A genetic model for colorectal tumorigenesis

Because most colorectal carcinomas appear to arise from adenomas, studies of different stages of colorectal neoplasia may shed light on the genetic alterations involved in tumor progression. We looked for four genetic alterations (ras-gene mutations and allelic deletions of chromosomes 5, 17, and 18) in 172 colorectal-tumor specimens representing various stages of neoplastic development. The specimens consisted of 40 predominantly early-stage adenomas from 7 patients with familial adenomatous polyposis, 40 adenomas (19 without associated foci of carcinoma and 21 with such foci) from 33 patients without familial polyposis, and 92 carcinomas resected from 89 patients. We found that ras-gene mutations occurred in 58 percent of adenomas larger than 1 cm and in 47 percent of carcinomas. However, ras mutations were found in only 9 percent of adenomas under 1 cm in size. Sequences on chromosome 5 that are linked to the gene for familial adenomatous polyposis were not lost in adenomas from the patients with polyposis but were lost in 29 to 35 percent of adenomas and carcinomas, respectively, from other patients. A specific region of chromosome 18 was deleted frequently in carcinomas (73 percent) and in advanced adenomas (47 percent) but only occasionally in earlier-stage adenomas (11 to 13 percent). Chromosome 17p sequences were usually lost only in carcinomas (75 percent). The four molecular alterations accumulated in a fashion that paralleled the clinical progression of tumors. These results are consistent with a model of colorectal tumorigenesis in which the steps required for the development of cancer often involve the mutational activation of an oncogene coupled with the loss of several genes that normally suppress tumorigenesis.

Genetic alterations during colorectal-tumor development.

Uncontrolled cell proliferation is the hallmark of cancer, and tumor cells have typically acquired damage to genes that directly regulate their cell cycles. Genetic alterations affecting p16(INK4a) and cyclin D1, proteins that govern phosphorylation of the retinoblastoma protein (RB) and control exit from the G1 phase of the cell cycle, are so frequent in human cancers that inactivation of this pathway may well be necessary for tumor development. Like the tumor suppressor protein p53, components of this "RB pathway," although not essential for the cell cycle per se, may participate in checkpoint functions that regulate homeostatic tissue renewal throughout life.

http://science.sciencemag.org/content/sci/274/5293/1672.full.pdf

Cancer Cell Cycles

Patterns of DNA methylation and chromatin structure are profoundly altered in neoplasia and include genome-wide losses of, and regional gains in, DNA methylation. The recent explosion in our knowledge of how chromatin organization modulates gene transcription has further highlighted the importance of epigenetic mechanisms in the initiation and progression of human cancer. These epigenetic changes -- in particular, aberrant promoter hypermethylation that is associated with inappropriate gene silencing -- affect virtually every step in tumour progression. In this review, we discuss these epigenetic events and the molecular alterations that might cause them and/or underlie altered gene expression in cancer.

The fundamental role of epigenetic events in cancer

Lessons from Hereditary Colorectal Cancer

Based upon observations on 48 cases of retinoblastoma and published reports, the hypothesis is developed that retinoblastoma is a cancer caused by two mutational events. In the dominantly inherited form, one mutation is inherited via the germinal cells and the second occurs in somatic cells. In the nonhereditary form, both mutations occur in somatic cells. The second mutation produces an average of three retinoblastomas per individual inheriting the first mutation. Using Poisson statistics, one can calculate that this number (three) can explain the occasional gene carrier who gets no tumor, those who develop only unilateral tumors, and those who develop bilateral tumors, as well as explaining instances of multiple tumors in one eye. This value for the mean number of tumors occurring in genetic carriers may be used to estimate the mutation rate for each mutation. The germinal and somatic rates for the first, and the somatic rate for the second, mutation, are approximately equal. The germinal mutation may arise in some instances from a delayed mutation.

/pdf/mutation-and-cancer-statistical-study-of-retinoblastoma-3lfgdufscj.pdf

Mutation and Cancer: Statistical Study of Retinoblastoma

https://cancerres.aacrjournals.org/content/canres/45/4/1437.full.pdf

Hereditary Cancer, Oncogenes, and Antioncogenes

Most cancers have many chromosomal abnormalities, both in number and in structure, whereas some show only a single aberration. In the era before molecular biology, cancer researchers, studying both human and animal cancers, proposed that a small number of events was needed for carcinogenesis. Evidence from the recent molecular era also indicates that cancers can arise from small numbers of events that affect common cell birth and death processes.

/pdf/two-genetic-hits-more-or-less-to-cancer-3njsu4dbcp.pdf

Two genetic hits (more or less) to cancer.

A model for carcinogenesis is presented that provides a framework for understanding the roles of "spontaneous" events, hereditary factors, and environmental agents in human carcinogenesis and for interpreting experimental carcinogenesis. This model incorporates two features: a) transition of target stem cells into cancer cells via an intermediate stage in two irreversible steps, and b) growth and differentiation of normal target and intermediate cells. Cast in mathematical terms, the model can be fitted to age-specific incidence data on human cancers of both children and adults and can illuminate the relative importance of agents that affect transition rates, tissue growth, and tissue differentiation. This is illustrated by application of the model to a) the epidemiology of lung cancer with emphasis on the role of cigarette smoking and b) the epidemiology of breast cancer with emphasis on the roles of hormones, radiation, and hereditary. The nature of the two events and of the intermediate stage is considered in light of hereditary conditions that predispose to cancer in humans. The modes of action of radiation and chemicals in carcinogenesis are discussed, as are predictions based on the model and amenable to experimental verification.

Mutation and Cancer: A Model for Human Carcinogenesis

The antioncogenes, or tumor suppressor genes, as negative regulators of cell division, stand in contrast to oncogenes. For most human cancers, the more frequently mutated genes are the antioncogenes, the principal exception being the leukemias and lymphomas. Persons heterozygous for germ-line mutations in antioncogenes are strongly predisposed to one or more kinds of cancer, and most dominantly inherited cancer is attributable to such heterozygosity. Seven antioncogenes have been cloned through the study of these persons, and several others have been mapped. An eighth one was mapped and cloned through the investigation of tumors and is not yet known in hereditary form. Three dominantly inherited forms of cancer are not attributable to mutations in antioncogenes. The corresponding nonhereditary forms of most cancers generally reveal abnormalities of the same antioncogenes that are found in the hereditary forms but may also show additional ones. Some cancers, especially the embryonal tumors of children, have a small number of antioncogene mutations; some others, such as most sarcomas, have more, and the common carcinomas have the most, reflecting a hierarchy of controls over growth of stem cell populations. Still more members of this gene category remain to be mapped and cloned through the study of cancer families and of tumors. The genes that have been cloned act at diverse points in the signal transduction pathway in cells, from the outer cell membranes to sites of gene transcription, in some cases as negative regulators of oncogene expression.

https://www.pnas.org/content/pnas/90/23/10914.full.pdf

Alfred G. Knudson

Papers

Mutation and Cancer: Statistical Study of Retinoblastoma

Hereditary Cancer, Oncogenes, and Antioncogenes

Two genetic hits (more or less) to cancer.

Mutation and Cancer: A Model for Human Carcinogenesis

Antioncogenes and human cancer