1. Autism and Fragile X Syndrome: Genetic and Neurobiological Causes
Presented by Taylor Beck
Dec. 4, 2006
MOL 90

2. What is Autism? Leo Kanner, 1943
A ‘triad’ of deficits (a hodgepodge of symptoms) comprising
Impaired social interaction
Impaired communication
restricted interests, repetitive behaviors, and obsessive-compulsive symptoms
Speech doesn’t develop or is so inflexible and unresponsive to context that it is unusable in normally paced conversation
Echolalia: parroting of other people’s words
“Parallel Play” at edge of group; No Pretend Play
Intense narrowly-focused interests (Baron-Cohen, 2002) concentrate on deterministic systems: videogames, toilets
Sensory hypersensitivity, motor incoordination

3. “Refrigerator Mom” theory of Autism (Bettelheim, 1967)
Autism considered an emotional disturbance resulting from bad “early attachment experiences”: Frigid parenting
Autistic kids “become” cold and unresponsive socially because they were raised by cold unloving mothers

4. But now… Developmental Neurobiology of Autism (Courchesne, 2005)
30% of autistics have macrocephaly: 5-10% enlarged brain
Brain size defined by head circumference, MRI, and postmortem brain weights, shows very abnormal development:
At birth, head circumference in ASD patients is below average
By 3-4 years old, ASD brain size exceeds normal by ~10% based on in vivo MRI studies and postmortem brain weights
By 6-7 years old, ASD may show only a small increase; However, the largest study suggests a persistent ~5% difference (Schultz et al, 2005)

5. Autistic Brain Development: Too Small Then Too Big
Age-Related Head Circumference
2-4 year olds: Regional Enlargement
Courchesne, 2004

6. Cerebellum Abnormalities
Increased Cerebellar white and gray matter (Courhcesne et al, 2001)
Reduced cerebellar vermis (gray matter) (Courchense et al, 2001)
Decreased numbers of cerebellar Purkinje cells (Lee, 2001)

7. Autism Spectrum Disorders (ASD): A Continuum of Behavioral Symptoms
Asperger Syndrome (Wing, 1981): Speech is normal, with social and narrow-interest symptoms of Autism
Pervasive Developmental Disorder- Not Otherwise Specified (PDD-NOS): Retarded social development
Autism’s symptoms are comorbid with many other mental illnesses
No two Autistic individuals are the same: Broad phenotypic range
e.g. Obsessions and“Intense narrow interests” range from the abstract (role playing games) to concrete/mundane (toilet)
ASD is best viewed not as a discrete pathology but as a continuum of symptoms which may share genetic and neurobiological causes with other mental illnesses including Fragile X mental retardation

8. 75% of Autistics are mentally retarded: Too many/ few synapses
30% of Autistics are epileptic: GABA/ Glutamate imbalance
30% have macrocephaly: 5-10% enlarged brain- too many cells
25% have high serotonin, as in Obsessive-Compulsive Disorder

9. The “Central Coherence” Problem in Autism and Autism Research
What is the deficit in Autism?
“Central Coherence” = Inability to bind together separate features into a single coherent concept or object (Frith, 1989)
“Theory of Mind” (Baron-Cohen, 1985)
Empathy/ Mirror Neuron System (Iacaboni & Dapretto, 2006)
Social Bonding: Vasopressin and Oxytocin (Insell, 2001)
Cerebellar abnormality: Increased cerebellar white matter; reduced vermis; 5-10% enlarged brain (Courchesne, 2001)
Abnormal neural connectivity: Indiscriminately high local connectivity coupled with low long-range computational connectivity-- Failure to differentiate signal from noise (Rubenstein & Merzenich, 2003)

10. Prevalence and Heritability?
~ 1 in 166 children, or 73 per 10,000, is affected with ASD (DiCicco-Bloom, 2006; Moldin, 2006)
Siblings born in families with an ASD child have a 50- to 100-fold greater chance of ASD, with a recurrence rate of 5-8% (Szatmari et al, 1998)
60-90% concordance rate in identical twins (Belmonte et al, 2004)
Broad variation of phenotypes suggests a large number of genetic and environmental biasing factors:
The same genes in different genetic and environmental contexts manifest different behavioral symptoms: No two autistics are the same

11. First Birthday Home Videos (Baranek, 1999; Osterling et al, 2002): How early can we see it?
Symptoms which differentiate Autistic children from Typical Development children at 24 months of age:
Responding to name (Baranek, 1999) *
Looking at others (Osterling et al, 2002) *
Smiling at others (Maestro et al, 2002)
Motor stereotypies/ Repetitive behaviors (Baranek, 1999)
“Buh buh duh buh”: Reactivity (Zwaigenbaum et al, 2005)
* Differentiate autistics from Delayed Development children
Subset of “late-onset” autistics show no symptoms at 1 year

