brds have an important role in the targeting of chromatin - modifying enzymes to specific 
         sites
. often they act with other protein - interaction modules to guarantee a high level of 
         targeting specificity for these essential enzymes .
for example , the methyltransferase ash1l 
         contains a combination of one brd and one plant homology domain ( phd ) , as well as a 
         bromo - adjacent homology domain ( bah ) ( ref .
, ash1l activates ultrabithorax expression , and mammalian 
         homologues have been associated with actively transcribed genes .
another example of a 
         multidomain methyltransferase containing a brd is the mixed lineage leukaemia ( mll ) gene 
         product ( ref .
21 ) , which is an essential gene and 
         acts as a key regulator of the expression of many genes .
mll is required for proper segment 
         identity in mammals , it displays haplo - insufficiency and regulates self - renewal of 
         haematopoietic stem cells by controlling hox ( homeobox ) gene expression ( refs 22 , 23 , 24 ) .
in addition , the hats crebbp and ep300 contain several protein - interaction modules , 
         including one brd , and zinc finger and kix domains ( ref . 25 ) .
both proteins share a high degree of sequence similarity and act as 
         transcriptional coactivators that control a large variety of biological processes , including 
         cell growth , genomic stability , development , neuronal plasticity and memory formation , as 
         well as energy homeostasis ( ref .
crebbp is a 
         coactivator of the camp response element - binding creb transcription factor .
the fundamental 
         role of crebbp is reflected by the severe phenotype of homozygous knockout mice , which die 
         in utero with signs of defective blood vessel formation in the central nervous system , 
         developmental retardation , and delays in both primitive and definitive haematopoiesis ( ref . 
          
similarly , homozygous deletion of 
           ep300 results in mice that die between days 9 and 11.5 of gestation as a 
         result of defects in neurulation , cell proliferation and heart development ( ref .
two additional hat - containing brds have been 
         reported and these interact with ep300 and crebbp : pcaf [ also known as k(lysine ) 
         acetyltransferase 2b ( kat2b ) ] and the related gcn5 .
both proteins acetylate histones and 
         transcription factors , and act as transcriptional coactivators . 
        
gcn5-knockout mice die during embryogenesis because of severe growth 
         retardation , failure in the development of dorsal mesoderm lineages and anterior neural tube 
         closure ( refs 29 , 30 ) .
by contrast , homozygous deletion of the closely related pcaf 
         gene does not show gross abnormalities , but leads to short - term memory deficits and an 
         exaggerated response to acute stress and conditioned fear , associated with increased plasma 
         corticosterone levels ( ref .
recent data identified evolutionarily conserved aaa atpase ancca ( aaa nuclear coregulator 
         cancer - associated protein)/atad2 as a protein required for recruitment of transcription 
         factors of the e2f family to their target sites , and as a transcriptional coregulator of 
         myc , oestrogen and androgen receptors ( ars ) .
atad2 associates through its brd with histone 
         h3 acetylated at lys  14 during late mitosis , regulating the expression of genes 
         required for cell cycle progression ( refs 32 , 33 , 34 ) .
dual brd proteins of the bet ( bromodomain and extra - terminal ) family also have a pivotal 
         role regulating the transcription of growth - promoting genes and cell cycle regulators .
the 
         bet family is represented by four members in humans ( brd2 , brd3 , brd4 and the 
         testis - specific isoform brdt ) , with each containing two n - terminal brds .
brd4 and brd2 are 
         key mediators of transcriptional elongation by recruiting the positive transcription 
         elongation factor complex ( p - tefb ) .
the p - tefb core complex is composed of cyclin - dependent 
         kinase-9 ( cdk9 ) and its activator cyclin t. cdk9 phosphorylates the rna polymerase ii 
         ( rnapii ) c - terminal domain , a region that contains 52 heptad repeats .
rnapii undergoes 
         sequential phosphorylation at ser5 during promoter clearance and at ser2 by p - tefb at the 
         start of elongation .
it has been shown that brd4 couples p - tefb to acetylated chromatin 
         through its brds .
interestingly , in contrast to other brd - containing proteins and 
         transcription factors , bet proteins remain associated with condensed and hypoacetylated 
         mitotic chromosomes ( ref .
