Aberrant expression of p16INK4a in human cancers - a new biomarker?

Abstract

The ARF and INK4a genes are located in the same CDKN2a locus, both showing its tumor suppressive activity. ARF has been shown to detect potentially harmful oncogenic signals, making incipient cancer cells undergo senescence or apoptosis. INK4a, on the other hand, responds to signals from aging in a variety of tissues including islets of Langerhans, neuronal cells, and cancer stem cells in general. It also detects oncogenic signals from incipient cancer cells to induce them senescent to prevent neoplastic transformation. Both of these genes are inactivated by gene deletion, promoter methylation, frame shift, and aberrant splicing although mutations changing the amino acid sequences affect only the latter. Recent studies indicated that polycomb gene products EZH2 and BMI1 repressed p16^INK4a expression in primary cells, but not in cells deficient for pRB protein function. It was also reported that that p14^ARF inhibits the stability of the p16^INK4a protein in human cancer cell lines and mouse embryonic fibroblasts through its interaction with regenerating islet-derived protein 3γ. Overexpression of INK4a is associated with better prognosis of cancer when it is associated with human papilloma virus infection. However, it has a worse prognostic value in other tumors since it is an indicator of pRB loss. The p16^INK4a tumor suppressive protein can thus be used as a biomarker to detect early stage cancer cells as well as advanced tumor cells with pRB inactivation since it is not expressed in normal cells.

Keywords: ARF; CDKN2a; DMTF1; PRC; RB; biomarker; cancer; expression; p16INK4a; prognosis.

Figure 1

The structure for the human p15^INK4b-p14^ARF-p16^INK4a locus. The genomic structure is well-conserved between human and mice, and thus gene knockout studies have been extensively conducted in mice. The distance between exon 1β and exon 1α is 19.4 kbp in humans and 12.4 kbp in mice. The exon 1α is 3.8 kbp upstream of exon 2 in humans; 5.2 kbp in mice. The ARF-INK4a (CDKN2a) locus is located 11.5 kbp apart from the genomic locus for CDKN2b that encodes for p15^INK4b in humans. All of p15^Ink4b, p19^Arf, and p16^Ink4a genes act as tumor suppressors as reported by Krimpenfort et al. (10, 28). The reverse triangles indicate the position of Dmp1-binding sites (mouse), which is different on the Arf genomic locus (red: −1060, −183, and +290 from 5′ end of Arf cDNA) from that on Ink4a (pink: −3482, −3390, −3292, −3171, −162 from 5′ end of Ink4a cDNA). The DMP1 consensus is located −2.3 kb and −0.31 kb of ARF and −4.04 kb and −1.40 kb of INK4a in humans. Both of these are Dmp1 target genes (Dmp1 reviewed in , –98) although the mode of regulation is different (36).

Figure 2

PRC1 and PRC2 repress transcription from the ARF-INK4a locus. Both the expression of p14^ARF and p16^INK4a are regulated by promoter hypermethylation through proteins of the polycomb repressor complex (PRC1) and PRC2 complexes (–47). The polycomb group of transcriptional repressor proteins was originally characterized in drosophila for maintenance proteins of pluripotency (47). PRC1 functions as a repressor of the maintenance complex while PRC2 functions as an initiator of transcriptional repression. Thus the methylation mediated by PRC2 is a prerequisite for the binding of PRC1 to the chromatin (48). The PRC1 complex is comprised of Polycomb (CXB2, 4, 6, 8), Bmi1, HPH (HPH1-3), and RING (RING1 and 2) proteins. The most important mechanism for transcriptional repression by the PRC1 complex involves mono-ubiquitination of histone H2A K119 by histone H2A ubiquitin ligase (48). The PRC2 complex is composed of three core proteins, EZH2 which mediates tri-methylation of histone H3K27, EED, SUZ12, and RbAp46 (49).