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Epigenetic regulation of chronic pain
Chronic pain arising from peripheral inflammation and tissue or nerve injury is a common Lingli Liang1, Brianna Marie
clinical symptom. Although intensive research on the neurobiological mechanisms Lutz2, Alex Bekker1
of chronic pain has been carried out during previous decades, this disorder is still & Yuan-Xiang Tao*,1,3,4,5
1
Department of Anesthesiology, New
poorly managed by current drugs such as opioids and nonsteroidal anti-inflammatory
Jersey Medical School, Rutgers, The State
drugs. Inflammation, tissue injury and/or nerve injury-induced changes in gene University of New Jersey, 185 S. Orange
expression in sensory neurons of the dorsal root ganglion, spinal cord dorsal horn Avenue, MSB F-548, Newark,
and pain-associated brain regions are thought to participate in chronic pain genesis; NJ 07103, USA
however, how these changes occur is still elusive. Epigenetic modifications including
2
Rutgers Graduate School of Biomedical
Sciences, New Jersey Medical School,
DNA methylation and covalent histone modifications control gene expression. Recent
Rutgers, The State University of New
studies have shown that peripheral noxious stimulation changes DNA methylation Jersey, Newark, NJ 07103, USA
and histone modifications and that these changes may be related to the induction 3
Department of Cell Biology & Molecular
of pain hypersensitivity under chronic pain conditions. This review summarizes the Medicine, New Jersey Medical School,
current knowledge and progress in epigenetic research in chronic pain and discusses Rutgers, The State University of New
Jersey, Newark, NJ 07103, USA
the potential role of epigenetic modifications as therapeutic antinociceptive targets 4
Department of Neurology &
in this disorder. Neuroscience, New Jersey Medical
School, Rutgers, The State University of
Keywords: chronic pain • DNA methylation • dorsal root ganglion • histone acetylation New Jersey, Newark, NJ 07103, USA
• histone methylation • inflammatory pain • neuropathic pain • spinal cord 5
Department of Physiology &
Pharmacology, New Jersey Medical
School, Rutgers, The State University of
Chronic pain is a major public health prob- rotransmitters, neuromodulators and struc- New Jersey, Newark, NJ 07103, USA
lem that affects approximately 30% of the tural proteins in primary sensory neurons of *Author for correspondence:
general population in the USA. It is a cause of dorsal root ganglion (DRG), spinal cord and Tel.: +1 973 972 9812
Fax: +1 973 972 1644
grave physiological and psychological distress other pain-related regions in the brain [2–4] . yt211@ njms.rutgers.edu
in those affected, and it places significant These changes contribute to the induction
pressures on the healthcare system. About and maintenance of chronic pain; however,
100 billion US dollars are spent on chronic how these changes are regulated by peripheral
pain-related healthcare expenses, and many noxious stimuli is still not fully understood.
patients experience a loss of productivity [1] . Recent studies have suggested that the
Chronic pain usually arises from inflam- mechanism for gene regulation involves
mation or tissue and nerve injury. Although epigenetic modifications. Environmental
intensive research on the neurobiological toxins, medications, diet and psychological
mechanisms of chronic pain has been carried stress alter epigenetic processes such as DNA
out during previous decades, this disorder is methylation, covalent histone modifica-
still poorly managed by current drugs such as tion (e.g., acetylation and methylation), and
opioids and nonsteroidal anti-inflammatory noncoding RNA expression. Accumulating
drugs, which are ineffective and/or produce evidence demonstrates that these processes
severe side effects [2] . Peripheral inflamma- play an important role in synaptic plastic-
tion and nerve injury produce transcriptional ity during memory formation as epigenetic
and translational changes in the expression changes correlate with hippocampal activ-
part of
of receptors, enzymes, ion channels, neu- ity [5–10] . Given that peripheral and central
10.2217/EPI.14.75 © 2015 Future Medicine Ltd Epigenomics (2015) 7(2), 235–245 ISSN 1750-1911 235Review Liang, Lutz, Bekker & Tao
sensitization under chronic pain conditions share promotes gene transcription. In contrast, histone
common mechanisms with the neuronal plasticity of deacetylation tightly condenses chromatin resulting in
memory formation, it is very likely that similar epigen- gene silencing (Figure 1) [16] .
etic mechanisms occur under both conditions. Indeed,
peripheral inflammation and nerve injury drive Effect of HDAC inhibitors on inflammatory pain
changes in DNA methylation, histone modifications Evidence from a pharmacological study has shown that
and noncoding RNAs in pain-related regions [8,9,11–14] . HDAC inhibitors can relieve inflammatory pain [20–22] .
