Missense mutations in hMLH1 associated with colorectal cancer.

One of the most prevalent hereditary syndromes associated with colorectal cancer is hereditary nonpolyposis colorectal cancer (HNPCC). The inherited gene defects in HNPCC have been shown to reside in DNA mismatch repair genes, mostly hMSH2 or hMLH1. Most HNPCC patients are heterozygous with regard to the relevant mismatch repair gene; they have one normal and one mutated allele, and mismatch repair in normal somatic cells is functional. Cancer predisposition in HNPCC is believed to be associated with the loss of the wild-type allele in somatic cells, resulting in defective DNA mismatch repair. This gives rise to DNA microsatellite instability (MSI), an increased somatic mutation rate, and eventually, to the accumulation of mutations in genes involved in colorectal carcinogenesis. In support of this theory, colorectal tumors in HNPCC patients and in mice deficient for hMSH2 or hMLH1 show MSI. Here, we describe two missense mutations in hMLH1 exon 16 associated with colorectal cancer. Interestingly, the tumors do not show MSI. This raises some potentially important issues. First, even microsatellite-negative colorectal tumors can be associated with germline mutations and these will be missed if an MSI test is used to select patients for mutation screening. Second, the lack of MSI in these cases suggests that the mechanism involved in carcinogenesis could be different from that generally hypothesized.

Microsatellite instability and mismatch repair gene inactivation in sporadic pancreatic and colon tumours.

Genomic instability has been proposed as a new mechanism of carcinogenesis involved in hereditary non-polyposis colorectal cancer (HNPCC) and in a large number of sporadic cancers like pancreatic and colon tumours. Mutations in human mismatch repair genes have been found in HNPCC patients, but their involvement in sporadic cancer has not been clarified yet. In this study we screened 21 pancreatic and 23 colorectal sporadic cancers for microsatellite instability by ten and six different microsatellite markers respectively. Microsatellite alterations were observed at one or more loci in 66.6% (14/21) of pancreatic cancers and in 26% (6/23) colon tumours, but all the pancreatic and half of the colon samples showed a low rate of microsatellite instability. All the unstable samples were further analysed for mutations in the hMLH1 and hMSH2 genes and for hypermethylation of the hMLH1 promoter region. Alterations in the hMLH1 gene were found only in colorectal tumours with a large presence of microsatellite instability. None of the pancreatic tumours showed any alteration in the two genes analysed. Our results demonstrate that microsatellite instability is unlikely to play a role in the tumorigenesis of sporadic pancreatic cancers and confirm the presence of mismatch repair gene alterations only in sporadic colon tumours with a highly unstable phenotype.

Tumorigenesis in colorectal tumors from patients with hereditary non-polyposis colorectal cancer.

Tumorigenesis of colorectal cancer in patients with hereditary non-polyposis colorectal cancer (HNPCC) has been postulated to follow a different pathway from that of sporadic colorectal tumors. A characteristic of HNPCC-associated tumors is the replication error phenotype. We studied tumorigenesis in 8 fresh-frozen and 67 paraffin-embedded colorectal tumors derived from 29 families with HNPCC or a familial aggregation of colorectal cancer. By using intragenic markers, inactivation of the wild-type allele of hMLH1 was shown to occur through loss of heterozygosity and not through a somatic point mutation. Microsatellite instability is very common and occurs early in almost all colorectal tumors from HNPCC patients. Transforming growth factor beta type II receptor (T beta RII) mutations occur in these tumors at a high frequency. Of colorectal cancers from families with HNPCC, 63% have frameshift mutations in T beta RII, compared with 10% of sporadic colorectal cancers. APC and K-RAS mutations appear to be as frequent in the HNPCC tumors as in the sporadic counterpart.

Mutation screening in the hMLH1 gene in Swedish hereditary nonpolyposis colon cancer families.

Hereditary nonpolyposis colorectal cancer is caused by heritable defects in the DNA mismatch repair genes hMLH1, hMSH2, hPMS1, and hPMS2. We have used denaturing gradient gel electrophoresis to analyze the 19 exons and exon-intron borders of hMLH1 in 39 Swedish hereditary nonpolyposis colorectal cancer families. Germline mutations were found in eight of these families: two splice mutations affecting exons 3 and 7, respectively, and six missense mutations, of which, four were in exon 2 and one each were in exons 1 and 16. The relatively high number of missense mutations raises several important clinical and technical issues. Such alterations can be identified only when using methods that target DNA or mRNA sequence alteration because they do not cause protein truncations detected by in vitro translation assays. Furthermore, the relationship between these missense mutations and the predisposition to colon cancer is difficult to determine without additional information; thus, genetic counseling based on mutation data is difficult.

Sublocalization of a locus at 3p21.3-23 predisposing to hereditary nonpolyposis colon cancer.

Hereditary nonpolyposis colon cancer (HNPCC) is a heterogeneous disease caused by at least three different genes on chromosomes 2 and 3, and one or more additional chromosomes. We used 19 dinucleotide markers in order to sublocalize further the 3p locus linked to HNPCC, and to order the markers into one map resulting in a panel of markers suitable for linkage studies. Human chromosome 3 mouse hybrids were used to determine the chromosomal position of the markers. Haplotype analysis in two families where the disease is linked to chromosome 3p21-23 was used in order to sublocalize further the region in which the gene is located. Based on our results, the gene has now been localized to the region 3p21.3-23.

Genetic mapping of a second locus predisposing to hereditary non-polyposis colon cancer.

Hereditary colon cancer is caused by mutations in several different loci. The APC gene on chromosome 5 causing adenomatous polyposis coli represents a minority of the inherited colon cancer cases, while hereditary-non polyposis colon cancer (HNPCC) may cause five percent of all human colon cancer. One gene causing HNPCC was recently mapped to chromosome 2 but the same study also showed that at least one additional locus may cause HNPCC. We now present tight linkage between a polymorphic marker on the short arm of chromosome 3 and the disease locus, and find that these families also manifest signs of a general DNA replication disorder.