The INK4A/ARF locus and its two gene products

https://doi.org/10.1016/S0959-437X(99)80004-5Get rights and content

Abstract

The INK4A/ARF locus on chromosome 9 is one of the sites mutated most frequently in human cancer. Two genes comprising overlapping reading frames encoding p16INK4a and p19ARF have been discovered at this locus and, remarkably, both play an important role in regulating cell growth, survival and senescence.

Introduction

Complex metazoans with long lifespans must maintain the ability to repopulate depleted or damaged tissues by continuous cell division throughout their existence but, at the same time, must regulate this cell division tightly to prevent unrestrained growth (i.e. cancer). Recent research has demonstrated two pathways that are critical in this regard: the retinoblastoma protein (pRB) pathway (Figure 1a) regulates passage through the cell cycle whereas the p53 pathway (Figure 1b) induces growth arrest or apoptosis in response to either DNA damage or inappropriate mitogenic signaling (reviewed by Sherr [1] and Levine [2]). It has become clear in the last several years that both of these pathways are perturbed in the majority of human cancers and that these pathways are intimately linked. Our recent understanding of the INK4A/ARF tumor suppressor locus and the two proteins it encodes, p16INK4a and p19ARF, has provided insight into the coordinate regulation of these pathways but this new knowledge has generated more questions than answers. In this review, we examine our present understanding of p16INK4a and p19ARF, reanalyze their role in neoplasia, and discuss future directions for research.

Section snippets

Unusual genomic structure

The short arm of chromosome 9 is frequently mutated in many human cancers. p16INK4a, identified through its ability to inhibit cyclin-dependent kinase 4 (cdk4) [3] was shown to reside at 9p21, along with a highly homologous gene p15INK4b 4, 5. Frequent point mutations of p16INK4a, but not p15INK4b, were found in melanoma-prone kindreds, and homozygous deletions of the locus were noted in a variety of tumor cell lines [4]. Shortly after, a second transcript was identified at the INK4A/ARF locus 6

p16INK4a: a bona fide tumor-suppressor gene

With the appreciation that three distinct proteins are encoded by a single region so commonly deleted in primary human tumors comes the question of which protein(s) — p16INK4a, p19ARF, p15INK4b or some combination thereof — confers the tumor-suppressor activity of 9p21. A growing body of evidence suggests that the answer is at least both p16INK4a and p19ARF, with their relative importance perhaps dictated by species and cell type specific factors. Although mutations of p15INK4b have been found

p19ARF: a critical component of the p53 pathway

As stated, p19ARF was shown to induce cell-cycle arrest, without evidence of direct inhibition of known cdks [9]. The capacity of ARF to arrest cell-cycle progression and block myc/ras transformation was subsequently shown to be p53-dependent as cells genetically (by knockout) or functionally (by SV40 T antigen expression) deficient for p53 are refractory to ARF inhibition 30••, 31••, 32••, 33•, 34•. ARF was, however, able to prevent mdm2/ras transformation [32••], which seems inconsistent with

What is the relative importance of the two gene products?

Mice generated through targeted disruption of exons 2 and 3 of INK4A/ARF are viable but highly tumor prone [53]. The majority of homozygous null animals develop sarcomas or lymphomas, generally within 9 months of life. These animals are also highly sensitive to carcinogens. Mouse embryo fibroblasts (MEFs) from null animals grow more rapidly and are more readily immortalized than MEFs from wild-type or heterozygous animals. Furthermore, null MEF’s are transformed by oncogenic H-ras alone, in

Unanswered questions

Although the appreciation that the INK4A/ARF locus encodes two proteins may partially explain the higher rate of mutation of this locus compared to other cdk inhibitors, and also elucidates the complex regulation of p53 function, several mysteries remain. An obvious question raised by the ARF null mice is what is the phenotype of p16INK4a-only knockout mice? It is also not clear if there are ARF functions distinct from the stabilization of p53. The modest overlap of p19ARF and p53 mutations

Conclusions

To summarize again, the INK4A/ARF locus encodes two gene products, p16INK4a and p19ARF, both of which play an important role in regulating cell growth and senescence. p16INK4a is an integral component of the pRB pathway, and germline mutations of p16INK4a are associated with familial melanoma and pancreatic adenocarcinoma. p19ARF functions to sense oncogenic mitogenic stimuli, stabilize p53, and facilitate p53-mediated arrest or apoptosis in cells undergoing unscheduled proliferation. Targeted

Acknowledgements

We would like to thank M Serrano and C Sherr for communicating unpublished data. We would like to thank L Chin, S Grossman, W Kaelin, B Rollins, and N Schreiber-Agus for critical reading of the manuscript. N Sharpless is a Howard Hughes Physician Postdoctoral Fellow. R DePinho is an American Cancer Society Research Professor and is supported by National Institutes of Health grants RO1 HD28317 and RO1 EY11267.

