National Cancer Institute
Posted Date: Jan 26, 2016
Expert-reviewed information summary about the genetics of colorectal cancer, including information about specific genes and family cancer syndromes. The summary also contains information about screening for colorectal cancer and research aimed at prevention of this disease. Psychosocial issues associated with genetic testing and counseling of individuals who may have hereditary colorectal cancer syndrome are also discussed.
This PDQ cancer information summary for health professionals provides comprehensive, peer-reviewed, evidence-based information about the genetics of colorectal cancer. It is intended as a resource to inform and assist clinicians who care for cancer patients. It does not provide formal guidelines or recommendations for making health care decisions.
This summary is reviewed regularly and updated as necessary by the PDQ Cancer Genetics Editorial Board, which is editorially independent of the National Cancer Institute (NCI). The summary reflects an independent review of the literature and does not represent a policy statement of NCI or the National Institutes of Health (NIH).
Genetics of Colorectal Cancer
Many of the medical and scientific terms used in this summary are found in the NCI Dictionary of Genetics Terms. When a linked term is clicked, the definition will appear in a separate window.
Many of the genes described in this summary are found in the Online Mendelian Inheritance in Man (OMIM) database. When OMIM appears after a gene name or the name of a condition, click on OMIM for a link to more information.
Colorectal cancer (CRC) is the third most commonly diagnosed cancer in both men and women.
Estimated new cases and deaths from CRC in 2016:
About 75% of patients with CRC have sporadic disease with no apparent evidence of having inherited the disorder. The remaining 25% of patients have a family history of CRC that suggests a hereditary contribution, common exposures among family members, or a combination of both. Genetic mutations have been identified as the cause of inherited cancer risk in some colon cancerâprone families; these mutations are estimated to account for only 5% to 6% of CRC cases overall. It is likely that other undiscovered genes and background genetic factors contribute to the development of familial CRC in conjunction with nongenetic risk factors.
(Refer to the PDQ summaries on Colorectal Cancer Screening; Colorectal Cancer Prevention; Colon Cancer Treatment; and Rectal Cancer Treatment for more information about sporadic CRC.)
Natural History of CRC
Colorectal tumors present with a broad spectrum of neoplasms, ranging from benign growths to invasive cancer and are predominantly epithelial-derived tumors (i.e., adenomas or adenocarcinomas).
Pathologists have classified the lesions into the following three groups:
Research, however, suggests increased CRC risk in some families who have multiple members affected with juvenile polyposis, Peutz-Jeghers syndrome, and hyperplastic polyposis.
Epidemiologic studies have shown that a personal history of colon adenomas places one at an increased risk of developing colon cancer.
Two complementary interpretations of this observation are as follows:
More than 95% of CRCs are carcinomas, and about 95% of these are adenocarcinomas. It is well recognized that adenomatous polyps are benign tumors that may undergo malignant transformation. They have been classified into three histologic types, with increasing malignant potential: tubular, tubulovillous, and villous. While there is no direct proof that most CRCs arise from adenomas, adenocarcinomas are generally considered to arise from adenomas, based upon the following important observations:
The following three characteristics of adenomas are highly correlated with the potential to transform into cancer:
In addition, removal of adenomatous polyps is associated with reduced CRC incidence. While most adenomas are polypoid, flat and depressed lesions may be more prevalent than previously recognized. Large, flat, and depressed lesions may be more likely to be severely dysplastic, although this remains to be clearly proven. Specialized techniques may be needed to identify, biopsy, and remove such lesions.
Molecular Events Associated With Colon Carcinogenesis
The transition from normal epithelium to adenoma to carcinoma is associated with acquired molecular events. This tumor progression model was deduced from comparison of genetic alterations seen in normal colon epithelium, adenomas of progressively larger size, and malignancies. At least five to seven major deleterious molecular alterations may occur when a normal epithelial cell progresses in a clonal fashion to carcinoma. There are at least two major pathways by which these molecular events can lead to CRC. While the majority of CRCs are due to events that result in chromosomal instability (CIN), 20% to 30% of CRCs display characteristic patterns of gene hypermethylation, termed CpG island methylator phenotype (CIMP), of which a portion display microsatellite instability (15% of CRCs).
The spectrum of somatic mutations contributing to the pathogenesis of CRC is likely to be far more extensive than previously appreciated. A comprehensive study that sequenced more than 13,000 genes in a series of CRCs found that tumors accumulate an average of approximately 90 mutant genes. Sixty-nine genes were highlighted as relevant to the pathogenesis of CRC, and individual CRCs harbored an average of nine mutant genes per tumor. In addition, each tumor studied had a distinct mutational gene signature.
