CDKN2A, also known ascyclin-dependent kinase inhibitor 2A, is agene which in humans is located atchromosome 9, band p21.3.[5] It is ubiquitously expressed in many tissues and cell types.[6] The gene codes for twoproteins, including theINK4 family memberp16 (or p16INK4a) andp14arf.[7] Both act astumor suppressors by regulating thecell cycle. p16 inhibits cyclin dependent kinases 4 and 6 (CDK4 andCDK6) and thereby activates theretinoblastoma (Rb) family of proteins, which block traversal fromG1 toS-phase. p14ARF (known as p19ARF in the mouse) activates thep53 tumor suppressor. Somatic mutations ofCDKN2A are common in the majority of human cancers, with estimates thatCDKN2A is the second most commonly inactivated gene in cancer afterp53.Germline mutations ofCDKN2A are associated with familialmelanoma,glioblastoma andpancreatic cancer.[8] TheCDKN2A gene also contains one of 27SNPs associated with increased risk ofcoronary artery disease.[9]
TheCDKN2A gene resides on chromosome 9 at the band 9p21 and contains 8exons.[10] This gene encodes two proteins,p16 andp14ARF, which are transcribed from the same second and third exons but alternative first exons: p16 from exon 1α and ARF from exon 1β. As a result, they are translated from differentreading frames and therefore possess completely differentamino acid sequences.[11] In addition to p16 and ARF, this gene produces 4 otherisoforms throughalternative splicing.[12]
This protein belongs to the CDKN2cyclin-dependent kinase inhibitor family.[12] p16 comprises fourankyrin repeats, each spanning a length of 33 amino acid residues and, in thetertiary structure, forming ahelix-turn-helix motif. One exception is the second ankyrin repeat, which contains only one helical turn. These four motifs are connected by three loops such that they are oriented perpendicular to the helical axes.
According to itssolvent-accessible surface representation, p16 features clustered charged groups on its surface and a pocket located on the right side with anegatively charged left inner wall and apositively charged right inner wall.[13]
P14ARF is a central actor of the cell cycle regulation process as it participates to the ARF-MDM2-p53 pathway and the Rb-E2F-1 pathway.[15] It is the physiological inhibitor of MDM2, an E3 ubiquitin ligase controlling the activity and stability of P53, and loss of P14ARF activity may have a similar effect as loss of P53.[16] P14ARF induces cell cycle arrest inG2 phase and subsequentapoptosis in a P53-dependent and P53-independent manner, and thus is regarded as a tumor suppressor.[17][18][19][20] In addition, P14ARF could down-regulate E2F-dependent transcription and plays a role in the control of the G1 to Sphase transition as well.[21]
P16 interacts with Rb and controls the G1 to S transition. It binds toCDK4/6 inhibiting its kinase activity and prevents Rb phosphorylation. Therefore, Rb remains associated with transcription factor E2F1, preventing transcription of E2F1 target genes which are crucial for the G1/S transition. During this process, a feedback loop exists between P16 and Rb, and P16 expression is controlled by Rb.[22][23] P16/Rb pathway collaborates with the mitogenic signaling cascade for the induction ofreactive oxygen species, which activates theprotein kinase C delta, leading to an irreversible cell cycle arrest. Thus P16 participates not only in the initiation but also in the maintenance of cellular senescence, as well in tumor suppression.[24][25] On the other hand, some specific tumors harbor high levels of P16, and its function in limitation of tumorigenic progression has been inactivated via the loss of Rb.[25][26]
In human cancer cell lines derived from various tumor types, a high frequency of genetic andepigenetic alterations (e.g.,promoterhyper-methylation,homozygous deletion or mutation) in theCDKN2A gene has been observed. Accordingly, epigenetic/genetic modulation of changes inCDKN2A might be a promising strategy for prevention or therapy of cancer.
