Although a role of p16 in this regulation has been presumed, there is no proof so far that loss of this tumor suppressor gene really affects E2F-mediated regulations.
Because of p18's structural and functional homology to p16 and p15, we hypothesized that it may also function as a tumor suppressor gene in some lymphoid malignancies.
Pancreatic carcinoma cells lines are known to have a high incidence of homozygous deletion of the candidate tumor suppressor gene p16 (MTS1/CDKN2), which resides in the chromosome 9p21 region.
Finally, because of the recently documented inverse relationship between expression of p16 and pRb protein in a variety of tumor cell lines, we analysed some of the p16-positive and negative melanoma cell lines for the presence of pRb protein.
This altered expression is not associated with frequent p16 mutation or gene loss, suggesting that alternative mechanisms of gene inactivation and/or altered regulation occur in the majority of these tumors.
Using Southern blotting analysis and multiplex PCR, aberrant methylation of the 5' CpG island of the p16 gene was found in a NPC xenograft (xeno-666) and 6 (22%) of 27 primary tumors, but not in normal tissues of the nasopharynx.
The p16 gene (MTS1 or CDK4I) encoding an inhibitor of cyclin-dependent kinase 4 (cdk4), has been reported to be deleted in various tumor cell lines, including lines derived from leukemic cells.
The p15 (MTS-2) gene, another candidate tumor suppressor gene located in the vicinity of the p16 gene, to which it shows structural and functional similarity, was also found to be deleted in 4 cases.
These data suggest that: (a) inactivation of the p16 tumor suppressor gene is a frequent event in squamous cell carcinomas of the head and neck; (b) p16 is inactivated by several distinct and exclusive events including homozygous deletion, point mutation, and promoter methylation; and (c) immunohistochemical analysis for expression of the p16 gene product is an accurate and relatively simple method for evaluating p16 gene inactivation.
Furthermore, we suggest that the p16 protein loss may contribute to the following: (1) early-stage hepatocarcinogenesis, because it was observed in 22% of early stage tumors; and (2) tumor progression, because it occurred approximately twice as often in advanced rather than in early stage tumors (40%).
We have demonstrated that the p16 gene is not deleted in sporadic, hereditary, malignant or benign forms of phaeochromocytomas, and therefore probably does not play a role in the pathogenesis of this tumour.
Given the co-existence of p16 abnormalities in primary tumours and cell lines observed in other tumour types, this high frequency of deletion suggests that p16 is a key tumour suppressor gene in the genesis of differentiated thyroid cancer.
These observations indicate that inactivating mutations or homozygous deletions of the p16 and p15 genes occur uncommonly, if ever, in parathyroid adenomas; however, loss of a different tumor suppressor gene (or genes) on 9p appears to contribute to the pathogenesis of a significant percentage of these tumors.
Because the MTAP gene is located adjacent to the tumor-suppressor gene p16 on chromosome 9p21 and more than 60% of T-cell ALL (T-ALL) patients have deletion in the p16 gene, we examined the status of the MTAP gene in T-ALL patients.
The frequencies of mutations in the adenomatous polyposis coli (APC). p53, and p16 (MTS1; multiple tumor suppressor 1/CDK4I; cyclin-dependent kinase 4 inhibitor) tumor suppressor genes were investigated in 23 oral squamous cell carcinomas (SCCs).
Our data point to p53 and p16 as gene targets of oral carcinogenesis, with chemicals in the betel quid possibly functioning in these tumors as carcinogens.
The tumor suppressors p16Ink4/CDKN2 and pRb were lost in 17 and 4 cases, respectively, due to various genetic mechanisms, including transcriptional block of p16 and nonsense mutations of RB1.
However, in some of the neoplasms with frequent 9p21 loss of heterozygosity (LOH), a structurally and functionally normal p16 is found suggesting that this gene might not be the primary or only target for inactivation.
A candidate tumor suppressor gene, p16INK4a, was mapped to this region and is frequently inactivated by several different mechanisms in many tumor types, including non-small cell lung cancer, but not in small cell lung cancer (SCLC). p16 functions as a cyclin/CDK inhibitor to prevent phosphorylation of pRB.
Analysis of p16 protein expression in 23 specimens revealed 1 tumor with abnormal staining, while Rb protein expression was determined to be normal in all 25 tumors tested.