This web page was produced as an assignment for an undergraduate course at Davidson College.
Prevalence
According
to
statistics obtained from the Centers for Disease Control and Prevention
(CDC), about 1 in 88 children in the United States have some form of
ASD. The condition is about five times more common in males. A ten-fold
increase in the number of autism diagnoses has occurred over the last 40
years, a trend only partially explained by the improved awareness and
diagnosis techniques for the condition. This makes autism the fastest
growing developmental disability in the United States. No known cure for
autism exists [2], but some pharmaceuticals
and behavioral interventions have shown promise in reducing the severity
of behavioral symptoms of the disease [8].
Causes
Genetic
There
are
many forms of autism and likewise, there appear to be no singular causes
of the disorder. Among neuropsychiatric disorders, however, autism is
among the most influenced by genetics. In fact, heritability of the
condition (the extent of differences in a trait that is due to genetic
differences) has been reported to be as high as 90% [3]. Siblings of
autistic children are 25 times more likely than the general population
to be autistic, and while 20-30% of dizygotic twins both have autism,
about 60% f monozygotic twins both have autism, suggesting a substantial
influence of genetic on the etiology of the disease [4]. The inheritance
patterns of autism and families varies substantially [7].
Certain specific genes and other genetic loci have been
implicated in the etiology of autism through genome-wide linkage
analyses, of genome wide association (GWA) studies [5]. GWA studies
analyze the differential genetic profile between healthy persons and
individuals with a certain condition such as autism, typically looking
for differences at the level of single nucleotides between the two
groups. No more than 1% of autism cases can be explained by a single
mutation in any of these candidate genes, suggesting a great variety in
nature of the mutations of these genes [6], indeed about 200 genes have
been identified that are disproportionately associated with autism [7].
A technique called chromosome microarray analysis which can analyzes the
composition of many genes at once, can utilize this information acquired
in the lab to help physicians diagnose autism [7]
Environmental
Myriad possible contributors to autism pathology outside of
genetics have also been identified, toxins in the atmosphere, factors in
the prenatal environment, complications at birth and parental age. The
exact contributions of environmental toxins have been difficult to
study, but the contributions of prenatal environment are better
understood [9, 10]. One interesting discovery made in the past few years
is that parental age at the time of conception also appears to play role
in the etiology of autism. Mothers over 40, for example are about 51%
more likely to give birth to an autistic child than a mother 25-29 years
old, and men over 40 were 78% more likely to father an autistic child
than a father under 25 years old [10]. This contribution by the father
may be due to an increased accumulation of mutations in gametes
occurring over life, or modification of DNA at the epigenetic level.
Epigenetic changes can include methylation of nucleotides or
deacetylation of histones, both processes of which may inhibit gene
transcription [10]. There is no well-supported evidence that modern
vaccines have contributed to the prevalence of autism [11].
References
[2]
http://www.cdc.gov/media/releases/2012/p0329_autism_disorder.html
[4] Geschwind, D. H. (2009). Advances in autism.
Annual Reviews of Medicine, 60,
367-80.
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Email Questions or Comments to gasmith@davidson.edu.