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Genetic and environmental influences on the transition from acute to chronic pain
Ze’ev Seltzer, DMDProfessor of Genetics
Canada Senior Research ChairUniversity of Toronto CTR for the Study of Pain
Ze’ev Seltzer, DMDProfessor of Genetics
Canada Senior Research ChairUniversity of Toronto CTR for the Study of Pain
Pathophysiology of the transition from acute to chronic pain
Comparative pain genetics
Animal models
Heritability and genetic assays
Expected gains for pain medicine
Presentation outline
Brief update
Pathophysiology of the transition
from acute to chronic pain
L4
L5
L6
DRGs
Sciatic N
L3
Saphenous N Totally
denervated
limb
Spinal gate
Brain
Dorsal roots
What triggers the transition? - I
• Electrical signal (“Injury discharge”; online/msec)
Injury discharge
3. Cut
Recording from a single
neuron
Peripheral nerve 1. Noxious & nonnoxious
stimuli
2. Electrical shock
to determine
fiber class: Aβ-δ; C
time (sec)200 400 600 800 1000
Nerve cut
100
200
300
400
500
33% of cut C-fibers
25% of cut A-fibers
Fir
ing
fre
qu
en
cy
(Hz)
Time after cut (sec)
0
חיתוך
Fir
ing
fre
qu
en
cy
(Hz)
120
60
Time after cut (min)Nerve cut
0
Prolonged “saw tooth” type
12% of cut afferents
Lasts hours
Injury discharge
Sackstein et al (1999)
Injury discharge triggers neuropathic pain - I
Local anesthetic
Neu
rop
ath
ic p
ain
sco
re
Preemptive analgesia
Vhcl
Efficacy of preemptive analgesia in humans is debated in the literature
Seltzer et al (1990a)
L4
L5
L6
DRGs
Sciatic N
L3
Saphenous NSpinal gate
Dorsal roots
What triggers the transition? - II
• Chemical signal(s) (Neurotrophic factors: e.g., NGF; hrs/days)
NGF
Partially
denervated
limb
Neuroplastic changes following nerve injury
DRGs
Sciatic N
Saphenous N
Partially
denervated
limbPNS changes:
• Neuroma formation
Partially
denervated
limb
Neuroplastic changes following nerve injury
DRGs
Sciatic N
PNS changes: • Neuroma formation• Spontaneous firing Production of novel
Na+ channels
Assembly in neuroma and DRG
Neuroplastic changes following nerve injury
DRGs
Sciatic N
Partially
denervated
limbPNS changes:
• Neuroma formation• Spontaneous firing • Firing induced by
• chemical mediators (histamine, bradykinin…)• Mechanical stimuli (e.g., malfitting prosthesis)• electrical stimuli (cross-talk, ephapses)
Partially
denervated
limb
Neuroplastic changes following nerve injury (cont.)
DRGs
Sciatic N
Saphenous N
Sympathetic-Sensory link Via NA & α2-AR
Partially
denervated
limb
Neuroplastic changes following nerve injury (cont.)
DRGs
Sciatic N
Saphenous N
CNS changes: • Loss of Io & IIo / microglia reaction / astroglia • Loss of receptors (e.g., opioid rec.) • mediators + phenotypic switch (e.g., GABA depolarizes)• Rewiring of the pain network:
• tuning curves; novel modalities• RFs• segmental disinhibition• central sensitization• reduced efficacy of descending inhibition
Partially
denervated
limb
Neuroplastic changes following nerve injury (cont.)
DRGs
Sciatic N
Saphenous N
CNS changes: • Loss of Io & their terminals / microglia reaction / astroglia • Loss of receptors (e.g., opioid rec.) • mediators + phenotypic switch (e.g., GABA depolarizes)• Rewiring of the pain network:
• tuning curves; novel modalities• RFs• segmental disinhibition• central sensitization• reduced efficacy of descending inhibition
Comparative approach:
Animal models used in pain genetics
Genetic selection based on spontaneous neuropathic pain
0-23-5
6-89-11
0
20
40
60
80
100
BC (F1xHA) BC (F1xLA)F1(HAxLA)
%
0-23-5
6-89-11
0
20
40
60
80
100
Unselected SabraLAHA
%
Neuropathic pain scores
Neuropathic pain scores
Devor and Raber 1990, 2005
0
2
4
6
8
10
Generation
d70
Sco
re
High Pain - HALow Pain - LA
Wistar
Sabra
Lewis
-~hundreds
Currently generation 53N
euro
pat
hic
pai
n s
core
Neuropathic pain levels are strain specific / 2 species
Var Pain = Var Gen + Var Env
Rats
0
2
4
6
8
10
LEW LA FSL SAB SD GEPR FIS HA
Au
toto
my
sco
re o
n d
63N
euro
pat
hic
pai
n s
core
s
Seltzer et al (2001)
Seltzer & Shir (1998)Mice (AXB-BXA Recombinant strains)
Parental strains
PSL modelPartial Sciatic tight Ligation
L4
L5
L6
Dorsal roots
DRGs
Spinal nerves
Sciatic N
L3
Seltzer, Dubner, Shir (1990)
Stimulus-evoked chronic pain is also determined genetically
