Abstract
Hox genes play key roles in axial patterning and regulating the regional identity of cells and tissues in a wide variety of animals from invertebrates to vertebrates. Nested domains of Hox expression generate a combinatorial code that provides a molecular framework for specifying the properties of tissues along the A–P axis. Hence, it is important to understand the regulatory mechanisms that coordinately control the precise patterns of the transcription of clustered Hox genes required for their roles in development. New insights are emerging about the dynamics and molecular mechanisms governing transcriptional regulation, and there is interest in understanding how these may play a role in contributing to the regulation of the expression of the clustered Hox genes. In this review, we summarize some of the recent findings, ideas and emerging mechanisms underlying the regulation of transcription in general and consider how they may be relevant to understanding the transcriptional regulation of Hox genes.
Highlights
Animals display remarkable variety in their body plans and there is great interest in understanding the degree to which conserved and distinct mechanisms underlie this diversity in the formation and elaboration of basic body plans in animal evolution
There is emerging evidence for a deeply conserved regulatory network, involving transcription factors (TFs) and signaling pathways, that governs patterning along the anterior–posterior (A–P) body axis [1,2,3,4,5,6]
Despite very different morphologies among chordates, many key TFs and components of major signaling pathways (e.g., Wnts and FGFs), known to regulate developmental processes, have been shown to be aligned along the A–P axis. This suggests that regulatory interactions between signaling pathways and core TFs set up a conserved gene regulatory network (GRN) that guides the formation of the basic body plan and patterning of the A–P axis
Summary
Animals display remarkable variety in their body plans and there is great interest in understanding the degree to which conserved and distinct mechanisms underlie this diversity in the formation and elaboration of basic body plans in animal evolution. Genes in the four mammalian Hox clusters are all transcribed in the same 5 to 3 direction with respect to transcription, and the order of Hox genes in each cluster on a chromosome corelates with their temporal and spatial expression domains and functions along the A–P axis of developing embryos (Figure 1) These nested domains of expression generate a combinatorial Hox code, which provides a molecular framework that serves as a key regulatory step in specifying regional identities and properties of tissues. It will be interesting and important to understand how this newly emerging picture of the dynamic molecular mechanisms governing transcription plays a role in modulating the inputs controlling the coordinated expression of the clustered Hox genes. Enhancers serve to stimulate transcription by integrating a variety of different regulatory inputs and binding sites for TFs to confer precise temporal, spatial and cell-type specific gene expression programs. TThheessee qquueessttiioonnss aarree rreelleevvaanntt ttoo uunnddeerrssttaannddiinnggtthheerreegguullaattiioonnoofftthheeHHooxxcclluusstteerrssbbee‐ccaauussee ooff tthheehhiigghhggeenneeddeennssitiytyaannddcocmomppacatcnt antautrueroefotfhtehcelucslutesrtse.rTs.hTeheenhenanhcaenrcseerms ebmedbdeedd‐ dweidthwinitahnidn flaanndkfilnagnkaninigndaniviindduiavliHduoxalclHuosxtecrlcuasntedricsapnladyisseplleacytisveelepcrteifveerepnrceefse,rceonmcepse, tciotimon‐ pbeettiwtieoennbgeetwneesenangdenceasnarnedgucalanterebgoutlhatneebaortahdnjaecaernatdgjaecneenst ogrenaecst moroarcet gmloobrealglyloobnalolythoenr ogtehneersginentehse icnomthpelecxo.mFpolrexe.xFamorpelex,aimnpthlee, minotuhsee mHooxubsecoHmoxpblecxo, mthpelreexa, rtehethrereaerRe AthRrEees RinAtRhEesminidtdhelemoifdtdhlee colfutshteerc, ltuwstoeru, ptwstoreuapmstarenadmoanneddoonwendsotrweanmstroefamHooxfbH4 o(Fxbig4u(rFeig2uAre), 2wAh)i,cwh hpiacrhtipciapratitceipinatme eindimateindgiaittisnrgeistpsornesspeotonsReAtobRyArebgyulraetginuglamtinugltimplueltciopdleincgodainndglaonndg lnoonng-cnoodni‐ncgod(linncgR(NlnAcRs)NtArasn) stcrrainpstcsri[p33ts,3[53,33,73,58,13]7. ,8O1]n.eOonfethoef steheRsAe RREAsR(EDsE(-DREA‐RREA)RiEs)ains aensseesnsteianlticaisl ceilsemeleemnteonftaonf aRnAR-dAe‐pdeenpdeenndteenntheannhcaenr,cwerh, iwchhiucnhduenrgdoeergsoeepsigeepnigeteincemtiocdmifiocda‐itfiiocnasti,oannsd, ains dreiqsurierqeudirteodcotoorcdoionradtientahte tghleobgalol breagl urelgatuiolantiofnHoof xHbogxebngeesniens hinemheamtoaptoipeotic‐ estiecmsstecmelslsce[3ll4s].[3T4h].isTfhuins cfutinonctaiol nroallerfoolre afonreannheanhcearnrcaeirsreasimseasnmyaqnuyeqstuioesntsiornegs arredgianrgditnhge tmhecmhaenchisamnsistmhrsotuhgrhouwghicwhhtihcehDthEe-RDAER‐REApRarEticpiapratticeispianterseginularetignuglastoinmgasnoymtranysctrriapnts‐, shcorwipttsa, rhgoewts taarregseetsleacrtedsealnecdtetdheanddyntahme idcysnoafmthices porfotcheessp.roWcehsys,. iWn hcyo,nitnracsot,ndtroasot,thdeor oetnhhearnecnehrsanecmerbseedmdebdedindetdheinHtohxebHcoluxsbtcelruosntelryoanplpyeaaprpteoawr toorkwornkaosninagsliengnleeanr eaadrjaacdejna‐t cgeennteg[e3n7e,6[23,872,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,48]2?–84]?
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