Hypothesis: Autism may be caused by Thyroid deficiency early in Pregnancy

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Hypothesis Autism may be caused by Thyroid
deficiency early in Pregnancy

• Gustavo Román MD

Professor of Neurology School of
Medicine University of Texas Health Science
Center San Antonio TX 78232 USA ROMANG_at_UTHSCSA.EDU

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Román G.C. Nutritional Disorders of the Nervous
System.
In, Shils ME, Shike M, Ross AC, Caballero B,
Cousins RJ, eds.  MODERN NUTRITION IN HEALTH
AND DISEASE. 10th edition (50th Anniversary
Edition), Philadelphia Lippincott Williams
Wilkins Chapter 88, pp 1362-1380, 2005.
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Alterations of cortical neuronal migration and
cerebellar Purkinje cells occur in autism.
Neuronal migration, via reelin regulation,
requires triiodothyronine (T3) produced by
deiodination of thyroxine (T4) by fetal brain
deiodinases. Experimental animal models have
shown that transient intrauterine deficits of
thyroid hormones (as brief as 3 days in mice)
result in permanent alterations of cerebral
cortical architecture reminiscent of those
observed in autism. Early (weeks 812) maternal
hypothyroxinemia with low T3 in the fetal brain
could alter neuronal migration producing
morphological brain changes leading to autism.
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Autism Incidence, Rochester MN
Barbaresi et al. Arch Pediatr Adolesc Med
20051593744
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Autism Incidence Rates
Barbaresi et al. Arch Pediatr Adolesc Med
20051593744.
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Research Hypothesis
GIVEN THAT

• Thyroid hormone is critical in early pregnancy
  for normal neuronal migration and for the normal
  development of the brain cortex
• Brain neuropathology in autism consists of
  alterations of neuronal migration affecting
  cerebral cortex (grey matter) and cerebellum

Thyroid hormone early in pregnancy may be
important in autism
THEN
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The Important Function of the Thyroid Gland in
Pregnancy
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Maternal T4 is critical for normal cortical
migration
Diet lacking Iodine
Thyroid gland Iodine
Anti-thyroid products
Thyroid hormones T3 T4
Weeks 12-24
Photo from Brain Research 1973621-35
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Thyroid Hormones
I
I
I
I
I
5-iodothyronine deiodinase
T3 is the active hormone
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Neuronal Cortical Migration

• Neocortical neurons migrate from their site of
  production in the periventricular germinative
  zone towards the cortical plate
• In humans, migration of neocortical neurons
  occurs mostly between weeks 12 and 24 of
  gestation
• Migration requires thyroid hormones, mainly
  triiodothyronine (T3) the active hormone
  produced by deiodination of maternal thyroxine
  (T4) by fetal brain deiodinases (D2)
• This complex process also involves molecules to
  control initiation of migration signaling
  molecules (mainly reelin) stop signals and
  others

Curr Opin Neurol 200619135140
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Abnormal Neuronal Migration

• Neurologic disorders such as lissencephaly,
  cortical heterotopias, and microcephaly are
  associated with gene products critical for
  neuronal migration
• Mutant mice provide insight into the genetic
  regulation of radial neuronal migration e.g. the
  reeler mouse has upside down cortical layers
  due to abnormal neuronal migration from lack of
  reelin
• Lissencephaly (smooth brain) results from
  sporadic mutations in the LIS1 gene
• Cocaine exposure of the developing fetus
  decreases GABA neuron migration to the cortex

Métin et al. J Neuroscience 2008 28 (46) 11746
11752
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Neuronal Cortical Migration
Iodine
T4 T3 reelin
T3R
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periventricular germinative zone
Primitive ventricle
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I II-III IV V VI
Late-born neurons migrate past early-born
neurons, forming six layers. Cortical neurons
migrate along scaffolds established by radial
glia, which are attached to the basal lamina of
the ventricular zone and extend through to layer
I. Cajal-Retzius neurons are the first to
populate the mantle of the cortex, and they
produce and secrete reelin, which is involved in
guidance of migrating neurons as they climb the
scaffold. Migration stops with phosphorilation
of Dab1.
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Autism Neuropathology

• Increased brain size and white matter (arcuate
  fibers)
• Irregular laminar pattern, thickened cortex
• Increased neuronal density
• Narrowing of minicolumnar width
• Altered Purkinje and granule cells in the
  cerebellum

