Population genetics of colonic cancer

A New Polygenic Model for Nonfamilial Colorectal Cancer Inheritance Based on the Genetic Architecture of the Azoxymethane-Induced Mouse Model

The azoxymethane model of colorectal cancer (CRC) was used to gain insights into the genetic heterogeneity of nonfamilial CRC. We observed significant differences in susceptibility parameters across 40 mouse inbred strains, with 6 new and 18 of 24 previously identified mouse CRC modifier alleles detected using genome-wide association analysis. Tumor incidence varied in F1 as well as intercrosses and backcrosses between resistant and susceptible strains. Analysis of inheritance patterns indicates that resistance to CRC development is inherited as a dominant characteristic genome-wide, and that susceptibility appears to occur in individuals lacking a large-effect, or sufficient numbers of small-effect, polygenic resistance alleles. Our results suggest a new polygenic model for inheritance of nonfamilial CRC, and that genetic studies in humans aimed at identifying individuals with elevated susceptibility should be pursued through the lens of absence of dominant resistance alleles rather than for the presence of susceptibility alleles.

Colonic screening and surveillance

Screening for colorectal cancer has not obtained worldwide acceptance in spite of its proven survival benefit for average-risk persons and some high-risk groups. The incidence of and mortality from colorectal cancer are worrying in Europe as well as in the USA, Australia and Japan. The best evidence-based studies are those published on screening using faecal occult blood tests, endoscopic methods and different tumour markers having been evaluated to a lesser degree. Feasibility studies are necessary before massive screening can be undertaken because the results obtained from randomized studies may not be reproduced to a satisfactory degree in average- as well as high-risk populations. Primary prevention by dietary intervention and drugs has been studied in great detail, so far without any major breakthrough. This chapter will address different screening methods in populations with a varying risk of colorectal cancer, together with providing a short review of prevention and intervention strategies.

Epidermal naevus with Darier's disease-like changes in a patient with Gardner's syndrome

We describe the unique presentation of a linear epidermal nevus with histologic features of Darier's disease occurring in a patient with Gardner's syndrome. Classification of localized forms of Darier's disease as an epidermal nevus or as a genetic mosaicism remains controversial. The association of this disorder with Gardner's syndrome has not been described in the literature before.

Risk factors for colorectal cancer in subjects with family history of the disease

The relationship between lifestyle factors, past medical conditions, daily meal frequency, diet and the risk of 'familial' colorectal cancer has been analysed using data from a case-control study conducted in northern Italy. A total of 1584 colorectal cancer patients and 2879 control subjects were admitted to a network of hospitals in the Greater Milan area and the Pordenone province. The subjects included for analysis were the 112 cases and the 108 control subjects who reported a family history of colorectal cancer in first-degree relatives. Colorectal cancer cases and control subjects with family history were similarly distributed according to sex, age, marital status, years of schooling and social class. Familial colorectal cancer was associated with meal frequency, medical history of diabetes (relative risk, RR = 4.6) and cholelithiasis (RR = 5.2). Significant positive trends of increasing risk with more frequent consumption were observed for pasta (RR = 2.5, for the highest vs the lowest intake tertile), pastries (RR = 2.4), red meat (RR = 2.9), canned meat (RR = 1.9), cheese (RR = 3.5) and butter (RR = 1.9). Significant inverse associations and trends in risk were observed for consumption of poultry (RR = 0.4), tomatoes (RR = 0.2), peppers (RR = 0.3) and lettuce (RR = 0.3). Significant inverse trends in risk with increasing consumption for beta-carotene and ascorbic acid were observed (RR = 0.5 and 0.4 respectively, highest vs lowest intake tertile). These results suggest that risk factors for subjects with a family history of colorectal cancer in first-degree relatives are not appreciably different from recognized risk factors of the disease in the general population.

Molecular biology of colon polyps and colon cancer

From a histologic and endoscopic standpoint, colon and rectal cancer (CRC) begins as a small neoplastic polyp which progressively enlarges and transforms through a dysplasia stage into invasive cancer. Recently, molecular abnormalities underlying the adenomacarcinoma progression have been defined. The adenomatous polyposis coli (APC) gene and mismatch repair genes are found to be dysfunctional early in the neoplastic process; either as inherited or somatic mutations. Subsequently, polyps progress to cancer along one of two paths depending on which gene is abnormal. When the APC gene is the initial mutation tumor development follows the "loss of heterozygocity" (LOH) pathway. If mismatch repair genes are altered, the "replication error" (RER) pathway is followed. Somatic mutations of the K-ras oncogene and the MCC, DCC, and p53 tumor suppressor genes accumulate in the LOH pathway and mark the progression through polyp stages. Microsatellite instability is a characteristic of the RER pathway but the precise genes involved in this pathway currently are not known. Defining these pathways has led to a new classification scheme for CRC with resultant changes in our clinical approach to screening, surveillance, and treatment.

Genetic epidemiology of colorectal cancer

Genetic epidemiological methods have played an integral role in the characterisation of the genetic susceptibilities to colorectal cancer. Classic epidemiological approaches, such as case-control and prospective cohort studies, that utilise family history information have laid the foundation for the more specialised family-based genetic methods, segregation analysis and linkage analysis. The genetic epidemiology of colorectal cancer can be characterised by several themes: the consistently increased risk of colorectal cancer in first-degree relatives of patients with colorectal cancer; genetic predisposition to some, if not the majority of colorectal neoplasms; and genetic heterogeneity of the inherited colorectal cancer syndromes. With the rapid development of molecular genetic techniques, new opportunities for further research include studies to estimate the proportion of colorectal cancer that is accounted for by genetic susceptibility, the number of loci that may be involved, and most importantly, gene-environment interaction studies, not only of the inherited syndromes, but of common colorectal cancer.

Role of clinical criteria in the diagnosis of hereditary non-polyposis colorectal cancer (HNPCC): results of a multivariate analysis

Hereditary non-polyposis colorectal cancer (HNPCC) is an autosomal dominant disease characterized by early-onset colorectal tumors, primarily in the right colon, that are frequently associated with other cancers. In the absence of a reliable biomarker, clinical criteria for diagnosing HNPCC have been proposed by the International Collaborative Group on HNPCC (ICG-HNPCC). However, these criteria are often too restrictive for application in small families. We analyzed 6 clinical/pathological features suggestive of genetic colon-cancer risk in 970 colorectal-cancer patients defined according to the ICG criteria as HNPCC (96) or non-HNPCC (874). Logistic regression analysis was used to determine their relative potentials for predicting the diagnosis of HNPCC. The most predictive were then used to estimate HNPCC risk levels within the non-HNPCC group. Multivariate analysis showed the following associations with HNPCC diagnosis: vertically transmitted cancer ("highly predictive"), early-onset (< 50 yrs) colorectal cancer, aggregation of tumors in the nuclear pedigree and proximal-colon tumors (the last 3 considered "predictive"). Re-evaluation of all families revealed that 76 non-HNPCC families (8.9% of the whole series) satisfied our highly predictive vertical-transmission criterion plus at least one of the other "predictive" criteria. The presence of 2 consecutive generations affected by colorectal cancer or early primaries seems to be a major risk indicator of hereditary colorectal cancer. Using this approach we identified a large group of families that require further evaluation, although they do not currently meet the ICG-HNPCC criteria for HNPCC.