326 New strategies for prevention of colon cancer—paradigm: Familiar polyposis coli, hereditary nonpolypoid colon cancer (HNPCC) and ulcerative colitis

Abstract

Familial colorectal cancer can be divided into two distinct classes, familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC). Identification of gene loci assigned to FAP (APC gene) and HNPCC (mismatch repair genes) has allowed molecular analyses of affected patients as well as individuals at risk within those families. We established a presymptomatic molecular diagnosis for FAP families registered at our hospital. Since the majority of mutations identified to date lead to truncated proteins we used a non-radio-active protein truncation test (PTT) as a screening method. According to this assay five overlapping segments of the APC coding sequence were amplified by PCR and subsequently transcribed and translated <i>in vitro</i> in a rabbit reticulocyte lysate using biotinylated t-RNA Lys. Labelled proteins were separated by PAGE, transferred to nylon membranes, and detected by streptavidin-alkaline phosphatase complex in a colour reaction. We have started analyzing HNPCC patients at the molecular level. Selection of patients was not based on strict Amsterdam criteria but rather on early age of onset of disease (<50 years) or anamnestic criteria. Seven colorectal cancer patients—three of them matching the Amsterdam criteria—were analyzed for mutations within the complete coding sequence of the hMutS2 mismatch repair gene by PTT and direct sequencing. No mutations were found although microsatellite instabilities could be demonstrated in three patients. Since microsatellite instabilities are indicative of replication error caused by a defective mismatch repair system our future sequence analyses will be extended to the other known mismatch repair genes. While in the 80's we still considered the subtotal colectomy and ileorectal anastomosis in FAP as a good alternative to a restorative proctocolectomy in cases of few rectal polyps, we nowadays almost exclusively perform a mucosal proctectomy and ileal pouch-anal anastomosis with equal functional results. Even singular polyps in the rectum are for us a reason to go for this far more radical and safe procedure from the oncological point of view. And we are reinforced by our longterm experience with subtotal colectomies requiring secondary surgery or even developing metachronous rectal cancer despite close follow-up of the rectum.