Summary: According to new research, one protein can regulate the expression of a large number of genes that modulate pain.
Source: Drexel University.
Chronic pain is one of the most prevalent, disabling and expensive public health crises in the United States. It affects more than 100 million Americans, with annual costs estimated at $635 billion, says a 2014 report from the American Pain Society.
Despite the enormous societal impact of chronic pain, present treatment options are limited to nonsteroidal anti-inflammatory drugs (NSAIDS), opioids, anticonvulsants and antidepressants, which provide pain relief to only about half of patients. Furthering the problem, prescription opioid overdoses have quadrupled since 1999. There were more than 45,000 painkiller-related deaths in 2014 alone, according to the Centers for Disease Control and Prevention.
Now, researchers at Drexel University College of Medicine are aiming to identify new molecular mechanisms involved in pain. Their latest study, published this month in Epigenetics & Chromatin, shows how one protein — acting as a master controller — can regulate the expression of a large number of genes that modulate pain.
“The hope is that understanding this pathway would lead to the identification of genes that we can target with drugs as non-addictive options for treating chronic pain,” said Melissa Manners, PhD, lead author of the study under the supervision of principal investigator Seena Ajit, PhD, an assistant professor in the College of Medicine.
In their effort to identify new drug targets, the research team explored how gene expression can be regulated by factors other than genetics. The field of epigenetics is gaining more attention, as researchers realize that genetics alone cannot explain all disease mechanisms.
One protein with a critical role in mediating epigenetic changes is methyl-CpG-binding protein 2 (MeCP2). MeCP2 can regulate the expression of a large number of genes by binding to DNA. Mutations in this DNA-binding protein cause Rett syndrome, an autism spectrum disorder that predominantly affects girls.
Rett patients have a number of severe impairments that can hinder a child’s ability to speak, walk, eat and breathe. However, a puzzling effect of this genetic mutation is the patient’s higher threshold for pain, suggesting that the functional MeCP2 protein has a role in controlling pain perception.
The researchers hypothesized that nerve injury can induce differences in the binding pattern of MeCP2, which, in turn, can lead to changes in the expression of a large number of downstream genes that could ultimately cause pain.
Ajit and her team performed their study in dorsal root ganglia (DRG), a cluster of nerve cell bodies at the root of a spinal nerve.
The researchers found that MeCP2 levels are increased after nerve injury.
“But we didn’t know which genes are regulated by the protein and wanted to pursue an approach that has not been explored previously,” Ajit said. “It was a technically challenging study to perform, because of the small size of the DRG.”
To investigate the genes regulated by MeCP2 under a chronic neuropathic pain state, the research team performed a global study to identify the DNA binding pattern in the DRG. They wanted to identify the DNA sequences in the mouse genome specifically bound by MeCP2 protein, and the changes in the regions bound by MeCP2 after nerve injury.
After nerve injury, the binding pattern of MeCP2 shifted more toward regions of the DNA that code for proteins and small non-protein coding regulatory RNA molecules.
These results, combined with findings from the group’s previous studies, show that MeCP2 broadly binds to chromatin, and can therefore influence the expression of numerous target genes in the peripheral nervous system. The discovery provides the molecular basis for a better understanding of how MeCP2 can simultaneously regulate multiple genes involved in the pain pathway.
“It is difficult to alter the function of MecP2 with a drug because of its broad expression and role,” Ajit said. “Now that we have a molecular basis to link MeCP2 to nerve injury-induced neuropathic pain, we plan to further explore the targets of MeCP2 and understand how they regulate pain.”
Source: Lauren Ingeno – Drexel University
Image Source: This NeuroscienceNews.com image is adapted from the Drexel University press release.
Original Research: Full open access research for “Genome-wide redistribution of MeCP2 in dorsal root ganglia after peripheral nerve injury” by Melissa T. Manners, Adam Ertel, Yuzhen Tian and Seena K. Ajit in Epigenetics & Chromatin. Published online May 27 2016 doi:10.1186/s13072-016-0073-5
Genome-wide redistribution of MeCP2 in dorsal root ganglia after peripheral nerve injury
Methyl-CpG-binding protein 2 (MeCP2), a protein with affinity for methylated cytosines, is crucial for neuronal development and function. MeCP2 regulates gene expression through activation, repression and chromatin remodeling. Mutations in MeCP2 cause Rett syndrome, and these patients display impaired nociception. We observed an increase in MeCP2 expression in mouse dorsal root ganglia (DRG) after peripheral nerve injury. The functional implication of increased MeCP2 is largely unknown. To identify regions of the genome bound by MeCP2 in the DRG and the changes induced by nerve injury, a chromatin immunoprecipitation of MeCP2 followed by sequencing (ChIP-seq) was performed 4 weeks after spared nerve injury (SNI).
While the number of binding sites across the genome remained similar in the SNI model and sham control, SNI induced the redistribution of MeCP2 to transcriptionally relevant regions. To determine how differential binding of MeCP2 can affect gene expression in the DRG, we investigated mmu-miR-126, a microRNA locus that had enriched MeCP2 binding in the SNI model. Enriched MeCP2 binding to miR-126 locus after nerve injury repressed miR-126 expression, and this was not mediated by alterations in methylation pattern at the miR-126 locus. Downregulation of miR-126 resulted in the upregulation of its two target genes Dnmt1 and Vegfa in Neuro 2A cells and in SNI model compared to control. These target genes were significantly downregulated in Mecp2-null mice compared to wild-type littermates, indicating a regulatory role for MeCP2 in activating Dnmt1 and Vegfa expression. Intrathecal delivery of miR-126 was not sufficient to reverse nerve injury-induced mechanical and thermal hypersensitivity, but decreased Dnmt1 and Vegfa expression in the DRG.
Our study shows a regulatory role for MeCP2 in that changes in global redistribution can result in direct and indirect modulation of gene expression in the DRG. Alterations in genome-wide binding of MeCP2 therefore provide a molecular basis for a better understanding of epigenetic regulation-induced molecular changes underlying nerve injury.
“Genome-wide redistribution of MeCP2 in dorsal root ganglia after peripheral nerve injury” by Melissa T. Manners, Adam Ertel, Yuzhen Tian and Seena K. Ajit in Epigenetics & Chromatin. Published online May 27 2016 doi:10.1186/s13072-016-0073-5