The Neurobiology of Autism:

New Pieces of the Puzzle

Maria T. Acosta, MD, and Phillip L. Pearl, MD

Address
Department of Neurology, Children's National Medical Center,
111 Michigan Avenue NW, Washington, DC 20010-2970, USA.
E-mail: [e-mail]

Current Neurology and Neuroscience Reports 2003, 3:149–156
Current Science Inc. ISSN 1528–4042
Copyright © 2003 by Current Science Inc.

Introduction

Autism is a behaviorally defined syndrome characterized
by atypical social interaction, disordered verbal and non-
verbal communication, restricted areas of interest, and lim-
ited imaginative play. Although autism has been well
validated as a syndrome, it is heterogeneous in its presenta-
tion. Severity varies widely; intelligence ranges from
severely deficient to superior, and language from absent to
grammatically complex but dysprosodic. Even when intel-

ligence is average, cognitive profiles are frequently mark-
edly uneven. Significant deficits (eg, in social cognition)
may coexist with islets of strength or even superiority (eg,
visuospatial skills). In addition to the triad of core deficits,
multiple other domains, including motor, sensory, and
perceptual (eg, auditory, tactile), may be affected. The
developmental course varies, with regression in language
and social relatedness noted in perhaps one third of cases.
Epilepsy affects approximately one third of autistic chil-
dren as well, and subclinical epilepsy has been hypothe-
sized to play a role in clinical regression [1,2].

Prevalence is considered approximately four to five per

1000 people, but numbers differ according to criteria used
for diagnosis. Earlier identification and better availability
of services can increase the awareness of this entity [3].

Nosology: Autism as a Complex
Neurobiologic Disorder

Whether autism should be considered a single syndrome
with highly variable severity (ie, the autistic spectrum) or
an aggregate of specific disorders that share common fea-
tures is unresolved. It is clear today that autism is a syn-
drome with multiple etiologies. Recently, enormous
advances in genetics, neuroimaging, and neurotransmitter
studies have advanced our knowledge of the pathophysiol-
ogy of autism, although much uncertainty remains.

There is an effort to develop a validated typology based

on behavioral criteria, based on the hypothesis that dys-
function of a particular brain region or neural network
may produce reasonably predictable behavioral deficits. In
autism, this dysfunction would potentially be a result of
genetic and environmental factors, expressed morphologi-
cally as alterations in early brain development.

Advances in Genetics:
Genotype versus Phenotype

There is a body of evidence implicating genetic factors as
playing a primary role in the etiology of autism [4]. Multi-
ple twin and family studies demonstrate significant differ-
ences in monozygotic and dizygotic twin concordance
rates, suggesting an underlying genetic predisposition to
autism exceeding 90% [5]. In addition, the rate of autism
among a proband's siblings is 3% to 6%, or 50 to 100
times greater than the general population [6]. The signifi-

The neurobiologic basis of autism is reviewed, with discus-
sion of evidence from genetic, magnetic resonance imaging,
neuropathology, and functional neuroimaging studies.
Although autism is a behaviorally valid syndrome, it is
remarkably heterogeneous and involves multiple develop-
mental domains as well as a wide range of cognitive, lan-
guage, and socioemotional functioning. Although multiple
etiologies are implicated, recent advances have identified
common themes in pathophysiology. Genetic factors play a
primary role, based on evidence from family studies, identi-
fication of putative genes using genome-wide linkage analy-
ses, and comorbidities with known genetic mutations. The
RELN gene, which codes for an extracellular protein guiding
neuronal migration, has been implicated in autism. Numer-
ous neuropathologic changes have been described, includ-
ing macroencephaly, acceleration and then deceleration in
brain growth, increased neuronal packing and decreased
cell size in the limbic system, and decreased Purkinje cell
number in the cerebellum. Abnormalities in organization of
the cortical minicolumn, representing the fundamental sub-
unit of vertical cortical organization, may underlie the
pathology of autism and result in altered thalamocortical
connections, cortical disinhibition, and dysfunction of the
arousal-modulating system of the brain. The role of
acquired factors is speculative, with insufficient evidence to
link the measles-mumps-rubella (MMR) vaccine with autism
or to change immunization practices.

150

Pediatric Neurology

cant sibling risk versus the rapid fall-off in risk for distant
relatives is consistent with the involvement of multiple
susceptibility genes. The existing genetic models postulate
different numbers of interacting genes, ranging from less
than 10, with three loci providing the best fit [7], to more
than 15 genes, each with a minor effect [8].

Because autism is a complex disorder, it is clear that the

design of genetic studies plays an important role. One
source of discrepancy may be the heterogeneity in autism,
both genetic and phenotypic. Among these considerations,
two types of genetic studies have been designed. One
focuses on individuals with well-defined subtypes and
groups of patients that are homogeneous and, therefore,
reduce the "noise" of pooling subjects. This should facili-
tate detection of the genetic "signal" [9].

The other type of study design looks for a wide range

of clinical and subclinical forms of the disorder in rela-
tives of affected patients. For example, an increased fre-
quency of speech disorders has been found [7]. Also, an
increased prevalence of a series of personality traits (eg,
impulsive, aloof, shy, and eccentric) [10] and specific pat-
terns of social behavior have been identified [11]. As the
measurement and detection of these differences become
more refined, the information may be useful in genetic
analyses. Monozygotic twin studies have shown a concor-
dance rate as high as 60% for the diagnosis of autism.
Milder forms of social and communicative abnormalities
were found in up to 95% of monozygotic twins [5]. The
rate of these abnormalities among relatives of probands is
remarkably increased compared with the general popula-
tion, and falls rapidly with a decrease in the degree of
familial relationship [12].