These changes might be responsible for inflammation/ A 5-day subcutaneous treatment with either of the two
nerve injury-induced alterations of some pain-associ- HDAC inhibitors, MS-275 and SAHA, substantially
ated genes in central neurons. The evidence suggests reduced nociceptive behaviors in the second phase of
that modification of epigenetic processes participates the formalin test and led to an increase in mGluR2
in the mechanisms that underlie the induction and (but not mGluR1a, mGluR4 or mGluR5) in the DRG
maintenance of chronic pain. (Table 1) [21] . This antinociception could be abrogated
The role of noncoding RNAs including microR- by an mGlu2/3 receptor antagonist. The induction
NAs and long noncoding RNAs in chronic pain has of DRG mGlu2 receptors in response to suberoyl-
recently been discussed [8] . This article focuses on the anilide hydroxamic acid (SAHA) was associated with
evidence for the changes in DNA methylation and increased acetylation of p65/RelA on lysine 310, a
histone modification, mostly in DRG and spinal cord, process that enhances the transcriptional activity of
under chronic pain conditions. We explore how these p65/RelA at NF-κB-regulated genes [21] . Given that
changes are induced by peripheral noxious stimuli and transcription of the mGlu2 receptor gene is activated
how these epigenetic processes regulate pain-related by p65/RelA in DRG neurons, HDAC inhibition
genes. We finally deduce potential mechanisms of how may produce antinociception by upregulating mGlu2
the changes in DNA methylation and histone modifi- receptor expression in DRG. However, it is not clear
cation contribute to the development and maintenance whether formalin injection changes the histone acety-
of chronic pain. lation conditions and HDAC expression and activity
in the DRG.
Histone modification in chronic pain It was reported that complete Freund’s adjuvant
The process of histone modification (CFA)-induced peripheral inflammation increased
The nucleosome is the basic unit of chromatin, the levels of class IIa HDAC members (HDAC4, 5,
composed of about 140 base pairs of DNA wrapped 7, 9), but not class I HDAC members (HDAC1, 2, 3),
around a histone octamer. Histones are small, alka- in the spinal dorsal horn [20] . Intrathecal administra-
line proteins categorized into five major families: H1/ tion of HDAC inhibitors targeting class II (SAHA,
H5, H2A, H2B, H3 and H4. Histones H2A, H2B, TSA, LAQ824) or IIa (VPA, 4-PB) significantly
H3 and H4 are known as the core histones, while delayed the development of thermal hyperalgesia and
histones H1 and H5 are known as linker histones. attenuated existing thermal hyperalgesia in a CFA-
The N-terminal histone tail protrudes from the induced inflammatory pain model (Table 1) [20] . It
nucleosome and can be post-translationally modi- appears that class I and II or IIa HDAC members
fied, including acetylation, methylation, phosphory- function differentially in inflammatory pain models.
lation, citrullination, SUMOylation, ubiquitination A recent investigation from Zhang and colleagues
and ADP-ribosylation [15] . These modifications result revealed the epigenetic mechanism of inflamma-
in changes in the 3D chromatin structure and gene tory pain in central pain-modulating neurons. CFA
expression [15] . induced the increase of global histone H3 and H4
acetylation in brainstem nucleus raphe magnus
Histone acetylation & deacetylation in (NRM), a crucial supraspinal site for maintenance
chronic pain of pain hypersensitivity [22] . However, acetylated H3
Histone acetylation and deacetylation are the processes was reduced in the Gad2 gene promoter region which
by which the histones on lysine residues within the epigenetically suppresses the transcription of Gad2
N-terminal tail and on the surface of the nucleosome (encoding glutamic acid decarboxylase 65) and con-
core are acetylated by histone acetyltransferase (HAT) sequently causes impaired inhibitory function. Local
or deacetylated by histone deacetylases (HDACs) [16] . injection of HDAC inhibitors Trichostatin A (TSA)
Acetyl-Coenzyme A is the major source of the acetyl and SAHA into nucleus raphe magnus reversed
group in histone acetylation [16–19] . Conventionally, this effect and produced a similar analgesic effect
histone acetylation makes the condensed chroma- on CFA-induced inflammatory pain using systemic
tin into a more relaxed structure, and consequently administration (Table 1) [22] .
