Evaluating the Role of Susceptibility Inducing Cofactors and of Acetaminophen in the Etiology of Autism Spectrum Disorder

Abstract

More than 20 previously reported lines of independent evidence from clinical observations, studies in laboratory animal models, pharmacokinetic considerations, and numerous temporal and spatial associations indicate that numerous genetic and environmental factors leading to inflammation and oxidative stress confer vulnerability to the aberrant metabolism of acetaminophen during early development, leading to autism spectrum disorder (ASD). Contrary to this conclusion, multivariate analyses of cohort data adjusting for inflammation-associated factors have tended to show little to no risk of acetaminophen use for neurodevelopment. To resolve this discrepancy, here we use in silico methods to create an ideal (virtual) population of 120,000 individuals in which 50% of all cases of virtual ASD are induced by oxidative stress-associated cofactors and acetaminophen use. We demonstrate that Cox regression analysis of this ideal dataset shows little to no risk of acetaminophen use if the cofactors that create aberrant metabolism of acetaminophen are adjusted for in the analysis. Further, under-reporting of acetaminophen use is shown to be a considerable problem for this analysis, leading to large and erroneously low calculated risks of acetaminophen use. In addition, we argue that factors that impart susceptibility to acetaminophen-induced injury, and propensity for acetaminophen use itself, can be shared between the prepartum, peripartum, and postpartum periods, creating additional difficulty in the analysis of existing datasets to determine risks of acetaminophen exposure for neurodevelopment during a specific time frame. It is concluded that risks of acetaminophen use for neurodevelopment obtained from multivariate analysis of cohort data depend on underlying assumptions in the analyses, and that other evidence, both abundant and robust, demonstrate the critical role of acetaminophen in the etiology of ASD.

Keywords: acetaminophen; autism spectrum disorder; oxidative stress; paracetamol.

Figure 1

The role of acetaminophen (APAP) and of a complex array of interacting genetic, epigenetic, and environmental factors in the etiology of autism spectrum disorder (ASD). The list of preexisting and environmental factors comprising the multifactorial model is intended to be representative and is not an exhaustive list. A similar model for fetal alcohol spectrum disorder (FASD) is currently accepted, with alcohol replacing acetaminophen in that model.

Figure 2

Survey of inclusion of the “multifactorial model” regarding the etiology of autism spectrum disorder (ASD) in 100 recent reviews. The survey reflects one assessment of the popularity of the multifactorial model among experts in the field, and inclusion of 100 reviews was deemed adequate for that purpose. The survey does not represent an exhaustive review of the use of a multifactorial model for ASD induction. For example, no effort was made to expand the search using terms related to “cause” (e.g., etiology, pathogenesis, induction), which may have been useful in an exhaustive review. Full texts (n = 30) that were not either open access or available online through the University of North Carolina, Chapel Hill Library were considered unavailable and were excluded. The survey was conducted by coauthor WP using the result of a PubMed query made 23 January 2024. The publication dates of the included reviews ranged from February 2023 to January 2024. ASD: autism spectrum disorder.

Figure 3

In silico modeling of the risk of ASD with acetaminophen (APAP) use. On the top left are 10 normally distributed risk factors that, when added together (bottom left), define the risk for acetaminophen-induced injury. The top right panel shows acetaminophen use as a function of relative risk in a virtual environment. In the top right panel, the x-axis indicates the fractional rank of risk on a linear scale, with 1 being greatest risk, 0.5 having average risk, and 0 being lowest risk. Also in the top right panel, the number indicated in the middle of each line indicates the average use of APAP in the virtual environment. A four-to-one ratio of APAP usage for the virtual individuals with the highest risk compared to those with the lowest risk was used (top right panel). The bottom right panel shows the final assignments of ASD in the in silico population based on risk (bottom left panel) and APAP exposure (top right panel).

Figure 4

Results of Cox regression analysis with variable actual acetaminophen (APAP) use, 7.5% documented use, and 50% of all ASD induced by APAP. The fraction of actual APAP use documented in the analysis has the most profound effect on the calculated hazard ratios, although it is the inclusion of more cofactors in the Cox analysis that causes the HR to become insignificant.