~½ sciatic thickness trapped in a
ligature
Partially
denervated
limb
Pain abnormalities in the PSL model - I
Shir & Seltzer (1998)
Baseline
0
4
8
12
16
20
-24 -21 -18 -4 -1 1 7 14 21 28 35 42
Time (days)
Wit
hd
raw
al t
hre
sho
ld (
g)
Injured side
Sham side
Partial Sciatic Ligation
Intact
Wit
hd
raw
al t
hre
sho
ld (
g)
Time (days)
Intact PSL
Tactile allodynia:
Allodynia
Pain abnormalities in the PSL model - II
Shir & Seltzer (1998)
Baseline PSL
0
2
4
6
8
10
-24 -18 -4 -1 1 7 14 21 28 35 42
Time (days)
Res
pons
e du
ratio
n (s
ec)
Res
po
nse
du
rati
on
(se
c)
Time (days)
Intact PSL
Heat hyperalgesia:
Hyperalgesia
Touch INTACT
0
4
8
12
16
20
GEPR HA SAB SD FSL LA LEW FIS
Wit
hd
raw
al th
res
ho
ld (
g)
Noxious heat INTACT
0
0.5
1
1.5
2
2.5
3
FIS LA LEW SD HA FSL SAB GEPR
Re
sp
on
se
du
rati
on
(s
ec
)
Autotomy
0
2
4
6
8
10
12
HA FIS GEPR SD SAB FSL LA LEW
Au
toto
my
sc
ore
on
d6
3
Tactile allodynia PSL
0
4
8
12
16
20
HA GEPR FSL LA LEW SAB SD
Wit
hd
raw
al th
res
ho
ld (
g)
Heat hyperalgesia PSL
0
5
10
15
20
25
SD SAB LA LEW GEPR HA FSL
Re
sp
on
se
du
rati
on
(s
ec
)Spontaneous neuropathic pain
(self-mutilation)
Shir et al (2001)
Acute pain - Tactile allodynia - Heat hyperalgesia - Spontaneous painRATS (Mogil et al. in mice)
R2 = 0.002
R2 = 0.005
R2 = 0.09
0
2
4
6
8
10
12
0 2 4 6 8 10 12Disorder B
Dis
ord
er
A
Aut X HH
Aut X TA
HH X TA
Conclusions
1. Acute pain sensitivity does not predict levels of chronic pain (3 different chronic pain models, 2 stimulus modalities, 2 species, 2 research groups).
2. Levels of spontaneous chronic pain are not correlated with levels of stimulus-evoked chronic pain.
3. If these results are translatable to humans, genes are ‘syndrome-specific’. Pharmaco-genetic solutions will have to be tailored per syndrome.
Heritability of chronic pain
How much of the variance is accountable by genetics?
129 A
AKR
BALB/c
C3H/H
e
C57BL/6
C57BL/1
0C58
CBA
DBA/2RIII
S0
10
20
30
40
50
60
70VehicleAcetaminophen
* *
*
* *
To
tal
Nu
mb
er
of
Wri
the
s
– Nociception: 30-76% mean ~ 53%– Anti-nociception / analgesia: 23-68% mean ~ 45%– Neuropathic pain (SNL, Autotomy, PSL): ~ 30-70% mean ~50%
Heritability in rodents
Mogil et al (2002)
Pedigree analyses / twins studies: h2 ~ 0.2-0.7 (mean ~50%)
• Sciatica• Diabetic neuropathy• Carpal tunnel syndrome• “Burning feet” syndrome
• Post-herpetic neuralgia (HLA)
• CRPS (HLA)
• Fibromyalgia (HLA; 5HTTP1)
• Low back pain / Sciatica (GCH1; BDNF)
• Migraine (Cacna1a, ATP1A2, …)
• TMD - Temporo-Mandibular Pain Disorder (COMT)
• Phantom limb pain / stump pain (HLA, GCH1, GDNF)
• Post-Mastectomy Pain Syndrome (COMT, GCH1)
Heritability of pain in humans
Phenomics of chronic pain as a complex trait
IntensitySpatial
Temporal
Type
Beliefs about pain
Education
Past painfulexperiences
Impact on QoL
Catastro-phizing
Cognitive Affective /motivational
Sensory discriminative
Chronic pain
Other Aggravating /
Relieving factors
Different genes for different phenotypes
Chronic pain phenomics
Choosing the right phenotypes for genetics:
• Clinical relevance
• Mechanism-based
• N traits vs. multiple comparisons (“Bonferroni correction”)
• Pooling / Indexing / Loosing resolution
• Endophenotypes
The Human Pain Phenome Project
Detailed registry of previous chronic pain episodes
Aetiology and medical history
Detailed phenotypes
Tests (QST, electrodiagnosis, imaging, biochemistry)
Treatment effects
Additional traits: life style, personality / character
Bioinformatics / data mining
Expected gains in pain genetics
- Diagnostic kits
- Prognostic kits
- Preventive pain medicine
- Novel painkillers
- New mechanisms
- Gene therapy
- Better animal models
- Faster / cheaper clinical trials
- Diagnostic kits
- Prognostic kits
- Preventive pain medicine
- Novel painkillers
- New mechanisms
- Gene therapy
- Better animal models
- Faster / cheaper clinical trials
- Diagnostic kits
- Prognostic kits
- Preventive pain medicine
- Novel painkillers
- New mechanisms
- Gene therapy
- Better animal models
- Faster / cheaper clinical trials
United States Congress declared:2001-2010: The Decade of Pain Control and Research
The Human Genome Project has developed methodological templates that can be transposed immediately to pain genetics.
