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Scientific Update of ADHD

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1 Scientific Update of ADHD
Russell Schachar The Hospital for Sick Children Department of Psychiatry Brain and Behaviour Programme University of Toronto

2 Affiliations CIHR NINDS Lilly Purdue Frederick Shire
Barr, Kennedy, Ickowicz, Crosbie, Pakulak, Ornstein Noseworthy, Chevrier Robaey, Perusse NINDS Levin, Dennis, Barnes Lilly Purdue Frederick Shire

3 Outline Summary and overview current understanding Neuroscience 101
Genes Proteins Brain structure Cognitive function Behavior Caveats Caveats not an overview of ADHD not focused on treatment (RT)

4 ADHD g 1 g 2 g 3 hyp-imp inattention function environment
Cell membranes, transmitters, assemblies… Refer to this schematic several times. It is important to know how genes influence complex behaviours as a context for what is currently known about the etiology of ADHD. Implications Genes with a weak effect on behavioural manifestations of ADHD, may have stronger effects on fundamental processes Proteins g 1 g 2 g 3

5 Genetics

6 What are genes? DNA is specific sequence of nucleotide bases that encode instructions for proteins Genome is complete set of DNA

7 How do genes function? Many changes evident and passed on
Most changes cannot be seen by microscope Most base pairs are not involved in genes and are not functional Can be used to track functional changes May regulate gene expression or function Epigenetic factors affect gene function Many hereditary effects may be outside of genes

8 Neurotransmitter systems
Serotonin Noradrenalin Glutamate GABA Transporters and receptors variably distributed Dopamine transporter DAT not expressed in some parts of brain

9 Dopamine D4 48bp Repeat Variants: Pharmacologic differences not linearly correlated with number of repeats EC50 Asghari et al, 1995 Jovanovic et al, 1999 2R 4R 7R 10R Implication: the 7R allele has a blunted response to dopamine

10 Critical for controlled release of neurotransmitters into the synaptic cleft
Loss of expression of a single copy of the gene results in dysregulation in the controlled release of glutamate, dopamine, and serotonin in select brain regions SNAP-25 SNAP-25

11 Genes, proteins and neural development

12 Neural development Stem cells differentiate Half survive
Growth factors (sonic hedgehog, notch, BDNF) Half survive Development depends of where they end up, activity there (use it or lose it!) Rate of division/survival depends on experience, formation of synapses, integration into networks Loss of neurons normal (exaggerated in some diseases) Neurons likely regenerate and affected by experience (can teach old dog new tricks) Environment can affect gene expression

13 Brain tour

14 Phineas Gage

15 Prefrontal cortex Not involved in specific tasks
Executive control of behaviour, thought and affect Organization and planning for future action and social goals Balances perceptual, instinctual and motivational input Reflective: guided by internal states and intentions Control subordinate attention and motor processes

16 Prefrontal subcortical circuits
Begin and end in frontal cortex Pass through subcortical structures Reciprocal and interacting connections Excitatory and inhibitory neurotransmitters Separate yet overlapping and interacting Specific and intermingled mixture of deficits Evident in individuals with massive lesions

17 Dorsolateral circuit Organization, planning, attention
Lesions generate concrete thinking, inability to stop, shift set, filter and ignore distractions, plan and organize

18 Orbitofrontal circuit
Mediates socially appropriate behaviour Lesions lead to marked personality change, social disinhibition, explosiveness, tactlessness, lability, lack of interpersonal sensitivity

19 Anterior cingulate circuit
Motivation, balancing competing demands, performance monitoring Lesions result in akinetic mutism, apathy, lack of motivation, insensitive to errors

20 Frontal/ executive processes “wave of attention”
Encoding, maintaining, retrieving in working memory Preparing and anticipating Interference management Withholding of response tendency Maintain set Retract or inhibition Error detection Error correction Advances in conceptualization and measurement of EF problems of EF theories implies localization tightly relates symptoms and functions invokes homunculus mixes levels of analysis not all PFC functions are executive not all EF are PFC

21 Summary Considerable understanding of brain development and function
Differentiation in structure and function Cognitive deficits Genetic risks Structural and functional anomalies

22 ADHD genetics Highly genetic Not simple Mendelian inheritance
Multigenic Non-genetic factors contribute separately and through various combinations Disorder occurs when combination of genetic and non-genetic factors exceeds some threshold Nature of risk and mode of inheritance unknown

23 Genetics of ADHD % risk to family % concordance
Genetic risk high but not complete Incomplete penetrance Non genetic risks Variable expression

24 Genome Scan D19S229 D19S247 D19S204 D19S221 D19S179 D19S248 D19S178 D19S246 D19S180 D19S254 Systematically screen all of the chromosomes for linkage using DNA markers spaced at regular intervals

25 Genome scans for ADHD Fisher et al., 2002 Ogdie et al., 2003
UCLA 126 affected sib pairs no regions met genome-wide significance levels suggestive 5p12, 10q26, 12q23, 16p13 (Smalley, 2002) Ogdie et al., 2003 expansion of Fisher et al., sample sib pairs 17p11 (LOD 2.98), 16p13 Bakker et al., 2003 164 Dutch affected sib pairs regions with LOD scores > 3, 15q15.1, 7p13 LOD score > 2, 9q33.3

