ASH1L (also called huASH1, ASH1, ASH1L1, ASH1-like, or KMT2H) is a histone-lysine N-methyltransferase enzyme encoded by the ASH1L gene located at chromosomal band 1q22. ASH1L is the human homolog of Drosophila Ash1 (absent, small, or homeotic-like).
ASH1L (also called huASH1, ASH1, ASH1L1, ASH1-like, or KMT2H) is a histone-lysine N-methyltransferase enzyme encoded by the ASH1L gene located at chromosomal band 1q22. ASH1L is the human homolog of Drosophila Ash1 (absent, small, or homeotic-like).
Ash1 was discovered as a gene causing an imaginal disc mutant phenotype in Drosophila. Ash1 is a member of the trithorax-group (trxG) of proteins, a group of transcriptional activators that are involved in regulating Hox gene expression and body segment identity. [5] Drosophila Ash1 interacts with trithorax to regulate ultrabithorax expression. [6]
The human ASH1L gene spans 227.5 kb on chromosome 1, band q22. This region is rearranged in a variety of human cancers such as leukemia, non-Hodgkin’s lymphoma, and some solid tumors. The gene is expressed in multiple tissues, with highest levels in brain, kidney, and heart, as a 10.5-kb mRNA transcript. [7] Mutations in ASH1L in humans have been associated with autism, epilepsy, and intellectual disability. [8]
Human ASH1L protein is 2969 amino acids long with a molecular weight of 333 kDa. [9] ASH1L has an associated with SET domain (AWS), a SET domain, a post-set domain, a bromodomain, a bromo-adjacent homology domain, and a plant homeodomain finger (PHD finger). Human and Drosophila Ash1 share 66% and 77% similarity in their SET and PHD finger domains, respectively. [7] A bromodomain is not present in Drosophila Ash1.
The SET domain is responsible for ASH1L’s histone methyltransferase (HMTase) activity. Unlike other proteins that contain a SET domain at their C terminus, ASH1L has a SET domain in the middle of the protein. The crystal structure of the human ASH1L catalytic domain, including the AWS, SET, and post-SET domains, has been solved to 2.9 angstrom resolution. The structure shows that the substrate binding pocket is blocked by a loop from the post-SET domain, and because mutation of the loop stimulates ASH1L HMTase activity, it was proposed that this loop serves a regulatory role. [10]
ASH1L is ubiquitously expressed throughout the body. [11] [12] [13] [14] In the brain, ASH1L is expressed across brain areas and cell types, including excitatory and inhibitory neurons, astrocytes, oligodendrocytes, and microglia. [15] [16] [17] ASH1L also does not appear to show specificity to any brain region. In humans, ASH1L mRNA expression levels are fairly equal across all regions of cortex. [18] [19] Similarly, in mice, ASH1L protein is highly expressed in the hippocampus, thalamus, hypothalamus, motor cortex, and basolateral amygdala. [20] In humans, ASH1L expression peaks prenatally and decreases after birth, with a second peak in expression towards adulthood. [18] [19] In mouse, ASH1L is expressed in the developing central nervous system as early as embryonic day 8.5 [21] [13] and is still expressed throughout the adult mouse brain. [22] Overall, the expression of ASH1L in the brain is spatially and temporally broad.
The ASH1L protein is localized to intranuclear speckles and tight junctions, where it was hypothesized to function in adhesion-mediated signaling. [7] ChIP analysis demonstrated that ASH1L binds to the 5’-transcribed region of actively transcribed genes. The chromatin occupancy of ASH1L mirrors that of the TrxG-related H3K4-HMTase MLL1, however ASH1L’s association with chromatin can occur independently of MLL1. While ASH1L binds to the 5’-transcribed region of housekeeping genes, it is distributed across the entire transcribed region of Hox genes. ASH1L is required for maximal expression and H3K4 methylation of HOXA6 and HOXA10. [23]
A Hox promoter reporter construct in HeLa cells requires both MLL1 and ASH1L for activation, whereas MLL1 or ASH1L alone are not sufficient to activate transcription. The methyltransferase activity of ASH1L is not required for Hox gene activation but instead has repressive action. Knockdown of ASH1L in K562 cells causes up-regulation of the ε-globin gene and down-regulation of myelomonocytic markers GPIIb and GPIIIa, and knockdown of ASH1L in lineage marker-negative hematopoietic progenitor cells skews differentiation from myelomonocytic towards lymphoid or erythroid lineages. These results imply that ASH1L, like MLL1, facilitates myelomonocytic differentiation of hematopoietic stem cells. [5]
The in vivo target for ASH1L’s HMTase activity has been a topic of some controversy. Blobel’s group found that in vitro ASH1L methylates H3K4 peptides, and the distribution of ASH1L across transcribed genes resembles that of H3K4 levels. [23] In contrast, two other groups have found that ASH1L’s HMTase activity is directed toward H3K36, using nucleosomes as substrate. [10] [24]
There are over 100 reported pathogenic, or disease-causing, variants in the ASH1L gene. [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] About half of the variants arise de novo, and half are inherited. Of the inherited variants, about half are maternally inherited and half are paternally inherited. Disease-causing variants may be missense, nonsense, or frameshift mutations. The missense mutations are distributed throughout the gene body without localizing to a known functional domain of ASH1L.