This is the time to:
• Establish new research teams • Support the collections of DNA samples / multicenter approach
• Finance genome-wide screens using microarray chips (1,000 samples X $ 500/ sample = $ 0.5 million / syndrome)
Proposed goal for 2010: First draft listing all major chronic pain genes in humans and mice.Given the right support – this is achievable !
Injury discharge triggers neuropathic pain - II
Electrical tetanization
Postoperative time (days)
C-fibers“Wind-up”
(0.5Hz)
C-fibers(0.1Hz)
A-fibers
CON
Neu
rop
ath
ic p
ain
sco
re
Local anesthetic
Seltzer et al (1990b)
Neuroplastic changes following nerve injury (cont.)
DRGs
Sciatic N
Spontaneous firing in intact DRGs
Partially
denervated
limb
Saphenous N
Activity in neuroma and DRG causes pain
Resection / RF / neurolysis of painful neuroma & GG - sometime successful
Devor et al (1999)
IntensitySpatial
Temporal
Type
Beliefs about pain
Education
Past painfulexperiences
Impact on QoL
Catastro-phizing
Cognitive Affective /motivational
Sensory discriminative
Chronic pain
Other Aggravating /
Relieving factors
Different genes for different phenotypes
Phenomics of chronic pain as a complex trait
Chronic pain phenomics
Choosing the right phenotypes for genetics:
• Clinical relevance
• Mechanism-based
• N traits vs. multiple comparisons (“Bonferroni correction”)
• Pooling / Indexing / Loosing resolution
• Endophenotypes
The Human Pain Phenome Project
Detailed registry of previous chronic pain episodes
Aetiology and medical history
Detailed phenotypes
Tests (QST, electrodiagnosis, imaging, biochemistry)
Treatment effects
Additional traits: life style, personality / character
Bioinformatics / data mining
Mogil et al (1999)
0
2
4
6
8
10
C5
7B
L/6
J
C5
8/J
AK
R/J
RIIIS
/J
12
9/J
BA
LB
/CJ
A/J
DB
A/2
J
SM
/J
CB
A/J
C3
H/J
day 3
5 S
co
re`
Mogil et al (1999)
R2 = 0.002
R2 = 0.005
R2 = 0.09
0
2
4
6
8
10
12
0 2 4 6 8 10 12Disorder B
Dis
ord
er
A
Aut X HH
Aut X TA
HH X TA
Spontaneous pain (self-mutilation)
Neu
rop
ath
ic p
ain
sco
re%
Hyp
eral
ges
ic m
ice
% A
llod
ynic
mic
eAcute pain - Tactile allodynia - Heat hyperalgesia - Spontaneous pain
MICE
Sensory-discriminative Affective
Cognitive
“Social”
pain genes
Thousands/~25K genes in the human genome encode the chronic pain network
Nocifension / reflexive
No• Most genes have been fixed throughout evolution
(e.g., “Painless” for noxious heat in the fly larva)
• While many have Single Nucleotide Polymorphisms (SNPs)
• Only a small fraction are functional, even fewer clinically relevant
• So how many will have to treated to treat a given pain syndrome?
Not known as of yetGuess: ~5 ‘major’ and up to ~15 ‘modifiers’ per syndrome
Shall we need to control thousands to treat pain ?
How many genes would have to be
pharmaco-genetically controlled to
provide solutions for a pain syndrome?
Neuroplastic changes following nerve injury (cont.)
Aδ-fibersDRGs
Sciatic N
Saphenous N
Collateral sprouting
Genetic and environmental influences on the transition from acute to chronic pain
Ze’ev Seltzer, DMDProfessor of Genetics
Canada Senior Research ChairUniversity of Toronto CTR for the Study of Pain
Ze’ev Seltzer, DMDProfessor of Genetics
Canada Senior Research ChairUniversity of Toronto CTR for the Study of Pain
• 1995 - suffered an accident at work– L. brachial plexus avulsion – L. hand numb and painful– L. hand paralysed at an awkward position
The case of Roni A. (male , age 44, contractor)
• 1997 – surgical relocation of the arm• 2002 – amputation of the hand
– Telescoping
– Triggering the phantom from the face/arm
– Exacerbation of pain
– When symp system aroused
– Changing weather
– When attempting to move phantom
No therapy has helped Roni get rid of the pain