Cytogenetic and molecular studies
Genome-wide linkage analyses of families with autism
have yielded positive signals for chromosomes 1, 2q, 7q, 9,
13, 15q, 16p, 17q, 19, 22, and X [13–16]. The best candi-
dates for regions containing an autism locus are chromo-
somes 2q and 7q [17,18]. An independent linkage study
identified a region in chromosome 7q31 as the locus
responsible for the speech-language disorder 1 (SPCH1)
[19]. The possible relationship between SPCH1 and autism
is supported by increased rates of language impairment in
relatives of autistic individuals [20,21]. Recently, the identi-
fication of the gene FOXP2, which is responsible for
SPCH1 and encodes a putative transcription factor, pro-
vides light on the neurodevelopmental process that culmi-
nates in speech and language [22]. Additional associations
in chromosome 7 have been found with another candidate
gene for autism, RELN, which encodes for reelin, an extra-
cellular protein guiding neuronal migration during brain
development. Recent studies have found a significant asso-
ciation between autistic disorder and a GGC repeat located
at 5’ on the RELN gene [23]. Reelin plays a pivotal role in
the development of the cerebral cortex, cerebellum, hip-
pocampus, and brain stem, which are structures associated

with alterations in autism [24]. However, data from differ-
ent groups have shown that changes in RELN expression
exhibit a broad phenotypic spectrum, including several
neuropsychiatric disorders [25,26], neuromuscular disor-
ders, and lissencephaly [27].

Approximately 10% to 15% of autistic individuals

demonstrate comorbility with known genetic conditions,
including tuberous sclerosis, neurofibromatosis, fragile X
syndrome, and chromosome abnormalities [28]. Cytoge-
netic screening for chromosomal abnormalities has dem-
onstrated chromosome 15 and X chromosome break-
points as the most frequently associated with autism [29].
Researchers have assumed that gene alterations arising
from such mutations, or the presence of duplicated or
missing genes, are directly responsible for the behavior
phenotype [9]. Special interest has developed in chromo-
some 15, as cytogenetic abnormalities in the Prader-
Willi/Angelman syndrome critical region (15q11-13)
have been described in several individuals with autism.
Markers across this region have been screened for evi-
dence of linkage association. Three of the gamma-ami-
nobutyric acid (GABA)-A receptor subunits are localized
in this critical region. Several studies found an association
with polymorphisms of a GABA-A receptor subtype,
GABRB3, and autism [30] or other genes nearby [31,32].
Other studies have not replicated these findings [33].
UBE3A, an Angelman syndrome gene, has been associ-
ated with autism [34,35]. Although it is premature to
consider these findings determinants in the etiology of
autism, they open additional opportunities for genetic
and neurotransmitter studies.

The higher incidence of autism in boys compared with

girls and the relatively high frequency of autistic character-
istics in patients with Fragile X syndrome have led to signif-
icant interest in the X chromosome. Family studies have
demonstrated the presence of an association between
known genes of major significance on the X chromosome
and autism [16,36]. Mutations in the X-linked gene
MECP2 have been identified in up to 80% of typical Rett
syndrome patients, with most of the mutations originating
de novo [37]. However mutations in MECP2 have been
observed in a variety of neurobehavioral problems, includ-
ing autistic syndrome, learning disabilities, and neonatal
encephalopathies [38,39•]. Table 1 shows a list of some of
the genes that have been associated with autism [39•]

Neuroimaging Studies:
Magnetic Resonance Imaging

Several neuroimaging studies have reported abnormalities
in specific brain regions, including cerebellum [40–44],
mesial temporal structures [45•,46], brain stem [47], basal
ganglia, and corpus callosum [48].

Increase in the overall size and weight of the brain in

individuals with autism compared with age-matched con-
trol subjects has been reported [49,50]. Brain size in chil-

The Neurobiology of Autism: New Pieces of the Puzzle • Acosta and Pearl

151

dren with autism appears to be normal at birth [43,51].
This conclusion is based on measures of head circumfer-
ence with an index that has been shown to be highly pre-
dictive of computed tomography (CT)-based or magnetic
resonance imaging (MRI)-based brain volume [52•,53].
By 2 to 4 years of age, however, 90% of autistic patients
have brain volumes that are larger than average. More-
over, 37% of 2- to 4-year-old autistic toddlers meet crite-
ria for developmental macroencephaly [51].

Several volumetric studies using MRI techniques have

explored this issue. Total brain volume by MRI volumetric
techniques was recently measured in a group of 67 autistic
individuals from 8 to 46 years of age and 83 normal con-
trol patients matched by sex, age, IQ, ethnicity, and socio-
economic status [54••]. All autistic individuals were non-
mentally retarded. An increase in total brain volume in
subjects with autism between 8 and 12 years of age was
found. No increase in brain volume was found for autistic
individuals 12 years or older. Similar findings in autistic
children aged 2 to 4 years has been reported [52•]. In this
study, 90% of autistic boys aged 2 to 4 years had more cere-
bral (18%) and cerebellar (39%) white matter and more
cerebral cortical grey matter (12%) than normal patients,
whereas older autistic children and adolescents did not
have such enlargement of grey and white matter. Addition-
ally, increased cerebral size in young autistic children
showed an anterior to posterior gradient, with frontal
lobes being the most enlarged and occipital lobes showing
the least effect. After a period of accelerated growth, cross-
sectional MRI data show slowed growth velocity through-
out cerebral and cerebellar regions, in contrast with nor-
mal subjects. For example, in normal children, frontal lobe

grey and white matter volumes increased by 19% and 46%,
respectively, from 2 to 4 years of age compared with 9 to 12
years, but increased by only 1% and 14%, respectively, in
autistic children during this same age span [55••,56••].

Results from multiple studies, therefore, support the

conclusions that autism is associated with acceleration in
brain growth during early childhood, and that this increase
in brain volume does not persist throughout the lifespan
[54••]. Tissue enlargement, however, does not appear to be
a global phenomenon in the developing autistic brain.
Variations have been observed in areas including the cere-
bellum, limbic system, and corpus callosum [43,56••,57].
In a study of boys with autism aged 2 to 3 years, there was
less grey matter in the cerebellum, a smaller ratio of grey to
white matter, and smaller vermian lobules VI-VII than in
normal control subjects [52•]. There are 16 MRI studies
reporting significantly reduced size of the cerebellar hemi-
spheres or vermis in patients with autism, making the cere-
bellum the most widely replicated site of MRI abnormality
in the autism literature [43]. These neuroimaging findings
correlate with post-mortem examinations in which 95% of
autistic cases have loss of cerebellar Purkinje neurons.

