Incasesoffamilialretinoblastoma,onedefectivecopyoftheRB geneispresentinthegermline,sothatonlyoneadditionalsomaticmutationisneededtocompletelyeliminateRBfunction.
RBexertsanti-proliferativeeffectsbycontrollingtheG1-to-Stransitionofthecellcycle.Initsactiveform,RBishypophosphorylatedandbindstoE2Ftranscriptionfactors.ThisinteractionpreventstranscriptionofgeneslikecyclinEthatareneededforDNAreplication,andsothecellsarearrestedinG1.
GrowthfactorsignalingleadstocyclinDexpression,activationofcyclinD–CDK4/6complexes,inactivationofRBbyphosphorylation,andthusreleaseofE2F.
Lossofcellcyclecontrolisfundamentaltomalignanttransfor-mation.AlmostallcancershaveadisabledG1checkpointduetomutationofeitherRB orgenesthataffectRBfunction,suchascyclinD,CDK4,andCDKIs.
ManyoncogenicDNAviruses,likeHPV,encodeproteins(e.g.,E7)thatbindRBandrenderitnonfunctional.
TP53: Guardian of the Genome
The p53-encoding tumor suppressor gene, TP53, is the most commonly mutated gene in human cancer. The p53 protein is a transcription factor that thwarts neoplastic transformation by three interlocking mechanisms: activation of temporary cell cycle arrest (termed quiescence), induction of permanent cell cycle arrest (termed senescence), or triggering of programmed cell death (termed apoptosis). If RB is a “sensor” of external signals, p53 can be viewed as a central monitor of internal stress, directing the stressed cells toward one of these pathways.
A variety of stresses trigger the p53 response pathways, including anoxia, inappropriate pro-growth stimuli (e.g., unbridled MYC or RAS activity), and DNA damage. By managing the DNA damage response, p53 plays a central role in maintaining the integrity of the genome, as discussed next.
In nonstressed, healthy cells, p53 has a short half-life (20 minutes) because of its association with MDM2, a protein that targets p53 for destruction. When the cell is stressed, for example, by an assault on its DNA, “sensors” that include protein kinases such as ATM (ataxia telangiectasia mutated) are activated. These activated sensors catalyze posttranslational modifications in p53 that release it from MDM2, increasing its half-life and enhancing its ability to drive the transcription of target genes. Hundreds of genes whose transcription is triggered by p53 have been found. These genes suppress neoplastic transformation by three mechanisms: p53-mediated cell cycle arrest may be considered the primordial response to DNA damage (
Fig. 6.21 ). It occurs late in the G1 phase and is caused mainly by p53-dependent transcription of the CDKI gene CDKN1A (p21). The p21 protein inhibits cyclin–CDK complexes and prevents phosphorylation of RB, thereby arresting cells in the G1 phase. Such a pause in cell cycling is welcome, because it gives the cells “breathing time” to repair DNA damage. The p53 protein also induces expression of DNA damage repair genes. If DNA damage is repaired successfully, p53 upregulates transcription of MDM2, leading to its own destruction and relief of the cell cycle block. If the damage cannot be repaired, the cell may enter p53induced senescence or undergo p53-directed apoptosis.
p53-induced senescence is a form of permanent cell cycle arrest characterized by specific changes in morphology and gene expression that differentiate it from quiescence or reversible cell cycle arrest. Senescence requires activation of p53 and/or Rb and expression of their mediators, such as the CDKIs. The mechanisms of senescence are unclear but seem to involve global chromatin changes, which drastically and permanently alter gene expression.