Cheah Peh Yean
Department of Colorectal Surgery, Singapore General Hospital, Singapore
Saw Swee Hock School of Public Health, National University of Singapore, Singapore
Duke-NUS Medical School, National University of Singapore
Colorectal Carcinomas (CRC) is the leading cancer in the developed world. In Singapore, it is the most frequent cancer for men and women combined and the second leading cause of cancer mortality.  A recent statistical study on the etiology of cancer has attributed cancer incidence to lifetime tissue specific progenitor or stem cell divisions.  The higher the number of divisions, the higher is the risk of stochastic replication error leading to mutated or defective genes and cancer. This seems to imply that cancer is mainly due to intrinsic risk or bad luck and not preventable. This is a very alarming view indeed. A recent article has already thrown doubt on the statistical interpretation of the earlier study.  In this article, I will discuss this contention from the perspective of CRC. The good news is that even in the familial CRC syndromes where the defective genes inherited play much more deterministic role, CRC is still preventable if gene testing and prophylactic colectomy is performed.
2. Familial Syndromes and familial clustering
Up to 20 % of CRC is familial i.e. it runs in the family. Up to 15% of these were considered familial syndromes (Fig. 1). The disease-causing genes for familial syndromes were discovered and well-studied since the late 1980s.  The two main autosomal dominantly inherited CRC are familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC) or Lynch Syndrome (LS). FAP patient develops hundreds to thousands of polyps (precursors to cancers) in the colon and rectum typically by the second or third decade of life. In contrast, Lynch Syndrome (LS) patient has predominantly right-colon tumor but few (less than 50) polyps due to accelerated disease progression. The adenomatous polyposis coli (APC) and mismatch repair genes (MMR) were identified to be the disease-causing genes for FAP and LS respectively (Table 1). Notably, the penetrance of APC in FAP is 100%. This means that an individual in the family who unfortunately inherited a copy of the mutated (defective) APC gene will develop CRC in his or her lifetime. The severity and time of CRC development is dependent on the mutation site to a certain extent. As the name implies, mismatch repair genes are involved in DNA repair when mismatch occurs during DNA replication. Defect in these genes lead to genomic instability and hence accelerated disease progression. The penetrance of the MMR genes (mainly MSH2 and MLH1) averages 80% implying that the incidence of CRC for a mutated MMR gene carrier is about 80%. 
The highly penetrant disease-causing genes for the other rarer syndromes, such as SMAD4 and BMPR1A for Juvenile Polyposis (JP) [6-7], LKB-1 for Peutz-Jeghers (PJ) , PTEN for Cowden Syndrome (CS) , BMPR1A and GRIM1 for Hereditary Mixed Polyposis (HMPS) [10-12], were also identified in the past two decades (Table 1). More recently, DNA polymerase delta (POLD1) and epsilon (POLE) were identified as new disease causing genes in a small subset of polyposis families without germline mutations in these other genes . Disease-causing genes for the two recessive syndromes, namely MutYH-associated polyposis (MAP)  and NTHL1-associated polyposis (NAP) , both in the base excision repair pathway, were also identified. In contrast to dominant syndrome patients, affected individuals in recessive syndrome have to inherit two copies of the defective genes from both parents to develop cancer; hence CRC risk for the next generation is marginal if any. Readers are encouraged to refer to comprehensive reviews for more detailed descriptions of the rare syndromes and the implicated pathways. [4,16]
For these familial syndromes, CRC incidence for the affected individuals is indeed due to the inherited defective genes or ‘bad luck’, although the term is used in a slightly different context from reference  which created such furore. Whether one inherits the good or bad gene is dependent on independent or stochastic shuffling of the chromosomes (on which the genes reside) during meiosis (Mendel’s Law). Nevertheless, CRC is still preventable in these individuals. The advent of molecular techniques and a well-documented pedigree from the Polyposis Registry enables presymptomatic detection of the mutated gene and this allows affected individual time to be counselled and prophylactic colectomy planned before cancer develops.  Genetic testing will also spare unaffected individuals the emotional turmoil of not knowing whether they have inherited the mutated gene, and the cost and risk of repeated invasive colonoscopies. CRC is one of the most preventable cancers if detected early since this is one organ we can ‘learn’ to live without. Individuals who have total colectomy i.e. the complete removal of the colon can learn to control their bowel in due time and hence enjoy reasonable quality of life after surgery. Nevertheless, the risk of secondary cancers from extracolonic manifestation is real and affected individuals and their doctors should remain vigilant.