TheCDKN2A gene is located on the chromosome 9p21 locus, which is intriguing for several reasons. First, this region is well known in cancer genetics as one of the most common sites of deletions leading to hereditary forms of cutaneous melanoma.[11][27] Second, genome wide association studies have reported a significant association of chromosome 9p21 with coronary artery disease and myocardial infarction[28] as well as the progression of atherosclerosis.[29]
CDKN2A is made up of four sections of exons – exon 1β, exon 1α, exon 2, and exon 3. These exons are used to create two proteins named p16 and p14ARF. Protein p16, created by exon 1α and exon 2, is responsible for tumor creation of genetic melanoma. When working normally, p16 binds to the cyclin dependent kinases CDK4 to inhibit their ability to create tumors, but when inactivated the suppression no longer occurs.[42] When a mutation occurs in protein p16, it prevents the protein kinase of CDK4, which results in the inactivation of the tumor suppressor gene.[42] This starts the development ofmelanoma.
Melanoma only occurs in a small proportion of the population. If only two family members have melanoma, there is a 5% chance somebody in the next generation will acquire the mutated gene. Also, there is a 20-40% chance of getting hereditary melanoma in a family if 3 or more people in the past generation had melanoma. For those who carry the hereditary mutated geneCDKN2A, acquiring skin cancer is a lot easier.[43] Those who have the gene are far more likely to get melanoma a second or third time compared to those who don't genetically have this gene.[44] The population that is affected by this mutation has a high familial history of melanoma or atypical moles and birth marks in large numbers, a history of primary melanoma/cancers in general,immunosuppression, skin that burns easily and doesn't tan, freckling, blue eyes, red hair, or a history of blistering.[43] People with these high risk factors are more likely to carry inherited mutations inCDKN2A.[44] For those who have a gene mutation, the severity is also dependent on the environmental surroundings. Out of those who carry the gene, those who express the phenotype and actually developed melanoma have a history of more sun exposure, and light skin compared to those who also had the gene but never actually developed melanoma.[44] This suggests that this gene co-works with ones surrounding environment. If two individuals are selected who carry theCDKN2A mutation, and both genetically have the same probability of acquiring skin cancer, but one is from Australia and the other is from Europe, there is a 58% the European will acquire cancer compared to a 91% chance the Australian will get it.[44] This is because the factors mentioned earlier pertaining to those who are more susceptible to the disease and also dependent on the amount of sunscreen one wears and the UV radiation potency in their environment.[43]
A multi-locus genetic risk score study based on a combination of 27 loci, including theCDKN2A gene, identified individuals at increased risk for both incident and recurrent coronary artery disease events, as well as an enhanced clinical benefit from statin therapy. The study was based on a community cohort study (the Malmo Diet and Cancer study) and four additional randomized controlled trials of primary prevention cohorts (JUPITER and ASCOT) and secondary prevention cohorts (CARE and PROVE IT-TIMI 22).[9]
Activation of theCDKN2A locus promotes thecellular senescence tumor suppressor mechanism, which is a permanent form of growth arrest. As senescent cells accumulate with aging, expression ofCDKN2A increases exponentially with aging in all mammalian species tested to date, and has been argued to serve as a biomarker of physiological age.[45] Notably, a recent survey of cellular senescence induced by multiple treatments to several cell lines does not identifyCDKN2A as belonging to a "core signature" of senescence markers.[46]
^Karayan L, Riou JF, Séité P, Migeon J, Cantereau A, Larsen CJ (February 2001). "Human ARF protein interacts with topoisomerase I and stimulates its activity".Oncogene.20 (7):836–48.doi:10.1038/sj.onc.1204170.PMID11314011.S2CID2801461.
^Kanellou P, Zaravinos A, Zioga M, Spandidos DA (June 2009). "Deregulation of the tumour suppressor genes p14(ARF), p15(INK4b), p16(INK4a) and p53 in basal cell carcinoma".The British Journal of Dermatology.160 (6):1215–21.doi:10.1111/j.1365-2133.2009.09079.x.PMID19298278.S2CID29291218.