Key changes in CIN cancers include widespread alterations in chromosome number (aneuploidy) and frequent detectable losses at the molecular level of portions of chromosome 5q, chromosome 18q, and chromosome 17p; and mutation of the KRAS oncogene. The important genes involved in these chromosome losses are APC (5q), DCC/MADH2/MADH4 (18q), and TP53 (17p), and chromosome losses are associated with instability at the molecular and chromosomal level. Among the earliest events in the colorectal tumor progression pathway is loss of the APC gene, which appears to be consistent with its important role in predisposing persons with germline APC mutations to colorectal tumors. Acquired or inherited mutations of DNA damage-repair genes also play a role in predisposing colorectal epithelial cells to mutations. Furthermore, the specific genes that undergo somatic mutations and the specific type of mutations the tumor acquires may influence the rate of tumor growth or type of pathologic change in the tumors. For example, the rate of adenoma-to-carcinoma progression appears to be faster in microsatellite-unstable tumors compared with microsatellite-stable tumors. Characteristic histologic changes such as increased mucin production can be seen in tumors that demonstrate microsatellite instability (MSI), suggesting that at least some molecular events contribute to the histologic features of the tumors.
The key characteristics of MSI cancers are that they are tumors with a largely intact chromosome complement and that, as a result of defects in the DNA mismatch repair (MMR) system, they more readily acquire mutations in important cancer-associated genes compared with cells that have an effective DNA MMR system. These types of cancers are detectable at the molecular level by alterations in repeating units of DNA that occur normally throughout the genome, known as DNA microsatellites.
The knowledge derived from the study of inherited CRC syndromes has provided important clues regarding the molecular events that mediate tumor initiation and tumor progression in people without germline abnormalities. Among the earliest events in the colorectal tumor progression pathway (both MSI and CIN) is loss of function of the APC gene product, which appears to be consistent with its important role in predisposing persons with germline APC mutations to colorectal tumors.
Family History as a Risk Factor for CRC
Some of the earliest studies of family history of CRC were those of Utah families that reported a higher number of deaths from CRC (3.9%) among the first-degree relatives of patients who had died from CRC than among sex-matched and age-matched controls (1.2%). This difference has since been replicated in numerous studies that have consistently found that first-degree relatives of affected cases are themselves at a twofold to threefold increased risk of CRC. Despite the various study designs (case-control, cohort), sampling frames, sample sizes, methods of data verification, analytic methods, and countries where the studies originated, the magnitude of risk is consistent.
Population-based studies have shown a familial association for close relatives of colon cancer patients to develop CRC and other cancers. Using data from a cancer family clinic patient population, the relative and absolute risk of CRC for different family history categories was estimated (see Table 1).
A systematic review and meta-analysis of familial CRC risk was reported. Of 24 studies included in the analysis, all but one reported an increased risk of CRC if there was an affected first-degree relative. The relative risk (RR) for CRC in the pooled study was 2.25 (95% confidence interval [CI], 2.00â2.53) if there was an affected first-degree family member. In 8 of 11 studies, if the index cancer arose in the colon, the risk was slightly higher than if it arose in the rectum. The pooled analysis revealed a RR in relatives of colon and rectal cancer patients of 2.42 (95% CI, 2.20â2.65) and 1.89 (95% CI, 1.62â2.21), respectively. The analysis did not reveal a difference in RR for colon cancer based on location of the tumor (right side vs. left side).
The number of affected family members and age at cancer diagnosis correlated with the CRC risk. In studies reporting more than one first-degree relative with CRC, the RR was 3.76 (95% CI, 2.56â5.51). The highest RR was observed when the index case was diagnosed in individuals younger than 45 years, for family members of index cases diagnosed at ages 45 to 59 years, and for family members of index cases diagnosed at age 60 years or older, respectively (RR, 3.87; 95% CI, 2.40â6.22 vs. RR, 2.25; 95% CI, 1.85â2.72 vs. RR, 1.82; 95% CI, 1.47â2.25). In this meta-analysis, the familial risk of CRC associated with adenoma in a first-degree relative was analyzed. The pooled analysis demonstrated an RR for CRC of 1.99 (95% CI, 1.55â2.55) in individuals who had a first-degree relative with an adenoma. This finding has been corroborated. Other studies have reported that age at diagnosis of the adenoma influences the CRC risk, with younger age at adenoma diagnosis associated with higher RR. As with any meta-analysis, there could be potential biases that might affect the results of the analysis, including incomplete and nonrandom ascertainment of studies included; publication bias; and heterogeneity between studies relative to design, target populations, and control selection. This study is reinforcement that there are significant associations between familial CRC risk, age at diagnosis of both CRC and adenomas, and multiplicity of affected family members.
When the family history includes two or more relatives with CRC, the possibility of a genetic syndrome is increased substantially. The first step in this evaluation is a detailed review of the family history to determine the number of relatives affected, their relationship to each other, the age at which the CRC was diagnosed, the presence of multiple primary CRCs, and the presence of any other cancers (e.g., endometrial) consistent with an inherited CRC syndrome. (Refer to the Major Genetic Syndromes section of this summary for more information.) Young subjects who report a positive family history of CRC are more likely to represent a high-risk pedigree than older individuals who report a positive family history. Computer models are now available to estimate the probability of developing CRC. These models can be helpful in providing genetic counseling to individuals at average risk and high risk of developing cancer. At least three validated models are also available for predicting the probability of carrying a mutation in a MMR gene.
