSI-H and MLH1 silencing. The TCGA CRC study also revealed that the hypermutated CRC had fewer APC, KRAS, and TP53 mutations, while these tumors also displayed higher numbers of BRAF and TGF-b pathway-related mutations, Table 1 [12]. In 2013, a proposal for a molecular classification of CRC was published by Domingo et al., based on 906 stages II and III CRCs [13]. Group 1: MSI-H and/or BRAF mutated; group 2: CIN and/or TP53 mutated with KRAS and PIK3Ca wild type status; group 3: CIN, KRAS and/or PIK3CA mutated; TP53 wild type status; group 4: CS, KRAS and/or PIK3CA mutated; TP53 wild type status; group 5: NRAS mutated; group 6: no mutations; group 7: other. Based on genetic and clinicopathological characteristics to classify CRC and to correlate with prognosis and response prediction, Sousa et al. defined three groups which correlated well with two of the known molecularly pathways, namely the CIN (hereby named CCS1) and the MSI (CCS2) group [14]. The third group overlaps partly with the CIMP group (CCS3), might be derived from the serrated pathway and is linked with poor prognosis. Importantly BRAF V600E mutations were found in 90% of CRC cases with sessile serrated adenoma (SSA) lesions and never in the conventional adenomas. The BRAF mutation

unmethylated. Conversely, genome-wide hypomethylation in non-promoter regions and hypermethylation in the promoter region of genes, particularly tumor suppressor genes, are often observed in cancer cells. According to the number of methylated markers, the CIMP phenotype can be also divided into CIMP-high and CIMP-low [5–8]. The BRAF oncogene mutation is often identified in CIMP-high CRC and is associated with increased cell growth, progression of carcinogenesis, and high colon cancer specific mortality. However CIMP-high tumors, regardless of BRAF mutation, are associated with reduced colon cancer mortality. CIMP-low, in contrast with CIMP-high, appears to have a different phenotype, with a low-level of DNA methylation [9,10] Fig. 1. Since the definition of the three pathways is not mutually exclusive, it is possible that tumors can exhibit features of multiple pathways. In 2007, Jeremy Jass proposed a molecular classification based on clinical, morphological, and molecular parameters, which included five subgroups [11]. Group 1 (12% of all CRCs): chromosomally stable (CS), MLH1 methylated, MSI-H, BRAF mutated, CIMP high status; group 2 (8% of all CRCs): CS, partially MLH1 methylated, microsatellite stable (MSS), MSI-L, BRAF mutated, CIMP high; group 3 (20% of all CRCs): CIN, MGMT methylated, MSS/MSI-L, KRAS mutated, CIMP low; group 4 (57% of all CRCs): CIN, MSS/MSI-L, CIMP negative; group 5 (3% of all CRCs): CS, MSI-H, CIMP negative, BRAF wild type.

Table 2 Overviews of CRC classification studies. (A and B) showing the clinical and biological characteristics and gene signatures of colon cancer subtypes (CCS): (A) De Sousa E. Melo et al. classification [14] and CRC assigner subtypes (B) Sadanandam et al. [16]. Subtypes

CCS1

CCS2

CCS3

(A) Sousa et al. classification [14] DFS Gene signature Precursor Wnt signaling

Good Epithelial Adenomatous adenomas High

Intermediate Inflammatory Unknown Low

Poor Mesenchymal SSA Low

CCS, colon cancer subtypes; DFS, disease-free survival; SSA, sessile serrated adenoma. (B) Sadanandam et al. classification [16] Subtypes

Stem-like

Transit amplifying RC-TA

DFS Gene signature Colon crypt top/base Wnt signaling

Poor Mesenchymal/stem cell Base High

Goblet cell

Enterocyte

Inflammatory type

Good Goblet cell Top Low

Intermediate Enterocyte cell Top Low

Intermediate Increased cytokines Unknown Unknown

CS-TA

Poor Good Heterogeneous with TA cell Top/base High/low

TA, transit-amplifying; CR-TA, cetuximab-resistant TA; CS-TA, cetuximab-sensitive TA; DFS, disease-free survival.

Please cite this article in press as: Mohammed AA et al. Molecular classification of colorectal cancer: Current perspectives and controversies, J Egyptian Nat Cancer Inst (2016), http://dx.doi.org/10.1016/j.jnci.2015.11.004

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A.A. Mohammed et al.

