Detection of APC mutations in stool DNA of patients with colorectal cancer by HD-PCR

In most cases the analysis of DNA mutations in presence of a high excess of wild type DNA fail because of the low sensitivity of the performed method for mutation detection. Here we describe the new high-sensitive and non-radioactive HD-PCR method (for HeteroDuplex-PCR). In opposite to the conventional analytical application the heteroduplex technique is performed to preparatively separate mutated from non-mutated PCR amplified DNA fragments. We used the new method to detect mutations in the tumor suppressor gene APC in stool DNA from patients with sporadic colorectal carcinomas. Since the alteration of the APC gene occurs early in most colorectal tumors, the detection of APC mutations in fecal tumor DNA by HD-PCR may be a powerful tool in non-invasive cancer diagnostics.

Loss of the PLA2G2A gene in a sporadic colorectal tumor of a patient with a PLA2G2A germline mutation and absence of PLA2G2A germline alterations in patients with FAP

The Min (multiple intestinal neoplasia) mouse with a germline mutation in the adenomatous polyposis coli gene serves as an animal model for familial adenomatous polyposis coli (FAP). The number and age at onset of colorectal adenomas varies in the offspring of Min mice crossed with other strains. The murine gene for the secretory phospholipase A2 (PLA2G2A) was found to be the main candidate for these variations. To test the hypothesis of a correlation between PLA2G2A gene alterations and human tumor development, we screened 14 patients with FAP and 20 patients with sporadic colorectal cancer for germline and somatic PLA2G2A gene mutations. None of the individuals with FAP showed PLA2G2A germline alterations. However, a germline mutation was observed in one patient with an apparently sporadic colorectal tumor; the wildtype allele was somatically lost in the tumor of this patient.

DNA alterations in sporadic colorectal tumors do not correlate with tumor staging diagnosed by the TNM system

Blood normal and tumor tissue samples of 23 patients with sporadic colorectal tumors were screened for DNA alterations in the tumor relevant genes APC, K-ras, DCC and p53. Six different microsatellite regions were analyzed for instability by a new developed non-radioactive method. Somatic DNA alterations were found in 17 tumor samples: 13 carried single or multiple changes in single genes; six carried alterations in microsatellites; two tumors showed tumor suppressor gene mutations in addition to microsatellite changes. We found no indications of correlations between current genetic models of colorectal tumor progression and the established TNM system for histopathological tumor classification.

Biochemical and biological consequences of changing the specificity of p21ras from guanosine to xanthosine nucleotides

The D119N mutation of p21ras was prepared by site-directed mutagenesis. Its nucleotide binding properties were investigated using fluorescently labelled guanosine and xanthosine nucleotides. Its affinity for guanosine nucleotides is severely reduced, with a concomitant increase in the affinity for xanthosine nucleotides, which leads to an almost complete reversal of base specificity. The protein is a GTPase as well as a XTPase and the hydrolysis reaction can be efficiently stimulated by GAP. Dissociation of XDP from the mutant is stimulated by the guanine nucleotide exchange factor Cdc25Mm in a similar manner to that of GDP from wildtype. The interaction of the mutant with the effector domain of c-Raf kinase or Ral-GEF is normal. In microinjection experiments in PC12 and NIH3T3 cells the protein behaves as an oncogenic mutant due to its high dissociation rate for GDP. However, when the protein is loaded with XDP before microinjection the onset of the oncogenic signal can be efficiently retarded. Thus, the protein behaves initially as wildtype and later as an oncogenic protein.

A method for preparation of fecal DNA suitable for PCR

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