26 Candidate gene study Case-control Ethnicity
Compare samples Ethnicity Associated characteristic or disorder Family-based Compare children and their parents or siblings

27 Dopamine, cognition and behaviour
Neurotoxin reduces DA in rats hyperlocomotion, learning problems DA depletion impairs working memory Impulsiveness associated with low extracellular DA Blocking DA reuptake makes DA more available and improves executive control DA particularly important uptake [18F]F-DOPA labels CAT terminals uptake significantly less in left and medial PFC of ADHD adults compared to controls

28 Drugs and ADHD

29 Psychostimulants and ADHD
54 – 75 % adults and children with ADHD responds to methylphenidate (0.6 mg/kg) (Spencer et al 2001) The behavioural modifications induced by stimulants occurs with the reaching of peak plasma level

30 Dopamine Transporter Some individuals with ADHD, have higher expression of the dopamine transporter. Possible mechanism of genetic susceptibility is over expression of the DAT protein Stimulants blockade dopamine transporter and temporarily correct the levels of dopamine. DAT Knock Out mouse difficulty shifting - perseverative errors novelty-driven hyperactivity spatial learning deficit

31 Synaptosomal-Associated Protein of 25 kDa (SNAP-25)
Rationale: mouse irradiation mutant strain Coloboma has a single copy of the SNAP-25 gene. The other copy has been deleted. 1) hyperactive 2) responsive to dextroamphetamine 3) not responsive to methylphenidate 4) delayed in some developmental milestones

32 A number of Genes Identified as linked to ADHD What now?
Confirm linkage in larger samples Determine how these genes contribute to ADHD Additional candidates e.g., neurotrophic factors

33 Neuroimaging Computed tomography (CT)
Series of x rays from different angles Positron emission tomography (PET) Inject radioisotope that emits positrons Water labelled with oxygen-15 measure changes in blood flow Deoxyglucose labelled with florine-18 which accumulates in active cells Magnetic resonance imaging (MRI) Magnets detect magnetic molecules fMRI detects changes in magnetic properties of hemoglobin as it carries O2 to active brain cells


35 Brain structure and ADHD

36 Inhibitory control Withholding and withdrawing of responses if intention or circumstances change or error is made Failure of inhibition results in errors & impulsiveness

37 Inhibition and ADHD Activates orbital, DLPF and basal ganglia
Rich in DA Disruption of DA by knock out or neurotoxins affects executive control DA tone associated with executive control Impulsiveness associated with low extracellular DA Blocking DA (mostly in basal ganglia) reuptake makes DA more available and improves executive control

38 Stop Signal Task * X/O fixation point
go stimulus: choice reaction time task 500 ms motor response 1000 ms Inter-trial interval = 3000 ms

39 Stop Signal Task * X/O stop signal delay (variable) stop signal 500 ms
RED SCREEN or TONE stop signal 1000 ms 500 ms

40 Latency of inhibition (SSRT) in ADHD and controls
SSRT (ms) Schachar et al., 2001

41 Inhibition & psychopathology
IQ, age, aggression, speed, reading Osterlaans et al., 1999

42 Impulsive Personality

43 Inhibition and Methylphenidate Response
T-score (Mean = 50, SD = 10) Tannock, Schachar & Logan, 1995


45 Inhibition in concordant and discordant siblings
SSRT ms Concordant Disconcordant Controls

46 Evidence of performance monitoring
Introspection Slowing following errors Slowing following correct responses Self-detected action slips or errors due to faulty knowledge (external feedback) Pattern seen in range of tasks Speeded choice response tasks Memory tasks Inhibition tasks

47 Relevance of performance monitoring to ADHD
Poorly regulated behaviour “often fails to give close attention to details or makes careless mistakes in schoolwork, work, or other activities” Inaccurate and variable task performance

48 Slowing after non-stopped responses in ADHD and controls

49 Executive control summary
New breed of measures of specific processes Cognitively and neurally distinct, multiple deficits characteristic of ADHD Functional assessment warranted

50 What do you mean “Cognitive tests are not diagnostic”?
Clinic sample (N = 100) ADHD controls 50/25 50/5 Of 30 cases with “diagnostic marker”, 25/30 =83% will be ADHD General population (N = 100) 5/ /19 Therefore, of every 21.5 cases with the “diagnostic marker”, 19/21.5 = 88% will be controls!

51 “Environmental” causes of ADHD/cognitive deficit
Traumatic closed head injury prematurity radiation Toxic alcohol (FAS) smoking Social / stress (alteration in DA, NA, S release in PFC) in animals & humans disrupts complex cognitive function maternal stress during pregnancy disrupted early care–giving

52 School policy dilemma ADHD is a learning problem associated with cognitive deficits and academic underachievement

53 Approach differs from RD
But is handled differently than learning disability Accommodation in classroom

54 Summary Heritable Genetic risks Structure Function Non-genetic factors
Educational system

55 Opportunities for participation
Children years Presumptive diagnosis of ADHD Two affected children and both parents Willing to give blood No exclusions Teju Pathare (416) One affect, sibling unaffected Tracee Francis (416)

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