Interestingly, another 5 - 10% of CRC still run in the family and like the familial syndromes, cancers are usually early onset. The clinical phenotype and inheritance mimics one of these familial syndromes yet the patients do not inherit any of these highly penetrant genes (Fig. 1). These are considered as familial clustering patients and the likely cause is variable as genetic heterogeneity could be the contributing factor. Recent research has enabled us to determine the likely genetic variants linked to the disease in some of these patients. [17-18]
Although CRC from the familial syndromes and familial clustering constitute only a minor percentage of the total CRC incidence worldwide, the detailed family pedigrees from polyposis registries illustrating clear mode of inheritance and relatively more assessable tissues (peripheral lymphocytes, polyp and tumor samples) from resected colon and rectum provide invaluable material for studies leading to breakthrough in our understanding of cancer biology and genetics.
3. Sporadic CRC
Majority (up to 80%) of the CRC however is sporadic i.e. occurs late in life (usually at or after the sixth decade). This implies that environmental insults (e.g. diet) play a more important role than inheritance. Nevertheless, studies in monozygotic twins have revealed that about 35% of CRC can be attributed to genetic susceptibility , suggesting that gene-environment interaction play a major role in sporadic CRC. For instance, genetic variants could influence how an individual responds to environmental insults and hence determine whether the afflicted cell or tissue is able to repair the damage or succumb to the insult. These risk variants can be at single nucleotide level known as single nucleotide polymorphism (SNP) or structural (chromosomal) level spanning up to several hundred kilobases.
One main distinction between familial and sporadic CRC is that contribution of each risk variant or locus to sporadic CRC would be small i.e. have small effect sizes and likely to be population specific, as indicated by recent genome-wide association studies (GWAS). These variants are also more likely to be in gene desert or regulatory regions instead of mutations in specific genes. We showed that only about half of the Caucasian-identified risk variants were associated with CRC among the Singapore Chinese population . Some of these risk variants were even found to be associated in the opposite direction in African Americans i.e. the risk allele in Caucasian was found to be the protective allele in African Americans.  The contrasting results could be due to different allelic frequencies and linkage disequilibrium blocks or genetic heterogeneity between populations. We have also identified several structural or copy number variants (deletion or amplification of chromosomal segments) that were not previously identified in the Caucasian populations.  This serves to highlight that a certain proportion of the risk variants contributing to the development of sporadic CRC probably evolved recently and hence rare and population-specific. Small effective sizes of these variants also necessitate large sample sizes and hence big data and complex bioinformatics analysis.
Metastasis or spread of the cancer to distant organs such as liver or lung is the major contributing factor to cancer mortality. Early stage CRC patients are considered curative after surgery. However, up to 25% still succumb to eventual metastasis and suffered from morbidity and untimely mortality. The genes that contribute to development of CRC are not necessarily the same as those that cause metastasis. Thus besides searching for variants to enable risk assessment, it is also important to identify genes that can serve as biomarkers distinguishing early stage CRC patients who are prone to metastasis from those who will remain metastasis-free. A signature with high positive predictive value will enable the patient to better weigh the risk to benefit ratio of adjuvant therapy as both chemo- and radiotherapy comes with inherent toxicity. We have recently identified a metastasis-prone signature on early stage Chinese CRC patients using genome-wide expression profiling on a cohort of local fresh frozen tumor samples.  We have also shown that the signatures trained on Caucasian populations have no predictive value on the Chinese samples. We have now translated this expression-based signature to formalin-fixed and paraffin-embedded (FFPE) samples, which are the way tumor samples are stored in the Pathology Department of hospitals worldwide. The successful translation of a metastasis-prone signature to these samples will have higher clinical utility.
For a small minority of individuals from familial syndrome families, CRC is indeed due to the inheritance of mutated genes or ‘bad luck’. Even then, CRC is preventable if genetic testing and prophylactic colectomy are performed. Genetic testing must be accompanied by proper counselling before and after the testing. For the majority of the population, sporadic CRC is certainly preventable by screening for risk variants and by alteration of undesirable behaviour or lifestyle. The latter was not discussed as this is beyond the scope of this article. Further, gene signature can serve as biomarkers to aid in metastasis prognostication and decision-making for further therapy. This will reduce cancer morbidity and mortality for the affected individuals and impact on cancer and public health management for the nation.
Works cited from the author’s laboratory are partially funded by the Biomedical Research Council, National Medical Research Council of Singapore and the SingHealth Foundation.
About the Author
- Singapore Cancer Registry Report. Trends in Cancer incidence in Singapore 2006-2010.
- Tomasetti C and Vogelstein B. Variation in cancer risk among tissues can be explained by the number of stem cell divisions. Science 2015, 347, 78–81.
- Wu S, Powers S, Zhu W et al. Substantial contribution of extrinsic risk factors to cancer development. Nature 2016, 529, 43-47.
- Cheah PY. Recent advances in colorectal cancer genetics and diagnostics. Crit Rev Oncol Hematol. 2009, 69, 45-55.
