org Ash1 absent, small, or homeotic discs 1: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References
Gene name - absent, small, or homeotic discs 1

Synonyms -

Cytological map position - 76B8--9

Function - enzyme

Keywords - trithorax group, histone methyl-transferase, chromatin

Symbol - ash1

FlyBase ID: FBgn0005386

Genetic map position - 3-46.6

Classification - histone methyl transferase, BAH (bromo-adjacent homology) domain, PHD-finger, SET domain

Cellular location - nuclear



NCBI links:   Precomputed BLAST |  Entrez Gene | UniGene

Recent literature
McCracken, A. and Locke, J. (2016). Mutations in ash1 and trx enhance P-element-dependent silencing in Drosophila melanogaster. Genome [Epub ahead of print]. PubMed ID: 27373142
Summary:
In Drosophila melanogaster, the mini-w+ transgene in Pci (a transgene inserted proximally on chromosome 4 between Ribosomal protein S3A (RpS3A) and cubitus interruptus (ci), is normally expressed throughout the adult eye; however, when other P or KP elements are present, a variegated-eye phenotype results, indicating random w+ silencing during development called P-element-dependent silencing (PDS). Mutant Su(var)205 and Su(var)3-7 alleles act as haplo-suppressors/triplo-enhancers of this variegated phenotype, indicating that these heterochromatic modifiers act dose dependently in PDS. Previously, a spontaneous mutation of P{lacW}ciDplac called P{lacW}ciDplacE1 (E1) was recovered that variegated in the absence of P elements, presumably due to the insertion of an adjacent gypsy element. From a screen for genetic modifiers of E1 variegation, this study describes the isolation of five mutations in ash1 and three in trx that enhance the E1 variegated phenotype in a dose-dependent and cumulative manner. These mutant alleles enhance PDS at E1, and in E1/P{lacW}ciDplac, but suppress position effect variegation (PEV) at In(1)wm4. This opposite action is consistent with a model where ASH1 and TRX mark transcriptionally active chromatin domains. If ASH1 or TRX function is lost or reduced, heterochromatin can spread into these domains creating a sink that diverts heterochromatic proteins from other variegating locations, which then may express a suppressed phenotype.
Huang, C., Yang, F., Zhang, Z., Zhang, J., Cai, G., Li, L., Zheng, Y., Chen, S., Xi, R. and Zhu, B. (2017). Mrg15 stimulates Ash1 H3K36 methyltransferase activity and facilitates Ash1 Trithorax group protein function in Drosophila. Nat Commun 8(1): 1649. PubMed ID: 29158494
Summary:
Ash1 is a Trithorax group protein that possesses H3K36-specific histone methyltransferase activity, which antagonizes Polycomb silencing. This study reports the identification of two Ash1 complex subunits, Mrg15 and Nurf55. In vitro, Mrg15 stimulates the enzymatic activity of Ash1. In vivo, Mrg15 is recruited by Ash1 to their common targets, and Mrg15 reinforces Ash1 chromatin association and facilitates the proper deposition of H3K36me2. To dissect the functional role of Mrg15 in the context of the Ash1 complex, this study identified an Ash1 point mutation (Ash1-R1288A) that displays a greatly attenuated interaction with Mrg15. Knock-in flies bearing this mutation display multiple homeotic transformation phenotypes, and these phenotypes are partially rescued by overexpressing the Mrg15-Nurf55 fusion protein, which stabilizes the association of Mrg15 with Ash1. In summary, Mrg15 is a subunit of the Ash1 complex, a stimulator of Ash1 enzymatic activity and a critical regulator of the TrxG protein function of Ash1 in Drosophila.