A developmental defect has been hypothesized in the

limbic system, given its mediation of memory, social, and
affective functions. A developmental MRI study of
patients ranging from ages 29 months to 42 years showed
a smaller cross-sectional area of the dentate gyrus [41].
Posterior regions of the corpus callosum were also
reduced in size in autism in a study of patients ranging
from 3 to 42 years of age [58].

In conclusion, recent MRI observations suggest abnor-

mal regulation of brain growth in autism, characterized by
early overgrowth followed by abnormally slow growth in
some regions, but premature arrest of growth in others.
This evidence raises a growth dysregulation hypothesis of
autism in which there is pathologic dysregulation in the
timing and amount of growth as well as cessation of
growth. Pathologic growth regulation during this critical
period of life could lead to widespread and pervasive con-
sequences for the functional differentiation of systems
mediating many neurobehavioral domains. Evidence con-
sistent with this prediction of aberrant functional organi-
zation in autism comes from recent functional MRI (fMRI)
studies of mature patients performing tasks involving
movement [59,60], face perception [59,60], processing
emotions [61], and visual attention [58].

This abnormal pattern of brain growth in autism

may also include a lack of normal acceleration in
growth that occurs in typically developing adolescents.
This period of maturation is associated with the emer-
gence of a second phase of higher-order abilities, par-
ticularly frontal lobe functions [62]. This means that
even though the brain size is "normalized" in adoles-
cents with autism, this normalization may be the result
of both early acceleration and later deceleration in
brain growth.

Table 1. Genes frequently associated with autism

Gene

Location

MECP2 (methyl-CpG binding protein - 2) Xq28
5-HTT (serotonin transporter)

17q11.1-q12

5-HTR 7 (serotonin receptor 7)

10q21-q24

GABRB3 (gamma-aminobutyric acid

receptor subunit beta 3)

15q11-q13

UBE3A/E6-AP (ubiquitin-protein ligase)

15q11-q13

HRAS1 (c-Harvey-ras-1)

11p15.5

HOXA-1 (Homebox A-1)

7p15.3

RAY1 or FAM4A1 (supressor of

tumorigenicity 7)

7q31.3

WNT2

7q31-33

RELN (reelin)

7q22

ALDH51A1 (succinic semialdehyde

dehydrogenase)

6p22

HLA genes

6p

GluR6 (glutamate receptor 6)

6q21

ASL (adenylosuccinate lyase)

22q13.3-q13.2

Mitochondrial transfer RNA (Lys)

Mitochondrial

DNA

HTR2A

13q

Bcl-2

18q 21.3

FOXP2 (speech-language disorder 1)

7q31

152

Pediatric Neurology

Elevated brain neurotrophins and neuropeptides

(brain-derived neurotrophic factor, neurotrophin 4/5,
vasoinhibitory peptide, and calcitonin gene-related pep-
tide) have been found in neonatal blood spots of individu-
als who later developed autism and mental retardation
[63••]. These and other growth factors play roles in neu-
ronal proliferation, migration, differentiation, growth, and
circuit organization. These elevations or others could be
the molecular basis for the early and accelerated brain
growth in autism. Neurotrophins typically stimulate the
growth of both neuronal and glial elements and endothe-
lium. The increased levels of neurotrophins suggest that
there is a premature over-expression of genes that leads to
the production of neurotropins and neuropeptides. Future
studies should examine possible relationships between
nerve growth factors and brain growth patterns.

Neuropathologic Studies: Morphologic
Developmental Alterations

Neuroanatomic abnormalities have been observed in the
limbic system and cerebellum. Three major neuropatho-
logic findings have been described. They are as follows: 1)
curtailed development of neurons in the forebrain limbic
system (anterior cingulate gyrus, hippocampus, subicu-
lum, entorhinal cortex, and mammillary body); 2) congen-
ital decrease in the number of Purkinje cells in the
cerebellum; and 3) age-related differences in cell size and
neuronal number in the cerebellar nuclei and the inferior
olivary nucleus of the brainstem, suggesting an evolving
process and disturbance in the synaptic relationships of
these nuclei [64,65].

Although gross neuropathologic changes are not typi-

cal in autism, abnormalities observed at the cellular level
may underlie the basis of clinical manifestations. Increased
neuronal cell density and decreased nerve cell size have
been found bilaterally in the amygdala, entorhinal cortex,
mammillary body, anterior cingulate gyrus, medial septal
nucleus, and hippocampal complex [66]. These changes in
the limbic system may be related to the emotional and
mood manifestations of the clinical syndrome. Although
dysmorphic neurons are not usually found, a simplified
dendritic pattern in hippocampal regions CA4 and CA1
has been described [46].

The minicolumn hypothesis
Microscopic examination of brain tissue in autistic patients
provides evidence for altered central nervous system (CNS)
development. The processes of neuronal genesis and
migration are accomplished mainly before birth in mam-
mals. Both events must occur at the proper tempo and in a
coordinated fashion for the critical events of neuronal dif-
ferentiation and synaptogenesis to occur properly. Recent
observations regarding minicolumnar organization pro-
vide important evidence about developmental alterations
in autism [67••,68]. The minicolumn, the fundamental

anatomic and physiologic unit of the cerebral cortex, is the
smallest level of cortical vertical organization.

Early in development, post-mitotic neurons leave the

ventral wall and travel in a path ending with their radial
arrangement within the cortical plate [69]. In the adult
brain, minicolumns appears as thin radial structures, rang-
ing from 30- to 60-

µ

m wide depending on the cortical

area. Each minicolumn contains a repetitive array of affer-
ent inputs, intrinsic microcircuitry, and efferent outputs
infusing the structure with a putative role as a physiologic
unit [70••].

