Juvenile polyposis syndrome (JPS) is an autosomal dominant predisposition to multiple juvenile polyps in the gastrointestinal tract. Germline mutations in the MADH4 or BMPR1A genes have been found to be causative of the disease in a subset of JPS patients. So far, no genotype-phenotype correlation has been reported. We examined 29 patients with the clinical diagnosis of JPS for germline mutations in the MADH4 or BMPR1A genes and identified MADH4 mutations in seven (24%) and BMPR1A mutations in five patients (17%). A remarkable prevalence of massive gastric polyposis was observed in patients with MADH4 mutations when compared with patients with BMPR1A mutations or without identified mutations. This is the first genotype-phenotype correlation observed in JPS.

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Germline mutations in human mismatch repair (MMR) genes yield a predisposition for the hereditary nonpolyposis colon cancer (HNPCC) syndrome. In contrast to hMLH1 and hMSH2, little is known about the overall involvement of hMSH6 in colorectal cancer. We investigated 82 tumors from patients who fulfilled the Bethesda guidelines for HNPCC as well as 146 sporadic tumors, analyzing microsatellite instability and expression of the 4 MMR proteins hMSH6, hMSH2, hMLH1 and hPMS2. Four tumors with lost expression and 1 tumor with cytoplasmic expression of hMSH6 were identified. Sequence analysis revealed germline mutations in 4 of the 5 patients, including 1 patient with sporadic disease. The lost or reduced expression of hMSH2 and hMLH1 was always identical to its heterodimerization partners, hMSH6 and hPMS2, respectively. Furthermore, hMSH2 expression was reduced upon hMSH6 deficiency. HNPCC is a high-penetrance, autosomal dominant cancer-susceptibility syndrome. Affected individuals are at increased risk for developing endometrial, small bowel, gastric, ovarian, ureteral and hepatobiliary carcinoma besides colorectal cancer. 1 A hallmark of most of these malignancies is contraction/expansion of simple sequence motifs, 2,3 which is termed microsatellite instability (MSI). Germline mutations in the human mismatch repair genes hMSH2, hMLH1, hPMS1 and hPMS2 have been found in HNPCC, with mutations in hMSH2 and hMLH1 accounting for the vast majority of genetic predispositions (reviewed in ref. 4). Moreover, about 10 -15% of sporadic colorectal cancers show MSI, 3 mostly based on epigenetic silencing of hMLH1 by hypermethylation of the promoter region. 5 Recently, germline mutations in another mismatch repair gene, hMSH6, have been included in the genetic predisposition of HNPCC or HNPCC-like cases. 6 The average age at onset of the disease has been shown to be about 10 years later than in hMSH2 and hMLH1 germline mutations carriers, which might reflect a lower penetrance of hMSH6 mutations. 7,8 Biochemical studies have revealed that the hMSH6 protein functions as a heterodimer complexed with hMSH2 (hMutS␣) in the recognition of base-base mispairs and small insertion-deletion loops (IDLs). For the repair of IDLs there is a functional overlap with the hMSH2-hMSH3 (hMutS␤) complex. 9,10 Mutations of hMSH6 in cell lines and tumors are accompanied by MSI primarily at mononucleotide repeats. 6,11,12 The overlapping substrate specificities for IDLs of mutS␣ and mutS␤ and the microsatellite stable (MSS) phenotype of the tumors developed in Msh6 knockout mice 13 suggest that hMSH6-deficient colorectal tumors may evolve without showing MSI. There have been mixed results on whether hMSH6 participates in colorectal tumors with a low level of MSI (MSI-L; instability in up to 30%). 14,15 Finally, little is known about the involvement of hMSH6 in colorectal tumors of patients not fulfilling the Bethesda guidelines.We thus investigated the frequency at which abolished hMSH6 protein expression is involved with the generation of colorectal c...