Offspring autism risk and advancing parental age (differences)

Parental age at offspring conception/birth in relation to offspring autism risk has been a recurrent theme in autism research circles for quite a few years now. I've covered it more than once on this blog (see here for example) and the various suggestions that advancing parental age in particular, might elevate the risk of offspring autism.

Set in this context, the paper by Sven Sandin and colleagues [1] (open-access) (a name not unfamiliar to this blog) adds to the research evidence based on their analysis of some 5.7 million children born between 1985 - 2004 resident in one of five countries (Denmark, Israel, Norway, Sweden and Western Australia). Including data on some 30,000 children diagnosed with an autism spectrum disorder (ASD): "Parental ages, sex and birth year were obtained from birth or civil registers."

After quite a bit of statistical modelling and controlling for various potentially confounding variables, several findings were reported pertinent to the authors' data being "the strongest evidence to date supporting the hypothesis that advanced parental ages at the time of birth are independently associated with risk for ASD in the offspring." Outside of "no support for any modification by the sex of the child" researchers also noted a "combined parental age effect" whereby there "was a joint effect of maternal and paternal age with increasing risk of ASD for couples with increasing differences in parental ages."

A few of the finer details of this study have been covered elsewhere (see here). I'll draw your attention to one or two statistics unearthed during the study:

• "relative to fathers aged 20–29 years, fathers 50 years or older had a statistically significantly increased risk for offspring with ASD (RR=1.66 95% CI:1.49–1.85)",
• "Relative to mothers aged 20–29 years, mothers younger than 20 years had a statistically significantly increased risk for offspring with ASD (RR=1.18 95% CI:1.08–1.29)" and 
• the "lowest risk corresponded to couples that generated the majority of births, specifically, 29–39-year-old fathers and 25–35-year-old mothers." 

Those estimates of relative risk (RR) statistics translate into an estimated 66% increased risk for offspring autism if a dad was over 50 years old compared with a dad in their 20s, an 18% increased risk for offspring autism for teen mums compared to 20-something mums and the lowest statistical risk of offspring autism being reported when dads conceive in their 30s coupled with a mid-20 to mid-30 year old mum. The authors also note that "Similar patterns of association, but with slightly higher RRs for the highest parental ages, were evident for AD [autistic disorder]" so completing the message about older parental ages at conception and differing parental ages being relevant across the autism spectrum.

Accepting that this was a huge study in terms of participant numbers and spanning different geographical locations, the authors rightly offer a few words of caution about their methods and data. So: "we lack information about potentially confounding variables such as SES [socio-economic status] and parental psychiatric history" is something to keep in mind [2]. Further: "We cannot rule out the possibility that other factors associated with parental age (for example, length of marriage or partnership, obstetric complications, gestational age and birth weight) have an important role in explaining our results" and "We did not have individual level information on co-morbid ID [intellectual disability] in ASD cases." I'd also suggest that given the growing emphasis on autism or ASD not existing in some sort of diagnostic vacuum (see here) one might reasonably ask whether other comorbidity outside of ID might also play a role in risk estimates.

As to the possible mechanism(s) of effect, well, the authors go through the usual older parents - older sperm and eggs mantra although perhaps bypassing an emerging area outside of just de novo mutations based on the role of epigenetic mechanisms (see here). They do suggest that the 'difference in parental age' factor might suggest "that the increase in risk is not attributable to advancing parental age per se, and that the risk increase cannot be explained solely by an accumulation of point mutations or other genomic alterations in the parents" but say little more on the basis of their collected data.

I might be wrong but I also didn't seem too much in the way of discussion of how parental nutrition might impact on offspring autism risk as per the proposed factor from other work by authors on the Sandin paper in relation to the inter-pregnancy interval (IPI) and autism risk (see here and see here). Although the idea that parental age might affect autism offspring risk, I'd be minded to suggest that this is only the first stage in a journey towards elucidating the particular mechanisms of any effect.

Music: The Pixies - Gigantic.

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[1] Sandin S. et al. Autism risk associated with parental age and with increasing difference in age between the parents. Mol Psychiatry. 2015 Jun 9.

[2] Lehti V. et al. Maternal socio-economic status based on occupation and autism spectrum disorders: A national case-control study. Nord J Psychiatry. 2015 Mar 3:1-8.

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Sandin S, Schendel D, Magnusson P, Hultman C, Surén P, Susser E, Grønborg T, Gissler M, Gunnes N, Gross R, Henning M, Bresnahan M, Sourander A, Hornig M, Carter K, Francis R, Parner E, Leonard H, Rosanoff M, Stoltenberg C, & Reichenberg A (2015). Autism risk associated with parental age and with increasing difference in age between the parents. Molecular psychiatry PMID: 26055426