GTMB 7 - Gene Therapy & Molecular Biology

Spencer and Davie: Dynamic histone acetylation and its involvement in transcription1999). The presence of acetylated histones along apromoter may increase the affinity of the SWI/SNFcomplex to this gene region. In support of this, SWI/SNFwas recruited to a promoter by a transactivator, however,its retention was enhanced when the histones along thisregion were acetylated (Hassan et al, 2001). Incubation ofthese nucleosomal arrays with SAGA and NuA4 increasedthis retention (Hassan et al, 2001). Furthermore, histoneacetyltransferases have been shown to increase the rate ofgene induction by accelerating ATP-dependent chromatinremodeling (Barbaric et al, 2001). The order ofrecruitment for chromatin-remodeling activities and thefunction of these complexes in gene activation orrepression is most likely gene-specific, and dependent onthe combination of transcription factors bound to thepromoter.IX. The effect of acetylation on nonhistoneproteinsHistone acetyltransferases can also acetylatetranscription factors (p53, ACTR, EKLF, estrogenreceptor, MyoD, GATA-1, E2F1), non-histonechromosomal proteins (HMG), components of thetranscription machinery (TFIIE, TFIIF), the nuclear importprotein importin, tubulin, and flap endonuclease-1 (Fen-1),an enzyme involved in DNA metabolism (Bannister et al,2000; Chen et al, 1999; Imhof et al, 1997; Munshi et al,1998; Hasan et al, 2001; Wang et al, 2001; Polesskaya etal, 2000; Herrera et al, 1999; Zhang and Bieker, 1998;Hung et al, 1999; L'Hernault and Rosenbaum, 1985;Martinez-Balbas et al, 2000). The acetylation of p53 andMyoD increases their binding affinity for DNA (Gu andRoeder, 1997; Polesskaya et al, 2000). As well, acetylationof E2F1 extends the half-life of this protein (Martinez-Balbas et al, 2000). Thus, along with modifying chromatinstructure, acetyltransferases may function in transcriptionby altering the DNA-binding properties of transcriptionfactors or enhancing the stability of transcription factors.The acetylation of HMGI(Y) plays an important rolein viral-induced interferon β gene activation as well as theinactivation of this event (Parekh and Maniatis, 1999).Upon infection, the enhanceosome assembles at theinterferon gene promoter with the help of HMGI(Y). Atthe same time, CBP and P/CAF are recruited to theinterferon β gene promoter where they acetylate H3 andH4 and, in combination with the enhanceosome, activatetranscription of the interferon β gene. Following induction,CBP acetylates HMGI(Y) which decreases its DNAbinding affinity and causes the disruption of theenhanceosome complex. In addition, p300 binds toestrogen receptor α in the absence of estrogen andacetylates lysine residues within the hinge/ligand bindingdomain of this receptor. This event suppresses thesensitivity of the receptor to ligand (Wang et al, 2001).The evidence from these studies suggests that the theory ofacetylation stimulating transcriptional activity is notalways true.Acetyltransferases may also function in otherbiological processes. The acetylation of flap endonuclease-1 by p300 reduces its ability to bind DNA, as well as itsnuclease activity, while acetylation of importin-alpha byCBP promotes its interaction with importin-beta in vitro(Hasan et al, 2001; Bannister et al, 2000). Furthermore, theacetylation of ACTR by another acetyltransferase suggeststhat acetylation may be a cascading event involved insignal transduction (Kouzarides, 2000; Marmorstein andRoth, 2001).X. Global versus targeted histoneacetylationNumerous studies have displayed an enrichment ofacetylated H3 and H4 along the promoter regions oftranscriptionally active genes. For example, activation ofthe human interferon gene induces H3 and H4hyperacetylation over 2-3 nucleosomes within thepromoter region (Parekh and Maniatis, 1999). Likewise,the yeast Gcn5 histone acetyltransferase complexacetylates histones only in the HO gene promoter (Krebset al, 1999). Hormone-mediated transcriptional activationalso involves the H3 and H4 hyperacetylation over thepromoter regions of hormone-responsive genes (Chen etal, 1999; Sewack G.F. et al, 2001). A similar scenariooccurs for histone deacetylation where the yeastSin3-Rpd3 histone deacetylase complex deacetylateshistones over a 1-2 nucleosome range within the promoterof a repressed gene (Kadosh and Struhl, 1998).In a recent study, the CpG island of thetranscriptionally active chicken carbonic anhydrase genewas associated with higher levels of acetylated histonescompared to the near-by promoter region (Myers et al,2001). The acetylation of H3 and H4 along this gene wasgreatest at the CpG island and showed a drastic drop atapproximately 1.5 kilobases into the transcribed region.Similarly, the chicken thymidine kinase gene displayedelevated levels of hyperacetylated histones along its CpGisland (Crane-Robinson et al, 1999). High levels ofhyperacetylated histones were also mapped to the chickenGAPDH promoter, which is located within a CpG island(Myers et al, 2001). The regions downstream of thispromoter that do not contain CpG islands displayed asharp drop in the levels of hyperacetylated H3 and H4. Aswell, chromatin fragments containing CpG islands areenriched in highly acetylated H3 and H4 isoforms (Taziand Bird, 1990). These findings suggest that histonehyperacetylation is a feature of CpG islands. In a recentstudy, acetylated histones were mapped to CpG islandslocated both within the promoter and regions downstreamfrom the transcription start site of a reporter gene (Cervoniand Szyf, 2001). The significance of histone acetylationalong CpG islands is not known. However, whenassociated with acetylated histones, a methylated DNAsequence will become demethylated (Cervoni and Szyf,2001). Because the interaction of demethylase with DNAis thought to be the limiting step in DNA demethylation,the acetylation of histones associated with CpG islandsmay increase the accessibility of demethylase to its targetDNA sequence (Cervoni and Szyf, 2001).However, histone hyperacetylation does not alwaysappear to be promoter- or CpG island-targeted. H46