Gastric adenocarcinoma and proximal polyposis of the stomach (GAPPS) is an autosomal-dominant cancer-predisposition syndrome with a significant risk of gastric, but not colorectal, adenocarcinoma. We mapped the gene to 5q22 and found loss of the wild-type allele on 5q in fundic gland polyps from affected individuals. Whole-exome and -genome sequencing failed to find causal mutations but, through Sanger sequencing, we identified point mutations in APC promoter 1B that co-segregated with disease in all six families. The mutations reduced binding of the YY1 transcription factor and impaired activity of the APC promoter 1B in luciferase assays. Analysis of blood and saliva from carriers showed allelic imbalance of APC, suggesting that these mutations lead to decreased allele-specific expression in vivo. Similar mutations in APC promoter 1B occur in rare families with familial adenomatous polyposis (FAP). Promoter 1A is methylated in GAPPS and sporadic FGPs and in normal stomach, which suggests that 1B transcripts are more important than 1A in gastric mucosa. This might explain why all known GAPPS-affected families carry promoter 1B point mutations but only rare FAP-affected families carry similar mutations, the colonic cells usually being protected by the expression of the 1A isoform. Gastric polyposis and cancer have been previously described in some FAP-affected individuals with large deletions around promoter 1B. Our finding that GAPPS is caused by point mutations in the same promoter suggests that families with mutations affecting the promoter 1B are at risk of gastric adenocarcinoma, regardless of whether or not colorectal polyps are present.

Approximately 1-2% of colorectal cancers (CRC) arise because of germline mutations in DNA mismatch repair genes, referred to as Lynch syndrome. These tumours show microsatellite instability (MSI) and loss of expression of mismatch repair proteins. Presymptomatic identification of mutation carriers has been demonstrated to improve survival; however, there is concern that many are not being identified using current practices. We evaluated population-based MSI screening of CRC in young patients as a means of ascertaining mutation carriers. CRC diagnosed in patients aged <60 years were identified from pathology records. No prior information was available on family history of cancer. PCR techniques were used to determine MSI in the BAT-26 mononucleotide repeat and mutation in the BRAF oncogene. Loss of MLH1, MSH2, MSH6 and PMS2 protein expression was evaluated in MSI1 tumours by immunohistochemistry. MSI1 tumours were found in 105/1,344 (7.8%) patients, of which 7 were excluded as possible Lynch syndrome because of BRAF mutation. Of the 98 ''red flag'' cases that were followed up, 25 were already known as mutation carriers or members of mutation carrier families. Germline test results were obtained for 35 patients and revealed that 22 showed no apparent mutation, 11 showed likely pathogenic mutations and 2 had unclassified variants. The proportion of MSI1 cases in different age groups that were estimated to be mutation carriers was 89% (<30 years), 83% (30-39), 68% (40-49) and 17% (50-59). We recommend MSI as the initial test for population-based screening of Lynch syndrome in younger CRC patients, regardless of family history. ' 2008 Wiley-Liss, Inc.Key words: HNPCC; colorectal cancer; microsatellite instability; BRAF Hereditary non-polyposis colorectal cancer (HNPCC), or Lynch syndrome, is mainly caused by mutations in the DNA mismatch repair (MMR) genes MLH1, MSH2, MSH6 and PMS2. 1,2 It accounts for approximately 1-2% of all colorectal cancers (CRC) and is also associated with an increased risk of endometrial and other extracolonic cancers. As a consequence of deficient MMR, almost all CRCs from patients with Lynch syndrome show microsatellite instability (MSI) in their tumour DNA. The MSI phenotype is characterized by ubiquitous changes in the length of nucleotide repeat sequences, with mononucleotide repeat tracts being particularly susceptible to deletions. 3 MSI is almost always accompanied by a loss of expression of MMR proteins. The most commonly observed combinations are loss of MLH1 with PMS2 and of MSH2 with MSH6, although other rarer patterns of loss have been reported. 4,5 Approximately 10% of sporadic CRCs also exhibit MSI and these tumours occur most frequently in the proximal colon of older women. 3 The large majority of sporadic MSI1 CRCs arise because of acquired, methylation-induced transcriptional silencing of MLH1 gene expression. 6 The MSI phenotype cannot therefore be used alone as a specific marker for Lynch syndrome. However, the presence of a hot-spot point mutation (V600E) in the BRAF onc...

The aim of this study was to determine the frequency of microsatellite instability (MSI ؉ )