is an early event in the serrated pathway and its forced expression will lead to a state of dormancy known as senescence. In SSA, BRAF mutations were found either in early hyperplastic polyps (the serrated precursors) or in the advanced dysplastic serrated polyps, confirming its role in neoplastic progression [15]. In other study done by Sadanandam et al., 5 different types of CRCs (stem-like, transit amplifying [TA], enterocyte, goblet-like, and inflammatory type) were identified based on gene expression profiles, of which the TA type could be further subdivided into 2 sub-groups based on different responses to epidermal growth factor receptor (EGFR)-targeted therapy [16] Table 2A and B. Over the last decade, it has been established that other pathways are involved in the pathogenesis of CRC, including microRNA (miRNA) and inflammatory pathways.miRNA are a class of short non-coding RNAs which regulate protein expression by inhibiting mRNA translation, in particular of genes involved in cell growth, proliferation, invasion, metastasis, risk and prognosis, even though as diagnostic biomarkers. In contrast to the original report that microRNA expression

levels are globally reduced in cancer; as miR-143 and miR-145 suggested that they were tumor suppressors, more microRNAs have been found to have elevated expression in CRC compared to those with reduced levels [17]. The connection between inflammation and tumorigenesis is established and is also likely to be involved with other sporadic as well as heritable CRC. The molecular mechanisms by which inflammation promotes cancer development are still being uncovered, but recent work has elucidated the role of distinct immune cells, cytokines, and other immune mediators in virtually all steps of colon tumorigenesis, including initiation, promotion, progression, and metastasis. This happens by promoting anti-apoptotic system, increasing DNA-damage, production of angiogenic and lymphangiogenic growth factors, facilitating invasion and altering cell adhesion [18]. What further complicates this issue is the presence of many molecular classifications without sharp differences between them and although defined subtypes of CRC do exist, the molecular subgroups cannot be accurately distinguished histologically or clinically at this time. In contrast, the contribution of epigenetic modifications to the pathogenesis of CRC has

A

Kras mutaon 30% KRAS, NRAS, BRAF, PIK3CA wt 15% Others 14% RAS/RAF mut+ PIK3CA mut 12% PIK3CA mut 10% BRAF mut 7% NRAS mut 6% HER2 amplificaon 4% MET amplificaon 2%

B KRAS mut 40%

MET amplificaon 15% Others 26%

KRAS mut 40% Others 26% MET amplificaon, 15% HER2 amplificaon, 12% EGFR S492R, 7%

Figure 2 Summarizes the resistance to anti-EGFR monoclonal antibodies in CRC, either de novo resistance (A), or acquired resistance (B). (A) De novo resistance to anti-EGFR monoclonal antibodies in CRC. (B) Acquired resistance to anti-EGFR monoclonal antibodies in CRC.

Please cite this article in press as: Mohammed AA et al. Molecular classification of colorectal cancer: Current perspectives and controversies, J Egyptian Nat Cancer Inst (2016), http://dx.doi.org/10.1016/j.jnci.2015.11.004

Molecular classification of colorectal cancer been highlighted by the identification of a subtype of CRC, the CIMP that has a distinct epigenome with a high frequency of methylated genes. Clinical application This knowledge has already translated into drug development and biomarker discovery. A noticeable example is the clinical use of monoclonal antibodies (mAbs); cetuximab/panitumumab, targeting EGFR and selection of patients based on downstream pathway aberrations, but unfortunately, clinical data suggests that the majority of patients whom initially respond to mAbs eventually acquire resistance [19] Fig. 2. Mechanisms of resistance to anti-EGFR moAbs in CRCs include, activating mutations of EGFR effectors, such as KRAS (by either point mutations or gene amplification), BRAF and PI3KCA, or PTEN loss of function, cause persistent activation of downstream signaling despite EGFR inhibition. KRAS mutations were found to emerge in a large proportion of tumor biopsies and circulating tumor DNA (ctDNA) from patients with secondary resistance to anti-EGFR mAbs [20]. However, not all hotspot mutations in the KRAS gene may have the same biologic behavior. KRAS G13D allele is reported to bear a weaker transforming and pathway activation potency. Retrospective data, suggest only the G13D mutation is sensitive to anti EGFR mAbs, whereas the other mutation subtypes were associated with no response [21,22]. Moreover, other genomic events that have been associated with resistance to anti-EGFR mAbs include HER2 and MET aberrant activation (by either receptor gene amplification or high ligand levels) of alternative receptors, such as HER2 or MET, can bypass EGFR inhibition and mediate downstream pathway activation [23,24]. Even so, we have a subset of CRC patients with KRAS wild-type who progressed after targeted EGFR. The predominant role of BRAF mutations and acquired EGFR ectodomain mutation (S492R) that prevents cetuximab binding in this scenario is likely to be the main contributor of this worse outcome [25]. In addition, Polymorphisms of genes encoding the Fcy receptors (Fc fragment of IgG receptor 2A (FCGR2A) and 3A (FCGR3A)), which influence their affinity for the Fc fragment, have been linked to the pharmacodynamics of mAbs. Patients with KRAS mutated tumors and the FCGR2A R/R polymorphism responded poorly when treated with chemotherapy only, and experienced the most benefit of the addition of cetuximab in terms of response rate [26]. Prospective validation of these findings is underway. The clinical trials in advanced CRC are selecting patients according to KRAS, BRAF, NRAS and PIK3CA mutation status. Based on preclinical data and early clinical trials, KRAS wild-type and quadruple-negative tumors are particularly sensitive to dual EGFR targeting (ERBB tyrosine kinase inhibitors added to anti-EGFR mAbs) and this strategy is undergoing clinical validation and offers promising treatment opportunities for those patients [27]. Also, second-generation anti-EGFR mAbs with increased receptor internalization/ degradation potency and agents engineered to induce enhanced antibody dependent cell-mediated cytotoxicity (ADCC) are now available for clinical testing [28,29].