In autism, more numerous, smaller [67••,68], and

less compact minicolumns have been described
[52•,55••,65]. This may imply defects in the prolifera-
tion of neuronal precursor cells or environmental-
induced changes affecting minicolumnar architecture.
Altered minicolumnar organization has also been
described in Down syndrome, where minicolumns are of
normal width, but the radial structures attain adult mini-
columnar size earlier than normal. This is consistent with
the accelerated aging observed in this condition.

The larger brain size in autistic children, coupled with

the observation that cortical minicolumns are smaller and
more numerous, results in a novel cytoarchitectural
arrangement of a relative and absolute increase in columns
per brain surface area, or processing units [67••,71]. This
configuration may originate during the genesis of neurons
and the minicolumn. As cortical surface area has increased
evolutionarily with essentially constant size of the minicol-
umn, the result is an increase in the number and complex-
ity of processing units. During this slow process
throughout evolution, selection pressures would have
occurred to benefit the organism, or at least not prove mal-
adaptive [70••]. In autism, a significant increase in pro-
cessing units may occur as an acute event, not subject to
normal selection pressures. If thalamic terminations
remain the same in autism and minicolumns are smaller,
then more minicolumns will be innervated per thalamic
afferent terminal than in the normal brain. The failure to
assimilate extra processing units may result in cortical
"noise" that then overtaxes the system [67••].

Among several conceptual classifications, autism has been

considered a disorder of the arousal-modulating system of the
brain. Accordingly, autistic individuals experience a chronic
state of over-arousal and subsequently exhibit abnormal
behaviors. The arousal theory is consistent with a reduction in
inhibitory interneuronal activity. The cortex contains inhibi-
tory cells that define minicolumnar organization. Lateral
inhibition caused by GABAergic neurons helps to ensure indi-
vidual minicolumn discreteness and, during development,
compels adjacent minicolumns into establishing connections
with functionally dissimilar sets of thalamic neurons. A lack
of this inhibition would grossly alter the connective patterns
between thalamic input and the cerebral cortex. The result
would affect the ability to discriminate between competing
types of sensory information [67••].

The Neurobiology of Autism: New Pieces of the Puzzle • Acosta and Pearl

153

Other developmental alterations
Other studies have reported dysregulation of the proteins
reelin and Bcl-2, with reduction of these two proteins in
autistic cerebellar tissue compared with human control tis-
sue [24]. Reelin, a signaling protein that guides neuronal
migration in the developing fetus, as well as a cellular sig-
naling system subserving cognition in adult brain [26], is
the product of the RELN gene, a candidate gene for autism
[23,72•]. The Bcl-2 protein governs programmed cell
death, or apoptosis, in the developing and maturing
human brain. Reductions in these neuroregulatory pro-
teins may explain the neuronal migration and cell density
abnormalities observed in autism, and are intriguing in
light of the aforementioned putative peptide biomarkers
found in newborns [63••].

Hippocampal studies in autistic individuals have

shown that cells in this region are often smaller, packed
more densely, and have less dendritic branching than is
expected [73]. Proteomic studies have demonstrated
changes in protein in frontal cortex. Reduction or absence
of

α

B-crystallin in autistic brains has been reported [74].

Crystallin belongs to a group of small heat shock proteins
and functions as a molecular chaperone.

Functional Neuroimaging
and Neurotransmitter Studies:
Serotoninergic Dysfunction

Serotonin, like other monoamine transmitters, has been
shown to play a role in regulating brain development
[75••]. It influences the processes of neurogenesis, neu-
ronal differentiation, neuropil formation, axon myelina-
tion, and synaptogenesis. Removal of serotonin during
very early fetal development in rats causes a permanent
reduction in the number of neurons in adult brain hippoc-
ampus and cortex [75••]. At a later time in development,
serotonin plays a role in dendritic development, including
overall dendritic length, spine formation, and branching in
both hippocampus and cortex. Additionally, serotonin lev-
els can be affected by multiple and nonspecific post-natal
factors, including hypoxia, viral infections, malnutrition
[76], social enrichment, and stress. Drugs, including
cocaine, nicotine, and alcohol, can alter serotonin levels.

Increased evidence points to an imbalance in seroto-

nin levels as important for the etiology of autism
[75••,77–79]. Although several reports conclude that
dysgenesis of several brain systems occurs in autism
[52•,55••,65,66,67••,68], only recently has a link been
made between the role of serotonin in brain development
and autism [75••,77,80]. When assessing the effects of
serotonin on cortical growth, both an abundance and a
paucity may be deleterious. Depletion of serotonin
results in a significant delay in maturation of the soma-
tosensory cortex [81,82]. In contrast, excessive serotonin
during early development results in hyperinnervation and
expansion of cortical architecture [83]. Early serotoniner-

gic afferents to the cortex influence neural stem cell pro-
liferation [84,85] and may contribute to an increased
number of cortical minicolumns. Further studies on the
role of serotonin in prenatal cortical development are
needed, as abnormal neurogenesis or migration may
underlie the macrocephaly associated with autism.

Whole blood serotonin levels are increased in patients,

and serotonin transporter inhibitors reduce rituals and
aggression in autism. Comparative studies of monoamine
levels in autistic children versus control patients suggest
abnormal maturation of the serotoninergic system during
child development [86].

Chugani et al. [87] demonstrated developmental

changes in serotonin synthesis capacity in children using
alpha[C-11]methyl-L-tryptophan and positron emission
tomography. For nonautistic children, serotonin synthe-
sis capacity was greater than 200% of adult values until
the age of 5 years, and then declined toward adult values.
In autistic children, serotonin synthesis capacity
increased gradually between the ages of 2 and 11 years to
values 1.5 times of adult normal values. These findings
suggest that humans undergo a period of high brain sero-
tonin-synthesis capacity during childhood, and that this
developmental process is disrupted in autistic children,
with some brain regions more severely impacted [77,87].
Changes in serotonin synthesis and receptor density with
age suggest that serotonin plays an important role in
brain development [88].