236 Epigenomics (2015) 7(2) future science groupEpigenetic regulation of chronic pain Review
A
Condensed chromatin: transcription repression
Histone
Histone tail
Me Me Me HDAC
DNA
Me
Me Me
Promoter regions
B
Loose chromatin: transcription activation
HAT
HAT HAT
Ac
Ac Ac
Promoter region
TF
DNA
Figure 1. Histone modification regulates gene expression. (A) Methylation of histones and deacetylation of histones
with HDAC results in a condensed chromatin. Under this condition, TFs cannot bind to the promoter region of the
gene, thereby gene transcription is repressed. (B) Histone acetylation with HAT results in loose chromatin that allows
the TF to bind to the promoter region of the gene. Consequently, gene transcription is activated.
Ac: Acetylation; HAT: Histone acetyltransferase; HDAC: Histone deacetylase; Me: Methylation; TF: Transcription factor.
Effect of HDAC inhibitors on visceral pain sciatic nerve (Table 1) [24] . However, how HDAC inhi-
One study on visceral pain also provided evidence to bition affects TNF-α expression under CCI-induced
support the involvement of central epigenetic mecha- neuropathic pain conditions is unknown. Intrathecal
nisms in pain [23] . Intracerebroventricular adminis- pretreatment with class I HDAC inhibitors (MS-275
tration of TSA significantly attenuated water avoid- or MGDC0103) attenuated mechanical and thermal
ance stress-induced visceral hypersensitivity in rats hypersensitivity in models of traumatic nerve injury
(Table 1) [23] . However, the targets of histone acetylation and antiretroviral drug (stavudine)-induced periph-
were not known in this study. eral neuropathy (Table 1) [25] . This analgesic effect
may be related to the increase in global H3K9ac in
Effects of HAT inhibitors & HDAC inhibitors on the spinal cord but not in DRG, suggesting that any
neuropathic pain potential mechanism could be found in the CNS [25] .
Several groups reported that the HDAC inhibitors Interestingly, the acetylation changes at the promot-
had an antinociceptive effect in neuropathic pain. ers of some pain-related genes, like μ opioid receptor,
Oral administration of sodium butyrate, an HDAC Kv4.3, Nav1.8, and brain-derived neurotrophic factor
inhibitor, not only attenuated chronic constriction (BDNF), in DRG neurons have been reported in a
injury (CCI)-induced pain hypersensitivity but also neuropathic pain model (Table 1) [26 -29] . Nerve injury-
reduced the CCI-induced increase in TNF-α in the induced reductions of histone H3 and H4 acetylation
future science group www.futuremedicine.com 237Review Liang, Lutz, Bekker & Tao
at the promoter regions of μ receptor, Nav1.8, and symptoms associated with neuropathic pain [27,28] ,
Kv4.3 silence their expression in DRG and may rep- whereas nerve injury-promoted increases in histone H3
resent an underlying cause of common negative and H4 acetylation at the promoter regions of BDNF
Table 1. Summary of studies on histone acetylation and deacetylation.
Pain model Changes of Tissue Inhibitors Nociceptive behavior Target Ref.
acetylation or response to inhibitors genes
enzyme expression
Formalin NA DRG MS-275, SAHA (sc.) Second phase (↓) mGluR2/3 [21]
CFA HDAC4, 5, 7, 9 (↑), Spinal SAHA, TSA, LAQ824, Thermal (↓) NA [20]
HDAC1, 2, 3 (-) Dorsal horn VPA, 4-PB (it.)
