P53 mutations in colorectal cancer assessed in both genomic DNA and cDNA as compared to the presence of p53 LOH

Mutations in p53 is a most common genetic alteration in human cancer and has been related to outcome, also in colorectal cancer. However, published results are not unanimous on this point without clear-cut explanations. Different analytical methods have been applied which could explain some discrepancies. Another factor of importance may be the source of nucleic acids used for identification of sequence alterations. Therefore we compared mutations in exons 7 and 8 of p53 found in genomic DNA, with mutations in cDNA from the same patients and evaluated to what extent LOH and mutations in p53 occurred simultaneously, which are prerequisites for a complete loss of p53 function according to the classic concept. cDNA and genomic DNA from tumor tissue from 123 patients were used. Thirty-four mutations were found in both tumor cDNA and genomic DNA. Twenty missense mutations were the same in both cDNA and genomic DNA. Two missense mutations, one 1 bp deletion and one nonsense mutation were only found in genomic DNA, while nine missense mutations and one 9 bp deletion were found in cDNA only. Statistical analysis showed no significant difference among mutations in cDNA and genomic DNA (p=1.00). Only 2 patients (5%) had both LOH and mutations in exons 2-11 of p53. One patient had LOH without p53 mutation, while 18 patients (44%) showed p53 mutation without LOH and 20 patients (49%) had wt p53 without LOH. Forty-four patients (52%) were non-informative for LOH of the entire cohort consisting of 85 patients. Our findings suggest that different results reported on the role mutated p53 may have in colorectal cancer are probably not explained by the source of nucleic acids (RNA vs DNA) for sequence determinations. Rather, unknown information about simultaneous but different presence of p53 LOH in previously published reports is more likely. However, an unexpectedly infrequent occurrence (5-10%) of complete and isolated p53 ablation (LOH + mutation) would demand considerably larger patient populations (1500-2000 patients) than ever published for relating p53 function to survival in patients with colorectal cancer. Therefore, the role of p53 dysfunction in progression of colorectal cancer remains uncertain.