These results suggest that p16INK4 in HCC is inactivated predominantly by posttranscriptional regulation rather than by genomic aberrations and lack of transcription.
On the other hand, genetic alterations of the cyclin D1 and p16INK4A genes were not so frequent, but appeared to be associated with the aggressive behavior of the tumor, which suggests that disruption of the cell cycle-related genes results in the progression of HCC.
Because the selective expression of p57KIP2 in liver, and because p16CDKN2/MTS1/INK4A has been found altered in many primary tumors, we undertook the present study to determine the presence of alterations in these genes in a group of hepatocellular carcinomas (HCC).
These results suggest that inactivation of the CDKN2 gene in HCC is a frequent event in which homozygous deletions are the most common mechanism of CDKN2 inactivation.
These genes were selected because: 1) CDKN2A and CDKN2B are very frequently inactivated in human cancers; 2) cyclin Ds are overexpressed in several tumors and 3) CDKN2A is posttranscriptionally silenced in hepatocellular carcinomas.
No methylated p16 sequences were detected in the peripheral plasma/serum of the six HCC cases without these changes in the tumor, in 38 patients with chronic hepatitis/cirrhosis, or in 10 healthy control subjects.
Our data also suggest that inactivation of the p16 and the p15 genes and the possibility of other unknown tumor-suppressor genes located on these defined deleted regions of chromosome 9 may be involved in the pathogenesis of HCC.
A strong correlation was found between the extent of methylation and the degree of expression of p16(INK4) in tumor tissues, indicating that epigenetic change due to extensive CpG methylation is the main cause of inactivation of p16(INK4) in hepatocellular carcinoma.
We analyzed 23 hepatocellular carcinomas from German patients for homozygous deletions of CDKN2 by coamplification with the human tyrosine hydroxylase (TH) gene and for CDKN2 mutations by PCR-single strand conformation polymorphism analysis and direct DNA sequencing.
These results suggest that the aberrant p16INK4A RNA transcript can be transcribed from the methylated p16INK4A gene, and endogenous reactivation of functional p16INK4A mRNA by a demethylating agent can restore the pRB pathway in HCC, and foster the terminal differentiation of the malignant cells.
Expression of p16 and p15 protein decreased in HCC when LOH occurred at locus D9S1752 (p15 gene locus) or at locus D9S1747 and D9S1748 (p16 gene is located between these 2 loci).
Our results suggest that methylation of the p16 promoter and the resulting loss of p16 protein expression are early events in a subset of hepatocarcinogenesis and that their detection is useful in the follow up of patients with a high risk of developing HCC, such as those with hepatitis B or C viral infections.