Comment on: A Danish population-based twin study on autism spectrum disorders.

12 May

There has been much discussion of twin studies in autism research for a long time. The reason is that if is found that “identical” (monozygotic) twins are often both autistic, that points to genetics as a major influence on the development of autism. For many years it was thought that this rate, the concordance, was about 90%. In other words, if one child is autistic, 90% of the time the other child is autistic. This was based on a number of older, small studies. More recently, a relatively large study showed a lower concordance: about 77% for ASD and 60% for autism. From this the authors claimed that the genetic contribution to autism risk was lower than previously thought, and that the environmental contribution was higher (about 55% environmental contribution).

A study just out from Denmark claims a concordance more in line with the older studies–95%. In A Danish population-based twin study on autism spectrum disorders., the authors write:

Genetic epidemiological studies of Autism Spectrum Disorders (ASDs) based on twin pairs ascertained from the population and thoroughly assessed to obtain a high degree of diagnostic validity are few. All twin pairs aged 3-14 years in the nationwide Danish Twin Registry were approached. A three-step procedure was used. Five items from the “Child Behaviour Checklist” (CBCL) were used in the first screening phase, while screening in the second phase included the “Social and Communication Questionnaire” and the “Autism Spectrum Screening Questionnaire”. The final clinical assessment was based on “gold standard” diagnostic research procedures including diagnostic interview, observation and cognitive examination. Classification was based on DSM-IV-TR criteria. The initial sample included 7,296 same-sexed twin pairs and, after two phases of screening and clinical assessment, the final calculations were based on 36 pairs. The probandwise concordance rate for ASD was 95.2 % in monozygotic (MZ) twins (n = 13 pairs) and 4.3 % in dizygotic (DZ) twins (n = 23 pairs). The high MZ and low DZ concordance rate support a genetic aetiology to ASDs.

This study is relatively small with only 13 “identical” twin pairs. Also, the concordance for “fraternal” (dizygotic) twins is relatively low at 4.3%. Sibling concordance is estimated at about 20%, so 4.3% raises a bit of a red flag. Of course the recent larger twin study is not without some controversy itself.

In the end, I doubt this new study will have much influence on the online parent community discussions (which are in themselves far from the most productive or important discussions on the topic. Just the apparently most vocal). We are left with there being some genetic contribution and some environmental contribution to autism risk. In other words, it remains important to put effort into both areas of research.


By Matt Carey

About these ads

4 Responses to “Comment on: A Danish population-based twin study on autism spectrum disorders.”

  1. brian May 12, 2013 at 18:10 #

    Discussion of the Hallmeyer twin study almost universally neglects that its conclusions were based on modeling, and are therefore dependent on the assumptions used in the model.

    Notably, Hallmeyer et al. used a multifactorial model assuming that numerous, normally-distributed factors of small effect can accumulate to cross some threshold value and produce the diagnosed condition. While there is evidence that this would apply to many cases of ASD, it is equally clear that some genetic factors with large effect, such as the de novo copy number variations that are associated with numerous other cases of ASD, would not be considered by this model.

    Accordingly, since the model’s assumptions have limited correspondence to what is known regarding the genetics of ASD, the study should be understood as just a good faith attempt rather than some sort of absolute determination of the environmental and genetic contributions to ASD.

    • brian May 12, 2013 at 19:41 #

      BTW, post-twinning copy-number variation is common. Discordant twin pairs resulting from such DNA sequence changes within hours of fertilization would have been considered to be due to environmental effects by Hallmeyer et al.

  2. Todd W. May 13, 2013 at 21:39 #

    Can’t access the full study at the moment. Any word on why the significant drop from initial screening to reported study groups?

Trackbacks/Pingbacks

  1. Comment on: A Danish population-based twin study on autism spectrum disorders. – Left Brain Right Brain | Twins Foundation Blog - May 12, 2013

    […] Comment on: A Danish population-based twin study on autism spectrum disorders.Left Brain Right BrainThere has been much discussion of twin studies in autism research for a long time. The reason is that if is found that “identical” (monozygotic) twins are often both autistic, that points to genetics as a major influence on the development of autism … […]

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

Join 1,028 other followers

%d bloggers like this: