Hereditary colorectal cancer syndromes

Abstract

Tumors of large bowel continue to be one of the leading causes of morbidity and mortality with about 300,000 new cases and 200,000 deaths per year in Europe and USA despite recent technological advancements. In sharp contrast with these discouraging data, the basic knowledge of colorectal neoplasms has grown remarkably in the last decades especially with the genetic elucidation of the two inherited cancer-predisposition syndromes, familial polyposis (FAP) and hereditary non polyposis colorectal cancer (HNPCC). Recognition of the genetic component of CRC is growing; gene mutations responsible for cell transformation can be present as inherited germline defect or arise in somatic cells as consequence of environmental insults. The two main hereditary syndromes, FAP and HNPCC, account for about 6-10% of CRCs, remaining cases are attributed to so called sporadic cancer. Although the timescale of the appearance and risk of recurrence of the hereditary and sporadic forms are quite different, they share a common pathway: the adenoma to carcinoma sequence. In 1990 Fearon and Vogelstein proposed a multistep model for the molecular events underlying colorectal tumorigenesis. The model was based on two assumptions: the first one is that the tumors are clonal, the second assumption is that the colorectal tumorigenesis occurs as succession of a series of events that can be described as dyplasia-carcinoma sequence or adenoma-carcinoma sequence. The initial alterations which are not detectable on histologic examination, are subtle changes in the normal balance between cell growth and cell death. With progression precursors to adenoma, the foci of aberrant cripts become detectable. Few adenomas progress to carcinoma, however if the progression of these lesions remain unchecked, there is an increased risk of tumor diffusion. As the cells need time to accumulate the genetic defects including mutational activation of oncogenes and inactivation of tumor suppressor genes to undergo full malignant transformation, CRC occurs mainly in the elderly. If one of these defects are present at birth as germline mutations, fewer mutational events will be requested to reach malignant transformation and the disease will appear earlier().