Casanova MF Brain Pathol 200717422433. Bailey
et al. A clinicopathological study of autism.
Brain 1998121889905.
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Autism Neuropathology
Bailey et al. Brain 1998121 889905.
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Reelin-Dab Signaling System

• Reelin, an extracellular glycoprotein secreted by
  Cajal-Retzius neurons of layer I and by GABAergic
  neurons, binds to membrane receptors on migrating
  neurons phosphorylating the Disabled homolog-1
  (Dab1) to guide cells to their destination
• Reelin receptors include the apolipoprotein E
  receptor 2 (ApoER2), the very-low-density
  lipoprotein receptor (VLDL-R) and the a3ß1
  integrin protein

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Reelin signaling system in autism
Reelin is secreted by Cajal-Retzius cells and
binds to receptors VLDLR, ApoER2, and a3ß1
integrin. Following binding of Reelin to its
receptors, Dab-1 protein is oligomerized and
phosphorylated. In autistic brain, Reelin
signaling system appears to be impaired in three
steps 1) Reelin ligand is not produced
adequately as evident by reductions in mRNA and
protein levels in superior frontal cortex and
cerebellum 2) Reelin receptor VLDLR mRNA is
upregulated potentially in response to reduced
levels of its ligand, Reelin 3) Dab-1 mRNA is
also reduced, potentially due to reduction in
levels of Reelin, which normally activates Dab-1
phosphorylation via a positive-feedback loop.
Alternatively, Dab-1 levels may be reduced in
response to increases in levels of VLDLR acting
via a negative-feedback loop. Alterations in
levels of Reelin, its receptor VLDLR, and adaptor
protein Dab-1 interfere with the Reelin signaling
system affecting long-term potentiation, synaptic
plasticity, cognition,and memory modalities
involved in autism
Fatemi et al. Reelin signaling is impaired in
autism. Biol Psychiatry 200557777
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Abnormal reelin in Autism

• Significant reduction of reelin protein
• Decreased reelin mRNA and Dab 1 mRNA in superior
  frontal cortex and cerebellum
• Reelin receptor VLDLR mRNA is upregulated
• In autistic children, some polymorphisms of the
  reelin (RELN) gene in chromosome 7 could confer
  susceptibility to environmental factors

Fatemi et al. Reelin signaling is impaired in
autism. Biol Psychiatry 2005 57 777 Persico
et al for the Collaborative Linkage Study of
Autism. Reelin gene alleles and haplotypes as a
factor predisposing to autistic disorder. Mol
Psychiatry 20016150 Zhang et al. Reelin gene
alleles and susceptibility for autism spectrum
disorders. Mol Psychiatry 20027 1012 Skaar et
al. Analysis of the RELN gene as a genetic risk
factor for autism. Mol Psychiatry 2005 10
56371.
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Hypothyroidism and reelin-dab

• Hypothyroidism reduces reelin expression
• Enhances Dab1 expression
• Alterations of the reelin-dab system may explain
  the migratory alterations observed in
  experimental models of congenital hypothyroidism
  and maternal hypothyroxinemia

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Environmental Factors Affecting Thyroid Function
during Pregnancy
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Dietary Iodine Thyroid Function

• Iodine is essential to produce thyroid hormones.
  Lack of iodine in the diet is the most important
  worldwide cause of hypothyroidism, goiter and
  other iodine deficiency disorder
• Iodine deficiency is increasingly found among
  women of reproductive age in the USA and in
  Europe
• Women of reproductive age are at highest risk due
  to the effects of the thyroid on ovulation,
  fertility, and pregnancy outcome

Hollowell JG, Haddow JE The prevalence of
iodine deficiency in women of reproductive age in
the United States of America. Public Health
Nutrition 2007101532-1539
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Thyroid hormone deficiency during critical
periods of brain development from the 2nd
trimester of pregnancy to early postpartum leads
to mental retardation, low intellectual capacity,
psychomotor delay, and deafnessAttention
deficit hyperactivity disorder (ADHD) has also
been reported in mothers with thyroid deficiency
during pregnancy
Thyroid, Pregnancy and Cognition
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Thyroid Hormones
5-iodothyronine deiodinase
T3 is the active hormone
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Maternal T4 is the only source of fetal T3

• T3 is the active thyroid hormone
• T3 is produced locally in the fetal brain by
  deiodination of maternal T4
• Iodothyronine deiodinases (D2) are important for
  the spatial and temporal presence of T3 in
  different fetal brain areas