5 There are several studies which have revealed that resistance to anti-EGFR therapeutics may be due to HER3 signaling pathways activation and compensatory PI3K pathway activation which have been translated into clinical trials. Preclinical studies have indicated that the fully humanized mAb U3-1287 (patritumab) binding to the extracellular region of HER3 led to its internalization and subsequent degradation in models of breast, lung, and head and neck cancer [30]. Although BRAF inhibitors have demonstrated limited antitumor activity as single agents in BRAF V600E-mutated CRC [31], the results of clinical trials investigating BRAF inhibitors combined with anti-EGFR mAbs with or without a third agent (MEK or PI3K pathway inhibitors) are highly anticipated. With regards to NRAS-mutated CRC, clinical investigation is centered on PI3K pathway inhibitors in combination with MEK inhibitors or anti-EGFR mAbs [32,23,33]. Blockade of programed death 1 (PD-1) and its receptor has demonstrated impressive clinical benefit in a variety of solid tumors. Interestingly, one of the longest complete remissions achieved with anti-PD-1 mAb therapy was documented in a patient with advanced MSI CRC [34–36]. We summarized some clinical trials in CRC in Table 3. Moving to clinical implications, in the observational study regarding aspirin, the benefits of the adjuvant aspirin belonged to those who were COX-2-positive or PIK3CA mutation negative [37]. The role of the inflammatory mediator was controversial. Clinical trials using anti-inflammatory drugs should indicate the therapeutic efficacy of anti-inflammatory biologics, such as anti-TNF, anti-IL-6, and anti-IL-1, considering that the anti-inflammatory drugs target myeloid and lymphoid cells, which do not carry oncogenic mutations and, therefore, do not undergo rapid evolution [18]. In a retrospective multicenter cohort of 782 patients with stage I–IV CRC who underwent surgery between 1987 and 2007 in seven centers suggested that PIK3CA mutations were associated with a good prognosis in patients with MSS stage I–III CC and so those with high-risk stage II or stage III PIK3CA-mutated MSS CRC may not require adjuvant chemotherapy [38]. Currently, monitoring genetic changes in a tumor requires multiple biopsies. In the future, it might just be a matter of drawing a tube of blood, what is called liquid biopsies or ctDNA, which is a specific cancer biomarker that can be detected, measured, and tracked. Preliminary data suggest ctDNA is detectable at diagnosis in the majority of patients with non-metastatic CRC. The potential for ctDNA as a CRC screening tool, and as a prognostic marker for patients with early stage cancer, should be further explored [39]. It is recognized now that MSI-positive CRC is a distinct subgroup of CRC with a favorable stage-adjusted prognosis compared with MSS CRC patients. The role of MSI for response to 5-fluorouracil (5-FU) in the adjuvant setting is conflicting. Ribic et al. showed that MSI tumors did not seem to benefit from 5-FU based adjuvant chemotherapy and were possibly even harmed [40], which is corresponding to the study done by Sargent et al. [41], in contrast, in the PETACC3 study and Sinicrope et al., suggesting that the improved prognosis of MSI tumors was maintained under 5-FU treatment [42,43]. On the other hand, Schetter et al., revealed that miR-21 expression is associated with therapeutic outcome to 5FUbased therapies in both American and Chinese cohorts.

Please cite this article in press as: Mohammed AA et al. Molecular classification of colorectal cancer: Current perspectives and controversies, J Egyptian Nat Cancer Inst (2016), http://dx.doi.org/10.1016/j.jnci.2015.11.004

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A.A. Mohammed et al. Table 3

Selected clinical trials in advanced CRC based on biological hallmarks.