Global histone H3 NRM TSA and SAHA Thermal (↓) GAD65 [22]
and H4 acetylation (NRM)
(↑)
Water avoidance stress NA NA TSA (icv.) Visceral NA [23]
hypersensitivity(↓)
CCI NA Sciatic Sodium Thermal (↓), TNF-α [24]
nerve butyrate(oral) mechanical (↓), cold(↓)
Traumatic nerve injury Global H3K9ac(↑) Spinal cord MS-275, MS-275 or Thermal (↓), NA [25]
and stavudine-induced MGDC0103 (it.) mechanical (↓)
peripheral neuropathy
SNL HDAC1(↑) Acetyl Spinal Baicalin Thermal (↓), [41]
H3 (↓) dorsal horn mechanical (↓)
Acetylation changes DRG NA NA μ receptor, [27–29]
on promoter of Nav1.8,
genes Kv4.3, BDNF
H3K9 ac(↑) on Injured Anacardic acid (ip.) Thermal (↓), MIP-2 and [31,32]
promoter of MIP-2 sciatic mechanical (↓) its receptor
and CXCR2 nerve CXCR2
CCI p300 (↑) Spinal cord p300 shRNA or Thermal (↓), COX-2 [33,34]
C646 (it.) mechanical (↓)
H4 acetylation at Spinal cord Cdk5 [30]
Cdk5 promoter (↑)
Sirt (↓), Acetyl H3(↑) Spinal cord Resveratrol Thermal (↓), NA [35]
mechanical (↓)
Incision NA NA Anacardic acid (ip.) Mechanical (↓), thermal NA [36,37]
(-)
SAHA Mechanical (↓), thermal NA
(-)
Morphine Sirt (↓), Acetyl H3(↑) Spinal cord Resveratrol ↓Tolerance NA [40]
NA NA Curcumin ↓mechanical allodynia, NA [39]
thermal hyperalgesia,
tolerance, and physical
dependence
SAHA ↑mechanical allodynia, NA
thermal hyperalgesia,
tolerance, and physical
dependence
BDNF: Brain-derived neurotrophic factor; CCI: Chronic constriction injury; CFA: Complete Freund’s adjuvant; COX-2: Cyclooxygenase-2; CXCR2: Chemokine CC motif
receptor 2; DRG: Dorsal root ganglion; HDAC: Histone deacetylase; icv.: Intracerebroventricular injection; ip.: Intraperitoneal injection; it.: Intrathecally;
MIP-2: Macrophage inflammatory protein 2; NA: Not applicable; NRM: Nucleus raphe magnus; SAHA: Suberoylanilide hydroxamic acid; sc.: Subcutaneous injection;
SNL: Spinal nerve ligation, TSA: Trichostatin A; VPA: Valproic acid.
238 Epigenomics (2015) 7(2) future science groupEpigenetic regulation of chronic pain Review
in DRG and in histone H4 acetylation at the promoter Given that the degree of histone acetylation is con-
regions of Cdk5 in spinal cord upregulates the expres- trolled by the enzymes HATs and HDACs, conven-
sion of BDNF and CdK5, respectively, and may con- tionally, HAT inhibitors or HDAC activators should
tribute to the induction or maintenance of neuropathic have opposite effects compared with HDAC inhibi-
pain (Table 1) [29,30] . tors. Based on previous observations described above,
Interestingly, data from other groups showed that how HATs and HDACs are involved in neuropathic
HAT inhibitors also had an antinociceptive effect in pain is still elusive. The role of histone acetylation
neuropathic pain. The HAT inhibitor anacardic acid and deacetylation in neuropathic pain remains to be
relieved spinal nerve ligation (SNL)-induced neuro- verified.