35 ) , suggesting a role in 
         epigenetic memory ( refs 36 , 37 ) .
homeostasis of bet expression levels is important for cell cycle 
         control because both inhibition of brd4 by microinjected specific antibodies and 
         overexpression of brd4 lead to cell cycle arrest in the g2 m and g1s phases , respectively 
         ( refs 38 , 39 ) , and genetic knockdown of brd4 in cultured human cells significantly reduces 
         cell growth ( ref .
brd2 associates with the e2f 
         transcription factors and with the swi / snf ( switch mating type / sucrose nonfermenting ) 
         complex to regulate the expression of diverse genes ( ref .
brd2 can function as a transcriptional coactivator or corepressor in a promoter - specific or 
         tissue - specific manner .
deletion of either brd2 or brd4 in mice is lethal , and brd4 mice also show severe developmental defects ( refs 43 , 44 , 45 ) .
mutagenesis of the brd2 promoter region resulted in mice 
         that expressed reduced levels of brd2 without causing gross developmental abnormalities . 
        
the testis - specific bet family member brdt is 
         essential for normal spermatogenesis , and specific deletion of the first brd in mice results 
         in abnormal spermatids and sterility ( ref .
47 ) . in 
         agreement with studies in mice , altered histone modifications have been observed in the 
           brdt promoter region of subfertile patients ( ref .
48 ) , and genome - wide association studies linked polymorphism in 
           brdt to sterility in european men ( ref .
tandem brds are also present in taf1 [ rnapii , tata box binding protein ( tbp)-associated 
         factor , 250  kda formerly called tafii250 ] , the largest subunit of the general 
         transcription factor tfiid .
taf1 binds to the core promoter sequence encompassing the 
         transcriptional start site , and also interacts with other transcriptional regulators , 
         thereby modulating the rate of transcription initiation ( ref .
it acts as a general transcriptional activator and as such 
         regulates a variety of essential biological processes , including myogenesis , dna - damage 
         response , the cell cycle and apoptosis ( refs 51 , 
           52 , 53 , 
           54 ) .
the c - terminal tandem brds have been shown 
         to specifically recognise the diacetylated histone h4 tail at k5/k12 or k8/k16 , as well as 
         diacetylated p53 at k373/k382 at the p21 promoter ( refs 55 , 56 ) .
taf1l is x - linked and might act as a functional substitute for taf1 
         during male meiosis , when sex chromosomes are transcriptionally silenced . similarly to taf1 , 
         taf1l can bind to the tata - binding protein ( tbp ) and can functionally substitute for taf1 in 
         a temperature - sensitive hamster cell line ( ref .
members of this 
         family are involved in a variety of cellular processes , including cell cycle progression , 
         signal transduction , apoptosis and gene regulation ( refs 58 , 59 ) .
, brwd1 is widely expressed , and its expression levels are dynamic during mouse 
         development .
it associates with the swi / snf complex component and functions as a 
         transcriptional regulator involved in chromatin remodelling ( ref .
however , 
         in drosophila , brwd3 function has been genetically linked to the 
         jak  stat pathway ( ref .
single brd modules are present in some members of the tripartite motif ( trim ) family of 
         transcriptional regulators ( ref .
trim proteins 
         are characterised by the presence of a ring finger , one or two zinc - binding motifs named 
         b - boxes , and an associated coiled - coil region ( ref .
trim24 ( tif1 ) , for instance , contains an n - terminal trim domain , a 
         nuclear receptor ( lxxll ) interaction motif and a c - terminal phd - brd ( ref .
66 ) and mediates ligand - dependent activation of ar and 
         the retinoic acid receptor ( rar ) , and has been shown to interact with other nuclear 
         receptors such as thyroid , vitamin d3 and oestrogen receptors ( ref .
trim28 ( tif1 ) is a corepressor for 
         krppel - associated box - domain - containing zinc finger proteins ( ref .
trim28 associates with heterochromatin - associated factors hp1 , hp1 and 
         hp1 to promote the silencing of euchromatic genes ( ref .
69 ) , and recruitment of trim28 to centromeres is required for induction 
         of the parietal and visceral endoderm differentiation pathways ( refs 70 , 71 , 72 ) .
interestingly , the phd domain of the trim28 corepressor functions 
         as an intramolecular e3 ligase , leading to sumoylation of the adjacent brd .
sumoylation is 
         required for trim28-mediated gene silencing , suggesting that the tightly linked phd - brd 
         module functions as an intramolecular ubiquitin - like modifier ( sumo ) e3 ligase ( refs 73 , 74 ) .
formation of transcription regulatory complexes of 
         smad4 with receptor - phosphorylated smad2 and smad3 is a key event in canonical tgf 
         signalling .
consequently , depletion of trim33 in human cell lines inhibits smad4-dependent 
         cell proliferation by competing with smad4 for selective binding to receptor - phosphorylated 
         smad2 and smad3 ( ref .
mice deficient in trim33 
         die in utero , demonstrating that trim33 has an important role in development ( ref .
the relatively poorly studied trim66 
         ( tif1 ) is mainly expressed in testis and , similarly to trim24/33 , associates with 
         heterochromatin - associated factors ( hps ) but not with nuclear receptors , and functions as a 
         transcriptional silencer ( ref .
the trim family member pml ( promyelocytic leukaemia protein trim19 ) has no brd itself but 
         associates with sp proteins , a family of three proteins in humans ( sp100 , sp110 and sp140 ) 
         that all contain a phd - brd tandem module n - terminally flanked by a sand dna - binding domain . 
        
the complex of pml and sp100 is found in nuclear bodies , which are nuclear structures that 
         have been associated with the pathogenesis of acute promyelocytic leukaemia ( ref .
nuclear bodies are implicated in the regulation of 
         many cellular functions , including chromatin organisation ( ref .
80 ) , dna repair and genome stability ( refs 81 , 82 ) , as well as regulation 
         of transcription ( refs 83 , 84 , 85 ) .
in addition , the 
         nuclear body is a target of autoantibodies in patients with primary biliary cirrhosis ( ref . 
          
however , little is known about the precise 
         mechanisms whereby nuclear body proteins exert their functions .
brds have an essential role in the assembly and correct targeting of swi / snf complexes , 
         which are particularly rich in brd interaction modules .
swi / snf complexes , also called 
         brahma - associated factors ( bafs ) , remodel chromatin structure , contributing to either 
         transcriptional activation or repression of target genes , depending on the composition of 
         the various complexes .
the components of swi / snf complexes were originally identified in 
         screens for mutants that result in defects in mating - type switching in yeast or that were 
         unable to grow on sucrose ( refs 88 , 89 , 90 ) . 
        
microarray studies later showed that swi / snf functions as a transcriptional regulator that 
         affects about 5% of all genes in yeast ( ref .
mammalian swi / snf complexes have a key role in cell differentiation and proliferation , and 
         represent an essential component of the embryonic stem cell core pluripotency 
         transcriptional network ( refs 92 , 93 ) .
all swi / snf complexes contain a core subunit , 
         which alters chromatin structure in an atp - dependent manner , resulting in an open and 
         accessible conformation with increased affinity for transcription factors ( ref .
these two proteins are mutually exclusive in swi / snf complexes and have been 
         named after the drosophila homologue brahma as brg1 ( brahma - related gene-1 , 
         smarca4 ) and the related protein brm ( smarca2 ) .
brg1 and brm contain a c - terminal brd that 
         has been implicated in the recognition of acetylated lysines within histone h3 and h4 tails 
         ( ref .
several swi / snf complexes have been 
         shown to mediate critical interactions between a number of hormone and other nuclear 
         receptors ( refs 96 , 97 , 98 , 99 ) . in addition , brg1 has been shown to associate with rb proteins , 
         inducing cell cycle arrest and transcriptional repression in an hdac - dependent manner . 
        
brg1/hdac - containing complexes have been shown to repress expression of genes involved in 
         cell cycle regulation ( refs 100 , 101 ) .
the chromatin - remodelling activity of brg1 has 
         also been shown to be important for traversal of the nucleosome by rnapii ( ref .
the swi / snf complex pbaf ( polybromo - associated 
         brg1-associated factor ) is characterised by the presence of the polybromo protein ( pb1 ) 
         ( also called baf180 ) ( refs 103 , 104 ) .
pb1 is required for ligand - dependent 
         transactivation by nuclear hormone receptors and contains six brds , two bromo - associated 
         domains ( bah ) and a homeobox dna - binding domain .
pbaf complexes , but not baf , activate 
         vitamin - d - receptor - dependent transcription in response to vitamin d , and mice lacking pb1 
         have defects in heart development ( ref .