Figure 1 shows the types of colon cancer cases that arise in various family risk settings.
Figure 1. The fractions of colon cancer cases that arise in various family risk settings. Reprinted from , Vol. 119, No. 3, Randall W. Burt, Colon Cancer Screening, Pages 837-853, Copyright (2000), with permission from Elsevier.
Inheritance of CRC Predisposition
Several genes associated with CRC risk have been identified; these are described in detail in the Colon Cancer Genes section of this summary. Almost all gene mutations known to cause a predisposition to CRC are inherited in an autosomal dominant fashion. To date, at least one example of autosomal recessive inheritance, MYH-associated polyposis (MAP), has been identified. (Refer to the MYH-Associated Polyposis [MAP] section of this summary for more information.) Thus, the family characteristics that suggest autosomal dominant inheritance of cancer predisposition are important indicators of high risk and of the possible presence of a cancer-predisposing mutation. These include the following:
Hereditary CRC has two well-described forms: FAP (including an attenuated form of polyposis [AFAP]), due to germline mutations in the APC gene, and Lynch syndrome (LS) (also called hereditary nonpolyposis colorectal cancer [HNPCC]), which is caused by germline mutations in DNA MMR genes. ( Figure 2 depicts a classic family with LS, highlighting some of the indicators of high CRC risk that are described above.) Many other families exhibit aggregation of CRC and/or adenomas, but with no apparent association with an identifiable hereditary syndrome, and are known collectively as familial CRC.
Figure 2. Lynch syndrome pedigree. This pedigree shows some of the classic features of a family with Lynch syndrome, including affected family members with colon cancer or endometrial cancer and a younger age at onset in some individuals. Lynch syndrome families may exhibit some or all of these features. Lynch syndrome families may also include individuals with other gastrointestinal, gynecologic, and genitourinary cancers, or other extracolonic cancers. As an autosomal dominant syndrome, Lynch syndrome can be transmitted through maternal or paternal lineages, as depicted in the figure.
Difficulties in Identifying a Family History of CRC Risk
The accuracy and completeness of family history data must be taken into account in using family history to assess individual risk in clinical practice, and in identifying families appropriate for cancer research. A reported family history may be erroneous, or a person may be unaware of relatives with cancer. In addition, small family sizes and premature deaths may limit how informative a family history may be. Also, due to incomplete penetrance, some persons may carry a genetic predisposition to CRC but do not develop cancer, giving the impression of skipped generations in a family tree.
Accuracy of patient-reported family history of colon cancer has been shown to be good, but it is not optimal. Patient report should be verified by obtaining medical records whenever possible, especially for reproductive tract cancers that may be relevant in identifying risk of LS. (Refer to the Accuracy of the Family History section in the PDQ summary on Cancer Genetics Risk Assessment and Counseling for more information.)
Other Risk Factors for CRC
Other risk factors that may influence the development of adenomatous polyps and CRC risk include diet, use of nonsteroidal anti-inflammatory drugs (NSAIDs), cigarette smoking, colonoscopy with removal of adenomatous polyps, and physical activity. Even in LS, a hereditary form of colon cancer, cigarette smoking has been identified as a risk factor for the development of colorectal adenomas. (Refer to the Lynch Syndrome [LS] section of this summary for more information).
(Refer to the PDQ summary on Prevention of Colorectal Cancer for more information.)
In practical terms, knowing that a person is at an increased risk of CRC because of a germline abnormality is most useful if the knowledge can be used to prevent the development of cancer or cancer-related morbidity and mortality once it has developed. While one can also use the information for family planning, decisions about work and retirement, and other important life decisions, prevention is usually the central concern.
This section covers screening: testing in the absence of symptoms for CRC and its precursors (i.e., adenomatous polyps) to identify people with an increased probability of developing CRC. Those with abnormalities should undergo diagnostic testing to see whether they have an occult cancer, followed by treatment if cancer or a precursor is found. Taken together, this set of activities is aimed at either preventing the development of CRC by finding and removing its precursor, the adenomatous polyp, or increasing the likelihood of cure by early detection and treatment.
In the context of high-risk syndromes such as LS or FAP, surveillance implies examining patients in whom adenoma or cancer occurrence is highly probable, and the examination is done for early detection. It is not screening in the traditional sense. The meaning of the terms screening versus surveillance has evolved over time and their usage in this summary may not be consistent with other oncologic and epidemiologic contexts.
Primary prevention (eliminating the causes of CRC in people with genetically increased risk) is addressed later in this section.
Currently, there are no published randomized controlled trials of surveillance in people with a genetically increased risk of CRC and few controlled comparisons. While a randomized trial with a no-surveillance arm is not feasible, there is a need for well-designed studies comparing various surveillance methods or differing periods of time between procedures. An observational study that compared surveilled subjects with unsurveilled (by choice) controls evaluated a 15-year experience with 252 relatives at risk of LS, 119 of whom declined surveillance. Eight of 133 (6%) in the surveilled group developed CRC, compared with 19 in the unsurveilled group (16%, P = .014). In general, however, people with genetic risk have been excluded from the trials of CRC screening that have been published thus far, so it is not possible to estimate effectiveness by subgroup analyses. Therefore, prevention in these patients cannot be based on strong evidence of effectiveness, as is ordinarily relied on by expert groups when suggesting screening or surveillance guidelines.
