Friday 26 June 2015

Early sex differences are not autism-specific

The title of this post mirrors the title of the paper published by Daniel Messinger and colleagues [1] (open-access available here) that reported on "younger sibling sex differences and proband sex differences on the odds of ASD [autism spectrum disorder] in a large sample of prospectively followed high-risk siblings."

Researchers found that alongside "a three-to-one male:female odds ratio in ASD recurrence... the emergence of ASD symptoms in high-risk siblings—both with and without eventual ASD outcomes—occurs in the context of naturally occurring sex-related variability." Further that their results cast "doubt on a female protective effect among high-risk ASD siblings."

Based on participant data derived from the Baby Siblings Research Consortium (BSRC) [2], a collaborative initiative focused on identifying the earliest signs and symptoms of autism, researchers assessed cognitive functioning and autism symptom severity in over 1800 infants, over 1200 of whom were categorised as 'high risk' insofar as being a "younger sibling of a proband with an ASD diagnosis." Of those 1241 high-risk siblings "252 had ASD outcomes" on the basis of a clinical best estimate diagnosis derived from the various data sources available for each participant.

Results: well, there were quite a few of them as previously discussed but I'm going to pick out a few highlights:

  • "The male rate of ASD recurrence in the high-risk siblings was approximately 1 in 4 (26.7 %) while the female rate was 1 in 10 (10.3 %)." Further: "The overall—combined male and female—ASD recurrence rate of 19.5 % yielded an ASD outcome for approximately one in five high-risk siblings." This is an important addition to the previous research done on sibling recurrence rates (see here) bearing in mind the potential effect of variables such as reproductive stoppage and multiplex status.
  • "Challenging accounts of greater female affectedness, there was no evidence that girls exhibited lower levels of cognitive functioning or higher levels of symptom severity than boys." The idea that there may be a female phenotype of autism has gained significant research traction in recent times (see here) partially based on the idea that it may take 'more genetic issues to trigger autism in girls than boys'. Alongside the idea that females may somehow possess greater protection against autism is the suggestion that autism when it does appear in females, may show differences in terms of severity perhaps as a function of that larger genetic load required. Messinger et al however, report that things might not be so simple.
  • "Boys across all groups exhibited slower growth trajectories and lower levels of cognitive performance than girls in fine motor, visual reception, receptive and expressive language functioning." Harking back to some previous musings on the 'fragile male' (see here) Messinger et al showed small but notable difference across the sexes and across ages suggesting female superiority of somewhere between "1.06 to 3.3 months on age-equivalent scores" compared to males. That's not to say males did not develop - everyone developed - rather that girls seemed to develop skills at a faster rate.
  • "With respect to the ASD symptom severity indices, males exhibited higher levels of repetitive behaviors than females, but there were no sex differences in social affect severity scores." This finding tallies with other research in this area (see here). The indication being that aspects such as stereotyped language, hand and finger mannerisms or complex mannerisms may be quite a bit more prevalent in 'boy autism' compared to 'girl autism'. More than that however, such repetitive behaviours might be more present in boys over girls (taking out the variable of autism diagnosis) "consistent with a male focus on regularity in the behavior of non-social objects and events." 'Boys and their toys' is a phrase that springs to mind.

The authors conclude that their results "suggest that male:female ASD differences are not ASD-specific but instead reflect more general sex differences reflected through a prism of autism-linked symptoms."

These are interesting results that gain some scientific traction as a function of the large participant numbers included for study and the prospective nature of the study initiative. The authors have made quite a bit of the fact that their results provide "no overall evidence of a female protective effect" in their high-risk siblings group but acknowledge the need for further investigations in this area specifically where female autism is present and how "female probands in multiplex families (two or more female siblings) [may] confer greater risk for ASD in successive offspring." I wonder if this might include some further thought on how sex differences in brain plasticity [3] might potentially be linked to autism?

Once again, autism reveals just how complicated a condition it is...

Music: The Slits - Typical Girls.


[1] Messinger DS. et al. Early sex differences are not autism-specific: A Baby Siblings Research Consortium (BSRC) study. Mol Autism. 2015 Jun 4;6:32.

[2] Messinger D. et al. Beyond autism: a baby siblings research consortium study of high-risk children at three years of age. J Am Acad Child Adolesc Psychiatry. 2013 Mar;52(3):300-308.

[3] Mottron L. et al. Sex differences in brain plasticity: a new hypothesis for sex ratio bias in autism. Molecular Autism. 2015. 6; 33.

---------- Messinger DS, Young GS, Webb SJ, Ozonoff S, Bryson SE, Carter A, Carver L, Charman T, Chawarska K, Curtin S, Dobkins K, Hertz-Picciotto I, Hutman T, Iverson JM, Landa R, Nelson CA, Stone WL, Tager-Flusberg H, & Zwaigenbaum L (2015). Early sex differences are not autism-specific: A Baby Siblings Research Consortium (BSRC) study. Molecular autism, 6 PMID: 26045943

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