12. Prospective Longitudinal Studies of high-risk family members (Szatmari et al, 1998)
At 12 months of age, atypical behaviors can distinguish siblings later diagnosed with ASD from other siblings and low-risk control infants
Visual attention (eye tracking)
Imitation
Social responses (respond to name; eye contact; smiling)

13. What is the deficit in Autism? Theory of Mind?
Theory of Mind: the ability to impute mental states to oneself and others: to understand that other people know, want, feel, and believe things (Premack & Woodruff, 1978)
‘Theory of mind’ is impossible without the ability to form second-order representations (Dennett, 1978; Pylyshyn, 1978)
Develops at age 2, along with emergence of pretend play (Leslie, 1984)
Absence of capacity to form second-order representations would lead to lack of theory of mind-- social ineptness and lack of pretend play in autistics: “Treat objects and people alike”

14. Does the autistic child have a “theory of mind”
Does the autistic child have a “theory of mind”? (Baron-Cohen et al, 1985)

15. Does the autistic child have a “theory of mind”
Does the autistic child have a “theory of mind”? (Baron-Cohen et al, 1985)
Naming Question: Sally and Anne- Which doll is which?
Belief Question: “Where will Sally look for the marble?”
Reality Question: “Where is the marble really?”
Memory Question: “Where was the marble at the beginning?”

16. Does the autistic child have a “theory of mind”
Does the autistic child have a “theory of mind”? (Baron-Cohen et al, 1985)
All subjects passed the Naming, Reality and Memory Questions-- without exception
Normal and Down’s Syndrome kids both passed the Belief Question, despite IQ differences
23 out of 27 normal children (85%)
12 out of 14 Down’s Syndrome children (86%)
Autistic children: 16 out of 20 (80%) fail the Belief Question
All 16 pointed to where the marble really was, rather than to some other location

17. Mirror Neurons in Monkeys and Humans
Macaque F5 Premotor neurons
fMRI: human Broca’s area (Brodmann’s 44 premotor) and inferior Parietal lobule show mirror properties: imitation > action> observation

18. Imitation and the Human Mirror Neuron System (Iacaboni, 1999)

19. Understanding Emotions in Others: The Mirror Neuron System in Autistics (Dapretto, 2005)

20. Face Processing in Autistics/ Asperger’s: Hypoactivation of the FFA (Schultz, 2005)
14 high-functioning autistics contrasted with 2 groups of 14 each age and IQ matched adults on object-discrimination task
Normal performance on object-recognition: Deficit is specific to faces.
All autistics socially impaired by the Autistic Diagnosis Observation Schedule, ADOS
Individual differences in FFA hypoactivation correlate inversely with time spent fixating on eye region (Dalton et al, 2005)

21. Attention and Autism: fMRI (Belmonte & Baron-Cohen, 2004)
Autistics show abnormally strong Parietal (sensory) activation during suppression of distractors, weak PFC and Medial Temporal (“Integrative” regions)
Over-processing at sensory level, under-processing at high level: the “Central Coherence” problem?
Non-autistic brothers share negative PFC and Medial Temporal cortex but not positive sensory (parietal) activation:
Low activity in integrative regions = a familial pattern of brain organization that puts individuals at risk?
Diagnostic role for fMRI in Autism? = Over-active parietal?

22. Social Bonding neuropeptides in voles: An animal model for autism
Social Bonding neuropeptides in voles: An animal model for autism? (Lim & Young, 2005)
Monogamous Prairie Vole vs. Polygamous Montane Vole
Oxytocin (OT) and vasopressin (AVP) receptors are highly expressed in social prairie vole but not montane vole mesolimbic reward structures: nucleus accumbens and ventral pallidum
Density of AVP receptors in male ventral pallidum and of OTRs in female nucleus accumbens correlates with individual monogamous social behavior (partner preference)

23. Vasopressin, Oxytocin and Social Bonding (Lim et al, 2005)
Pair-bond formation in monogamous males is facilitated by AVP and prevented by V1aR blocker (Young et al, 1999)
Overexpression of the vasopressin receptor gene V1aR in the ventral pallidum of the polygamous montane vole induces pair-bonding (Pitkow et al, 2001)
OT receptors in the nucleus accumbens facilitate “spontaneous” maternal behavior in individual female prairie voles (Olazabal & Young, 2006)
Virgin female prairie voles with higher OT-receptor density in the NA spent more time spontaneously grooming pups

