Prognosis value of aberrant p16^<INK4a> gene methylation in colorectal cancer

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Materials and MethodsThe subjects of our study comprised a series of 151 patients(mean age, 65.8 years ; range, 39-86) diagnosed and undergoing surgery for colorectal carcinoma. The median duration of follow-up was 79 months(range, 60-123). Slides stained with hematoxylin and eosin were examined, and pT, pN and pathological staging was completed according to the UICC/TNM classification. In addition, fresh tumor tissues and matched normal mucosa away from the tumor were currently obtained from 25 patients. Genomic DNA was extracted from formalin-fixed paraffin-embedded sections of 151 tumor and 10 normal tissues, with additional extraction from the frozen sections of 25 tumor tissues. Total RNA was extracted from the frozen sections of 25 tumor tissues and 4 matched normal mucosas. Bisulfite-modified DNA was amplified using specifically designed primers for the methylated and unmethylated p16 sequence. The real-time quantitative methylation specific PCR was performed, and the methylatio … More n index was calculated according to the equation : (concentration of methylated p16 sequence/concentrations of methylated plus unmethylated p16 sequence) X 100. Total RNA was employed to synthesize cDNA which was then used for real-time quantitative PCR. The primer pairs located in exons lα and 2 with flanking intron 1 of the p16 gene. Immunostaining was performed with monoclonal anti-p16(E6H4, Dako). In addition, image analysis was performed on p16 immunostained slides from randomly selected cases.Results Normal mucosa samples showed p16 methylation indices varying from 0 to 2%, and the indices of tumor samples represented 0 to 100%(median, 14.2% ; P=0.01) . Aberrant methylation of p16(methylation indices >2%) was detected in 100/151(66%) tumor samples. Loss of p16 expression(immunoreactivity <5%) was observed in 22/151(15%) cases. In image analyses, pl6-immunolabehng indices varied from 0 to 53% with no significant correlation to p16 methylation index. The expression level of p16 mRNA in normal tissues was consistently low whereas the value in tumor tissue was, in some cases, high level, which was statistically different(P=0.003). No significant correlation was observed between pl6 mRNA levels and methylation indiceswhereas p16 mRNA levels were positively correlated with p16-immunolabeling indices with statistically significant(P=0.043). Presence of p16 methylation was significantly associated with smaller tumor(P=0.048), but not with other clinicopathological features. In the Cox's regression model, present p16 methylation(P=0.007), high pT(P=0.007) and advanced tumor stage(P<0.001), proved to be independent predictors of short cancer-related survival. Cox's regression multivariate analysis also showed that p16 methylation(P<0.001), and advanced pN(P<0.001) and pT stages(P=0.014) predicted short recurrence-free survival. Less