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

References (81)

  • A Patino-Garcia et al.

    Analysis of the p16INK4 and TP53 tumor suppressor genes in bone sarcoma pediatric patients

    Cancer Genet Cytogenet

    (1997)
  • ML Avantaggiati et al.

    Recruitment of p300/CBP in p53-dependent signal pathways

    Cell

    (1997)
  • W Gu et al.

    Activation of p53 sequence-specific DNA binding by acetylation of the p53 C-terminal domain

    Cell

    (1997)
  • SR Grossman et al.

    p300/MDM2 complexes participate in MDM2-mediated p53 degradation

    Mol Cell

    (1998)
  • CJ Sherr

    Cancer cell cycles

    Science

    (1996)
  • M Serrano et al.

    A new regulatory motif in cell-cycle control causing specific inhibition of cyclin D/CDK4

    Nature

    (1993)
  • A Kamb et al.

    A cell cycle regulator potentially involved in genesis of many tumor types

    Science

    (1994)
  • GJ Hannon et al.

    p15INK4b is a potential effector of cell cycle arrest mediated by TGF-β

    Nature

    (1994)
  • S Stone et al.

    Complex structure and regulation of the p16(MTS1) locus

    Cancer Res

    (1995)
  • L Mao et al.

    A novel p16INK4a transcript

    Cancer Res

    (1995)
  • D Duro et al.

    A new type of p16INK4/MTS1 gene transcript expressed in B-cell malignancies

    Oncogene

    (1995)
  • DE Quelle et al.

    Alternative reading frames of the INK4a tumor suppressor gene encode two unrelated proteins capable of inducing cell cycle arrest

    Cell

    (1995)
  • HG Drexler

    Review of alterations of the cyclin-dependent kinase inhibitor INK4 family genes p15, p16, p18 and p19 in human leukemia-lymphoma cells

    Leukemia

    (1998)
  • JF Flores et al.

    Loss of the p16INK4a and p15INK4b genes, as well as neighboring 9p21 markers, in sporadic melanoma

    Cancer Res

    (1996)
  • CJ Hussussian et al.

    Germline p16 mutations in familial melanoma

    Nat Genet

    (1994)
  • A Kamb et al.

    Analysis of the p16 gene (CDKN2) as a candidate for the chromosome 9p melanoma susceptibility locus

    Nat Genet

    (1994)
  • AM Goldstein et al.

    Increased risk of pancreatic cancer in melanoma-prone kindreds with p16INK4 mutations

    N Engl J Med

    (1995)
  • EA Holland et al.

    Analysis of the p16 gene, CDKN2, in 17 Australian melanoma kindreds

    Oncogene

    (1995)
  • MG FitzGerald et al.

    Prevalence of germ-line mutations in p16, p19ARF, and CDK4 in familial melanoma: analysis of a clinic-based population

    Proc Natl Acad Sci USA

    (1996)
  • JF Flores et al.

    Analysis of the CDKN2A, CDKN2B and CDK4 genes in 48 Australian melanoma kindreds

    Oncogene

    (1997)
  • T Wolfel et al.

    A p16INK4a-insensitive CDK4 mutant targeted by cytolytic T lymphocytes in a human melanoma

    Science

    (1995)
  • L Zuo et al.

    Germline mutations in the p16INK4a binding domain of CDK4 in familial melanoma

    Nat Genet

    (1996)
  • K Guan et al.

    Growth suppression by p18, a p16INK4/MTS1- and p14INK4/MTS2-related CDK6 inhibitor, correlates with wild-type pRb function

    Genes Dev

    (1994)
  • M Serrano et al.

    Inhibition of ras-induced proliferation and cellular transformation by p16INK4

    Science

    (1995)
  • J Koh et al.

    Tumour-derived p16 alleles encoding proteins defective in cell-cycle inhibition

    Nature

    (1995)
  • N Dyson

    The regulation of E2F by pRB-family proteins

    Genes Dev

    (1998)
  • AA Russo et al.

    Structural basis for inhibition of the cyclin-dependent kinase Cdk6 by the tumour suppressor p16INK4a

    Nature

    (1998)
  • J Zhu et al.

    Senescence of human fibroblasts induced by oncogenic Raf

    Genes Dev

    (1998)
  • AW Lin et al.

    Premature senescence involving p53 and p16 is activated in response to constitutive MEK/MAPK mitogenic signaling

    Genes Dev

    (1998)
  • T Kamijo et al.

    Functional and physical interactions of the ARF tumor suppressor with p53 and Mdm2

    Proc Natl Acad Sci USA

    (1998)
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