pathic pain by suppressing the hyperacetylation of
histone H3 in the promoter region of macrophage Histone methylation & demethylation in
inflammatory protein 2 (MIP-2) and its receptor che- chronic pain
mokine CC motif receptor 2, resulting in the blockade Histone methylation is another process of histone
of SNL-induced upregulation of MIP-2 and chemo- modification by which methyl groups are transferred
kine CC motif receptor 2 in the injured sciatic nerve to amino acids of histone proteins in chromosomes
(Table 1) [31,32] . In another report, CCI increased the (Figure 1) . Histone methylation is catalyzed by S-ade-
expression of p300, a HAT E1A binding protein, in the nosylmethionine-dependent histone lysine methyl-
lumbar spinal cord (Table 1) [33,34] . Intrathecal admin- transferases and protein arginine methyltransferases,
istration of p300 shRNA or an inhibitor of p300 HAT whereas histone demethylation is catalyzed by histone
reversed CCI-induced mechanical allodynia and thermal N-methylated lysine residue demethylases and the pep-
hyperalgesia and suppressed the expression of cyclooxy- tidyl arginine deiminases [15,42] . Histone methylation
genase-2 (COX-2) in spinal cord (Table 1) [33,34] . Con- could repress or activate gene transcription depending
sistently, the intrathecal administration of resveratrol, on the sites and content being methylated. In general,
an activator of Sirt1 (a classic III HDAC), attenuated methylation of histone H3 at Lys9 or Lys27 (H3K9 or
CCI-induced mechanical allodynia and thermal hyper- H3K27) or histone H4 at Lys20 (H4K20) correlates
algesia, reversed the CCI-induced decrease in spinal with transcriptional repression, whereas methylation of
Sirt1, and blocked the CCI-induced increase in spinal H3K4, H3K36 and H3K79 correlates with enhanced
histone H3 acetylation [35] . The analgesic effect of HAT transcription [15] .
inhibition was also reported in other persistent pain Although histone methylation has been reported
conditions. In an incision model, injection of the HAT to participate in the mechanism of formation of long-
inhibitor anacardic acid intraperitoneally reduced inci- term memories and learning [7,43–45] , the role of histone
sion-induced pain hypersensitivity [36,37] . As expected, methylation in chronic pain is still unclear. Evidence
the HDAC inhibitor SAHA exacerbated mechanical indicates that histone methylation may be related to the
hypersensitivity after incision (Table 1) [36,37] . Given expression of chemokine (CC motif) ligands (CCLs),
that neuropathic pain and opioid tolerance/opioid- a class of small cytokines, in neuropathic pain [32,46] .
induced hyperalgesia share some common intracellular The peripheral nerve injury-induced reduction in
pathways in their mechanisms [38] , the evidence sug- H3K27me3 in the promoter region of monocyte che-
gests that histone modification is also involved in the motactic protein 3 (MCP-3, known as CCL7) might
development and maintenance of opioid tolerance and be responsible for the nerve injury-induced increase in
opioid-induced hyperalgesia. Indeed, daily administra- the expression of MCP-3 in spinal cord [46] . IL-6 may
tion of the HAT inhibitor curcumin with morphine for be involved in this response as the increased MCP-3
4 days reduced the development of morphine-induced expression was almost abolished in IL-6 knockout mice
mechanical allodynia, thermal hyperalgesia, tolerance with partial sciatic nerve ligation [46] . Peripheral nerve
and physical dependence [39] . Conversely, the HDAC injury also increased the mRNA levels of CCL2, CCL3
inhibitor SAHA enhanced these responses (Table 1) [39] . and their receptors (CCR2 and CCR1/CCR5, respec-
The intrathecal injection of resveratrol suppressed the tively) in the injured sciatic nerve. These increases
established morphine analgesic tolerance, reversed the could be related to the increased H3K4me3 in the pro-
morphine-induced decrease in spinal Sirt1, and attenu- moter regions of these cytokine genes [32] . An increase
ated the morphine-induced increase in spinal histone in global histone methylation was also observed in
H3 acetylation [40] . Interestingly, baicalin, a flavonoid spinal cord after intrathecal injection of pertussis
compound isolated from Huang Qin, ameliorated toxin, which induced significant thermal hyperalge-
SNL-induced neuropathic pain by suppressing HDAC1 sia [47] . However, whether these methylation sites are
expression and preventing histone-H3 acetylation in the really required for these changes in gene expression is
spinal cord dorsal horn (Table 1) [41] . unknown. Furthermore, whether nerve injury-induced
