miércoles, 13 de febrero de 2019

Genetics of Colorectal Cancer (PDQ®)—Health Professional Version - National Cancer Institute

Genetics of Colorectal Cancer (PDQ®)—Health Professional Version - National Cancer Institute

National Cancer Institute

Genetics of Colorectal Cancer (PDQ®)–Health Professional Version

Executive Summary

This executive summary reviews the topics covered in the PDQ summary on the genetics of colorectal cancer (CRC), with hyperlinks to detailed sections below that describe the evidence on each topic.
  • Inheritance and Risk
    Factors suggestive of a genetic contribution to CRC include the following: (1) a strong family history of CRC and/or polyps; (2) multiple primary cancers in a patient with CRC; (3) the existence of other cancers within the kindred consistent with known syndromes causing an inherited risk of CRC, such as endometrial cancer; and (4) early age at diagnosis of CRC. Hereditary CRC is most commonly inherited in an autosomal dominant pattern, although two syndromes are inherited in an autosomal recessive pattern (MUTYH-associated polyposis and NTHL1).
    At least three validated computer models are available to estimate the probability that an individual affected with cancer carries a pathogenic variant in a mismatch repair (MMR) gene associated with Lynch syndrome, the most common inherited CRC syndrome. These include the MMRpro, MMRpredict, and PREMM5 (PREdiction Model for gene Mutations) prediction models. Individuals with a quantified risk of 2.5% or greater on PREMM5 or 5% or greater on MMRpro and MMRpredict are recommended for genetic evaluation referral and testing.
  • Associated Genes and Syndromes
    Hereditary CRC has two well-described forms: (1) polyposis (including familial adenomatous polyposis [FAP] and attenuated FAP (AFAP), which are caused by pathogenic variants in the APC gene; and MUTYH-associated polyposis, which is caused by pathogenic variants in the MUTYH gene); and (2) Lynch syndrome (often referred to as hereditary nonpolyposis colorectal cancer), which is caused by germline pathogenic variants in DNA MMR genes (MLH1MSH2MSH6, and PMS2) and EPCAM. Other CRC syndromes and their associated genes include oligopolyposis (POLEPOLD1), NTHL1juvenile polyposis syndrome (BMPR1ASMAD4), Cowden syndrome (PTEN), and Peutz-Jeghers syndrome (STK11). Many of these syndromes are also associated with extracolonic cancers and other manifestations. Serrated polyposis syndrome, which is characterized by the appearance of hyperplastic polyps, appears to have a familial component, but the genetic basis remains unknown. The natural history of some of these syndromes is still being described. 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. In addition, most individuals with CRC diagnosed before age 50 years and without a family history of cancer do not have a pathogenic variant associated with an inherited cancer syndrome.
    Genome-wide searches are showing promise in identifying common, low-penetrance susceptibility alleles for many complex diseases, including CRCs, but the clinical utility of these findings remains uncertain.
  • Clinical Management
    It is becoming the standard of care at many centers that all individuals with newly diagnosed CRC are evaluated for Lynch syndrome through molecular diagnostic tumor testing assessing MMR deficiency. A universal screening approach to tumor testing is supported, in which all CRC cases are evaluated regardless of age at diagnosis or fulfillment of existing clinical criteria for Lynch syndrome. A more cost-effective approach has been reported whereby all patients aged 70 years or younger with CRC and older patients who meet the revised Bethesda guidelines are tested for Lynch syndrome. Tumor evaluation often begins with immunohistochemistry testing for the expression of the MMR proteins associated with Lynch syndrome or microsatellite instability (MSI) testingBRAF testing, and MLH1 hypermethylation analyses.
    Colonoscopy for CRC screening and surveillance is commonly performed in individuals with hereditary CRC syndromes and has been associated with improved survival outcomes. For example, surveillance of Lynch syndrome patients with colonoscopyevery 1 to 2 years, and in one study up to 3 years, has been shown to reduce CRC incidence and mortality. Extracolonic surveillance is also a mainstay for some hereditary CRC syndromes depending on the other cancers associated with the syndrome. For example, regular endoscopic surveillance of the duodenum in FAP patients has been shown to improve survival.
    Prophylactic surgery (colectomy) has also been shown to improve survival in patients with FAP. The timing and extent of risk-reducing surgery usually depends on the number of polyps, their size, histology, and symptomatology. For patients with Lynch syndrome and a diagnosis of CRC, extended resection is associated with fewer metachronous CRCs and additional surgical procedures for colorectal neoplasia than in patients who undergo segmental resection for CRC. The surgical decision must take into account the age of the patient, comorbidities, clinical stage of the tumor, sphincter function, and the patient’s wishes.
    Chemopreventive agents have also been studied in the management of FAP and Lynch syndrome. In FAP patients, celecoxib and sulindac have been associated with a decrease in polyp size and number. A double-blind, randomized, controlled trial evaluating the efficacy of sulindac plus an epidermal growth factor receptor inhibitor, erlotinib, versus placebo in FAP or AFAP patients with duodenal polyps suggested that erlotinib has the potential to inhibit duodenal polyps in FAP patients. An ongoing trial will determine whether lower doses of erlotinib alone will significantly reduce duodenal polyp burden. Aspirin use (600 mg daily) was shown to have a preventive effect on cancer incidence in Lynch syndrome patients in a large randomized trial; lower doses are being examined in an ongoing study.
    Novel therapies that stimulate the immune system have been evaluated in MMR-deficient tumors, including those related to Lynch syndrome. The dense immune infiltration and cytokine-rich environment in MMR-deficient tumors may improve clinical outcomes. A critical pathway responsible for mediating tumor-induced immune suppression is the programmed cell death-1 (PD-1)–mediated checkpoint pathway. Two phase 2 studies using anti–PD-1 immune checkpoint inhibitors (pembrolizumab and nivolumab) demonstrated favorable outcomes, including progression-free survival, radiographic response rates, and disease control rates in metastatic CRC with MMR deficiency and MSI that had progressed on prior cytotoxic chemotherapy. Pembrolizumab has shown similar benefit in other noncolorectal cancers with MMR deficiency and MSI, but not in tumors that are microsatellite stable.
  • Psychosocial and Behavioral Issues
    Psychosocial factors influence decisions about genetic testing for inherited cancer risk and risk-management strategies. Uptake of genetic counseling and genetic testing for Lynch syndrome and FAP varies widely across studies. Factors that have been associated with genetic counseling and testing uptake in Lynch syndrome families include having children, the number of affected relatives, perceived risk of developing CRC, and frequency of thoughts about CRC. Psychological studies have shown low levels of distress, particularly in the long term, after genetic testing for Lynch syndrome in both carriers and noncarriers. However, other studies have demonstrated the possibility of increased distress following genetic testing for FAP. Colon and gynecologic cancer screening rates have been shown to increase or be maintained among carriers of MMR pathogenic variants within the year after disclosure of results, while screening rates decrease among noncarriers. The latter is expected as the screening recommendations for unaffected individuals are those that apply to the general population. Studies measuring quality-of-life variables in FAP patients show normal-range results; however, these studies suggest that risk-reducing surgery for FAP may have negative quality-of-life effects for at least some proportion of those affected. Patients' communication with their family members about an inherited risk of CRC is complex; gender, age, and the degree of relatedness are some elements that affect disclosure of this information. Research is ongoing to better understand and address psychosocial and behavioral issues in high-risk families.