Given these considerations, clinical decisions are based on clinical judgment. These decisions take into account the biologic and clinical behavior of each kind of genetic condition, and possible parallels with patients at average risk, for whom screening is known to be effective.
The evidence base for the effectiveness of screening in average-risk people (those without apparent genetic risk) is the benchmark for considering an approach to people at increased risk. (Refer to the PDQ summary on Screening for Colorectal Cancer for more information.)
The fact that screening of average-risk persons reduces the risk of dying from CRC forms the basis for recommending surveillance in persons at a higher genetic risk of CRC. As logical as this approach seems, randomized trials of surveillance have not been performed in this special population, though observational studies performed on families with LS and FAP support the value of surveillance. These studies demonstrate a shift towards earlier stage at diagnosis and a corresponding reduction in CRC mortality among colonoscopy-detected cancers.
(Refer to the Major Genetic Syndromes section of this summary for more information about surveillance in high-risk populations.)
Widely accepted criteria (1â3 below) for appropriate screening apply as much to diseases with a strong genetic component (more than one affected first-degree relative or one first-degree relative diagnosed at younger than 60 years) as they do to other diseases. Additional criteria (4 and 5) were added below.
Of these criteria, the first and second are satisfied in genetically determined CRC. The harms of screening (criterion 4), especially major complications of diagnostic colonoscopy (perforation and major bleeding), are also known. Evidence that early intervention results in better outcomes (criterion 3) is limited but suggests benefit. One study in the setting of LS found earlier stage/local tumors in the screened individuals.
Clinical criteria may be used to identify persons who are candidates for genetic testing to determine whether an inherited susceptibility to CRC is present. These criteria include the following:
- A strong family history of CRC and/or polyps (especially oligopolyposis).
- Multiple primary cancers in a patient with CRC.
- Existence of other cancers within the kindred consistent with known syndromes causing an inherited risk of CRC, such as endometrial cancer.
- Early age at diagnosis of CRC.
When such persons are identified, options tailored to the patient situation are considered. (Refer to the Major Genetic Syndromes section of this summary for information on specific interventions for individual syndromes.)
At this time, the use of mutation testing to identify genetic susceptibility to CRC is not recommended as a screening measure in the general population. The rarity of mutations in the APC tumor suppressor gene and LS-associated MMR genes and the limited sensitivity of current testing strategies render general population testing potentially misleading and not cost effective.
Rather detailed recommendations for surveillance in FAP and LS have been provided by several organizations representing various medical specialties and societies. The following guidelines are readily available through the National Guideline Clearinghouse:
- American Cancer Society.
- United States Multisociety (American Gastroenterological Association and American Society for Gastrointestinal Endoscopy) Task Force on Colorectal Cancer.
- American Society of Colon and Rectal Surgeons.
- National Comprehensive Cancer Network.
- Gene Reviews.
The evidence bases for recommendations are generally included within the statements or guidelines. In many instances, these guidelines reflect expert opinion resting on studies that are rarely randomized prospective trials.
Primary Prevention of Familial CRC
Observational studies of average-risk people have suggested that the use of some drugs and supplements (NSAIDs, estrogens, folic acid, and calcium) might prevent the development of CRC. (Refer to the PDQ summary on Prevention of Colorectal Cancer for more information.) None of the evidence is convincing enough to lead expert groups to recommend these drugs and supplements specifically to prevent CRC, and few studies specifically enrolled people with an inherited predisposition for CRC. Although antioxidants are hypothesized to prevent cancer, a randomized controlled trial of antioxidant vitamins (beta carotene, vitamin C, and vitamin E) has shown no effect on CRC incidence.
Several components of diet and behavior have been suggested, with various levels of consistency, to be risk factors for CRC. (Refer to the PDQ summary on Prevention of Colorectal Cancer for more information.) These lifestyle factors may represent potential means of prevention. Expert groups differ on the interpretation of the evidence for some of these components.
Little is known about whether these same factors are protective in people with a genetically increased risk of CRC. In one case-control study, low levels of physical activity, high caloric intake, and low vegetable intake were significantly related to cancer risk in people with no family history of CRC but showed no relationship in people with a family history, despite adequate statistical power to do so.
Colon Cancer Genes
Major genes are defined as those that are necessary and sufficient for disease causation, with important mutations (e.g., nonsense, missense, frameshift) of the gene as causal mechanisms. Major genes are typically considered those that are involved in single-gene disorders, and the diseases caused by major genes are often relatively rare. Most pathogenic mutations in major genes lead to a very high risk of disease, and environmental contributions are often difficult to recognize. Historically, most major colon cancer susceptibility genes have been identified by linkage analysis using high-risk families; thus, these criteria were fulfilled by definition, as a consequence of the study design.