future science group www.futuremedicine.com 239Review Liang, Lutz, Bekker & Tao
changes in histone methylation contribute to neuro- DNA methytransferases & DNA methylation in
pathic pain remains to be investigated. A recent study chronic pain
showed that an increase in the expression of methyl- Effect of the DNMT inhibitors on
CpG-binding protein 2 (MeCP2) in mouse central inflammatory pain
nucleus of the amygdala (CeA) was caused by both Although the function of DNA methylation has been
CFA-induced chronic inflammatory pain and repeated reported in other pathological states, so far, only a few
morphine exposure [48] . The increased MeCP2 bound studies have demonstrated the potential role of DNA
to and repressed the transcriptional repressor histone methylation and the activity and expressional levels of
dimethyltransferase G9a, resulting in a reduction in DNMTs in pain. Cystathionine-β-synthase (Cbs) syn-
G9a-catalyzed repressive marker H3K9me2 and an thesizes hydrogen sulfide, an endogenous gas molecule,
increase in the expression of BDNF in CeA [48] . Over- which is necessary and sufficient to elicit mechanical
expression of CeA MeCP2 or knockdown of CeA pain hypersensitivity and increased excitability of DRG
G9a facilitated behavior of morphine reward, whereas neurons. Peripheral inflammation induced by CFA
knockdown of CeA MeCP2 inhibited behavior of mor- leads to demethylation of the cystathionine-β-synthase
phine reward [48] . Whether such epigenetic cascade (cbs) gene in DRG [58] . This demethylation may be
occurs in neuropathic pain needs to be confirmed. associated with the CFA-induced increase in expression
of Cbs mRNA and protein in DRG and could influ-
DNA methylation & chronic pain ence the induction of inflammation-induced mechani-
Process of DNA methylation cal hypersensitivity [58] . Given the fact that peripheral
In mammalian cells, DNA methylation is a biochemi- inflammation did not decrease DNMT expression and
cal process, in which a methyl group is added to the activity in DRG [12] , it is unclear how demethylation
fifth carbon of cytosine residues situated adjacent to occurs in DRG under chronic inflammatory pain con-
a guanine residue (CpG site). DNA sequences with ditions. The level of DNA methylation is controlled by
a high concentration of CpG residues are referred to both DNMTs and demethylation enzymes (e.g., ten-
as CpG islands and are generally located at the start eleven translocation dioxygenases). Whether periph-
of the gene sequence within the promoter region eral inflammation changes the expression and activity
(Figure 2) . The process of DNA methylation is medi- of DNA demethylation enzymes in DRG remains to
ated by a group of DNA methyltransferases (DNMTs) be determined. Interestingly, a recent study reported
that includes DNMT1, DNMT3a and DNMT3b; the CFA-induced hypermethylation of CpG islands in
this protein family also includes DNMT2, an inacti- the miR-129 promoter in spinal cord neurons [59] . This
vated isoform, and DNMT3L, which lacks the con- methylation may regulate chronic inflammatory pain
served catalytic domain [49–51] . DNMT1 maintains the by targeting CaMKIIγ [59] .
methylation of DNA that is already established at the
genome and is considered to be the primary mainte- Effect of the DNMT inhibitors on
nance DNMT [50,51] . Both DNMT3a and DNMT3b neuropathic pain
act as de novo methyltransferases and methylate In addition to peripheral inflammation, peripheral
unmethylated DNA [50,51] . Evidence has now expanded nerve injury caused by sciatic nerve CCI increased the
the role of DNMT1 to also include facilitation of de level of global DNA methylation in the spinal cord [60] .
novo DNA methylation by DNMT3a and DNMT3b Blocking spinal cord DNA methylation with intrathe-
at gene promoters [6,50,52–54] . cal 5-azacytidine attenuated CCI-induced thermal
DNA methylation interferes with gene transcrip- and mechanical pain hypersensitivities [60] . CCI also
tion by physically interfering with the binding of increased the level of DNA methylation in the proxi-
transcription factors and serving as docking sites mal promoter region of the μ opioid receptor gene in
for methyl-CpG-binding domain proteins (MBDs) DRG [61] . This increase may be related to the CCI-
(Figure 2) [55,56] . MBDs contain a specific domain of induced decrease in the analgesic effect of opioids [61] .
approximately 70 residues, the methyl-CpG-binding In the spared nerve injury-induced neuropathic pain
domain, which directly binds to one or more meth- model, DNMT1 and DNMT3a (but not DNMT3b)
ylated CpGs of a gene promoter. MBDs function transcripts were upregulated in the injured DRGs [62] .
