Beta-catenin mutations are more frequent in small colorectal adenomas than in larger adenomas and invasive carcinomas.

Loss of serine or threonine phosphorylation sites from exon 3 of beta-catenin has been identified in approximately half of colorectal tumors which lack adenomatous polyposis coli (APC) mutations, but the overall contribution of beta-catenin mutations to sporadic colorectal tumorigenesis is unclear. We therefore used PCR to amplify and sequence exon 3 of beta-catenin from 202 sporadic colorectal tumors. Exon 3 beta-catenin mutations were identified in 6 of 48 small (< 1 cm) adenomas, 2 of 82 large (> or =1 cm) adenomas, and 1 of 72 invasive carcinomas. Eight of the nine mutations, including all of those in the small adenomas and the invasive cancer, involved loss of serine or threonine phosphorylation sites. The percentage of beta-catenin mutations in small adenomas (12.5%) was significantly greater than that in large adenomas (2.4%) and invasive cancers (1.4%; P = 0.05 and P = 0.02, respectively). We conclude that mutation of beta-catenin can be an early, perhaps initiating, event in colorectal tumorigenesis. Small adenomas with beta-catenin mutations do not appear to be as likely to progress to larger adenomas and invasive carcinomas as other adenomas, however, with the result that beta-catenin mutations are only rarely seen in invasive cancers. This suggests that APC and beta-catenin mutations are not functionally equivalent, and that the APC gene may have other tumor suppressor functions besides the degradation of beta-catenin.

Alleles of APC modulate the frequency and classes of mutations that lead to colon polyps.

Most inherited mutant alleles of the adenomatosis polyposis coli gene (APC) cause the appearance of large numbers of colon polyps, the familial polyposis syndrome. (These mutant alleles are designated APCp alleles.) A subset of APC mutations, the attenuated or APC(AP) alleles, predispose to only a few colon polyps. This leads to the hypothesis that if mutation of the inherited normal allele is rate limiting in polyp development, the increased number of polyps associated with the APCp allele indicates that the frequency of mutations that can lead to polyp formation is higher among APCp carriers than among APC(AP) carriers. We have previously suggested that the APC protein might modulate the frequency of mutations, such as loss of heterozygosity (LOH), necessary for colon polyp formation. We thus reasoned that tumours from patients who carry an APC(AP) allele might show a reduced frequency of LOH compared with tumours from patients who carry an APCp allele. Loss of AAPC mutant alleles is designated as LOH(AP). Screening of tumours from APC(AP) carriers revealed a reduction of LOH compared with that of an unselected group of polyposis patients. In fact, no loss of the inherited APC(N) allele was observed, although sequencing showed that the inherited APC(N) allele had frequently undergone point mutations and small deletions in the tumours. A low frequency loss of the inherited APC(AP) allele was seen. These findings support the suggestion that the APC(AP) allele has residual gene activity and that this activity modulates the spectrum and frequency of mutations that lead to adenoma formation.

The inducible prostaglandin biosynthetic enzyme, cyclooxygenase 2, is not mutated in patients with attenuated adenomatous polyposis coli.

Germ-line mutations in the APC gene cause adenomatous polyposis coli (APC), a syndrome in which patients develop hundreds to thousands of precancerous adenomatous colorectal polyps. We described previously an attenuated form of APC (AAPC) resulting from very 5' mutations in APC in which affected patients exhibit fewer colorectal polyps and a later age of onset of colorectal cancer. However, because striking variations in colorectal polyp numbers occur among patients carrying identical AAPC mutations, alleles of another gene may modify the expression of the APC disease phenotype. We tested the hypothesis that loss of function of human cyclooxygenase 2 (COX-2), known to modify the APC phenotype in the Apc delta716 mouse, results in a decreased tumor burden in AAPC patients that develop very few colorectal polyps. Genomic DNA sequence analysis of human COX-2 revealed a silent mutation in exon 3 that was evenly distributed between two classes of patients with AAPC, those with small or large numbers of colorectal polyps. We also found no difference in levels of COX-2 mRNA in transformed blood lymphocytes among AAPC patients of either class or patients with classical APC, and no alterations that correlated with a lesser or greater number of colorectal polyps were detectable within approximately the first 1 kb of the promoter sequence. Therefore, mutation of the human COX-2 gene does not appear to be responsible for a low tumor burden among AAPC subjects.

A comparison of calcium phosphate coprecipitation and electroporation. Implications for studies on the genetic effects of DNA damage.

Plasmid-based transfection assays provide a rapid means to measure homologous and nonhomologous recombination in mammalian cells. Often it is of interest to examine the stimulation of recombination by DNA damage induced by radiation, genotoxic chemicals, or nucleases. Transfection is frequently performed by using calcium phosphate coprecipitation (CPP), because this method is well suited for handling large sample sets, and it does not require expensive reagents or equipment. Alternative transfection methods include lipofection, microinjection, and electroporation. Since DNA strand breaks are known to stimulate both homologous and nonhomologous recombination, the induction of nonspecific damage during transfection would increase background recombination levels and thereby reduce the sensitivity of assays designed to detect the stimulation of recombination by experimentally induced DNA damage. In this article, we compare the stimulatory effects of nuclease-induced double-strand breaks (DSBs) on homologous and nonhomologous recombination for molecules transfected by CPP and by electroporation. Although electroporation yielded fewer transfectants, both nonhomologous and homologous recombination were stimulated by nuclease-induced DSBs to a greater degree than with CPP. Ionizing radiation is an effective agent for inducing DNA strand breaks, but previous studies using CPP generally showed little or no stimulation of homologous recombination among plasmids damaged with ionizing radiation. By contrast, we found clear dose-dependent enhancement of recombination with irradiated plasmids transfected using electroporation. Thus, electroporation provides a higher signal-to-noise ratio for transfection-based studies of damage-induced recombination, possibly reflecting less nonspecific damage to plasmid DNA during transfection of mammalian cells.

Three secretory phospholipase A(2) genes that map to human chromosome 1P35-36 are not mutated in individuals with attenuated adenomatous polyposis coli.

Mutation of Pla2g2a, a secretory phospholipase A(2) gene, dramatically increases the number of intestinal polyps that develop in the multiple intestinal neoplasia (Min) mouse, a murine model for adenomatous polyposis coli in humans. We tested the hypothesis that mutation of the human homologue(s) of this gene might be responsible for the more severe phenotype (hundreds of polyps) seen in a subset of individuals with attenuated adenomatous polyposis coli (AAPC). DNA sequence analysis demonstrated that alterations of PLA2G2A, as well as related genes PLA2G2C and PLA2G5, were evenly distributed between three classes of AAPC subjects: those with small, intermediate, and large numbers of adenomatous colonic polyps. Among 67 additional unrelated AAPC subjects, a stop mutation in PLA2G2C did not correlate with an increased burden of adenomatous polyps. Therefore, mutation of the human homologue(s) of murine Pla2g2a does not appear to be responsible for phenotypic variation among subjects with AAPC.

Alternatively spliced adenomatous polyposis coli (APC) gene transcripts that delete exons mutated in attenuated APC.

Reverse transcription-PCR combined with either (a) restriction enzyme digestion and repeat PCR or (b) ligase chain reaction has identified two new alternatively spliced transcripts of the adenomatous polyposis coli (APC) gene. In one of these transcripts exons 1-4 and the first 16 bases of exon 5 are deleted; in the other exons 2-4 and the first 16 bases of 5 are deleted. Both transcripts use an intraexonic splice acceptor in exon 5. These transcripts delete exons mutated in attenuated APC (3 and 4) and could account for the reduction in severity of this variant.