The functions of the major colon cancer genes have been reasonably well characterized over the past decade. Three proposed classes of colon cancer genes are tumor suppressor genes, oncogenes, and DNA repair genes. Tumor suppressor genes constitute the most important class of genes responsible for hereditary cancer syndromes and represent the class of genes responsible for both familial adenomatous polyposis (FAP) and juvenile polyposis syndrome (JPS), among others. Germline mutations of oncogenes are not an important cause of inherited susceptibility to colorectal cancer (CRC), even though somatic mutations in oncogenes are ubiquitous in virtually all forms of gastrointestinal cancers. Stability genes, especially the mismatch repair (MMR) genes responsible for Lynch syndrome (LS) (also called hereditary nonpolyposis colorectal cancer [HNPCC]), account for a substantial fraction of hereditary CRC, as noted below. (Refer to the Lynch syndrome [LS] section in the Major Genetic Syndromes section of this summary for more information). MYH is another important example of a stability gene that confers risk of CRC based on defective base excision repair. Table 2 summarizes the genes that confer a substantial risk of CRC, with their corresponding diseases.
De Novo Mutation Rate
Until the 1990s, the diagnosis of genetically inherited polyposis syndromes was based on clinical manifestations and family history. Now that some of the genes involved in these syndromes have been identified, a few studies have attempted to estimate the spontaneous mutation rate ( de novo mutation rate) in these populations. Interestingly, FAP, JPS, Peutz-Jeghers syndrome, Cowden syndrome, and Bannayan-Riley-Ruvalcaba syndrome are all thought to have high rates of spontaneous mutations, in the 25% to 30% range, while estimates of de novo mutations in the MMR genes associated with LS are thought to be low, in the 0.9% to 5% range. These estimates of spontaneous mutation rates in LS seem to overlap with the estimates of nonpaternity rates in various populations (0.6% to 3.3%), making the de novo mutation rate for LS seem quite low in contrast to the relatively high rates in the other polyposis syndromes.
Next-Generation Sequencing and Novel CRC Susceptibility Genes
Next-generation sequencing (NGS) involves technological advances over the traditional capillary-based Sanger DNA sequencing that was used in the Human Genome Project to sequence the human genome. NGS dramatically decreases the time required for genomic sequencing by utilizing massively parallel multiplexing techniques. Comparisons of genomic sequencing results between individuals with and without CRC affords yet another method to identify CRC susceptibility genes.
Whole-genome sequencing (WGS) and whole-exome sequencing (WES) are currently being used to assess somatic alterations in tumors to inform prognosis and/or targeted therapeutics and to assess the germline to identify cancer risk alleles. (Refer to the Clinical Sequencing section in the PDQ Cancer Genetics Overview summary for more information.)
An example of the success of NGS in identifying CRC susceptibility genes is the discovery of POLE/POLD1 germline mutations in patients with adenomatous polyposis but no germline mutations in known CRC genes. (Refer to the Oligopolyposis section in the Major Genetic Syndromes section of this summary for more information about POLE/POLD1).
WES has also been used to identify new potential CRC predisposition variants. Forty-three patients with CRC from 29 families with strong disease aggregation and without mutations in known hereditary CRC genes underwent WES. Twenty-eight candidate variants were identified; family segregation and somatic studies were used to categorize the most interesting variants in CDKN1B, XRCC4, EPHX1, NFKBIZ, SMARCA4, and BARD1.
Genetic Polymorphisms and CRC Risk
It is widely acknowledged that the familial clustering of colon cancer also occurs outside of the setting of well-characterized colon cancer family syndromes. Based on epidemiological studies, the risk of colon cancer in a first-degree relative of an affected individual can increase an individualâs lifetime risk of colon cancer 2-fold to 4.3-fold. The relative risk (RR) and absolute risk of CRC for different family history categories is estimated in Table 1. In addition, the lifetime risk of colon cancer also increases in first-degree relatives of individuals with colon adenomas. The magnitude of risk depends on the age at diagnosis of the index case, the degree of relatedness of the index case to the at-risk case, and the number of affected relatives. It is currently believed that many of the moderate- and low-risk cases are influenced by alterations in single low-penetrance genes or combinations of low-penetrance genes. Given the public health impact of identifying the etiology of this increased risk, an intense search for the responsible genes is under way.
Each locus would be expected to have a relatively small effect on CRC risk and would not produce the dramatic familial aggregation seen in LS or FAP. However, in combination with other common genetic loci and/or environmental factors, variants of this kind might significantly alter CRC risk. These types of genetic variations are often referred to as polymorphisms. Most loci that are polymorphic have no influence on disease risk or human traits (benign polymorphisms), while those that are associated with a difference in risk of disease or a human trait (however subtle) are sometimes termed disease-associated polymorphisms or functionally relevant polymorphisms. When such variation involves changes in single nucleotides of DNA they are referred to as single nucleotide polymorphisms (SNPs).