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T3 receptors (TRs) in Fetal Brain

• During the second trimester of gestation T3
  increases in the fetal cerebral cortex
• T3 binds to nuclear thyroid hormone receptors
  (TRs) regulating gene expression in different
  brain regions, interacting with regulatory sites
  in target genes
• TRs are present in human cerebral cortex by 89
  weeks of gestation and increase 10-fold between
  10 and 18 weeks gestation

Obregon MJ et al. Ontogenesis of thyroid function
and interactions with maternal function. Endocr
Dev 20071086-98.
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Maternal T4 is Critical in Pregnancy

• Maternal TSH increases progressively during
  normal pregnancy causing a rise of serum
  thyroglobulin and thyroxine (T4) except in
  Iodine deficient women since this requires
  doubling of the iodine intake
• Transfer of maternal T4 to the fetus continues up
  to birth
• Free T4 for embryonic and fetal tissues depends
  on circulating maternal T4 therefore, low
  maternal T4 (maternal hypothyroxinemia) is
  harmful to the fetus

Escobar GM, Ares S, Berbel P, Obregón MJ, del Rey
FE. The changing role of maternal thyroid
hormone in fetal brain development. Semin
Perinatol 200832 380-6.
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Window of Time for Iodine and Thyroid on the
fetal brain

• Iodine treatment of pregnant mothers with severe
  dietary iodine deficiency is effective in
  preventing endemic cretinism but within a fairly
  narrow period that ends at the beginning of the
  3rd trimester of gestation
• Therefore, there is a critical in utero window
  of time for the effect of T4/T3 thyroid hormones
  on normal brain growth and development

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Female rats were fed a low-iodine diet (LID) and
made hypothyroxinemic (low T4, normal T3/TSH),
then treated or not with KI early in pregnancy
(E14-E16). Permanent alterations of cortical
cytoarchitecture of the cortex and hippocampus
were found. Lavado-Autric et al. Early
maternal hypothyroxinemia alters histogenesis and
cerebral cortex cytoarchitecture of the progeny.
J Clin Invest 2003111107382.
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LID KI
LID KI
LID
LID
Lavado-Autric et al. Early maternal
hypothyroxinemia alters histogenesis and cerebral
cortex cytoarchitecture of the progeny. J Clin
Invest 2003111107382.
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Severity of Thyroid Deficiency on the Fetal Brain

• Severe maternal hypothyroxinemia from dietary
  iodine deficiency results in endemic cretinism
  (EC)
• Clinical features of EC profound mental
  retardation, deafmutism, squint of eyes, signs
  of bulbar damage, spastic diplegia, pyramidal and
  extrapyramidal signs, abnormal gait
• Congenital hypothyroidism (CH) affects 1/3500
  newborns (0.03) as a result of thyroid
  dysfunction in the fetus and the newborn it is
  unrelated to dietary iodine deficiency
• CH causes mental retardation if untreated
• Maternal hypothyroxinemia is estimated to be
  150200 times more prevalent than CH (4.3-5.7 of
  all newborns)
• Developmental delay and lower intellectual
  function, ADHD

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2007 Nov18(9)338-43
Iodine deficiency causes maternal
hypothyroxinemia, which affects pregnant women
even in apparently iodine-sufficient areas, and
often goes unnoticed because L-thyroxine (T4)
levels remain within the normal range, and
thyroid-stimulating hormone (TSH) is not
increased. Even a mild hypothyroxinemia during
pregnancy increases the risk of
neurodevelopmental abnormalities, and
experimental data clearly demonstrate that it
damages the cortical cytoarchitecture of the
fetal brain. The American Thyroid Association
(ATA) recommends a supplement of 150 mg
iodine/day during pregnancy and lactation, in
addition to the use of iodized salt.
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TRENDS in Endocrinology and Metabolism 2007,
Vol.18, No.9
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Thyrotoxic environmental pollutants

• A number of environmental pollutants (man made or
  natural) are recognized endocrine disruptors
• Anti-thyroid agents occur with contamination of
  water sources and foodstuffs with herbicides such
  as 2,4-D, PCBs, PBBs, perchlorate, mercury, or
  coal derivatives such as resorcinol, anthracenes,
  and phthalates.

