Transferase & Demethylase Assays

Histone acetyltransferases (HATs), a group of transcription regulators, covalently modify the epsilon amino group of lysines found in the N-terminal tails of histones, or other proteins, by the addition of an acetyl group from acetyl coenzyme A. Acetylation of lysines inside histone tails generally leads to transcriptional activation and deacetylation results in silencing. In addition, the stability and function of many non-histone proteins are regulated by the acetylation state of specific lysine residues [1, 2]. Known protein families with histone acetyl transferase activity include GNAT (GCN5, PCAF), CBP/p300, TAFII250 (TFIID), SRC-1, ACTR and MYST [3,4,5,6,7,8]. HAT complexes are involved in such diverse processes as transcriptional activation, gene silencing, DNA repair and cell-cycle progression. Dysfunction of these enzymes is often associated with diseases, ranging from neurodegenerative disorders to cancer [9].

Methylation of proteins on the epsilon-amino group of lysine or the guanidine group of arginine using the S-adenosyl-L-methionine (SAM) as the methyl group donor has been described for some time [1,2]. Lysine can be mono-, di- and trimethylated, while arginine can be both monomethylated symmetrically or asymmetrically dimethylated.
Multiple lysine and arginine residues present in histone N-terminal tails, many of which are methylated in vivo, and the various degrees of methylation has the potential to encode a great deal of regulatory information into chromatin. Histone lysine methylation is catalyzed by a family of proteins that contain a SET domain and by yeast Dot1 and its mammalian homologue, DOT1L, which use a novel enzymatic domain [3,4]. Arginine methylation is performed by the PRMT class of histone methyltransferases[5].

LSD1, a flavin-containing amine oxidase homolog and component of various corepressor complexes, was the first enzyme demonstrated to be capable of lysine demethylation and is elevated in a number of cancers [1,2]. The identification of LSD1 demonstrated that methylation is reversible and opened the door to the identification of a much larger family of demethylase enzymes, namely the Jumonji C (JmjC) domain proteins, a large family comprising 28 enzymes in humans [3]. The recent identification of histone demethylase enzymes, and the characterization of their role in cancer in other diseases, provides a new avenue for research into epigenetic regulation and a set of novel targets for pharmacological intervention.

Gene silencing through methylation is a well conserved and important feature of gene expression in virtually all eukaryotic and many prokaryotic organisms. In animals, the process is complex and essential in growth and development. There is additional evidence that inappropriate silencing can lead to serious genetic defects as well as cancer. Specifi cally, there is new evidence to suggest that the balance between growth promoting (oncogenes) and growth suppressing (tumor suppressor) genes is altered by inappropriate methylation. This can lead to aggressive cell growth associated with cancer. Inappropriate silencing can be partially reversed using hypomethylating agents such as the drug aza-deoxycytidine (aza-dC). Aza-dC (also aza-C) is in fact a useful tool to explore methylation events in vivo with endogenous methylation enzymes (DNA methyltransferases or DNMTs).

The methylation of mammalian genomic DNA is catalysed by DNA methyltransferases (Dnmts), which play a special role in the initiation of chromatin remodelling and gene expression regulation. The mammalian Dnmts are Dnmt1, Dnmt3, Dnmt3A and Dnmt3B, which are responsible for methylation pattern acquisition during gametogenesis, embryogenesis and somatic tissue development. Dnmt1 and Dnmt3 proteins comprise two domains: the N-terminal “regulatory” domain and C-terminal “catalytic” domain; in contrast Dnmt2 has only the “catalytic” domain. The Dnmts use S-adenosyl-L-methionine (AdoMet) as a donor of methyl groups. The only modifi cation of mammalian genomic DNA is the methylation at the 5- position of the cytosine (C) residue within the cytosine-guanine dinucleotides (CpG) resulting in the formation of 5-methylcytosine (m5C).