as docking sites, in which they recruit other tran- Interestingly, the level of global DNA methylation was
scriptional corepressors, such as HDACs, to the tar- reduced in the prefrontal cortex and amygdala (but
geted gene for gene silencing, or coactivators, such as not the visual cortex and thalamus) following spared
CREB1, for transcription activation [55–57] . The ability nerve injury [63] . This reduction strongly correlated
to recruit these proteins may be why DNA methylation with the severity of pain behaviors [63] . It appears that
has such a profound effect on gene expression. nerve injury-induced changes in DNA methylation are
240 Epigenomics (2015) 7(2) future science groupEpigenetic regulation of chronic pain Review
A TSS Transcriptional
activation
TF RNAPII
CpG CpG CpG Gene
B DNMT
MBD TSS Transcriptional
repression
Me Me Me
CpG CpG CpG Gene
Me Me
Figure 2. DNA methylation represses gene transcriptional processes. (A) Without methylation at the CpG sites of
the gene promoter, the transcription factor and RNAPII bind to the promoter region of the gene, thereby gene
transcription is activated. (B) When methyl groups are added at the CpG islands by methyl-CpG-binding domain
protein-mediated DNA methyltransferases, the TF and RNAPII cannot bind the promoter region of the gene,
resulting in the repression of gene transcription.
CpG: -C-phosphate-G-; Me: Methylation; TF: Transcription factor; TSS: Transcription start site.
spatially different in the CNS and are implicated in dis- gene promoter was reported in patients experiencing
tinct functions in spinal and supraspinal levels under chronic low back pain associated with disc degenera-
neuropathic pain conditions. However, which type tion [65] . The endothelin B receptor gene promoter was
of cells in the nervous system expresses these changes heavily methylated in human oral squamous cell car-
is elusive. A recent study reported that DNMT1 was cinoma lesions, which are highly painful, whereas this
found in both neurons and satellite glial cells of DRG, promoter was not methylated in human oral dysplasia
DNMT3a in DRG satellite glial cells and DNMT3b in lesions, which are typically not painful [66] . A regula-
DRG neurons [62] , but the conclusion remains uncer- tory DNA methylation region in the CpG-island shore
tain because no specific neuronal and glial markers of the TRPA1 promoter was reported to have a pos-
were used [62] . Additionally, the specificity and selec- sible impact on TRPA1 gene expression and thermal
tivity of the antibodies used were not addressed [62] . sensitivity [67] . Joint resident synovial fibroblasts from
These earlier studies raise several unanswered ques- patients with rheumatoid arthritis exhibited a global
tions. For example, is the expression and/or activity hypomethylation or both hypomethylation and hyper-
of DNMTs and demethylation enzymes spatially and methylation patterns compared with patients with
temporally changed following peripheral inflammation osteoarthritis or healthy controls [68–70] . This hypo-
or nerve injury? If so, which type of DNMT? What methylation was identified in key genes relevant for
are the downstream targeted genes of DNMTs under rheumatoid arthritis, related to multiple pathways,
chronic pain conditions? Do DNMT inhibitors lead and associated with increased gene expression [70] . A
to side effects in addition to antinociception given that reduction of DNMT1 and an increase in the expres-
they are pharmacologically nonselective for specific sion of S-adenosyl methionine decarboxylase, spermi-
DNMTs? Broader future investigations are required. dine/spermine N1-acetyltransferase and polya-mine-
modulated factor1-binding protein1 may be associated
DNA methylation in patients with chronic pain with hypomethylation in rheumatoid arthritis [68,71] .