Polymorphisms underlying polygenic susceptibility to CRC are considered low penetrance, a term often applied to sequence variants associated with a minimal to moderate risk. This is in contrast to high-penetrance variants or alleles that are typically associated with more severe phenotypes, for example those APC or MMR gene mutations leading to an autosomal dominant inheritance pattern in a family. The definition of a moderate risk of cancer is arbitrary, but it is usually considered to be in the range of an RR of 1.5 to 2.0. Because these types of sequence variants are relatively common in the population, their contribution to total cancer risk is estimated to be much higher than the attributable risk in the population from the relatively rare syndromes such as FAP or LS. Additionally, polymorphisms in genes distinct from the MMR genes can modify phenotype (e.g., average age of CRC) in individuals with LS.
Low-penetrance variants have been identified in a number of strategies. Earlier studies focused on candidates genes chosen because of biologic relevance to cancer pathogenesis. More recently, genome-wide association studies ( GWAS) have been used much more extensively to identify potential CRC susceptibility genes. (Refer to the GWAS section of this summary for more information.) Another approach is to use meta-analyses of existing GWAS datasets to discover additional novel CRC susceptibility genes.
Major Genetic Syndromes
Originally described in the 1800s and 1900s by their clinical findings, the colon cancer susceptibility syndrome names often reflected the physician or patient and family associated with the syndrome (e.g., Gardner syndrome, Turcot syndrome, Muir-Torre syndrome, Lynch I and II syndromes, Peutz-Jeghers syndrome [PJS], Bannayan-Riley-Ruvalcaba syndrome, and Cowden syndrome). These syndromes were associated with an increased lifetime risk of colorectal adenocarcinoma. They were mostly thought to have autosomal dominant inheritance patterns. Adenomatous colonic polyps were characteristic of the first five, while hamartomas were found to be characteristic in the last three.
With the development of the Human Genome Project and the identification in 1990 of the adenomatous polyposis coli (APC) gene on chromosome 5q, overlap and differences between these familial syndromes became apparent. Gardner syndrome and familial adenomatous polyposis (FAP) were shown to be synonymous, both caused by mutations in the APC gene. Attenuated FAP (AFAP) was recognized as a syndrome with less adenomas and extraintestinal manifestations as having FAP mutation on the 3â and 5â ends of the gene. Turcot syndrome families were shown to be genetically part of FAP with medulloblastomas and Lynch syndrome (LS) with glioblastomas. Muir-Torre and LS were shown to have genetic similarities. MYH-associated polyposis (MAP) was recognized as a separate adenomatous polyp syndrome with autosomal recessive inheritance. Once the mutations were identified, the absolute risk of colorectal cancer (CRC) could be better assessed for mutation carriers (see Table 3).
With these discoveries genetic testing and risk management became possible. Genetic testing refers to searching for mutations in known cancer susceptibility genes using a variety of techniques. Comprehensive genetic testing includes sequencing the entire coding region of a gene, the intron- exon boundaries (splice sites), and assessment of rearrangements, deletions, or other changes in copy number (with techniques such as multiplex ligation-dependent probe amplification [MLPA] or Southern blot). Despite extensive accumulated experience that helps distinguish pathogenic mutations from benign variants and polymorphisms, genetic testing sometimes identifies variants of uncertain significance (VUS) that cannot be used for predictive purposes.
Familial Adenomatous Polyposis (FAP)
FAP is one of the most clearly defined and well understood of the inherited colon cancer syndromes. It is an autosomal dominant condition, and the reported incidence varies from 1 in 7,000 to 1 in 22,000 live births, with the syndrome being more common in Western countries. Autosomal dominant inheritance means that affected persons are genetically heterozygous, such that each offspring of a patient with FAP has a 50% chance of inheriting the disease gene. Males and females are equally likely to be affected.
Classically, FAP is characterized by multiple (>100) adenomatous polyps in the colon and rectum developing after the first decade of life (see Figure 3).
Figure 3. Multiple polyps in the colon of a patient with familial adenomatous polyposis shown endoscopically (left panel) and upon surgical resection (right panel).
Variant features in addition to the colonic polyps may include polyps in the upper gastrointestinal (GI) tract, extraintestinal manifestations such as congenital hypertrophy of retinal pigment epithelium, osteomas and epidermoid cysts, supernumerary teeth, desmoid formation, and other malignant changes such as thyroid tumors, small bowel cancer, hepatoblastoma, and brain tumors, particularly medulloblastoma (see Table 4).
FAP is also known as familial polyposis coli, adenomatous polyposis coli (APC), or Gardner syndrome (colorectal polyposis, osteomas, and soft tissue tumors). Gardner syndrome has sometimes been used to designate FAP patients who manifest these extracolonic features. However, Gardner syndrome has been shown molecularly to be a variant of FAP, and thus the term Gardner syndrome is essentially obsolete in clinical practice.
