HNPCC is the cause of about 5% of colon cancers in Finland. It is an autosomal dominant cancer syndrome, also known as the Lynch syndrome or the Cancer Family Syndrome. Being autosomal domintant, the disease affects 50% of the children of a affected father or mother.

The gene causing HNPCC has been identified and cloned. It is localizes on the cromosome 2p and 3p, and these findings will make it possible to investigate possible gene carriers for the presence of the defect gene, before they get any symtoms.

Almost 100% of those having the gene will get cancer, most commonly colon cancer, and clearly in a younger age than usual for colon cancer in general.

Like other colon cancers, HNPCC tumors develop from premalignant adenomas, and macroscopically they cannot be differentiated from sporadic colon cancer tumors. The tumors can be multiple, and the risk of a new cancer after that the initial cancer has been succesfully treated is hight.

HNPCC should be suspected, when
- at least three relatives have colorectal carcinoma, one of them should be a first-degree relative of the other two (and familial adenomatous polyposis should be excluded)
- at least two successive generations should be affected
- in at least one of the relatives, colorectal cancer should be found blow 50 years of age

These are the so-called Amsterdam criteria. All patients with HNPCC do not fulfil these criteria - especially if the families have been very small, and some relatives have died because of unrelated reasons in a very young age.

These pictures show a HNPCC tumor in the distal sigmoideum. Very often these tumors are located proximally to the splenic flexure.

HNPCC has been divided into two clinical variants, Lynch syndrome 1, where the patients have only colorectal carcinomas, and Lynch syndrome II, where some of the relatives have other carcinomas, especially endometrial carcinomas. In addition, there is another syndrome, the Muir-Torre syndrome, which also has a dominant inheritance and in addition cutaneous adenocarcinomas. This syndrome is very rare, and may have the same genetic background as HNPCC.