Other neurotransmitters
Other neurotransmitter alterations associated with autism
include glutamate and GABA. The glutamate receptor 6
(GluR6) gene is localized in 6q21 and has been implicated
in autism [89]. It is highly expressed in brain regions
involved in learning and memory, and in motor and moti-
vational aspects of behavior [17]. GABAergic interneurons
in mouse cortex originate from the ganglionic eminence of
the ventral telencephalon. Disturbances of interneuron
migration and integration have been implicated in a vari-
ety of developmental disorders, including cortical dyspla-
sia with epilepsy [90], schizophrenia [91], Tourette's
syndrome [92], and autism [67••,68].

Acquired Factors/Immunizations

A possible etiologic connection between viral infections
and autism has received recent attention. Despite a pro-
posed link between autism and several pathogens, includ-
ing rubella, cytomegalovirus, and herpes simplex [93,94], a
direct search for viral genome or protein in blood and cere-
brospinal fluid of patients with autism has not produced
consistent results [95]. Congenital rubella has been associ-
ated with autism, although less than 1% of patients with
autism have congenital rubella [96].

A speculative association between the measles-mumps-

rubella (MMR) vaccine and autism has been described as a

154

Pediatric Neurology

new, chronic inflammatory bowel syndrome with autistic-
like developmental regression [97]. Parallel epidemiologic
studies of central nervous system complications from
native and attenuated measles, mumps, and rubella viruses
show that these manifestations do not mimic autism [39].
Given that MMR vaccination is practically ubiquitous in
developed countries, and that the time of its administra-
tion overlaps with the onset of autism, a causal link
between these events cannot be confirmed or ruled out by
anything other than statistical evidence. Since the publica-
tion of the presumptive association between autism and
MMR, several large, epidemiologic studies have been devel-
oped. A large, 14-year retrospective study in Finland did
not find an association between MMR and cases of autism
[98]. Other investigators have likewise been unable to
demonstrate an association between autism and MMR
[99–101]. Recently, the American Academy of Pediatrics
published the results of a Consensus Development Confer-
ence regarding MMR and autism, concluding that an asso-
ciation is based on single cases, personal observations, and
isolated and circumstantial information [102••]. Separate
administration of measles, mumps, and rubella vaccines to
children provides no benefit over the administration of the
combination MMR vaccine, and may result in delayed or
missed immunizations [102••].

Conclusions

The past decade has provided enormous advances in the
scientific evidence surrounding the neurobiologic basis of
autism. Based on this evidence, autism may be hypothe-
sized to result from genetically determined prenatal alter-
at i ons i n br ai n devel opment . Mul t i pl e gene s,
neurotransmitter systems, and brain regions are impli-
cated. The timing of prenatal factors, both genetically and
environmentally based, appears to have a fundamental
role. Alterations in the minicolumnar organization of the
cerebral cortex appears to be a significant factor in the
pathogenesis of autism, and this process appears to be reg-
ulated by neurotransmitters such as serotonin.

Central nervous system development is not finished at

birth, and the post-natal onset of developmental arrest or
regression invokes the presence of external or newly
acquired cerebral insults. Yet, there is strong evidence from
genetic, neuroimaging, and neurotransmitter studies that
individuals with autism have prenatal onset or pre-pro-
grammed alterations in brain function and development.
Additional studies are needed to understand the early fac-
tors regulating nerve cell formation, migration, and corti-
cal col umn f or mat i on, as wel l as acqui r ed or
environmental factors that may interact with the earliest
events in brain development.

References and Recommended Reading

Papers of particular interest, published recently, have been
highlighted as:
•

Of importance

••

Of major importance

1.

Shinnar S, Rapin I, Arnold S, et al.: Language regression in
childhood. Pediatr Neurol 2001, 24:183–189.

2.

Tuchman RF, Rapin I: Regression in pervasive developmental
disorders: seizures and epileptiform electroencephalogram
correlates. Pediatrics 1997, 99:560–566.

3.

Yeargin-Allsopp M: Past and future perspectives in autism epi-
demiology. Mol Psychiatry 2002, 7(suppl 2):S9–S11.

4.

Lamb JA, Moore J, Bailey A, Monaco AP: Autism: recent molec-
ular genetic advances. Hum Mol Genet 2000, 9:861–868.

5.

Bailey A, Le Couteur A, Gottesman I, et al.: Autism as a strongly
genetic disorder: evidence from a British twin study. Psychol
Med 1995, 25:63–77.

6.

Rutter M: The Emanuel Miller Memorial Lecture 1998.
Autism: two-way interplay between research and clinical
work. J Child Psychol Psychiatry 1999, 40:169–188.

7.

Pickles A, Starr E, Kazak S, et al.: Variable expression of the
autism broader phenotype: findings from extended
pedigrees. J Child Psychol Psychiatry 2000, 41:491–502.

8.

Risch N, Spiker D, Lotspeich L, et al.: A genomic screen of
autism: evidence for a multilocus etiology. Am J Hum Genet
1999, 65:493–507.

9.

Spence MA: The genetics of autism. Curr Opin Pediatr
2001, 13:561–565.

10.

Murphy M, Bolton PF, Pickles A, et al.: Personality traits
of the relatives of autistic probands. Psychol Med
2000, 30:1411–1424.

11.

Constantino JN, Todd RD: Genetic structure of reciprocal
social behavior. Am J Psychiatry 2000, 157:2043–2045.

12.

Bailey A, Palferman S, Heavey L, Le Couteur A: Autism: the phe-
notype in relatives. J Autism Dev Disord 1998, 28:369–392.

13.

Smalley SL, Kustanovich V, Minassian SL, et al.: Genetic linkage
of attention-deficit/hyperactivity disorder on chromosome
16p13, in a region implicated in autism. Am J Hum Genet
2002, 71:959–963.