Most cases of FAP result from mutations of the APC gene on chromosome 5q21. Individuals who inherit a mutant APC gene have a very high likelihood of developing colonic adenomas; the risk has been estimated to be more than 90%. The age at onset of adenomas in the colon is variable: By age 10 years, only 15% of FAP gene carriers manifest adenomas; by age 20 years, the probability rises to 75%; and by age 30 years, 90% will have presented with FAP. Without any intervention, most persons with FAP will develop colon or rectal cancer by the fourth decade of life. Thus, surveillance and intervention for APC gene mutation carriers and at-risk persons have conventionally consisted of annual sigmoidoscopy beginning around puberty. The objective of this regimen is early detection of colonic polyps in those who have FAP, leading to preventive colectomy.
The early appearance of clinical features of FAP and the subsequent recommendations for surveillance beginning at puberty raise special considerations relating to the genetic testing of children for susceptibility genes. Some proponents feel that the genetic testing of children for FAP presents an example in which possible medical benefit justifies genetic testing of minors, especially for the anticipated 50% of children who will be found not to be mutation carriers and who can thus be spared the necessity of unpleasant and costly annual sigmoidoscopy. The psychological impact of such testing is currently under investigation and is addressed in the Psychosocial Issues in Hereditary Colon Cancer Syndromes section of this summary.
A number of different APC mutations have been described in a series of FAP patients. The clinical features of FAP appear to be generally associated with the location of the mutation in the APC gene and the type of mutation (i.e., frameshift mutation vs. missense mutation). Two features of particular clinical interest that are apparently associated with APC mutations are (1) the density of colonic polyposis and (2) the development of extracolonic tumors.
The APC gene on chromosome 5q21 encodes a 2,843-amino acid protein that is important in cell adhesion and signal transduction; beta-catenin is its major downstream target. APC is a tumor suppressor gene, and the loss of APC is among the earliest events in the chromosomal instability colorectal tumor pathway. The important role of APC in predisposition to colorectal tumors is supported by the association of APC germline mutations with FAP and AFAP. Both conditions can be diagnosed genetically by testing for germline mutations in the APC gene in DNA from peripheral blood leukocytes. Most FAP pedigrees have APC alterations that produce truncating mutations, primarily in the first half of the gene. AFAP is associated with truncating mutations primarily in the 5â and 3â ends of the gene and possibly missense mutations elsewhere.
More than 300 different disease-associated mutations of the APC gene have been reported. The vast majority of these changes are insertions, deletions, and nonsense mutations that lead to frameshifts and/or premature stop codons in the resulting transcript of the gene. The most common APC mutation (10% of FAP patients) is a deletion of AAAAG in codon 1309; no other mutations appear to predominate. Mutations that reduce rather than eliminate production of the APC protein may also lead to FAP.
Most APC mutations that occur between codon 169 and codon 1393 result in the classic FAP phenotype. There has been much interest in correlating the location of the mutation within the gene with the clinical phenotype, including the distribution of extracolonic tumors, polyposis severity, and congenital hypertrophy of the retinal pigment epithelium. The most consistent observations are that attenuated polyposis and the less classic forms of FAP are associated with mutations that occur in or before exon 4 and in the latter two-thirds of exon 15, and that retinal lesions are rarely associated with mutations that occur before exon 9. Exon 9 mutations have also been associated with attenuated polyposis. Additionally, individuals with exon 9 mutations tend not to have duodenal adenomas.
Researchers have found that dense carpeting of colonic polyps, a feature of classic FAP, is seen in most patients with APC mutations, particularly those mutations that occur between codons 169 and 1393. At the other end of the spectrum, sparse polyps are features of patients with mutations occurring at the extreme ends of the APC gene or in exon 9. (Refer to the Attenuated Familial Adenomatous Polyposis [AFAP] section of this summary for more information.)
APC gene testing is now commercially available and has led to changes in management guidelines, particularly for those whose tests indicate they are not mutation carriers. Presymptomatic genetic diagnosis of FAP in at-risk individuals has been feasible with linkage and direct detection of APC mutations. These tests require a small sample (<10 cc) of blood in which the lymphocyte DNA is tested. If one were to use linkage analysis to identify gene carriers, ancillary family members, including more than one affected individual, would need to be studied. With direct detection, fewer family membersâ blood samples are required than for linkage analysis, but the specific mutation must be identified in at least one affected person by DNA mutation analysis or sequencing. The detection rate is approximately 80% using sequencing alone.
Studies have reported whole exon deletions in 12% of FAP patients with previously negative APC testing. For this reason, deletion testing has been added as an optional adjunct to sequencing of APC. Furthermore, mutation detection assays that use MLPA are being developed and appear to be accurate for detecting intragenic deletions. MYH gene testing may be considered in APC mutationânegative affected individuals. (Refer to the Adenomatous polyposis coli [APC] section of this summary for more information.)
Patients who develop fewer than 100 colorectal adenomatous polyps are a diagnostic challenge. The differential diagnosis should include AFAP and MYH-associated colorectal neoplasia (also reported as MYH-associated polyposis or MAP). AFAP can be diagnosed by testing for germline APC gene mutations. (Refer to the Attenuated Familial Adenomatous Polyposis [AFAP] section in the Major Genetic Syndromes section of this summary for more information.) MYH-associated neoplasia is caused by germline homozygous recessive mutations in the MYH gene.