105 ) 
         because of impaired epithelial - to - mesenchymal transition and arrested maturation of the 
         epicardium as a result of the downregulation of fgf , tgf and vegf signalling ( ref .
pb1 also has a role in cell cycle regulation and 
         is a key regulator of senescence ( ref .
brds are present in chromatin - remodelling complexes of the iswi ( imitation swi ) family that 
         assemble into at least seven different complexes containing a central core atpase of the two 
         snf2-like mammalian homologues snf2l and snf2h of yeast iswi .
the iswi 
         complex nurf ( nucleosome remodelling factor ) contains the brd phd finger transcription 
         factor bptf .
bptf contains a c - terminal phd - brd and was identified as a highly expressed 
         protein in patients with alzheimer disease as fetal alz-50 reactive clone 1 , and in fetal 
         brain in patients with neurodegenerative diseases ( fetal alzheimer antigen , falz ) ( refs 
           108 , 109 ) . the phd domain in bptf associates with trimethylated histone h3 lys4 , an 
         interaction that is required for the recruitment of snf2l1 to promoters ( ref .
. the iswi complex acf / wcrf ( atp - utilising 
         chromatin remodelling and assembly factor / williams syndrome transcription factor ) contains 
         baz1 ( also called wcrf or acf1 ) , a protein of the baz ( brd adjacent zinc finger ) family , 
         which is represented by four related genes in humans ( baz1a , baz1b , baz2a and baz2b ) , with 
         similar domain organisation , including a phd - brd interaction module .
baz1a was first 
         identified in hela cell nuclear extract as a factor associating with snf2h forming a complex 
         with atp - dependent chromatin - remodelling activity ( ref .
later , the snf2h / baz1a remodelling activity was shown to be required for the 
         dna - replication machinery to penetrate condensed chromatin structures .
snf2h / baz1a is 
         particularly enriched in replicating pericentromeric heterochromatin , and knockdown of baz1a 
         by rnai impairs replication of condensed chromatin ( refs 112 , 113 ) .
baz2a ( tip5 , ttf-1-interacting protein 5 ) is a key subunit of the norc ( nucleolar 
         remodelling complex ) , which mediates transcriptional silencing of ribosomal rna ( ref .
interestingly , mutation of a tyrosine residue in 
         the baz2a brd in yeast impairs interaction with acetylated histones ( ref .
115 ) and the mutation y1775f represses norc 
         interaction with chromatin and rna polymerase i transcription ( ref .
a table containing all human brd proteins identified to date and 
         a phylogenetic tree of this protein family is shown in table 1 and figure 1a , respectively .
( a ) phylogenetic tree based on sequence alignments of predicted 
               brds . for targets with multiple brds , the domains have been numbered starting from the 
               n - terminus and the number is shown in parentheses .
118 ) with monoacetylated lys14 in histone h3 and 
               a diacetylated h4 peptide monoacetylated on both lys5 and lys8 .
( a ) phylogenetic tree based on sequence alignments of predicted 
               brds . for targets with multiple brds , the domains have been numbered starting from the 
               n - terminus and the number is shown in parentheses .
118 ) with monoacetylated lys14 in histone h3 and 
               a diacetylated h4 peptide monoacetylated on both lys5 and lys8 .
given the central role of brds in epigenetic gene regulation , it is surprising that only a 
         few substrates have been reported and mapped to specific sites . reported affinities range 
         from low micromolar to millimolar kd values , raising questions 
         regarding the physiological relevance of described weak in vitro substrate interactions , as 
         well as which additional factors contribute to binding specificity ( table 2 ) .
brds are often associated with other protein - interaction 
         modules , a mechanism that is thought to generate high target selectivity and increased 
         binding affinity with substrates owing to avidity that is generated on simultaneous binding 
         of several interaction domains .
this property led to the suggestion that epigenetic 
         regulation recognises patterns of post - translational modifications ( words ) rather than 
         single modifications ( letters ) ( ref .
, the reading process might require combinations of several modifications for 
         high - affinity interaction with a single brd .
recently , moriniere and coworkers showed that 
         the testis - specific bet isoform brdt requires the presence of several acetylation sites for 
         high - affinity binding to histone tails ( ref .
interestingly , both acetylated lysines interact with the same binding pocket in brdt ( fig .
it is also likely that other 
         post - translational modifications , such as phosphorylation and methylation , influence 
         substrate recognition , providing the basis for crosstalk of transcription control and 
         cellular signalling .
similarly , the related brd protein brd3 also requires two adjacent 
         acetylation sites for tight interaction with the transcription factor gata1 ( ref .
histone 
                 h3 residues 1 - 25 with single acetylations on k4 , k9 , k14 , k18 or k23.abbreviations : fa , stopped - flow fluorescence anisotropy ; fp , fluorescent 
                 polarization ; itc , isothermal titration calorimetry ; nmr , nuclear magnetic 
                 resonance ; spr , surface plasmon resonance .