14.

A genomewide screen for autism: strong evidence for linkage
to chromosomes 2q, 7q, and 16p. Am J Hum Genet
2001, 69:570–581.

15.

Gutknecht L: Full-genome scans with autistic disorder: a
review. Behav Genet 2001, 31:113–123.

16.

Shao Y, Wolpert CM, Raiford KL, et al.: Genomic screen and
follow-up analysis for autistic disorder. Am J Med Genet
2002, 114:99–105.

17.

Licinio J, Alvarado I: Progress in the genetics of autism. Mol
Psychiatry 2002, 7:229.

18.

Yu CE, Dawson G, Munson J, et al.: Presence of large deletions
in kindreds with autism. Am J Hum Genet 2002, 71:100–115.

19.

Fisher SE, Vargha-Khadem F, Watkins KE, et al.: Localization of
a gene implicated in a severe speech and language disorder.
Nat Genet 1998, 18:168–170.

20.

Folstein SE, Mankoski RE: Chromosome 7q: where autism
meets language disorder?. Am J Hum Genet 2000, 67:278–281.

21.

Warburton P, Baird G, Chen W, et al.: Support for linkage of
autism and specific language impairment to 7q3 from two
chromosome rearrangements involving band 7q31. Am J Med
Genet 2000, 96:228–234.

22.

Lai CS, Fisher SE, Hurst JA, et al.: A forkhead-domain gene is
mutated in a severe speech and language disorder. Nature
2001, 413:519–523.

23.

Persico AM, D'Agruma L, Maiorano N, et al.: Reelin gene alleles
and haplotypes as a factor predisposing to autistic disorder.
Mol Psychiatry 2001, 6:150–159.

24.

Fatemi SH, Stary JM, Halt AR, Realmuto GR: Dysregulation of
reelin and Bcl-2 proteins in autistic cerebellum. J Autism Dev
Disord 2001, 31:529–535.

The Neurobiology of Autism: New Pieces of the Puzzle • Acosta and Pearl

155

25.

Petek E, Windpassinger C, Vincent JB, et al.: Disruption of a
novel gene (IMMP2L) by a breakpoint in 7q31 associated
with Tourette syndrome. Am J Hum Genet 2001, 68:848–858.

26.

Fatemi SH, Kroll JL, Stary JM: Altered levels of Reelin and its
isoforms in schizophrenia and mood disorders. Neuroreport
2001, 12:3209–3215.

27.

Hong SE, Shugart YY, Huang DT, et al.: Autosomal recessive
lissencephaly with cerebellar hypoplasia is associated with
human RELN mutations. Nat Genet 2000, 26:93–96.

28.

Fombonne E, Chakrabarti S: No evidence for a new variant of
measles-mumps-rubella-induced autism. Pediatrics
2001, 108:E58.

29.

Gillberg C: Chromosomal disorders and autism. J Autism Dev
Disord 1998, 28:415–425.

30.

Buxbaum JD, Silverman JM, Smith CJ, et al.: Association
between a GABRB3 polymorphism and autism. Mol Psychiatry
2002, 7:311–0316.

31.

Menold MM, Shao Y, Wolpert CM, et al.: Association analysis
of chromosome 15 gaba receptor subunit genes in autistic
disorder. J Neurogenet 2001, 15:245–259.

32.

Martin ER, Menold MM, Wolpert CM, et al.: Analysis of linkage
disequilibrium in gamma-aminobutyric acid receptor
subunit genes in autistic disorder. Am J Med Genet
2000, 96:43–48.

33.

Maestrini E, Lai C, Marlow A, et al.: Serotonin transporter (5-
HTT) and gamma-aminobutyric acid receptor subunit beta3
(GABRB3) gene polymorphisms are not associated with
autism in the IMGSA families. The International Molecular
Genetic Study of Autism Consortium. Am J Med Genet
1999, 88:492–496.

34.

Nurmi EL, Bradford Y, Chen Y, et al.: Linkage disequilibrium at
the Angelman syndrome gene UBE3A in autism families.
Genomics 2001, 77:105–113.

35.

Slopien A, Rajewski A: Genetic studies in autistic disorders.
Psychiatry Pol 2000, 34:435–446.

36.

Schutz CK, Polley D, Robinson PD, et al.: Autism and the X
chromosome: no linkage to microsatellite loci detected using
the affected sibling pair method. Am J Med Genet
2002, 109:36–41.

37.

Amir RE, Zoghbi HY: Rett syndrome: methyl-CpG-binding
protein 2 mutations and phenotype- genotype correlations.
Am J Med Genet 2000, 97:147–152.

38.

Hammer S, Dorrani N, Dragich J, et al.: The phenotypic conse-
quences of MECP2 mutations extend beyond Rett syndrome.
Ment Retard Dev Disabil Res Rev 2002, 8:94–98.

39.• Korvatska E, Van de WJ, Anders TF, Gershwin ME: Genetic and

immunologic considerations in autism. Neurobiol Dis
2002, 9:107–125.

Overview of genetic and inmunologic aspects of autism.
40.

Kemper TL, Bauman ML: The contribution of neuropathologic
studies to the understanding of autism. Neurol Clin
1993, 11:175–187.

41.

Saitoh O, Karns CM, Courchesne E: Development of the hip-
pocampal formation from 2 to 42 years: MRI evidence of
smaller area dentata in autism. Brain 2001, 124:1317–1324.

42.

Courchesne E, Karns CM, Davis HR, et al.: Unusual brain
growth patterns in early life in patients with autistic disor-
der: an MRI study. Neurology 2001, 57:245–254.

43.

Courchesne E: Abnormal early brain development in autism.
Mol Psychiatry 2002, 7(suppl 2):S21–S23.

44.

Hardan AY, Minshew NJ, Harenski K, Keshavan MS: Posterior
fossa magnetic resonance imaging in autism. J Am Acad Child
Adolesc Psychiatry 2001, 40:666–672.