All affected humans are heterozygous for ASH1L mutations. A single pathogenic copy of ASH1L causes disease, which may be the result of two different genetic mechanisms: haploinsufficiency or dominant negative function. The ClinGen clinical genomics resource states that there is “Sufficient Evidence for Haploinsufficiency” in ASH1L. [48]
The most common phenotypes, or symptoms, related to ASH1L mutations are autism spectrum disorder (ASD), epilepsy, intellectual disability, and attention deficit hyperactivity disorder (ADHD). The Simons Foundation Autism Research Initiative (SFARI) gives ASH1L a score of 1.1, indicating that ASH1L is a high confidence autism gene with the best level of evidence linking it to autism. [8]
In biochemistry, the DNA methyltransferase family of enzymes catalyze the transfer of a methyl group to DNA. DNA methylation serves a wide variety of biological functions. All the known DNA methyltransferases use S-adenosyl methionine (SAM) as the methyl donor.
Histone methyltransferases (HMT) are histone-modifying enzymes, that catalyze the transfer of one, two, or three methyl groups to lysine and arginine residues of histone proteins. The attachment of methyl groups occurs predominantly at specific lysine or arginine residues on histones H3 and H4. Two major types of histone methyltranferases exist, lysine-specific and arginine-specific. In both types of histone methyltransferases, S-Adenosyl methionine (SAM) serves as a cofactor and methyl donor group.
The genomic DNA of eukaryotes associates with histones to form chromatin. The level of chromatin compaction depends heavily on histone methylation and other post-translational modifications of histones. Histone methylation is a principal epigenetic modification of chromatin that determines gene expression, genomic stability, stem cell maturation, cell lineage development, genetic imprinting, DNA methylation, and cell mitosis.
Histone H4 is one of the five main histone proteins involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal tail, H4 is involved with the structure of the nucleosome of the 'beads on a string' organization. Histone proteins are highly post-translationally modified. Covalently bonded modifications include acetylation and methylation of the N-terminal tails. These modifications may alter expression of genes located on DNA associated with its parent histone octamer. Histone H4 is an important protein in the structure and function of chromatin, where its sequence variants and variable modification states are thought to play a role in the dynamic and long term regulation of genes.
DNA (cytosine-5)-methyltransferase 1(Dnmt1) is an enzyme that catalyzes the transfer of methyl groups to specific CpG sites in DNA, a process called DNA methylation. In humans, it is encoded by the DNMT1 gene. Dnmt1 forms part of the family of DNA methyltransferase enzymes, which consists primarily of DNMT1, DNMT3A, and DNMT3B.
Enhancer of zeste homolog 2 (EZH2) is a histone-lysine N-methyltransferase enzyme encoded by EZH2 gene, that participates in histone methylation and, ultimately, transcriptional repression. EZH2 catalyzes the addition of methyl groups to histone H3 at lysine 27, by using the cofactor S-adenosyl-L-methionine. Methylation activity of EZH2 facilitates heterochromatin formation thereby silences gene function. Remodeling of chromosomal heterochromatin by EZH2 is also required during cell mitosis.
DNA (cytosine-5)-methyltransferase 3A (DNMT3A) is an enzyme that catalyzes the transfer of methyl groups to specific CpG structures in DNA, a process called DNA methylation. The enzyme is encoded in humans by the DNMT3A gene.
Histone-lysine N-methyltransferase 2A, also known as acute lymphoblastic leukemia 1 (ALL-1), myeloid/lymphoid or mixed-lineage leukemia1 (MLL1), or zinc finger protein HRX (HRX), is an enzyme that in humans is encoded by the KMT2A gene.
