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Subject: BALANCE TIPS TOWARD ENVIRONMENT AS HERITABILITY EBBS IN AUTISM?

U.S. Department of Health and Human Services 
NATIONAL INSTITUTES OF HEALTH NIH News 
National Institute of Mental Health (NIMH) <http://www.nimh.nih.gov/>
For Immediate Release: Tuesday July 5, 2011

CONTACT: Jules Asher, NIMH press office,
301-443-4536,<e-mail:[log in to unmask]>

BALANCE TIPS TOWARD ENVIRONMENT AS HERITABILITY EBBS IN AUTISM?
Non-inherited genetic factors also in the mix -- NIH-funded science

The largest and most rigorous twin study of its kind to date has found that
shared environment influences susceptibility to autism more than previously
thought.  

The study, supported by the National Institutes of Health, found that shared
environmental factors - experiences and exposures common to both twin
individuals - accounted for 55 percent of strict autism and 58 percent of
more broadly defined autism spectrum disorders (ASD)
(http://www.nimh.nih.gov/health/topics/autism-spectrum-disorders-pervasive-d
evelopmental-disorders/index.shtml). Genetic heritability accounted for 37
percent of autism and 38 percent of ASD. Random environmental factors not
shared among twins play a much smaller role. 

Earlier twin studies had estimated the genetic heritability of autism to be
as high as 90 percent, due to much lower estimates of concordance - both
members of a twin pair having the disorder - in fraternal twins. The new
study found such concordance to be four to five times higher.

"High fraternal twin concordance relative to identical twin concordance
underscores the importance of both the environment and moderate genetic
heritability in predisposing for autism," explained Joachim Hallmayer, M.D.,
of Stanford University, Palo Alto, Calif. a grantee of the NIH's National
Institute of Mental Health. "Both types of twin pairs are more often
concordant than what would be expected from the frequency of autism in the
general population. However, the high concordance among individuals who
share only half their genes relative to those who share all of their genes
implies a bigger role for shared environmental factors."

Hallmayer, senior co-investigator Neil Risch, Ph.D., of the University of
California, San Francisco, and colleagues report on findings of the
California Autism Twins Study (CATS) in the July 2011 issue of the Archives
of General Psychiatry. 

"These new findings are in line with other recent observations supporting
both environmental and genetic contributions to ASD, with the environmental
factors likely prenatal and the genetic factors highly complex and sometimes
not inherited
(http://www.nimh.nih.gov/about/director/2011/autism-spring.shtml)," said
NIMH director Thomas R. Insel, M.D. 

Studies are under way to determine if autism may be traceable, in part, to
environmental exposures
(http://www.niehs.nih.gov/health/topics/conditions/autism/index.cfm) early
during pregnancy.

The new study is the first to analyze a large sample of twins drawn from the
general population; previous twin studies have been based on more limited
samples, such as patients in treatment. It is also the first to employ the
latest standard in diagnosing autism, which requires structured clinical
assessments based on interviews with the parents as well as direct
observation of the child. 

Drawing upon state records, the researchers initially identified 1,156 twin
pairs, with at least one member affected by an ASD, born to California
mothers between 1987 and 2004. The children were all at least 4 years old,
an age when autism can be reliably diagnosed. Ultimately, this group was
winnowed to 192 twin pairs - 54 identical and 138 fraternal - for genetic
analysis. Since autism disproportionately affects males, males outnumbered
females by four to five times, with 80 of the pairs including both sexes. 

Concordance for ASD was 77 percent among identical male pairs, and 31
percent among fraternal male pairs. In females, concordance for ASD was more
closely spaced - 50 percent for identical and 36 percent for fraternal
pairs. By contrast, previous studies had found concordance rates for
fraternal twins that were much lower, ranging only in the single digits. 

"Spectrum disorders traditionally thought to have less genetic loading turn
out to stem from a similar mix of environmental and genetic heritability as
narrowly defined autism," noted Thomas Lehner, Ph.D., chief of the NIMH
Genomics Research Branch.

Yet, there can also be genetic influences that are not inherited from
parents. New evidence
(http://www.eurekalert.org/pub_releases/2011-06/yu-yrd060611.php) emerged
last month that rare, spontaneous mutations
(http://www.nimh.nih.gov/about/director/2010/in-search-of-the-missing-geneti
c-signals.shtml) occur at abnormally high rates in autism. 

"Such non-inherited genetic changes were proposed as a major mechanism of
autism susceptibility, based on the very low concordance among fraternal
twins found in earlier studies and evidence of increased risk associated
with older parental age," explained Risch. "In light of the high fraternal
twin concordance observed in our study, such new mutations may play a more
limited role, since they would primarily occur in only one member of a
fraternal pair, which would not lead to concordance." 

Also participating in the research were investigators at: Autism Genetic
Resource Exchange; California Department of Public Health; Kaiser
Permanente; University of California, Davis. The research was also funded by
Autism Speaks.

The mission of the NIMH is to transform the understanding and treatment of
mental illnesses through basic and clinical research, paving the way for
prevention, recovery and cure. For more information, visit the NIMH website
(http://www.nimh.nih.gov/index.shtml).

About the National Institutes of Health (NIH): NIH, the nation's medical
research agency, includes 27 Institutes and Centers and is a component of
the U.S. Department of Health and Human Services. NIH is the primary federal
agency conducting and supporting basic, clinical, and translational medical
research, and is investigating the causes, treatments, and cures for both
common and rare diseases. For more information about NIH and its programs,
visit <www.nih.gov>.
-------------------------

REFERENCE:
Genetic heritability and shared environmental factors among twin pairs with
autism. Hallmayer J, Cleveland S, Torres A, Phillips J, Cohen B, Torigoe T,
Miller J, Fedele A, Collins J, Smith K, Lotspeich L, Croen LA, Ozonoff S,
Lajonchere C, Grether JK, Risch N. Archives of General Psychiatry. 2011
July. 
  
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