Multistage carcinogenesis and the incidence of colorectal cancer

Abstract

We use general multistage models to fit the age-specific incidence of colorectal cancers in the Surveillance, Epidemiology, and End Results registry, which covers Ϸ10% of the U.S. population, while simultaneously adjusting for birth cohort and calendar year effects. The incidence of colorectal cancers in the Surveillance, Epidemiology, and End Results registry is most consistent with a model positing two rare events followed by a high-frequency event in the conversion of a normal stem cell into an initiated cell that expands clonally to give rise to an adenomatous polyp. Only one more rare event appears to be necessary for malignant transformation. The two rare events involved in initiation are interpreted to represent the homozygous loss of adenomatous polyposis coli gene function. The subsequent transition of a preinitiated stem cell into an initiated cell capable of clonal expansion via symmetric division is predicted to occur with a frequency too high for a mutational event but may reflect a positional effect in colonic crypts. Our results suggest it is not necessary to invoke genomic instability to explain colorectal cancer incidence rates in human populations. Temporal trends in the incidence of colon cancer appear to be dominated by calendar year effects. The model also predicts that interventions, such as administration of nonsteroidal anti-inflammatory drugs, designed to decrease the growth rate of adenomatous polyps, are very efficient at lowering colon cancer risk substantially, even when begun later in life. By contrast, interventions that decrease the rate of mutations at the adenomatous polyposis coli locus are much less effective in reducing the risk of colon cancer.T he first attempts to formulate a quantitative description of carcinogenesis reflecting essential biological processes on the pathway from a normal cell to a cancer cell go back almost half a century (1). Perhaps the best known model is due to Armitage and Doll (2), who noticed that the age-specific incidence of many carcinomas appeared to increase approximately with power of age, which could be related to the number of rate-limiting steps involved in the formation of a malignant tumor. However, it was also realized that a two-stage model with clonal expansion of intermediate cell populations could generate similar age-specific incidence curves (3). These considerations, combined with the idea of recessive oncogenesis first formulated by Knudson (4), led to the two-stage clonal expansion (TSCE) model, which explicitly incorporates clonal expansion as a stochastic process during carcinogenesis (5-7).Recent studies of the genetic profiles of various tumors suggest the involvement of several genes during tumorigenesis. A case in point is colorectal cancer, perhaps the best studied cancer in terms of the putative sequence of genetic events in its pathogenesis (8-12). Over the past 10 years, an impressive number of studies have been carried out identifying several molecular pathways involved in the development of colorectal cancer (see ref...

Evolutionary dynamics of tumor suppressor gene inactivation

“…Furthermore, measuring the inactivation kinetics of TSGs in animal models and comparing the data with our equations will reveal the relevant tissue architecture for the process of cancer initiation in various organs (38). In addition, our findings have implications for linking cancer incidence curves to the molecular events of cancer progression (35).…”

“…Because genetic instability is a feature of almost all late-stage cancer cells, the main question is whether genetic instability arises early or late during tumorigenesis. The problem is unresolved both from an experimental (28-31) and theoretical perspective (15,(32)(33)(34)(35). The present article contributes to the ongoing discussion by providing a quantitative framework for the kinetics of TSG inactivation.…”

Somatic evolutionary genomics: Mutations during development cause highly variable genetic mosaicism with risk of cancer and neurodegeneration

“…Meza et al (22) began with a model of colorectal cancer progression and incidence that they had previously studied (23). In that model, carcinogenesis progresses through four stages: two initial transitions, followed by a third transition that triggers clonal expansion, and then a final transition to the malignant stage.…”

Age-specific incidence of cancer: Phases, transitions, and biological implications

“…The incidence of colorectal cancer, for example, is showing a remarkable reduction following the increased adoption of colonoscopies, sigmoidoscopies, and fecal occult blood tests in the 1970s and 1980s (16, 17), whereas increasing trends in obesity (or body mass index) may well explain increasing rates of colon cancer in younger (postwar) cohorts (18). To disentangle the effects of age, period, and cohort on cancer incidence, a number of mathematical and computational approaches have been suggested (15,(19)(20)(21), but are rarely invoked.Despite these difficulties in identifying the natural course of the age-specific incidence, free of secular trends and factors, several firm predictions concerning the behavior of the age-specific incidence can be made, predictions based on mathematical analyses, sound statistical criteria, and biological plausibility. Here, we make the case that neoplastic progression that is initiated by the loss of a tumor suppressor (or "gatekeeper") gene in two rare rate-limiting events necessarily leads to a linearly increasing age-specific incidence above an age that is characteristic for the timescale of neoplastic progression, a behavior that reflects the steadily increasing risk of malignant transformations in premalignant clones whose incidence increases linearly in the target organ.…”