45.• Sparks BF, Friedman SD, Shaw DW, et al.: Brain structural

abnormalities in young children with autism spectrum
disorder. Neurology 2002, 59:184–192.

Supportive evidence of volumetric changes in brain development in
children with autism.
46.

Raymond GV, Bauman ML, Kemper TL: Hippocampus in
autism: a Golgi analysis. Acta Neuropathol (Berl)
1996, 91:117–119.

47.

Hashimoto T, Tayama M, Murakawa K, et al.: Development of
the brainstem and cerebellum in autistic patients. J Autism
Dev Disord 1995, 25:1–18.

48.

Hardan AY, Minshew NJ, Keshavan MS: Corpus callosum size
in autism. Neurology 2000, 55:1033–1036.

49.

Fidler DJ, Bailey JN, Smalley SL: Macrocephaly in autism and
other pervasive developmental disorders. Dev Med Child Neu-
rol 2000, 42:737–740.

50.

Fombonne E, Roge B, Claverie J, et al.: Microcephaly and mac-
rocephaly in autism. J Autism Dev Disord 1999, 29:113–119.

51.

Lainhart JE, Piven J, Wzorek M, et al.: Macrocephaly in chil-
dren and adults with autism. J Am Acad Child Adolesc Psychiatry
1997, 36:282–290.

52.• Courchesne E, Karns CM, Davis HR, et al.: Unusual brain

growth patterns in early life in patients with autistic
disorder: an MRI study. Neurology 2001, 57:245–254.

This study shows evidence of abnormal regulation of brain growth,
with early overgrowth and later abnormal slow growth.
53.

Lemons JA, Schreiner RL, Gresham EL: Relationship of brain
weight to head circumference in early infancy. Hum Biol
1981, 53:351–354.

54.•• Aylward EH, Minshew NJ, Field K, et al.: Effects of age on brain

volume and head circumference in autism. Neurology
2002, 59:175–183.

Supportive evidence for an abnormal pattern of brain development
early in life and during adolescence in autism.
55.•• Carper RA, Courchesne E: Inverse correlation between frontal

lobe and cerebellum sizes in children with autism. Brain
2000, 123(Pt 4):836–844.

This study shows evidence of concurrent abnormalities in both the
frontal lobe and the cerebellum.
56.•• Carper R, Moses P, Tigue Z, Courchesne E: Cerebral lobes in

autism: early hyperplasia and abnormal age effects. Neuroim-
age 2002, 16:1038.

Evaluation of volumetric variations in different brain regions. Mecha-
nisms that might account for early hyperplasia are discussed.
57.

Saitoh O, Karns CM, Courchesne E: Development of the hip-
pocampal formation from 2 to 42 years: MRI evidence of
smaller area dentata in autism. Brain 2001, 124:1317–1324.

58.

Egaas B, Courchesne E, Saitoh O: Reduced size of corpus
callosum in autism. Arch Neurol 1995, 52:794–801.

59.

Muller RA, Pierce K, Ambrose JB, et al.: Atypical patterns of
cerebral motor activation in autism: a functional magnetic
resonance study. Biol Psychiatry 2001, 49:665–676.

60.

Pierce K, Muller RA, Ambrose J, et al.: Face processing occurs
outside the fusiform 'face area' in autism: evidence from
functional MRI. Brain 2001, 124:2059–2073.

61.

Critchley HD, Daly EM, Bullmore ET, et al.: The functional
neuroanatomy of social behaviour: changes in cerebral
blood flow when people with autistic disorder process facial
expressions. Brain 2000, 123(Pt 11):2203–2212.

62.

Casey BJ, Giedd JN, Thomas KM: Structural and functional
brain development and its relation to cognitive develop-
ment. Biol Psychol 2000, 54:241–257.

63.•• Nelson KB, Grether JK, Croen LA, et al.: Neuropeptides and

neurotrophins in neonatal blood of children with autism or
mental retardation. Ann Neurol 2001, 49:597–606.

Evaluation of expression of neuropeptides and neurotrophins
in peripheral blood in children with autism drawn in the first days
of life.
64.

Kemper TL, Bauman M: Neuropathology of infantile autism.
J Neuropathol Exp Neurol 1998, 57:645–652.

65.

Bauman M, Kemper TL: Histoanatomic observations of the
brain in early infantile autism. Neurology 1985, 35:866–874.

66.

Kemper TL, Bauman ML: Neuropathology of infantile autism.
Mol Psychiatry 2002, 7(suppl 2):S12–S13.

67.•• Casanova MF, Buxhoeveden DP, Switala AE, Roy E: Minicolum-

nar pathology in autism. Neurology 2002, 58:428–432.

Description of differences between brain of autistic patients and con-
trol patients in the number and structure of minicolumns

156

Pediatric Neurology

68.

Casanova MF, Buxhoeveden DP, Switala AE, Roy E: Asperger's
syndrome and cortical neuropathology. J Child Neurol
2002, 17:142–145.

69.

Kornack DR, Rakic P: Radial and horizontal deployment of
clonally related cells in the primate neocortex: relationship
to distinct mitotic lineages. Neuron 1995, 15:311–321.

70.•• Buxhoeveden DP, Casanova MF: The minicolumn hypothesis

in neuroscience. Brain 2002, 125:935–951.

Review of the concept of minicolumn as an anatomic and
functional unit.

71.

Casanova MF, Buxhoeveden DP, Cohen M, et al.: Minicolumnar
pathology in dyslexia. Ann Neurol 2002, 52:108–110.

72.• Pickett J: Current investigations in autism brain tissue

research. J Autism Dev Disord 2001, 31:521–527.

Update on current autism brain research efforts.
73.

Blatt GJ, Fitzgerald CM, Guptill JT, et al.: Density and distribu-
tion of hippocampal neurotransmitter receptors in autism:
an autoradiographic study. J Autism Dev Disord
2001, 31:537–543.