^Huang Y, Tyler T, Saadatmandi N, Lee C, Borgstrom P, Gjerset RA (July 2003). "Enhanced tumor suppression by a p14ARF/p53 bicistronic adenovirus through increased p53 protein translation and stability".Cancer Research.63 (13):3646–53.PMID12839954.
^Li Y, Nichols MA, Shay JW, Xiong Y (December 1994). "Transcriptional repression of the D-type cyclin-dependent kinase inhibitor p16 by the retinoblastoma susceptibility gene product pRb".Cancer Research.54 (23):6078–82.PMID7954450.
^Takahashi A, Ohtani N, Yamakoshi K, Iida S, Tahara H, Nakayama K, Nakayama KI, Ide T, Saya H, Hara E (November 2006). "Mitogenic signalling and the p16INK4a-Rb pathway cooperate to enforce irreversible cellular senescence".Nature Cell Biology.8 (11):1291–7.doi:10.1038/ncb1491.PMID17028578.S2CID8686894.
^Ye S, Willeit J, Kronenberg F, Xu Q, Kiechl S (July 2008). "Association of genetic variation on chromosome 9p21 with susceptibility and progression of atherosclerosis: a population-based, prospective study".Journal of the American College of Cardiology.52 (5):378–84.doi:10.1016/j.jacc.2007.11.087.PMID18652946.
^Huang Q, Su X, Ai L, Li M, Fan CY, Weiss LM (October 2007). "Promoter hypermethylation of multiple genes in gastric lymphoma".Leukemia & Lymphoma.48 (10):1988–96.doi:10.1080/10428190701573224.PMID17852707.S2CID72186314.
^Robaina MC, Faccion RS, Arruda VO, de Rezende LM, Vasconcelos GM, Apa AG, Bacchi CE, Klumb CE (February 2015). "Quantitative analysis ofCDKN2A methylation, mRNA, and p16(INK4a) protein expression in children and adolescents with Burkitt lymphoma: biological and clinical implications".Leukemia Research.39 (2):248–56.doi:10.1016/j.leukres.2014.11.023.PMID25542698.
^Qureshi MA, Jan N, Dar NA, Hussain M, Andrabi KI (September 2012). "A novel p16(INK4A) mutation associated with esophageal squamous cell carcinoma in a high risk population".Biomarkers.17 (6):552–6.doi:10.3109/1354750X.2012.699556.PMID22724384.S2CID19678492.
^He D, Zhang YW, Zhang NN, Zhou L, Chen JN, Jiang Y, Shao CK (April 2015). "Aberrant gene promoter methylation of p16, FHIT, CRBP1, WWOX, and DLC-1 in Epstein-Barr virus-associated gastric carcinomas".Medical Oncology.32 (4) 92.doi:10.1007/s12032-015-0525-y.PMID25720522.S2CID38800637.
^Bhagat R, Kumar SS, Vaderhobli S, Premalata CS, Pallavi VR, Ramesh G, Krishnamoorthy L (September 2014). "Epigenetic alteration of p16 and retinoic acid receptor beta genes in the development of epithelial ovarian carcinoma".Tumour Biology.35 (9):9069–78.doi:10.1007/s13277-014-2136-1.PMID24913706.S2CID1766337.
^abTsao H, Niendorf K (November 2004). "Genetic testing in hereditary melanoma".Journal of the American Academy of Dermatology.51 (5):803–8.doi:10.1016/j.jaad.2004.04.045.PMID15523363.
^abcKefford R, Bishop JN, Tucker M, Bressac-de Paillerets B, Bianchi-Scarrá G, Bergman W, Goldstein A, Puig S, Mackie R, Elder D, Hansson J, Hayward N, Hogg D, Olsson H (November 2002). "Genetic testing for melanoma".The Lancet. Oncology.3 (11):653–4.doi:10.1016/s1470-2045(02)00894-x.PMID12424065.