- Chung DC, Rustgi AK. The hereditary nonpolyposis colorectal cancer syndrome: genetics and clinical implications. Ann Intern Med 2003, 138, 560-570.
- Howe JR, Roth S, Ringold JC, et al. Mutations in the SMAD4/DPC4 gene in juvenile polyposis. Science 1998, 280, 1086-1088.
- Howe JR, Bair JL, Sayed MG, et al. Germline mutations of the gene encoding bone morphogenetic protein receptor 1A in juvenile polyposis. Nat Genet 2001, 28, 184-187.
- Aretz S, Stienen D, Uhlhaas S, et al. High proportion of large genomic deletions and genotype-phenotype update in 80 unrelated families with Juvenile Polyposis Syndrome. J Med Genet 2007, 44, 702-709.
- Liaw D, Marsh DJ, Li J, et al. Germline mutations of the PTEN gene in Cowden disease, an inherited breast and thyroid cancer syndrome. Nat Genet 1997, 16, 64-67.
- Cheah PY, Wong YH, Chau YP, et al. Germline bone morphogenesis protein receptor 1A (BMPR1A) mutation causes colorectal tumorigenesis in Hereditary Mixed Polypsis Syndrome (HMPS). Am J Gastroenterology 2009, 104, 3027-3033.
- Jaeger E, Leedham S, Lewis A, et al. Hereditary mixed polyposis syndrome is caused by a 40-kb upstream duplication that leads to increased and ectopic expression of the BMP antagonist GREM1. Nat Genet 2012, 44, 699–705.
- Cheah PY, Lim KH, Koh PK, et al. The classification of intestinal polyposis. Nat Genet 2013, 45, 2.
- Palles C, Cazier J, Howarth KM et al. Germline mutations affecting the proofreading domains of POLE and POLD1 predispose to colorectal adenomas and carcinomas. Nat Genet 2013, 45, 136-146.
- Al-Tassan N, Chmiel NH, Maynard J, et al. Inherited variants of MYH associated with somatic G:C to T:A mutations in colorectal tumors. Nat Genet 2002, 30, 227-232.
- Weren RDA, Ligtenberg MJL, Kets CM et al. A germline homozygous mutation in the base-excision repair gene NTHL1 causes adenomatous polyposis and colorectal cancer. Nat Genet 2015, 47, 668-673.
- Tomlinson I. The Mendelian colorectal cancer syndromes. Annals of Clinical Biochemistry 2015, 52, 690–692.
- Thean LF, Loi C, Ho KS, et al. Genome-wide scan identifies a copy number variable region at chromosome 3q26 that regulates PPM1L in APC mutation-negative familial colorectal cancer patients. Genes Chrom & Cancer 2010, 49, 99-106.
- Thean LF, Wong YH, Loi C, et al. Manuscript in submission.
- Lichtenstein P, Holm NV, Verkasalo PK et al. Environmental and heritable factors in the causation of cancer - analysis of cohorts of twins from Sweden, Denmark and Finland. N Eng J Med 2000, 343, 78-85.
- Thean LF, Li HH, Teo YY, et al. Association of Caucasian-identified variants with colorectal cancer risk in Singapore Chinese. PLoS ONE 2012, 7, e42407.
- Kupfer SS, Anderson JR, Hooker S, et al. Genetic heterogeneity in colorectal cancer associations between African and European Americans. Gastroenterology 2010, 139, 1677-1685.
- Thean LF, Teo YY, Koh W et al. Manuscript in submission.
- Hong Y, Downey T, Eu KW, et al. A 'metastasis-prone' signature for early-stage mismatch-repair proficient sporadic colorectal cancer patients and its implications for possible therapeutics. Clin Exp Metastasis 2010, 27, 83-90.
Dr. Cheah Peh Yean is Co-Director and Senior Scientist of the Colorectal Cancer Research Laboratory at the Department of Colorectal Surgery, Singapore General Hospital. She is an Adjunct A/Prof in Saw Swee Hock School of Public Health as well as Duke-NUS Medical School, National University of Singapore. Dr. Cheah has been actively involved in colorectal cancer research, related to genomics and pathology for over 18 years. She is instrumental in positioning the laboratory at the forefront of colorectal cancer research in the region, focusing on the diverse ethnicity and genetic variation of the Asian populations, thus contributing towards better patient care and public health for Singapore and the region. Recent work has entailed the discovery of new tumor suppressors for APC mutation-negative familial CRC patients via high-density genotyping arrays; potential biomarkers to predict metastasis in early stage CRC via genome-wide expression, mutation and miRNA profiling as well as genome-wide association study to identify risk loci associated with differential response to environmental insults for sporadic CRC. She has held numerous competitive national grants as principal investigator since 1997. She is an academic editor for international peer-reviewed journal.