Perinatal exposure to potential endocrine disrupting chemicals and autism spectrum disorder: From Norwegian birth cohort to zebrafish studies

BACKGROUND

The etiology of autism spectrum disorder (ASD) is multifactorial, involving genetic and environmental contributors such as endocrine-disrupting chemicals (EDCs).

OBJECTIVE

To evaluate the association between perinatal exposure to 27 potential EDCs and ASD among Norwegian children, and to further examine the neurodevelopmental toxicity of associated chemicals using zebrafish embryos and larvae.

METHOD

1,199 mothers enrolled in the prospective birth-cohort (HUMIS, 2002-2009) study. Breastmilk levels of 27 chemicals were measured: polychlorinated biphenyls, organochlorine pesticides, polybrominated diphenyl ethers, and perfluoroalkyl substances as a proxy for perinatal exposure. We employed multivariable logistic regression to determine association, utilized elastic net logistic regression as variable selection method, and conducted an in vivo study with zebrafish larvae to confirm the neurodevelopmental effect.

RESULTS

A total of 20 children had specialist confirmed diagnosis of autism among 1,199 mother-child pairs in this study. β-Hexachlorocyclohexane (β-HCH) was the only chemical associated with ASD, after adjusting for 26 other chemicals. Mothers with the highest levels of β-HCH in their milk had a significant increased risk of having a child with ASD (OR = 1.82, 95 % CI: 1.20, 2.77 for an interquartile range increase in ln-transformed β-HCH concentration). The median concentration of β-HCH in breast milk was 4.37 ng/g lipid (interquartile range: 2.92-6.47), and the estimated daily intake (EDI) for Norwegian children through breastfeeding was 0.03 µg/kg of body weight. The neurodevelopmental and social behavioral effects of β-HCH were established in zebrafish embryos and larvae across various concentrations, with further analysis suggesting that perturbation of dopaminergic neuron development may underlie the neurotoxicity associated with β-HCH.

CONCLUSIONS

Prenatal exposure to β-HCH was associated with an increased risk of specialist-confirmed diagnoses of ASD among Norwegian children, and the EDI surpasses the established threshold. Zebrafish experiments confirm β-HCH neurotoxicity, suggesting dopaminergic neuron disruption as a potential underlying mechanism.

Phenotypic Characterization of Larval Zebrafish (Danio rerio) with Partial Knockdown of the cacna1a Gene

The CACNA1A gene encodes the pore-forming α1 subunit of voltage-gated P/Q type Ca²⁺ channels (Ca_v2.1). Mutations in this gene, among others, have been described in patients and rodents suffering from absence seizures and episodic ataxia type 2 with/without concomitant seizures. In this study, we aimed for the first time to assess phenotypic and behavioral alterations in larval zebrafish with partial cacna1aa knockdown, placing special emphasis on changes in epileptiform-like electrographic discharges in larval brains. Whole-mount in situ hybridization analysis revealed expression of cacna1aa in the optic tectum and medulla oblongata of larval zebrafish at 4 and 5 days post-fertilization. Next, microinjection of two antisense morpholino oligomers (individually or in combination) targeting all splice variants of cacna1aa into fertilized zebrafish eggs resulted in dose-dependent mortality and decreased or absent touch response. Over 90% knockdown of cacna1aa on protein level induced epileptiform-like discharges in the optic tectum of larval zebrafish brains. Incubation of morphants with antiseizure drugs (sodium valproate, ethosuximide, lamotrigine, topiramate) significantly decreased the number and, in some cases, cumulative duration of epileptiform-like discharges. In this context, sodium valproate seemed to be the least effective. Carbamazepine did not affect the number and duration of epileptiform-like discharges. Altogether, our data indicate that cacna1aa loss-of-function zebrafish may be considered a new model of absence epilepsy and may prove useful both for the investigation of Cacna1a-mediated epileptogenesis and for in vivo drug screening.