histone 
                 h3 residues 1 - 25 with single acetylations on k4 , k9 , k14 , k18 or k23 .
abbreviations : fa , stopped - flow fluorescence anisotropy ; fp , fluorescent 
                 polarization ; itc , isothermal titration calorimetry ; nmr , nuclear magnetic 
                 resonance ; spr , surface plasmon resonance .
many proteins that use brds for their recruitment to specific regulatory complexes have 
         been implicated in the development of cancer .
brd - containing proteins are usually 
         multicomponent , and often the reported disease association has not been directly linked to 
         defects in the brd module itself . however ,
a number of dominant oncogenic rearrangements and 
         correlation of overexpression of brd proteins with patient survival provide a strong case 
         for targeting brds in cancer .
genetic rearrangements of brd - containing proteins have been linked to the development of a 
         number of extremely aggressive tumours . a very aggressive poorly differentiated carcinoma 
         that originates mainly from midline locations such as the head , neck or mediastinum is nut 
         ( nuclear protein in testis ) midline carcinoma ( nmc ) ( ref . 132 ) .
nmcs are genetically characterised by translocations that involve the nut 
         protein with brd4 , brd3 or an unknown partner gene .
both brd4nut and 
         brd3nut fusion genes encode proteins composed of the n - terminal tandem brds and 
         almost the entire nut gene ( fig .
nut blocks cellular differentiation , and depletion of this oncogene by rnai 
         results in squamous differentiation and cell cycle arrest ( refs 133 , 134 ) . 
        
brd4nut specifically recruits cbp / p300 , leading to stimulation of cbp / p300 hat 
         activity , formation of nuclear foci and inactivation of p53 ( ref .
selective inhibition of brd4nut by recently developed 
         acetyl lysine competitive inhibitors results in epithelial differentiation , tumour shrinkage 
         and survival in brd4nut xenograft mice ( ref .
other domain types are 
               labelled directly in the figure and breakpoints are indicated by arrows .
other domain types are 
               labelled directly in the figure and breakpoints are indicated by arrows .
chromosomal translocations of crebbp with the mll protein and the monocytic leukaemia zinc 
         finger protein ( moz ) have been described in myeloid and lymphoid acute leukaemia and 
         myelodysplasia secondary to therapy with drugs targeting dna topoisomerase ii ( refs 137 , 138 ) 
           ( fig .
in addition , crebbp mutations have been identified in relapsed 
         acute lymphoblastic leukaemia ( ref .
140 ) and are 
         very common in diffuse large b - cell lymphoma and follicular lymphoma , constituting the major 
         pathogenetic mechanism shared by common forms of b - cell non - hodgkin 's lymphoma ( ref .
crebbp and the related hat ep300 are also highly 
         expressed in advanced prostate cancer , and expression levels have been linked with cancer 
         patient survival ( ref .
overexpression of several brd proteins has been reported in cancer and has been linked to 
         patient survival .
for instance , a recent study showed that atad2 is overexpressed in more 
         than 70% of breast tumours and that higher protein levels correlate with tumour histological 
         grades , poor overall survival and disease recurrence ( ref .
revenko and coworkers showed that atad2 is required for recruitment of 
         specific e2f transcription factors and for chromatin assembly of the host cell factor 
         1mll histone methyltransferase complex .
as a result of its association with the 
         mll methyltransferase , depletion of atad2 results in a marked decrease of trimethylation of 
         lys4 in histone h3 , which has been linked to transcriptional activation .
brd mutations 
         disable atad2 function as an e2f coactivator and its ability to promote cancer cell 
         proliferation ( ref .
the closely related 
         protein atad2b has recently been shown to be highly expressed in glioblastoma and 
         oligodendroglioma , as well as in breast carcinoma ( ref .
aberrant expression has also been reported for trim24 in breast cancer , and high expression 
         levels have been shown to negatively correlate with survival of breast cancer patients ( ref . 
          
, however , trim24 seems to 
         function as a liver - specific tumour suppressor ( ref .
trim24 also interacts with ar and enhances transcriptional activity of the ar 
         by dihydrotestosterone in prostate cancer cells ( ref .
these data suggest that trim24 function and its role in tumourigenesis might 
         be highly context dependent .
the testis - specific bet family member brdt is frequently overexpressed in non - small - cell 
         lung cancer ( ref .
