Enzyme Structure Reveals New Drug Targets For Cancer And HIV
If the genome is the parts list of the human cell, certain proteins are the production managers, activating and deactivating genes as needed. Scientists funded by the National Institute of General Medical Sciences (NIGMS), part of the National Institutes of Health, now have a clearer understanding of how a key protein controls gene activity and how mutations in the protein may cause disease. The work could provide new avenues to design drugs aimed at cancer, diabetes, HIV, and heart disease.
Control of gene expression occurs at four different levels starting with DNA-packaging into higher order structures controlled by ATP-dependent chromatin remodelling complexes (ATP-DRs), polycomb group (PcG) and trithorax group (TxG) genes. DNA is wrapped around nucleosomes that are assembled from dimers of the histones H2A, H2B, H3 and H4 which all contain tails that can be either acetylated (HATs) and demethylated (HDMTs) during active transcription or deactylated (HDACs) and highly methylated (HMTs) during repressed transcription. At the parts level of DNA control via DNA methylation is mediated by the DNA methylating enzymes (DNMTs), methyl-DNA binding proteins (eg MeCBPs), and cytosine deaminases which could act as demethylating enzymes. Finally transcriptional repression is also mediated by microRNAs. Viruses target DNA methyltransferase activity and p300/CBP histone acetyltransferase activity but may also target other epigenetic mechanisms to induce cancer or disease. (from Flanagan BJC 2006)
The investigators focused on a protein called p300/CBP that belongs to a family of enzymes known as histone acetyltransferases, or HATs. These enzymes activate genes by attaching chemicals called acetyl groups to histones, the spool-like proteins that hold DNA in a tightly wound form.
Mutations in p300/CBP are linked to a variety of cancers, including those of the colon, breast, pancreas, and prostate. Researchers believe that a substance that selectively inhibits p300/CBP might be the basis for an anticancer agent.
Nearly 10 years ago, Philip Cole, M.D., Ph.D., of the Johns Hopkins University School of Medicine and his coworkers designed a p300/CBP inhibitor. But the inhibitor is not active in the human body, so it has been used exclusively as a research tool.
In the new study, the investigators combined X-ray crystallography with detailed enzymology to understand how p300/CBP works.
Their three-dimensional crystal structure provides an image of how a key part of p300/CBP binds to the inhibitor. Their studies of numerous mutant versions of the enzyme reveal which amino acids in p300/CBP are essential for its activity.
The work has a number of clinical implications. Understanding the structure and behavior of p300/CBP will help scientists design a p300/CBP inhibitor that might function in human cells as an anticancer drug.
Proper functioning of p300/CBP is critical for insulin regulation and the health of heart cells. As a result, compounds that can regulate p300/CBP activity might be useful in the treatment of diabetes and heart disease.
In addition, HAT activity is necessary for the multiplication of HIV, leading at least one scientific group to suggest that targeting HATs or similar enzymes might be an new way to thwart the virus.
Finally, the article also shows that some p300/CBP mutations previously linked to certain cancers lie right where p300/CBP contacts the inhibitor. Studying how these mutations alter the enzyme’s function should shed light on why the mutations can lead to disease.
“This work illustrates how enzymology and structural biology can combine to yield both fundamental and practical insights about an important biomedical problem. The studies provide a new framework for understanding p300/CBP in health and disease,” said Jeremy M. Berg, NIGMS Director.Nature. 2008 Feb 14;451(7180):846-50.
Program in Gene Expression and Regulation, The Wistar Institute, 3601 Spruce Street, Philadelphia, Pennsylvania 19104, USA.
The transcriptional coactivator p300/CBP (CREBBP) is a histone acetyltransferase (HAT) that regulates gene expression by acetylating histones and other transcription factors. Dysregulation of p300/CBP HAT activity contributes to various diseases including cancer. Sequence alignments, enzymology experiments and inhibitor studies on p300/CBP have led to contradictory results about its catalytic mechanism and its structural relation to the Gcn5/PCAF and MYST HATs. Here we describe a high-resolution X-ray crystal structure of a semi-synthetic heterodimeric p300 HAT domain in complex with a bi-substrate inhibitor, Lys-CoA. This structure shows that p300/CBP is a distant cousin of other structurally characterized HATs, but reveals several novel features that explain the broad substrate specificity and preference for nearby basic residues. Based on this structure and accompanying biochemical data, we propose that p300/CBP uses an unusual ‘hit-and-run’ (Theorell-Chance) catalytic mechanism that is distinct from other characterized HATs. Several disease-associated mutations can also be readily accounted for by the p300 HAT structure. These studies pave the way for new epigenetic therapies involving modulation of p300/CBP HAT activity.