p53 mutations and DNA ploidy in colorectal adenocarcinomas

The p53 tumour suppressor gene has an important role in the the maintenance of genome stability and its mutational inactivation may be at the origin of aneuploidy in cancer cells. The aim of this study was to determine whether p53 mutations were associated to DNA aneuploidy, as assessed by flow cytometry, in colorectal adenocarcinomas. Analysis of p53 mutations spectrum of the sorted nuclei was done by Denaturing Gradient Gel Electrophoresis (DGGE) and DNA sequencing. Overall, we studied 20 adenocarcinomas, the corresponding control mucosa, and 7 lymph node metastases. Five tumours (25%) were DNA diploid, while 15 tumours (75%) were composed of DNA aneuploid and diploid subpopulations. DNA diploid control mucosa and adenocarcinomas showed no p53 mutations, while 60% of the tumours with DNA aneuploidy had p53 mutations. Therefore, p53 mutations occurred significantly more often in DNA aneuploid than in DNA diploid tumours (p < 0.04, Fisher's exact test). Incidences of DNA aneuploidy and p53 mutations in lymph node metastases were 60 and 86%, respectively. In all tumours showing a p53 mutation, the wild-type allele was not or only bearly visible in DNA aneuploid cells suggesting that, in such cells, aneuploidy is accompanied by complete p53 functional inactivation. The present observations suggest that p53 mutations may have a role in the origin of aneuploidy at late stages of colorectal carcinogenesis.

Heterogeneous p53 mutations in a Burkitt lymphoma from an AIDS patient with monoclonal c-myc and VDJ rearrangements

This study investigates the timing of p53 mutations detected in the malignant cells of a Burkitt's lymphoma cell line (BRG-P) with respect to other maturation or transforming events. The BRG-P cell line, derived from an AIDS patient, was of special value since it displayed subclones that had undergone an isotype switch from IgM to IgA1 (BRG-M and BRG-A cells). BRG-M and BRG-A cells were characterized by the same monoclonal c-myc and VDJ rearrangements and by the expression of Ig receptors with specificity for a 45 kDa protein of human breast cells. Analysis of p53 mutations in the different BRG subclones showed that 1) BRG-M cells displayed 2 different p53 mutations in trans; since the original BL cells also showed the same mutations, this finding indicated that both occurred in vivo; 2) one of the p53 alleles of BRG-A cells was lost, while the other showed a mutation different from those seen in BRG-M cells; and 3) all 3 mutations observed in BRG-M or BRG-A cells resulted in the functional inactivation of the transcriptional activation function of p53. Together, our data demonstrate that p53 mutations were relatively late events during lymphomagenesis. Moreover, in view of the role of p53 in cell apoptosis, it is conceivable that BRG cells were subjected to a strong selective pressure that favored p53 inactivation. Such inactivation was possibly required to counterbalance other potentially apoptotic events, including the presence of a deregulated c-myc oncogene and signals delivered by the host environment in situ.

Lack of mutations in K-ras codons 12 and 13 in human atherosclerotic lesions

In the framework of a project investigating the possible involvement of cancer biomarkers in human atherogenesis, we evaluated the occurrence of K-ras mutations in the DNA extracted from smooth muscle cells of abdominal aorta atherosclerotic lesions. The molecular analysis of the DNA from 32 surgical specimens, using PCR-based denaturing gradient gel electrophoresis (DGGE), did not reveal any variant in K-ras codons 12 and 13, which are the most frequently involved codons among the ras genes mutated in various types of human tumors. Analysis of the DNA extracted from four cell lines carrying known K-ras mutational alleles showed typically positive DGGE patterns. Thus, on the whole, the conclusions of this study and of previous studies using the same biological material are consistent with the occurrence of DNA adducts in human atherosclerotic lesions but in the absence of p53 involvement or of K-ras mutations in codons 12 and 13. The search for candidate genes which may possibly be involved in the atherogenetic process warrants further studies.

p53 is frequently mutated in Barrett's metaplasia of the intestinal type

Barrett's Esophagus (BE) is a complication of gastroesophageal reflux in which the normal squamous epithelium of the lower esophagus is replaced by metaplastic tissue. The clinical significance of this condition is the associated predisposition to adenocarcinomas (ADCs). Three types of BE have been characterized: the gastric fundic (F) type, the gastric cardial (C) type, and the intestinal (I) type. The latter is the most closely associated with the development of ADCs; the causes of this bias remain unknown. To determine whether p53 and/or K-ras gene alterations (a) are present in preneoplastic lesions and (b) are associated with a specific histotype, we performed PCR-based denaturing gradient gel electrophoresis (DGGE) analysis of exon 1 (codons 12-13) of K-ras gene and of exons 5-8 of the p53 gene in biopsies obtained from 30 patients with BE of the I type (9 patients), combined I type (I + C +/- F; 10 patients) and non-I type (C, F, or C + F; 11 patients). None of the cases under study revealed K-ras mutations, whereas biopsies from 12 patients showed at least one p53 DGGE variant. Four patients showed the exact same variants in leukocytes also (polymorphisms), whereas eight cases revealed specific DGGE variants only in biopsies. The molecular characterization of these variants revealed that four of them showed a single base pair substitution, and four showed multiple mutations. Of 17 somatic mutations, all but 1 were base pair substitutions located mainly in exons 7 and 8. The majority of these mutations were GC targeted (13 of 16; 81%), 54% (7 of 13) of which were transitions occurring at CpG sites. All somatic mutations were found in BE with at least one I component. The association with the histotype was statistically significant (P < 0.03; pure I type versus non-I type; P < 0.04, combined I type versus non-I type; Fisher's exact test). Loss of heterozygosity in the vicinity of the p53 locus was evaluated by PCR using a highly polymorphic variable number of tandem repeats marker on 25 out of 30 cases. Ninety-two % of the cases analyzed were informative, and none of them showed LOH. In conclusion, we showed that p53 mutations are frequently observed in specimens from BE patients of the I-type, whereas no involvement of K-ras (exon 1) mutational activation was observed. In light of the key roles that the p53 protein plays in controlling cell cycle and cell diploidy, this result may suggest why this type of metaplasia is the most closely associated to the development of ADCs.