Colorectal cancer: lessons for genetic counselling and care for families

Genetic predisposition as a basis for chemoprevention, surgical and other interventions in colorectal cancer

Strategies of cancer prevention are generally developed with the population at large in mind. However, special attention is warranted for those persons with rare genetic traits associated with a greatly elevated risk of developing colorectal cancer (CRC) and some other malignancies: Orphan diseases demand Orphan preventive measures! Recent advances in modern genetics have enhanced our understanding of several genes and the specific germ-line mutations responsible for colorectal carcinogenesis. A number of features provide evidence for a genetic predisposition to CRC. These include typical clinical and histological features of a particular syndrome, a familial aggregation of CRC and associated malignancies, young age at onset of CRC, occurrence of multiple neoplasias and/or unusual localisation of the tumour (e.g., right side of the colon). In hereditary colorectal cancer, genetic testing can easily be demonstrated as cost-effective.

Familial colorectal cancer in Ashkenazim due to a hypermutable tract in APC

Approximately 130,000 cases of colorectal cancer (CRC) are diagnosed in the United States each year, and about 15% of these have a hereditary component. Two well-defined syndromes, familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC), account for up to 5% of the total new cases of CRC. Truncating APC mutations are responsible for FAP, and defective mismatch repair genes cause HNPCC. However, the genes responsible for most of the familial cases are unknown. Here we report a mutation (T to A at APC nucleotide 3920) found in 6% of Ashkenazi Jews and about 28% of Ashkenazim with a family history of CRC. Rather than altering the function of the encoded protein, this mutation creates a small hypermutable region of the gene, indirectly causing cancer predisposition.