147 ) , but the functional consequences of 
         brdt overexpression have not been investigated so far . the role of brd4 in cancer is better 
         understood .
brd4 has been shown to be a key regulator of cell cycle control and 
         transcriptional elongation of growth - promoting genes .
in particular , the key role of brd4 in 
         the recruitment of p - tefb ( cdk9/cyclint ) to transcriptional start sites provides an 
         alternative strategy to targeting cdk9 , which emerged as a validated target in chronic 
         lymphocytic leukaemia ( ref .
148 ) . in breast 
         cancer , however , brd4 has been identified as an inherited susceptibility 
         gene for disease progression and its expression levels have been associated with patient 
         survival ( ref .
brd4 and brd2 also have a key 
         role for the transmission of tumour viruses during mitosis by providing a chromatin anchor 
         to viral episomes .
for instance , during latent viral infection of herpes viruses associated 
         with development of kaposi sarcoma , the transmission of viral genomes to daughter cells 
         during mitosis is mediated by the episome 's latency - associated nuclear antigen 1 , which is 
         tethered to chromatin through its interaction with brd4 ( ref .
also , papilloma viruses that have been linked to the development 
         of cervical cancers and epstein  barr viruses associate with brd4 in order to 
         anchor their viral genomes to mitotic chromosomes ( refs 151 , 152 ) .
transcriptional control of proinflammatory cytokines is the central mechanism in the 
         aetiology of inflammatory disease , and given the success of hdac inhibitors in this area , it 
         is likely that selective brd inhibitors will modulate these processes .
a first example has 
         been provided by the recent pan - bet inhibitor ibet , which leads to the disruption of 
         chromatin complexes responsible for the expression of inflammatory genes and conferred 
         protection against lipopolysaccharide - induced endotoxic shock and bacteria - induced sepsis 
         ( ref .
three sites of polymorphism in 
           brd2 have recently been linked to rheumatoid arthritis ( ref .
154 ) and brd2-hypomorphic mice are 
         severely obese and have reduced inflammation in fat tissue ( ref .
the brd - containing hats ep300/crebbp have been proposed as therapeutic targets in 
         inflammatory diseases such as lung inflammation and asthma ( ref .
activation of proinflammatory genes is intimately linked to 
         activation of nuclear factor b ( nfb ) .
the activated p65 subunit 
         translocates to the nucleus , where its affinity to its target genes and transcriptional 
         activity is regulated by acetylation by ep300/crebbp .
compounds that inhibit nfb 
         acetylation such as the natural product gallic acid have anti - inflammatory properties ( refs 
           156 , 157 ) .
ep300 and pcaf also regulate inflammatory responses through their regulation 
         of cyclooxygenase-2 ( cox2 ) expression .
cox2 is a key enzyme of prostaglandin biosynthesis 
         that is well established as a major player in inflammatory response and a clinically 
         successful target for the development of anti - inflammatory drugs .
stimulation by bacterial 
         lipopolysaccharides and other cytokines leads to increased binding of pcaf and ep300 to the 
           cox2 promoter , and its activation .
conversely , inhibitors of ep300 have 
         been shown to reduce cox2 protein levels and promoter activities ( ref .
the concerted activation of several proinflammatory genes is regulated by the swi / snf class 
         of atp - dependent remodelling complexes , which make the promoters of inflammatory genes 
         permissive for transcriptional induction .
the presence of the catalytic atpase subunit brg1 
         at the promoter of proinflammatory genes such as il6 has been shown to be 
         necessary for activation of these genes , and termination of transcriptional activation is 
         regulated by proteasomal degradation of brg1 , ensuring a timely and adequate immune response 
         ( ref .
although experimental data are still 
         missing , it is intriguing to speculate that removal of brg1 from promoter regions might have 
         an effect on inflammatory conditions .
increasing evidence points to the fact that epigenetic targets have a role in the molecular 
         manifestation of stress and related disorders .
because brd inhibitors have only just been 
         discovered , no study has addressed the role of brd inhibition in neurological disorders so 
         far .
however , several studies report important functions of brd - containing proteins in 
         several diseases .
trim28 , for instance , is highly expressed in the mouse hippocampus and 
         cerebellum .
inducible deletion of trim28 in the forebrain of adult mice 
         resulted in stress - related behaviour and cognitive impairment of these mice similar to 
         effects observed in behavioural disorders such as borderline personality or bipolar 
         disorder .
