 The
Twin Autism Project is a relatively new project and
is currently in its first funding cycle; therefore,
published data on the full sample of twins is not yet
available. However, preliminary results from the project
were presented at the Behavior Genetics Association's
35th Annual meeting in June 2005.
Twin Concordance for the Autism Spectrum Based
on Community Diagnoses and Screening of a Birth Cohort
Inferences about the degree of genetic influence on
individual differences in the autism spectrum depend
crucially on the relative risk to siblings and the differential
concordance of monozygotic (MZ) versus dizygotic (DZ)
cotwins. Previous European twin studies that used older
diagnostic criteria for autism revealed concordance
rates suggesting strong genetic influence; however,
concordance rates for the spectrum using current diagnostic
criteria are still unknown. A decrease of relative risk
rates for siblings over time suggests that the autism
spectrum, as currently diagnosed, is heritable, but
not as strongly heritable as classic autism was. Our
sample comprised 2808 individual twins screened at 2
years of age (mean 27 mos.) after being recruited from
1998 to 2003 state birth records. Our 35 screening items
were drawn from an autism screener combined with items
from the Infant Toddler Social Emotional Assessment.
The screener items were divided into four subscales
measuring speech, social behavior, restricted interests,
and motor impairment. We focused on twins who had extreme
scores (top 5%) on both the social and speech subscales
(social-speech score). A probandwise analysis of the
top 5% of social-speech scores revealed an MZ concordance
rate in the .60s and a DZ rate in the .40s. An independent
case-finding approach has identified 95 twin pairs in
the state of Wisconsin in which at least one twin has
an autism spectrum diagnosis. Of these 95 twin pairs,
we identified 13 individual twins with a later autism
spectrum diagnosis who had previously been administered
the screener. Eight of the 13 scored in the top 4% of
social-speech scores and four others placed in the top
12%. The pairwise concordance rates for 79 of the 95
twin pairs (zygosity of twins or diagnosis of cotwin
were unknown for 16 pairs) were 77% for MZ and 28% for
DZ pairs. Although more thorough analysis and in-depth
study of both twins with extreme screener scores and
the twin pairs found through case-finding is still underway,
the preliminary evidence suggests that the autism spectrum
is heritable, although not as strongly heritable as
the earlier European twin studies of autism indicated.
Kees, E.K., Hefter, R.L., Klaver, J., Schweigert,
S.A., Arneson, C., Gernsbacher, M.A., & Goldsmith,
H.H. (2005, June). Twin concordance for the autism spectrum
based on community diagnoses and screening of a birth
cohort. Behavior Genetics, 35, 809. (abstract). |
