Fetal and postnatal metal dysregulation in autism

Abstract

Genetic and environmental factors contribute to the etiologies of autism spectrum disorder (ASD), but evidence of specific environmental exposures and susceptibility windows is limited. Here we study monozygotic and dizygotic twins discordant for ASD to test whether fetal and postnatal metal dysregulation increases ASD risk. Using validated tooth-matrix biomarkers, we estimate pre- and post-natal exposure profiles of essential and toxic elements. Significant divergences are apparent in metal uptake between ASD cases and their control siblings, but only during discrete developmental periods. Cases have reduced uptake of essential elements manganese and zinc, and higher uptake of the neurotoxin lead. Manganese and lead are also correlated with ASD severity and autistic traits. Our study suggests that metal toxicant uptake and essential element deficiency during specific developmental windows increases ASD risk and severity, supporting the hypothesis of systemic elemental dysregulation in ASD. Independent replication in population-based studies is needed to extend these findings.

Figure 1. Roots of Autism and ADHD Twin Study in Sweden (RATSS).

Recruitment, case ascertainment and collection of teeth for measurement of metals.

Figure 2. Overview of tooth-matrix biomarkers applied in this study.

(a) Plane in which teeth were sectioned. (b) Laser ablation-inductively coupled plasma mass spectrometry analysis. The surface of dentine was ablated using a laser and the ejected material transferred to a mass spectrometer. Neonatal line and other histological features are used to select sampling locations adjacent to the dentine-enamel interface extending from the dentine horn to the tooth cervix (dashed black line). (c) Output data where developmental timing is assigned to elemental profiles in each sample, reconstructing the history of elemental uptake over the prenatal and early childhood periods. The illustrations were created by Jill K Gregory from the Icahn School of Medicine at Mount Sinai.

Figure 3. Pre- and postnatal differences in metal uptake between non-ASD control and ASD discordant and concordant twin pairs.

Patterns in lead, manganese and zinc distribution over the developmental period studied are shown for (a) non-ASD control monozygotic (MZ) twin pair, (b) ASD discordant MZ and dizygotic (DZ) twin pairs and (c) ASD concordant MZ twin pair. Red lines indicates ASD twins and blue/green, control co-twins. Grey dashed line denotes time of birth. Element levels are reported as metal ion counts (²⁰⁸Pb, ⁵⁵Mn, or ⁶⁶Zn) relative to an internal standard (⁴³Ca).

Figure 4. Developmental periods of maximal metal dysregulation in ASD discordant versus non-ASD control twin pairs.

DLMs for zinc, manganese and lead. Black line represents mean of paired differences in metal levels between twins discordant for ASD (3 MZ; 4 DZ) and non-ASD control pairs (9 MZ; 10 DZ). Data are shown as 95% bands unadjusted (grey band) and adjusted (blue band) for intra-twin correlations. Grey vertical lines are Holm–Bonferroni corrected family-wise 95% CIs for 11 comparisons (every fifth week). Values above zero represent increased levels in ASD cases compared to their non-ASD sibling after taking into account average difference in control twins. A critical window is defined as an area of the DLM adjusted for intra-twin correlations (blue band) that does not include zero. Models were adjusted for sex, zygosity, gestational age, the average birth weight of the twin pairs, and the s.d. of the birth weight in the twin pairs.

Figure 5. Developmental periods of maximal metal dysregulation in ASD discordant versus ASD concordant twin pairs.

DLMs for zinc, manganese and lead. Black line represents mean of paired differences in metal levels between twins discordant for ASD (3 MZ; 4 DZ) and ASD concordant pairs (5 MZ; 1 DZ). Data are shown as 95% bands unadjusted (grey band) and adjusted (blue band) for intra-twin correlations. Grey vertical lines are Holm–Bonferroni corrected family-wise 95% CIs for 11 comparisons (every fifth week). Values above zero represent increased levels in ASD cases compared to their non-ASD twins after taking into account average difference in concordant twins. A critical window is defined as an area of the DLM adjusted for intra-twin correlations (blue band) that does not include zero. Models were adjusted for sex, zygosity, gestational age, the average birth weight of the twin pairs, and the s.d. of the birth weight in the twin pairs.

Figure 6. Association of elemental concentrations with measures of autism traits and clinical severity.

(a) Correlation of manganese, lead and zinc with Social Responsiveness Scale-2 (SRS-2) at different developmental time points. (b) The correlation of these elements with Autism Diagnostic Observation Schedule-2 (ADOS-2). Data are shown as 95% bands unadjusted (grey band) and adjusted (blue band) for intra-twin correlations. Grey vertical lines are Holm–Bonferroni corrected family-wise 95% CIs for 11 comparisons (every fifth week). Values above zero represent positive correlation between metal uptake and clinical index score. A critical window is defined as an area of the intra-twin adjusted correlation (blue band) that does not include zero. Models are adjusted for gender, gestational age and birth weight.