Presymptomatic genetic testing removes the necessity of annual screening of at-risk individuals who do not have the familial gene mutation. For at-risk individuals who have been found to be definitively mutation-negative by genetic testing, there is no clear consensus on the need for or frequency of colon screening, though all experts agree that at least one flexible sigmoidoscopy or colonoscopy examination should be performed in early adulthood (by age 18â25 years). Colon adenomas will develop in nearly 100% of persons who are APC gene mutation positive; risk-reducing surgery comprises the standard of care to prevent colon cancer after polyps have appeared and are too numerous or histologically advanced to monitor safely using endoscopic resection.
Individuals at risk of FAP, because of a known APC mutation in either the family or themselves, are evaluated for onset of polyposis by flexible sigmoidoscopy or colonoscopy. Once an FAP family member is found to manifest polyps, the only effective management to prevent CRC is eventual colectomy. Prophylactic surgery has been shown to improve survival in patients with FAP. If feasible, the patient and his/her family members should be included in a registry because it has been shown retrospectively that registration and surveillance reduce CRC incidence and mortality. In patients with classic FAP identified very early in their course, the surgeon, endoscopist, and family may choose to delay surgery for several years in the interest of achieving social milestones. In addition, in carefully selected patients with AFAP (those with minimal polyp burden and advanced age), deferring a decision about colectomy may be reasonable with surgery performed only in the face of advancing polyp burden or dysplasia.
The recommended age at which surveillance for polyposis should begin involves a trade-off. On the one hand, someone who waits until the late teens to begin surveillance faces a remote possibility that a cancer will have developed at an earlier age. Although it is rare, CRC can develop in a teenager who carries an APC mutation. On the other hand, it is preferable to allow people at risk to develop emotionally before they are faced with a major surgical decision regarding the timing of colectomy. Therefore, surveillance is usually begun in the early teenage years (age 10â15 years). Surveillance has consisted of either flexible sigmoidoscopy or colonoscopy every year. If flexible sigmoidoscopy is utilized and polyps are found, colonoscopy should be performed. Historically, sigmoidoscopy may have been a reasonable approach at the time in identifying early adenomas in a majority of the patients. However, colonoscopy must be considered the tool of choice in light of (a) improved instrumentation for full colonoscopy, (b) sedation, (c) recognition of AFAP, in which the disease is typically most manifest in the right colon, and (d) the growing tendency to defer surgery for a number of years. Individuals who have tested negative for an otherwise known family mutation do not need FAP-oriented surveillance at all. They are recommended to undergo average-risk population screening. In the case of families in which no family mutation has been identified in an affected person, then clinical surveillance is warranted. Colon surveillance should not be stopped in persons who are known to carry an APC mutation but who do not yet manifest polyps, since adenomas occasionally are not manifest until the fourth and fifth decades of life. (Refer to the Attenuated Familial Adenomatous Polyposis [AFAP] section of this summary for more information.) (Refer to the PDQ summary on Colorectal Cancer Screening for more information on these methods.)
In some circumstances, full colonoscopy may be preferred over the more limited sigmoidoscopy. Among pediatric gastroenterologists, tolerability of endoscopic procedures in general has been regarded as improved with the use of deeper intravenous sedation.
Table 7 summarizes the clinical practice guidelines from different professional societies regarding diagnosis and surveillance of FAP.
Once an FAP family member is found to manifest polyposis, colectomy is the only effective management. Patient and doctor should enter into an individualized discussion to decide when surgery should be performed. It is useful to incorporate into the discussion the risk of developing desmoid tumors after surgery. Timing of risk-reducing surgery usually depends on the number of polyps, their size, histology, and symptomatology. Once numerous polyps have developed, surveillance colonoscopy is no longer useful in timing the colectomy because polyps are so numerous that it is not possible to biopsy or remove all of them. At this time, it is appropriate for patients to consult with a surgeon who is experienced with available options, including total colectomy and postcolectomy reconstruction techniques. Rectum-sparing surgery, with sigmoidoscopic surveillance of the remaining rectum, is a reasonable alternative to total colectomy in those compliant individuals who understand the consequences and make an informed decision to accept the residual risk of rectal cancer occurring despite periodic surveillance.
Surgical options include restorative proctocolectomy with IPAA, subtotal colectomy with ileorectal anastomosis (IRA), or total proctocolectomy with ileostomy (TPC). TPC is reserved for patients with low rectal cancer in which the sphincter cannot be spared or for patients on whom an IPAA cannot be performed because of technical problems. There is no risk of developing rectal cancer after TPC because the whole mucosa at risk is removed. Whether a colectomy and an IRA or a restorative proctocolectomy is performed, most experts suggest that periodic and lifelong surveillance of the rectum or the ileal pouch be performed to remove or ablate any polyps. This is necessitated by case series of rectal cancers arising in the rectum of FAP patients who had subtotal colectomies with an IRA in which there was an approximately 25