chromatin immunoprecipitation experiments confirmed changes in histone methylation 
         and acetylation patterns in the promoter regions of trim28 target genes such as 
           mkrn3 and pcdhb6 ( refs 160 , 161 ) .
two other brd - containing proteins , smarca2 ( brm ) and brd1 , have been identified in 
         genome - wide association studies as susceptibility genes for schizophrenia and bipolar 
         disorder in several independent studies , but the molecular mechanisms are still unclear 
         ( refs 162 , 163 ) . in addition , low levels of smarca2 have been found in the post - mortem 
         prefrontal brains of schizophrenic patients , and the gene expression profiles in the 
         diseased brains match those after downregulation of smarca2 in cells and in smarca2-knockout 
         mice , which show impaired social interaction and prepulse inhibition .
interestingly , smarca2 
         expression can be increased in the mouse brain on application of antipsychotic drugs , 
         providing further evidence of the potential of this protein as a target for the treatment of 
         schizophrenia ( refs 164 , 165 ) .
mutations in crebbp , and less frequently in ep300 , are 
         the genetic background for rubinstein  taybi syndrome ( rts ) , a rare human genetic 
         disorder characterised by mental retardation and physical abnormalities ; many patients with 
         rts have either breakpoints or microdeletions in chromosome 16p13.3 where the 
         crebbp gene is located , but also heterozygous point mutations can lead to 
         rts ( refs 166 , 167 , 168 ) .
several of the pathological 
         features can be mirrored by heterozygous crebbp - deficient mice strains 
         ( refs 169 , 170 , 171 ) .
although the precise mechanisms 
         underlying the disease are not yet understood , it is thought that the hat activity of crebbp 
         and reduced transcriptional activity result in altered synaptic plasticity , which ultimately 
         influences long - term memory , leading to mental retardation ( ref .
ep300 also has a role in the aetiology of amyotrophic lateral 
         sclerosis , alzheimer disease and huntington disease .
huntington disease is a polyq disease 
         in which polyglutamine repeats are added to the huntingtin protein , causing its 
         translocation to the nucleus and formation of aggregates .
crebbp and pcaf interact directly 
         with huntingtin aggregates , resulting in their depletion ( refs 173 , 174 ) .
indeed , hdac 
         inhibitors have long been used as mood stabilisers and are studied for the treatment of 
         huntington and alzheimer diseases ( ref .
brds share a conserved fold that comprises a left - handed bundle of four alpha helices 
           ( z , a , 
           b , c ) , linked 
         by highly variable loop regions ( za and bc loops ) , which form the rim of the 
         substrate - binding pocket and determine substrate recognition ( refs 55 , 127 ) ( fig . 3a ) . despite the conservation of the overall brd 
         fold , the surface and loop regions of brds
cocrystal structures with peptidic substrates have 
         demonstrated that the acetyl lysine is recognised by a central deep hydrophobic cavity , 
         where it is anchored by a hydrogen bond to an asparagine residue present in most brds ( ref . 
          
acetylation of lysine residues neutralises 
         the charge of the  - amino group . as a consequence , the central cavity of acetyl 
         lysine binding sites in brds is quite hydrophobic and particularly rich in aromatic 
         residues ; it also has sufficient size to accommodate potent acetyl lysine competitive 
         ligands .
these properties make brds attractive targets for the design of pharmacologically 
         active molecules that compete with protein interactions mediated by these modules . figure 3structural overview of a bromodomain and binding mode of bromodomain 
                 inhibitors .
the main structural 
               elements as well as the acetyl lysine binding site residues are labelled .
( b ) 
               superimposition of a diacetylated bet substrate peptide and the inhibitor jq1 . 
               inhibitor and
peptide molecules are shown in stick representation and are coloured 
               according to atom types .
( c ) binding of jq1 to the bromodomain of brd4 . conserved 
               water molecules in the active site are highlighted and hydrogen bonds are shown as 
               dashed lines .
the main structural 
               elements as well as the acetyl lysine binding site residues are labelled .
( b ) 
               superimposition of a diacetylated bet substrate peptide and the inhibitor jq1 . 
              
inhibitor and peptide molecules are shown in stick representation and are coloured 
               according to atom types .