[Note: 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.]
[Note: 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.]
[Note: A concerted effort is being made within the genetics community to shift terminology used to describe genetic variation. The shift is to use the term “variant” rather than the term “mutation” to describe a genetic difference that exists between the person or group being studied and the reference sequence. Variants can then be further classified as benign (harmless), likely benign, of uncertain significance, likely pathogenic, or pathogenic (disease causing). Throughout this summary, we will use the term pathogenic variant to describe a disease-causing mutation. Refer to the Cancer Genetics Overview summary for more information about variant classification.]
Colorectal cancer (CRC) is the third most commonly diagnosed cancer in both men and women.
Estimated new cases and deaths from CRC in 2019 in the United States:[1]
  • New cases: 145,600.
  • Deaths: 51,020.
About 75% of patients with CRC have sporadic disease with no apparent evidence of having inherited the disorder. The remaining 10% to 30% of patients have a family history of CRC that suggests a hereditary contribution, common exposures or shared risk factors among family members, or a combination of both.[2Pathogenic variants in high-penetrancegenes have been identified as the cause of inherited cancer risk in some colon cancer–prone families; these are estimated to account for only 5% to 6% of CRC cases overall.[3,4]
In addition, pathogenic variants in lower penetrance genes may contribute to familial colon cancer risk. In such cases, gene-gene and gene-environment interactions may contribute to the development of CRC.
(Refer to the PDQ summaries on Colorectal Cancer ScreeningColorectal Cancer PreventionColon Cancer Treatment; and Rectal Cancer Treatment for more information about sporadic CRC.)