WD repeat-containing protein 5 is a protein that in humans is encoded by the WDR5 gene.
Histone-lysine N-methyltransferase SETD7 is an enzyme that in humans is encoded by the SETD7 gene.
SET domain containing 2 is an enzyme that in humans is encoded by the SETD2 gene.
CpG-binding protein (CGBP) also known as CXXC-type zinc finger protein 1 (CXXC1) or PHD finger and CXXC domain-containing protein 1 (PCCX1) is a protein that in humans is encoded by the CXXC1 gene.
Histone-lysine N-methyltransferase 2D (KMT2D), also known as MLL4 and sometimes MLL2 in humans and Mll4 in mice, is a major mammalian histone H3 lysine 4 (H3K4) mono-methyltransferase. It is part of a family of six Set1-like H3K4 methyltransferases that also contains KMT2A, KMT2B, KMT2C, KMT2F, and KMT2G.
Histone-lysine N-methyltransferase KMT5B is an enzyme that in humans is encoded by the KMT5B gene. The enzyme along with WHSC1 is responsible for dimethylation of lysine 20 on histone H4 in mouse and humans.
Trithorax-group proteins (TrxG) are a heterogeneous collection of proteins whose main action is to maintain gene expression. They can be categorized into three general classes based on molecular function:
Euchromatic histone-lysine N-methyltransferase 1, also known as G9a-like protein (GLP), is a protein that in humans is encoded by the EHMT1 gene.
Cancer epigenetics is the study of epigenetic modifications to the DNA of cancer cells that do not involve a change in the nucleotide sequence, but instead involve a change in the way the genetic code is expressed. Epigenetic mechanisms are necessary to maintain normal sequences of tissue specific gene expression and are crucial for normal development. They may be just as important, if not even more important, than genetic mutations in a cell's transformation to cancer. The disturbance of epigenetic processes in cancers, can lead to a loss of expression of genes that occurs about 10 times more frequently by transcription silencing than by mutations. As Vogelstein et al. points out, in a colorectal cancer there are usually about 3 to 6 driver mutations and 33 to 66 hitchhiker or passenger mutations. However, in colon tumors compared to adjacent normal-appearing colonic mucosa, there are about 600 to 800 heavily methylated CpG islands in the promoters of genes in the tumors while these CpG islands are not methylated in the adjacent mucosa. Manipulation of epigenetic alterations holds great promise for cancer prevention, detection, and therapy. In different types of cancer, a variety of epigenetic mechanisms can be perturbed, such as the silencing of tumor suppressor genes and activation of oncogenes by altered CpG island methylation patterns, histone modifications, and dysregulation of DNA binding proteins. There are several medications which have epigenetic impact, that are now used in a number of these diseases.
H3K4me3 is an epigenetic modification to the DNA packaging protein Histone H3 that indicates tri-methylation at the 4th lysine residue of the histone H3 protein and is often involved in the regulation of gene expression. The name denotes the addition of three methyl groups (trimethylation) to the lysine 4 on the histone H3 protein.
H3K27me3 is an epigenetic modification to the DNA packaging protein Histone H3. It is a mark that indicates the tri-methylation of lysine 27 on histone H3 protein.
H3K9me2 is an epigenetic modification to the DNA packaging protein Histone H3. It is a mark that indicates the di-methylation at the 9th lysine residue of the histone H3 protein. H3K9me2 is strongly associated with transcriptional repression. H3K9me2 levels are higher at silent compared to active genes in a 10kb region surrounding the transcriptional start site. H3K9me2 represses gene expression both passively, by prohibiting acetylation as therefore binding of RNA polymerase or its regulatory factors, and actively, by recruiting transcriptional repressors. H3K9me2 has also been found in megabase blocks, termed Large Organised Chromatin K9 domains (LOCKS), which are primarily located within gene-sparse regions but also encompass genic and intergenic intervals. Its synthesis is catalyzed by G9a, G9a-like protein, and PRDM2. H3K9me2 can be removed by a wide range of histone lysine demethylases (KDMs) including KDM1, KDM3, KDM4 and KDM7 family members. H3K9me2 is important for various biological processes including cell lineage commitment, the reprogramming of somatic cells to induced pluripotent stem cells, regulation of the inflammatory response, and addiction to drug use.
H3K4me1 is an epigenetic modification to the DNA packaging protein Histone H3. It is a mark that indicates the mono-methylation at the 4th lysine residue of the histone H3 protein and often associated with gene enhancers.