Expression of Tumor Suppressor Gene p16^INK4 Products in Primary Gastric Cancer

Abstract

Recent studies have shown that the cyclin-dependent kinase (CDK) inhibitor p27^Kip1 represents an indicator for patients’ outcome in several human malignancies including gastric cancer. However, the clinicopathologic value of another class of CDK inhibitor, p16^INK4, has not been determined. In a retrospective study, we examined the expression of p16^INK4 by immunohistochemical assay of 80 samples of primary gastric cancers and their adjacent nonneoplastic mucosas. Less than 10% of non-tumor gastric mucosal cells were p16^INK4 positive, whereas the expression of p16^INK4 in gastric cancer cells varied widely from 0 to 100% (mean, 24.5%). The expression of p16^INK4 was not seen in 11.3% (9/80) of the cancer cases, but in 65% (52/80) this protein was even overexpressed when compared with the nonneoplastic mucosa. A clinicopathologic survey indicated that a low or no expression of p16^INK4 was associated with poorly differentiated carcinoma (p = 0.0133), but the level of expression did not correlate with other parameters including patients’ prognosis or with the expression of the pRb protein. In an effort to explore the underlying mechanism for the p16^INK4-negative cases, a prospective study was also performed on 20 cases of gastric cancer to compare the level of the p16^INK4 protein with the methylation status of the p16^INK4 promoter. Gastric cancer tissues with methylation expressed significantly lower levels of the p16^INK4 protein (p = 0.0013) and two of them lacked p16^INK4 expression altogether, whereas all the cancer tissues without methylation expressed it. These findings suggest that the p16^INK4 protein may be associated with differentiation of gastric cancer tissues and that methylation of the p16^INK4 promoter may, in part, account for the loss of p16^INK4 expression.

Differences in proximal (cardia) versus distal (antral) gastric carcinogenesis via retinoblastoma pathway

“…The following scenario is therefore likely: p16 is expressed with a low frequency in the normal state, which may be related to a relatively low proliferative status. In support of this is the limited expression in glandular (proliferative area) rather than surface epithelium (quiescent area) seen here and also noted by others [17,18] . In tumours there may be loss of negative feedback through decrease in pRb expression which causes p16 overexpression and other studies have also shown an inverse relationship between expression of the two molecules [9,10] .…”

“…However, a recent immunohistochemical study found a progressive p16 decrease and a pRb increase from gastritis to dysplasia [12] in samples from the cardia. This is surprising in view of the opposite trend in distal gastric samples seen here and also by others [17] . The reason for this discrepancy is uncertain.…”

“…Other studies on (distal) gastric carcinogenesis have shown a similar p16 'overexpression' in gastric carcinomas [16,17] with weak staining in non-involved mucosa. However, a recent immunohistochemical study found a progressive p16 decrease and a pRb increase from gastritis to dysplasia [12] in samples from the cardia.…”

Relationship between inactivation of p16 gene and gastric carcinoma

“…Inactivation of p16 gene correlates with hypermethylation of 5' promotor regions CpG island in GC. Some studies have found that hypermethylaion in the promotor regions CpG island is an important mechanism for p16 gene inactivation in GC, whereas deletion and point mutation of the gene are rarely seen [49][50][51][52][53][54][55][56] . The CpG island hypermethylation occurred early in multistep gastric carcinogenesis tends to accumulate along the process [55] .…”

Azacitidine induces demethylation of p16INK4a and inhibits growth in adult T-cell leukemia/lymphoma

“…MSP distinguishes unmethylated from methylated alleles in a given gene based on sequence changes after bisulfite treatment of DNA using primers designed for either methylated or unmethylated DNA (28). The CpGenome™ DNA Modification kit (Chemicon International, Inc., Temecula, CA) was used for MSP, as recommended by the supplier (29). Genomic DNA was treated with sodium bisulfite.…”