BIOLOGICAL OVERVIEW

Members of the Polycomb group of repressors and trithorax group of activators maintain heritable states of transcription by modifying nucleosomal histones or remodeling chromatin. Although tremendous progress has been made toward defining the biochemical activities of Polycomb and trithorax group proteins, much remains to be learned about how they interact with each other and the general transcription machinery to maintain on or off states of gene expression. The trithorax group protein Kismet (KIS) is related to the SWI/SNF and CHD families of chromatin remodeling factors. KIS promotes transcription elongation, facilitates the binding of the trithorax group histone methyltransferases ASH1 and TRX to active genes, and counteracts repressive methylation of histone H3 on lysine 27 (H3K27) by Polycomb group proteins. This study sought to clarify the mechanism of action of KIS and how it interacts with ASH1 to antagonize H3K27 methylation in Drosophila. Evidence is presented that KIS promotes transcription elongation and counteracts Polycomb group repression via distinct mechanisms. A chemical inhibitor of transcription elongation, DRB, had no effect on ASH1 recruitment or H3K27 methylation. Conversely, loss of ASH1 function had no effect on transcription elongation. Mutations in kis cause a global reduction in the di- and tri-methylation of histone H3 on lysine 36 (H3K36) - modifications that antagonize H3K27 methylation in vitro. Furthermore, loss of ASH1 significantly decreases H3K36 dimethylation, providing further evidence that ASH1 is an H3K36 dimethylase in vivo. These and other findings suggest that KIS antagonizes Polycomb group repression by facilitating ASH1-dependent H3K36 dimethylation (Dorighi, 2013).

Since KIS promotes transcription elongation, promotes ASH1 binding and counteracts Polycomb repression, it is suspected that these activities might be functionally interdependent. However, the loss of ASH1 function leads to an increase in repressive H3K27 trimethylation without affecting transcription elongation. Furthermore, the treatment of salivary glands with the elongation inhibitor DRB did not affect the level of ASH1 or H3K27me3 associated with polytene chromosomes. It is therefore concluded that KIS promotes transcription elongation and antagonizes Polycomb repression via distinct mechanisms (Dorighi, 2013).

These findings suggest that the major mechanism by which KIS antagonizes Polycomb group repression is by promoting the association of the trithorax group histone methyltransferases ASH1 and TRX with chromatin. Recent biochemical studies have suggested several mechanisms by which ASH1 and TRX counteract Polycomb repression. A histone modification catalyzed by TRX in vitro (H3K4 trimethylation) disrupts interactions between PRC2 and its nucleosome substrate. H3K4me3 directly interferes with the binding of the PRC2 subunit NURF55 (CAF1) to nucleosomes and inhibits the catalytic activity of E(Z) allosterically through interactions with the SU(Z)12 subunit of PRC2. The relevance of this modification to TRX function in vivo is not clear, however, as the bulk of H3K4 trimethylation in Drosophila is catalyzed by the histone methyltransferase SET1. Another mechanism by which TRX counteracts Polycomb repression was suggested by its physical association with the histone acetyltransferase CBP in the TAC1 complex. The acetylation of H3K27 by CBP directly blocks the methylation of this residue by PRC2. It is therefore tempting to speculate that the diminished binding of TAC1 to active genes contributes to the increased methylation of H3K27me3 observed in kis mutants (Dorighi, 2013).

Other histone modifications, including both the di- and tri- methylation of H3K36, also block the catalytic activity of PRC2 in vitro. In Drosophila, H3K36 trimethylation is catalyzed by SET2, which associates with the elongating RNA Pol II via its phosphorylated CTD. In this way, H3K36me3 becomes concentrated at the 3' ends of genes where it plays a role in preventing cryptic initiation. Consistent with its role in transcription elongation, kis mutations decreased the level of H3K36me3 on polytene chromosomes. Interestingly, H3K36me3 blocks the methylation of H3K27 at genes expressed in the C. elegans germline. Thus, H3K36 trimethylation might represent a conserved mechanism for antagonizing PRC2 function to maintain appropriate patterns and steady-state levels of transcription. Transcription- dependent H3K36 trimethylation is unlikely to be the sole mechanism by which KIS counteracts Polycomb repression, however, because blocking transcription elongation with DRB did not increase the level of H3K27me3 on polytene chromosomes. Furthermore, ash1 mutants display elevated levels of H3K27 methylation without a reduction in H3K36 trimethylation or transcription elongation, suggesting that additional mechanisms exist to counteract repressive H3K27 methylation (Dorighi, 2013).

An antagonism between H3K36 dimethylation and H3K27 trimethylation was suggested by the recent discovery that H3K36me2 inhibits PRC2 function in vitro. This finding, together with recent evidence that ASH1 dimethylates H3K36 in vitro, prompted an investigation of whether ASH1 also dimethylates H3K36 in vivo. The chromosomal distributions of ASH1 and H3K36me2 overlap significantly, consistent with their localization at the 5' end of active genes. Furthermore, the levels of H3K36me2 on the polytene chromosomes of both ash1 and kis mutant larvae were significantly reduced, consistent with the role of KIS in promoting ASH1 binding. Taken together, these observations strongly suggest that KIS antagonizes Polycomb repression by promoting the ASH1-dependent dimethylation of H3K36 (Dorighi, 2013).