24. “Pair Bonding” = Social Bonding?
The lab measure of “pair bonding” is really a measure of social preference-- the “monogamous” or “social” vole chooses to spend more time in the cage of the familiar mate: is not “indifferent” to other individuals-- Does not “Treat others as objects” (Autism?)
The Vasopressin/ Oxytocin “Social-Bonding” system is one basic neurobiological mechanism that might underlie autism

25. Prairie Vole to Human?
V1aR gene: polymorphic repetitive element determines receptor expression pattern-- Individual differences in length of unstable element correlate with individual differences in social behavior (Hammock & Young, 2005)
avpr1a: Human vasopressin receptor gene also contains polymorphic repetitive elements
Two independent studies report alleles of this polymorphism associated with ASD (Kim et al, 2001; Wassink et al, 2004)

26. Oxytocin knock-out mice don’t cry for mom or approach her
OT knock-out pups do not show normal ultrasonic distress calls when separated from their mother (Winslow et al, 2000) and increased latencies to approach mother
Consistent with “Refrigerator mom” behavior of ASD children
ASD children have decreased plasma Oxytocin (Modahl, 1998)
Oxytocin as therapy for social deficits in ASD? (Holland et al, 2003)

27. Genetics of Autism?
ENGRAILED-2, a cerebellar developmental patterning gene recently associated with ASD (Gharani et al, 2004)
EN-2 deletion or over-expression produces Purkinje cell deficits in cerebellum; diminished posterior vermis in mice also seen in human autistics
Human EN-2 localizes close to chromosomal 7 region identified by several ASD linkage scans
EN-2 overexpression in neural precursors maintains precursor proliferation and reduces neuronal differentiation

28. Fragile X and Autism
75% of autistics are mentally retarded; Fragile X syndrome is the most common form of inherited MR
While ~5% of autistics have Fragile X, 1/3 of Fragile X patients are autistic; Fragile X shares many symptoms with autism:
Epilepsy: 30% of autistics are epileptic
Social impairment/ Anxiety
Echolalia/ Delayed speech development
Sensory hypersensitivity and motor incoordination
Small Cerebellar vermis; Large caudate nucleus
If Autism is viewed as a “spectrum” of behavioral symptoms rather than a discrete pathology, Fragile X overlaps this continuum

29. Fragile X
In contrast to Autism, Fragile X is a one-gene disorder: caused by CGG repeat expansion of the X-linked FMR1gene, and the loss of FMRP, an RNA-binding protein produced in response to the activation of group 1 mGlu receptors
A “Spectrum Condition” like Autism:
Variable phenotype: Severity of retardation/social/ motor/ anxiety deficits varies: Mild form is just learning disability
Full mutation (>200 repetitions) causes methylation of FMR1, no production of mRNA-- no FMRP
“Pre-mutation” (>55 repetitions): Disruption in translation not transcription-- FMRP scarce despite abundant mRNA
= Same phenotype by different mechanisms

30. Don’t look for “lesions”!: Network Interactions
Monotonicity and Independence: “We assume that (I) an abnormal loss of function in a gene or cellular process ought to produce a phenotype opposite to that found in the case of an abnormal gain of function, and (II) this relation between dosage and phenotype is the same regardless of the individual’s genetic, environmental or developmental context. We make these assumptions for the same practical reason that a physicist posits a frictionless surface, or a novelist invents thematic characters and plots: they simplify complex relationships for which we have no exact models, and they are often close enough to reality to make useful predictions about real-world processes. They are, however, fictions.” (Belmonte & Bourgeron, 2006)
Variable phenotype AND variable cause is the norm in developmental disorders

31. Context Matters: Many Routes to Dysfunction
SLC6A4 Serotonin Membrane Transporter: Genetic link to Autism
Association with Autism studies split two ways: Preferential transmission of either long or short alleles of SLC6A4 promoter. Relationship of gene dosage to genetic and environmental modifiers-- Role of SLC6A4 may differ for Autism subpopulation with elevated platelet serotonin
Rare SLC6A4 functional alleles have been associated with neuropsychiatric disorders: Obsessive-Compulsive Disorder, anorexia and Asperger syndrome
Role is Dimensional (Quantitative properties shared by distinct disorders) rather than Categorical (qualitative, essential properties unique to one disorder)

32. Fragile X and Autism: Overlapping Symptoms- Common Genetics?

33. Noisy networks: Loud and silent both = NOISY
Signal vs. Noise is equivalently disrupted in an over-connected and an under-connected network or neurons or genes
Treat the effects instead of the causes
e.g. Fragile X treatment with GluR5 antagonist does nothing to reverse the pathology (Under-inhibition of GluR-mediated LTD due to lack of FMRP) but instead prevents these receptors from being activated in the first place
Dissociation between pathological processes and therapeutic targets
Many routes to dysfunction = many routes to cure

34. mGluR theory of Fragile X: Implications for Autism?