Alterations of DNA methylation have also been observed Additionally, promoter methylation states of the death
in patients with painful diseases. Women with fibro- receptor 3, IL-6, IL-10, IL-R2 and chemokine ligand
myalgia showed significant differences in DNA meth- 12 genes were altered in blood mononuclear cells and
ylation patterns compared with age-matched healthy synovial fibroblasts in rheumatoid arthritis [42,72] . It
controls, when genomic DNA isolated from whole appears that DNA methylation has the potential to
blood was examined [64] . Fibromyalgia-associated serve as a biomarker for some types of painful disorders
genes with differential methylation include BDNF, (e.g., autoimmune disorders or inflammation).
histone deacetylase 4, N-Acetyltransferase 15, protein
kinase C alpha and protein kinase G1 [64] . Increased MBDs in chronic pain
methylation at the extracellular matrix protein As discussed above, DNA methylation-triggered gene
SPARC (Secreted Protein, Acidic, Rich in Cysteine) transcriptional changes require a family of MBDs. The
future science group www.futuremedicine.com 241Review Liang, Lutz, Bekker & Tao
MBD family is composed of MeCP2 and MBD1–4. depends on the behavioral observations following phar-
Each of these proteins, with the exception of MBD3, is macological inhibitor administration. These inhibitors
capable of binding specifically to methylated DNA [73] . have varying degrees of specificity and selectivity for
Accumulating evidence indicates that MeCp2 may be the corresponding epigenetic enzymes. They may also
related to chronic pain. MeCP2 is associated closely exert their effects through nonepigenetic mechanisms,
with Rett syndrome, a neurodevelopmental disorder, resulting in potential side effects. For example, HATs
which is primarily caused by mutations in the MeCP2 and HDACs are not histone specific and can also
locus and patients display decreased pain sensitiv- acetylate and deacetylate, respectively, other targets in
ity [74,75] . Preclinical studies showed that the expres- the cytoplasm. These factors call for careful interpreta-
sion of MeCP2 and the level of its phosphorylation tion of current findings claiming the role of a particu-
were increased in the superficial dorsal horn under lar epigenetic enzyme in chronic pain. Therefore, the
CFA-induced inflammatory pain conditions [76,77] . development of pharmacologic inhibitors for isoform
This phosphorylation is controlled by a descending or subtype-specific epigenetic enzymes and/or the use
serotonergic pathway as serotonergic depletion pre- of targeted genetic inhibition of isoform or subtype-
vented CFA-induced MeCP2 phosphorylation [76] . It specific epigenetic enzymes will be required. More-
has been demonstrated that once MeCP2 is phosphor- over, whether these epigenetic enzymes are activated
ylated, it can be dissociated from the promoter regions by peripheral noxious insults and how their activation
of genes that have been repressed [78] . Therefore, the contributes to chronic pain remain to be investigated.
CFA-induced increase in MeCP2 phosphorylation Given that chronic pain remains a challenging condi-
may facilitate gene expression in the superficial dorsal tion to manage and that the contribution of epigen-
horn under inflammatory pain conditions. However, etic mechanisms underlying this disorder is becoming
the significance of how this increased phosphorylation increasingly recognized, it is conceivable that the sig-
is related to descending inhibitory serotonergic func- nificance of histone modification and DNA methyla-
tion in this model is unclear. The changes in MeCP2 tion in chronic pain will become even more apparent
expression following peripheral nerve injury are incon- in the coming years.
sistent. An increase in MeCP2 expression was observed
in the spinal cord of CCI rats [60] , whereas a decrease Financial & competing interests disclosure
was detected in the superficial dorsal horn of rats This work was supported by grants from the NIH (NS072206,
after spared nerve injury [77] . The role of MeCP2 in HL117684 and DA033390) and the Rita Allen Foundation. The
neuropathic pain remains to be further clarified. authors have no other relevant affiliations or financial involve-
ment with any organization or entity with a financial inter-
Conclusion & future perspective est in or financial conflict with the subject matter or materials
The evidence described above suggests that histone discussed in the manuscript apart from those disclosed.
modifications and DNA methylation in DRG and spi- No writing assistance was utilized in the production of this
nal cord are involved in chronic pain. The conclusion manuscript.
Exclusive summary
Histone modification in chronic pain
• Histone deacetylase inhibitors reduce inflammatory pain and visceral pain.
• Both histone acetyltransferase inhibitors and histone deacetylase inhibitors show antinociceptive effects in
neuropathic pain.
• Peripheral nerve injury or inflammation alters histone methylation in the promoter regions of some pain-
related genes.
DNA methylation & chronic pain
• DNA methyltransferase inhibitors blocks inflammatory pain.
• DNA methyltransferase inhibitors attenuates neuropathic pain.
• Alterations of DNA methylation have been found in patients with some painful diseases.
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