74.

Junaid MA, Pullarkat RK: Proteomic approach for the elucida-
tion of biological defects in autism. J Autism Dev Disord
2001, 31:557–560.

75.•• Whitaker-Azmitia PM: Serotonin and brain development: role

in human developmental diseases. Brain Res Bull
2001, 56:479–485.

Overview of the role of serotonin in prenatal and post-natal
brain development.
76.

Barreto Medeiros JM, Cabral Filho JE, De Souza SL, et al.: Early
malnourished rats are not affected by anorexia induced by a
selective serotonin reuptake inhibitor in adult life. Nutr
Neurosci 2002, 5:211–214.

77.

Chugani DC, Muzik O, Behen M, et al.: Developmental
changes in brain serotonin synthesis capacity in autistic and
nonautistic children. Ann Neurol 1999, 45:287–295.

78.

Tordjman S, Gutknecht L, Carlier M, et al.: Role of the seroto-
nin transporter gene in the behavioral expression of autism.
Mol Psychiatry 2001, 6:434–439.

79.

Carlsson ML: Hypothesis: is infantile autism a hypo-
glutamatergic disorder? Relevance of glutamate-serotonin
interactions for pharmacotherapy. J Neural Transm
1998, 105:525–535.

80.

Donovan SL, Mamounas LA, Andrews AM, et al.: GAP-43 is
critical for normal development of the serotonergic innerva-
tion in forebrain. J Neurosci 2002, 22:3543–3552.

81.

Blue ME, Erzurumlu RS, Jhaveri S: A comparison of pattern
formation by thalamocortical and serotonergic afferents in
the rat barrel field cortex. Cereb Cortex 1991, 1:380–389.

82.

Bennett-Clarke CA, Leslie MJ, Lane RD, Rhoades RW: Effect of
serotonin depletion on vibrissa-related patterns of thalamic
afferents in the rat's somatosensory cortex. J Neurosci
1994, 14:7594–7607.

83.

Cases O, Vitalis T, Seif I, et al.: Lack of barrels in the soma-
tosensory cortex of monoamine oxidase A- deficient mice:
role of a serotonin excess during the critical period. Neuron
1996, 16:297–307.

84.

Lauder JM, Sze PY, Krebs H: Maternal influences on
tryptophan hydroxylase activity in embryonic rat brain.
Dev Neurosci 1981, 4:291–295.

85.

Lauder JM, Wallace JA, Krebs H: Roles for serotonin in neuro-
embryogenesis. Adv Exp Med Biol 1981, 133:477–506.

86.

Martineau J, Barthelemy C, Jouve J, et al.: Monoamines (sero-
tonin and catecholamines) and their derivatives in infantile
autism: age-related changes and drug effects. Dev Med Child
Neurol 1992, 34:593–603.

87.

Chugani DC, Muzik O, Rothermel R, et al.: Altered serotonin
synthesis in the dentatothalamocortical pathway in autistic
boys. Ann Neurol 1997, 42:666–669.

88.

Chugani DC: Role of altered brain serotonin mechanisms in
autism. Mol Psychiatry 2002, 7(suppl 2):S16–S17.

89.

Jamain S, Quach H, Quintana-Murci L, et al.: Y chromosome
haplogroups in autistic subjects. Mol Psychiatry
2002, 7:217–219.

90.

Roper SN, Eisenschenk S, King MA: Reduced density of parval-
bumin- and calbindin D28-immunoreactive neurons in
experimental cortical dysplasia. Epilepsy Res 1999, 37:63–71.

91.

Benes FM, McSparren J, Bird ED, et al.: Deficits in small inter-
neurons in prefrontal and cingulate cortices of schizo-
phrenic and schizoaffective patients. Arch Gen Psychiatry
1991, 48:996–1001.

92.

Leckman JF, Riddle MA: Tourette's syndrome: when
habit-forming systems form habits of their own? Neuron
2000, 28:349–354.

93.

Stubbs EG, Ash E, Williams CP: Autism and congenital
cytomegalovirus. J Autism Dev Disord 1984, 14:183–189.

94.

Gillberg IC: Autistic syndrome with onset at age 31 years:
herpes encephalitis as a possible model for childhood
autism. Dev Med Child Neurol 1991, 33:920–924.

95.

Van Gent T, Heijnen CJ, Treffers PD: Autism and the immune
system. J Child Psychol Psychiatry 1997, 38:337–349.

96.

Fombonne E: The epidemiology of autism: a review. Psychol
Med 1999, 29:769–786.

97.

O'Leary JJ, Uhlmann V, Wakefield AJ: Measles virus and
autism. Lancet 2000, 356:772.

98.

Peltola H, Patja A, Leinikki P, et al.: No evidence for measles,
mumps, and rubella vaccine-associated inflammatory bowel
disease or autism in a 14-year prospective study. Lancet
1998, 351:1327–1328.

99.

Taylor B, Miller E, Farrington CP, et al.: Autism and measles,
mumps, and rubella vaccine: no epidemiological evidence
for a causal association. Lancet 1999, 353:2026–2029.

100. Dales L, Hammer SJ, Smith NJ: Time trends in autism and in

MMR immunization coverage in California. JAMA
2001, 285:1183–1185.

101. Kaye JA, Mar Melero-Montes M, Jick H: Mumps, measles, and

rubella vaccine and the incidence of autism recorded by
general practitioners: a time trend analysis. BMJ
2001, 322:460–463.

102.••Halsey NA, Hyman SL: Measles-mumps-rubella vaccine and

autistic spectrum disorder: report from the New Challenges
in Childhood Immunizations Conference convened in Oak
Brook, Illinois, June 12-13, 2000. Pediatrics 2001, 107:E84.

Conclusions from American Academy of Pediatrics Consensus Devel-
opment Conference entitled, "New Challenges in Childhood Inmuni-
zations." Available evidence does not support the hypothesis that the
measles-mumps-rubella vaccine causes autism.