NCT ID NCT02227667

NCT02442414

NCT01376505

NCT01304602

NCT01776307

NCT02512172

NCT02537418

NCT01802320

NCT01937715

NCT01252628

Trial description  To evaluate the efficacy of MEDI4736 in immunological subsets of advanced colorectal cancer  Drug: MEDI4736; Durvalumab: Fc optimized monoclonal antibody directed against programed cell death-1  To determine the maximum tolerated dose of KBP-5209 as a single agent for patients with advanced colorectal cancer after failure of standard chemotherapy  Drug: KBP-5209; Pirotinib: is a second-generation, irreversible pan-EGFR tyrosine kinase inhibitor  To evaluate the side effects and best dose of vaccine therapy in treating patients with advanced colorectal cancer  Biological: HER-2 vaccine  To evaluate the efficacy of BKM120 in patients with advanced colorectal cancer after failure or intolerance of at least one line of therapy  Drug: BKM120; Buparlisib: PI3K inhibitor  To evaluate the efficacy of BBI608 in combination with cetuximab, panitumumab or capecitabine in patients with advanced colorectal cancer after failure of at least 2 regimens containing 5-Fluorouracil, oxaliplatin, or irinotecan  Drug: BBI608; is a cancer stem cell inhibitor  To evaluate the safety and effectiveness of the combination of intravenous romidepsin and/or 5-azacitidine with IV MK-3475 in patients with Microsatellite stable  Drug: MK-3475; antibody directed against programed cell death-1  To evaluate the side effects and best dose of durvalumab alone or combined with tremelimumab in advanced colorectal cancer after failure of standard chemotherapy  Drug: Durvalumab: antibody directed against programed cell death-1: Tremelimuma: antibody activate the immune system through blocking the cytotoxic T lymphocyteassociated antigen 4 (CTLA-4)  For patients advanced colorectal cancer after failure or intolerance of at least one line of therapy, with KRAS-wild type, enriched for PTEN loss and PIK3CA mutation  Drug: MK2206; Akt inhibitor  To evaluate safety and efficacy of PF-05212384 with FOLFIRI regimen in advanced colorectal cancer  Drug: PF-05212384; pan-class I isoform PI3K/mTOR inhibitor  To evaluate the safety and efficacy of a cetuximab and PX-866 combination treatment in advanced colorectal cancer  Drug: PX-866; PI3K inhibitor

Study type and design

Trial phase

Last updated

Interventional

Phase II

April 23, 2015

Interventional

Phase I

May 8, 2015

Interventional

Phase I

May 18, 2015

Interventional

Phase I

March 31, 2015

Interventional

Phase II

August 28, 2015

Interventional

Phase I

July 27, 2015

Interventional

Phase I

August 31, 2015

Interventional

Phase II

August 24, 2015

Interventional

Phase II

August 18, 2015

Interventional

Phase I

June 16, 2015

Information based on clinical trials listing on clinicaltrials.gov. NCT ID National Clinical Trials Identifier.

This association, suggests that increased miR-21 expression is in part responsible for the resistance to 5FU [44]. Although the result of NSABP C-08 (adjuvant study) was negative, the authors revealed retrospectively a statistically significant survival benefit from the addition of bevacizumab in MMR defective (dMMR) tumors in contrast with no benefit in patients with MMR proficient tumors (pMMR) [45]. This may be related to dMMR tumor cells, because of their hypermutated status and high immunogenicity, at the micrometastatic level they have to evade attack from the immune system in order to progress and VEGF-A is one of the main tumor-derived soluble factors that can create an immune suppressive microenvironment. Finally, anatomically and embryologically, CRC is also divided into proximal colon cancer (right from the splenic flexure), distal colon cancer (left from the splenic flexure) and rectal cancer. The proximal colon originates from the mid gut, while the distal colon and rectum arise from the hindgut. Also the nourishing arteries and the innervation differ between left and right colon [46], which is reflected on differences in biol-

ogy, prognosis and response to treatment in CRC originating in the left versus right side of the colon. For example, BRAF mutations seemed mainly prognostic in left, but not in rightsided tumors. Also benefit from cetuximab in KRAS wild type patients seemed restricted to left sided tumors [47]. Conclusion The evolution of the genomic and epigenomic features of CRC, together with the identification of molecular subtypes based on gene expression profiles; reflects the complexity and heterogeneity of the disease. Biologic hallmarks of CRC include deficient mismatch repair, chromosomal instability, and epithelial–mesenchymal transition at variable levels of activation. Each of these classification modalities provides a guide for patient stratification and for rational design of drugs targeting specific pathways. There is a growing need for comprehensive disease classification system that links clinical and molecular features to

Please cite this article in press as: Mohammed AA et al. Molecular classification of colorectal cancer: Current perspectives and controversies, J Egyptian Nat Cancer Inst (2016), http://dx.doi.org/10.1016/j.jnci.2015.11.004

Molecular classification of colorectal cancer

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personalize the treatment and assess if there is a relationship between these subtypes and survival endpoints hoping to reduce disease burden in the future.

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Conflict of interest

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The authors certify that there is no actual or potential conflict of interest in relation to this article.

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Please cite this article in press as: Mohammed AA et al. Molecular classification of colorectal cancer: Current perspectives and controversies, J Egyptian Nat Cancer Inst (2016), http://dx.doi.org/10.1016/j.jnci.2015.11.004