The differences in the chromosomal distributions of ASH1 and H3K36me2 and the residual H3K36me2 observed in ash1 mutants are probably due to the presence of another H3K36 dimethylase (MES-4) in Drosophila. In addition to dimethylating H3K36, MES-4 is required for SET2-dependent H3K36 trimethylation in vivo, as revealed by RNAi knockdown of MES-4 both in larvae and in cultured cells. By contrast, this study failed to observe a significant reduction in H3K36me3 levels in ash1 mutant larvae. The findings suggest that ASH1 and MES-4 play non-redundant roles in H3K36 methylation in vivo (Dorighi, 2013).

It is becoming increasingly clear that multiple mechanisms (including the trimethylation of H3K4, the di- and tri-methylation of H3K36 and the acetylation of H3K27) antagonize repressive H3K27 methylation catalyzed by Polycomb group proteins. The current findings suggest that KIS plays a central role in coordinating these activities. By facilitating the binding of TRX and ASH1, KIS promotes H3K27 acetylation and H3K36 dimethylation in the vicinity of active promoters. By stimulating elongation, KIS promotes H3K36 trimethylation over the body of transcribed genes. Thus, KIS appears to counteract Polycomb group repression by promoting multiple histone modifications that inhibit H3K27 methylation by the E(Z) subunit of PRC2 (Dorighi, 2013).

Haploinsufficiency for CHD7, a KIS homolog in humans, is the major cause of CHARGE syndrome, a serious developmental disorder affecting ~1 in 10,000 live births (Janssen, 2012). Infants born with CHARGE syndrome often have severe health complications due to defects in the development of tissues derived from the neural crest, including coloboma of the eye, cranial nerve abnormalities, ear defects and hearing loss, congenital heart defects, genital abnormalities and narrowing or blockage of the nasal passages. Based on the phenotypes associated with kis mutations in Drosophila, it seems likely that some of these defects may stem from changes in gene expression resulting from loss of transcription elongation and inappropriate gene silencing by Polycomb group proteins. The current findings suggest that changes in histone H3 modifications resulting from the loss of CHD7 function might contribute to the broad spectrum of developmental defects associated with CHARGE syndrome (Dorighi, 2013).

Noncoding RNAs of trithorax response elements recruit Drosophila Ash1 to Ultrabithorax

Please note that the Sanchez-Elsner paper has been retracted (see retraction report)

Homeotic genes contain cis-regulatory trithorax response elements (TREs) that are targeted by epigenetic activators and transcribed in a tissue-specific manner. The transcripts of three TREs located in the Drosophila homeotic gene Ultrabithorax mediate transcription activation by recruiting the epigenetic regulator Ash1 to the template TREs. TRE transcription coincides with Ubx transcription and recruitment of Ash1 to TREs in Drosophila. The SET domain of Ash1 binds all three TRE transcripts, with each TRE transcript hybridizing with and recruiting Ash1 only to the corresponding TRE in chromatin. Transgenic transcription of TRE transcripts restores recruitment of Ash1 to Ubx TREs and restores Ubx expression in Drosophila cells and tissues that lack endogenous TRE transcripts. Small interfering RNA-induced degradation of TRE transcripts attenuates Ash1 recruitment to TREs and Ubx expression, which suggests that noncoding TRE transcripts play an important role in epigenetic activation of gene expression (Sanchez-Elsner, 2006).


GENE STRUCTURE

cDNA clone length - 7053

Bases in 5' UTR - 548

Exons - 6

Bases in 3' UTR - 525


PROTEIN STRUCTURE

Amino Acids - 2144

Structural Domains

The primary translation product of the 7.5-kb ash1 transcript is predicted to be a basic protein of 2144 amino acids. The ASH1 protein contains a SET domain, a PHD finger and a BAH (Bromo adjacent homology) domain (see NCBI Conserved Domain Summary. These motifs are found in the products of some trithorax group and Polycomb group genes (Tripoulas, 1996).


absent, small, or homeotic discs 1: Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

date revised: 16 February 2003

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