Rogan Ragan. Int J Hyg Environ Health
2007210659, Pediatrics 2003112247 Minerva
Pediatr 20065847 Clin Med Res 20031281 Res
Environ Health 200722213
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Polychlorinated biphenyls (PCBs)
Kuroda Y. Effects of endocrine disrupters on
brain development. Development of novel assay
systems for risk assessment. Environ Sci
2003102333.
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Flavonoids inhibit Deiodinase
Food Chem Toxicol 200240913917
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Flavonoids inhibit thyroid D1 activity
Food Chem Toxicol 200240913917
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Flavonoids and Thyroid

• Flavonoids are universally present in plants,
  fruits, grains and vegetables.
• Flavonoids are present in high concentrations in
  various staple foods such as millet, soy, beans
  and nuts.
• Following ingestion, flavonoid glycosides are
  hydrolyzed by intestinal microbial glycosidases
  to flavonoids aglycones, and absorbed to undergo
  further metabolism.
• Each metabolic step increases their antithyroid
  effects (Gaitan, 1996).

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Flavonoids Structures
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Soy (Glycine max)

• Genistein and daidzein from soy are important
  plant isoflavonoids with antithyroid effects
• Both inhibit thyroperoxidase, the enzyme that
  catalyzes iodination and thyroid hormone
  biosynthesis
• Genistein and other flavonoids inhibit
  5'-deiodinases in neural tissues as a result of
  conformational changes in the enzyme

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D1 inhibition by flavonoids

• Catechin from tea (Camellia sinensis)
• Quercetin (found in apples, onions, red grapes,
  citrus fruits, broccoli, cherries, berries, and
  prickly-pear cactus)
• Kaempferol (from Delphinium, Witch-hazel, and
  grapefruit)
• Rutin (from buckwheat)
• Baicalein (from the roots of baikal or Chinese
  skullcap (Scutellaria baicalensis Georgi)
• Effects are worsened by iodine deficiency
• Some anthocyanins, catechins and tannins from
  nuts exhibit goitrogenic effects

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Plants with hypothyroid effects

• Millets such as pearl millet (Pennisetum
  glaucum) and fonio millet (Digitaria exilis),
  bugleweed (Lycopus virginicus), gypsywort
  (Lycopus europaeus), water horehound (Lycopus
  lucidus or Lycopus americanus), gromwell
  (Lithospermum ruderale), European gromwell
  (Lithospermum officinale), lemonbalm (Melissa
  officinalis), and perhaps rosemary (Rosmarinus
  officinalis) and sage (Salvia officinalis)

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Structure of millet flavonoids with anti-thyroid
peroxidase effects
Nutrition 199612100-106
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Hypothesis Summary

• Transient hypothyroxinemia (low T4) early in
  pregnancy (weeks 812).
• Low T4 may result from dietary iodine deficiency
  or from maternal exposure to antithyroid
  substance (s), or both.
• Exposures may include
• dietary use of soybean products (genistein and
  daidzein) or other plant products containing
  isoflavonoids or thiocyanate which inhibit
  thyroperoxidase and/or tissue deiodinases leading
  to low fetal T3 or,
• contamination of water sources and foodstuffs
  with thyrotoxic environmental pollutants (man
  made or natural), including herbicides such as
  2,4-D, PCBs, PBBs, perchlorate, mercury, or coal
  derivatives such as resorcinol, anthracenes, and
  phthalates.
• Low maternal T4 plus inactivation of brain
  deiodinases by any one of the above mechanisms
  would lower fetal T3 tissue levels resulting in
  abnormal neuronal migration via reelin
  dysregulation.
• reelin-dab signaling abnormalities occur in
  autistic brains low reelin protein, reelin mRNA,
  and Dab 1 mRNA, with increase of the reelin
  receptor and some polymorphisms of the reelin
  (RELN) gene

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Take-home message

• The American Thyroid Association (ATA) recommends
  a supplement of 150 mg iodine/day during
  pregnancy and lactation, in addition to the use
  of iodized salt.
• T3, T4, fT4, TSH and iodine in urine must be
  determined in women considering conception, as
  early as possible in the pregnancy, and during
  the 2nd and 3rd trimesters
• However, the American College of Obstetricians
  and Gynecologists. does not recommend routine
  screening for subclinical hypothyroidism because
  there is not good evidence that identification
  and treatment improves maternal or infant
  outcomes (Casey Leveno Obstet Gynecol
  2006108128392).