( c ) binding of jq1 to the bromodomain of brd4 . conserved 
               water molecules in the active site are highlighted and hydrogen bonds are shown as 
               dashed lines .
potent and very selective inhibitors have recently been published for bet brds ( refs 136 , 153 , 
           178 ) .
all inhibitors that have been published so 
         far are based on a triazolo - diazepine scaffold that successfully mimics interactions 
         observed in bet peptide complexes ( fig .
interestingly , a number of tightly bound and conserved water molecules remain in cocrystal 
         structures of bet triazolo - diazepine complexes , which interact with the inhibitor through a 
         network of hydrogen bonds ( fig .
two bet 
         inhibitors have been studied in two different disease models , providing compelling support 
         of bet brds as targets in drug discovery .
the inhibitor jq1 has been studied in midline 
         carcinoma where inhibition of brd4nut led to terminal differentiation , cell cycle 
         arrest and apoptosis of carcinoma cells , and significant reduction of tumour growth in 
         patient - cell - line - derived xenograft models ( ref .
the inhibitor ibet led to significant reduction of the expression of 
         proinflammatory genes in activated macrophages , and conferred protection against 
         lipopolysaccharide - induced endotoxic shock and bacteria - induced sepsis , supporting 
         inhibition of bet brds as a strategy for the generation of immunomodulatory drugs ( ref . 
          
acetyl lysine mimetic inhibitors have also been reported in the case of crebbp , competing 
         for its interaction with p53 .
these inhibitors were identified by nmr screening using a 
         library of compounds that consists of one aromatic ring connected to an 
           nhcoch3 group by different types of linkers ( ref .
the same laboratory also reported a series of 
         cyclic peptides with improved binding affinities over natural substrates ( ref .
179 ) and azobenzene - based inhibitors such as 
         4-hydroxyphenylazo - benzenesulfonic acid ( ms456 ) and ischemin .
ischemin binds to the brds of 
         crebbp with a dissociation constant ( kd ) of 
         19  m and shows at least fivefold selectivity over other human brds .
the 
         binding mode of ischemin in crebbp is shown in figure 
         3d . in cellular assays
ischemin alters post - translational modifications of p53 and 
         histones , inhibits p53 interaction with cbp and transcriptional activity in cells , and 
         prevents apoptosis in ischaemic cardiomyocytes ( ref .
early lead compounds such as n1-aryl - propane-1,3-diamine have 
         also been identified for pcaf ( ref .
a 
         summary of the chemical structures of the currently most advanced brd inhibitors is shown in 
           figure 4 .
targeting brds for the development of protein - interaction inhibitors has recently emerged 
         as a strategy for the design of pharmacologically active reagents .
the relatively weak 
         interaction of brds with their substrates , the diversity and physicochemical properties of 
         the acetyl lysine binding site , and the large number of available crystal structures will 
         facilitate the rational design of such inhibitors . however , brds usually constitute only one 
         of the interaction domains found in brd - containing proteins , and whether selective 
         inhibition of the acetyl lysine interaction alone will result in the desired phenotype needs 
         to be investigated in future research projects . the large number of diseases that have been 
         linked to brd - containing proteins and the success of particular hdac inhibitors indicate 
         that brd inhibitors will find a large number of applications in pharmaceutical sciences and 
         basic research .
 acetylation of lysine residues is a post - translational modification with broad relevance 
           to cellular signalling and disease biology . 
 enzymes that  write 
           ( histone acetyltransferases , hats ) and  erase ( histone deacetylases , 
           hdacs ) acetylation sites are an area of extensive research in current drug development , 
           but very few potent inhibitors that modulate the  reading process 
           mediated by acetyl lysines have been described . 
 the principal readers of 
              - n - acetyl lysine ( kac ) marks are 
           bromodomains ( brds ) , which are a diverse family of evolutionary conserved 
           protein - interaction modules . 
 the conserved brd fold contains a deep , largely hydrophobic 
           acetyl lysine binding site , which represents an attractive pocket for the development of 
           small , pharmaceutically active molecules . 
 proteins that contain brds have been implicated 
           in the development of a large variety of diseases . 
 recently , two highly potent and 
           selective inhibitors that target brds of the bet ( bromodomains and extra - terminal ) family 
           provided compelling data supporting targeting of these brds in inflammation and in an 
           aggressive type of squamous cell carcinoma . 
 it is likely that brds will emerge alongside 
           hats and hdacs as interesting targets for drug development for the large number of 
           diseases that are caused by aberrant acetylation of lysine residues . 
Role of BRD proteins in chromatin biology
BRD substrates
BRDs as therapeutic targets in cancer
BRDs as therapeutic targets for the treatment of inflammation
BRDs as therapeutic targets for the treatment of neurological diseases
Development of BRD inhibitors
Research in progress and outstanding research questions