Colorectal Polyps as Precursors to Colorectal Cancer (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).
Transformation of any polyp into cancer goes through the adenoma-carcinoma sequence. Polyps that have traditionally been considered nonneoplastic include those of the hyperplastic, juvenile, hamartomatous, inflammatory, and lymphoid types. However, in certain circumstances, hamartomatous and juvenile polyps can progress into cancer.
Research, however, does suggest a substantial risk of colon cancer in individuals with juvenile polyposis syndrome and Peutz-Jeghers syndrome, although the nonadenomatous polyps associated with these syndromes have historically been viewed as nonneoplastic.[5-7]
Epidemiologic studies have shown that a personal history of colon adenomas places one at an increased risk of developing colon cancer.[8]
Two complementary interpretations of this observation are as follows:
  1. The adenoma may reflect an innate or acquired tendency of the colon to form tumors.
  2. Adenomas are the primary precursor lesion of colon cancer.
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. Adenocarcinomas are generally considered to arise from adenomas,[9-13] based upon the following important observations:
  1. Benign and malignant tissue occur within colorectal tumors.[14]
  2. When patients with adenomas were followed for 20 years, the risk of cancer at the site of the adenoma was 25%, a rate much higher than that expected in the normal population.[15]
The following three characteristics of adenomas are highly correlated with the potential to transform into cancer:[14]
  1. Larger size.
  2. Villous pathology.
  3. The degree of dysplasia within the adenoma.
In addition, removal of adenomatous polyps is associated with reduced CRC incidence.[16,17] 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.[18,19] Specialized techniques may be needed to identify, biopsy, and remove such lesions.[20]

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 percentage of deaths from CRC (3.9%) among the first-degree relatives(FDRs) of patients who had died from CRC than among sex-matched and age-matched controls (1.2%).[21] This difference has since been replicated in numerous studies that have consistently found that FDRs 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.[22-27]
A systematic review and meta-analysis of familial CRC risk has been reported.[28] Of 24 studies included in the analysis, all but one reported an increased risk of CRC if there was an affected FDR. The relative risk (RR) for CRC in the pooled study was 2.25 (95% confidence interval [CI], 2.00–2.53) if there was an affected FDR. 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 an RR in relatives of colon and rectal cancer patients of 2.42 (95% CI, 2.20–2.65) and 1.89 (95% CI, 1.62–2.21), 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 FDR with CRC, the RR was 3.76 (95% CI, 2.56–5.51). The highest RR was observed when the index case was diagnosed in individuals younger than 45 years (RR, 3.87; 95% CI, 2.40–6.22) compared with family members of index cases diagnosed at ages 45 to 59 years (RR, 2.25; 95% CI, 1.85–2.72), and to family members of index cases diagnosed at age 60 years or older (RR, 1.82; 95% CI, 1.47–2.25). In this meta-analysis, the familial risk of CRC associated with adenoma in an FDR was analyzed. The pooled analysis demonstrated an RR for CRC of 1.99 (95% CI, 1.55–2.55) in individuals who had an FDR with an adenoma.[28] This finding has been corroborated.[29] 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.[30,31] 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.

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