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

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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

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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