The Mediating Role Of Placental Inflammation In The Association Between Serum Thallium Levels During Pregnancy And Cognitive Development Among Preschool Children: A Birth Cohort Study

Background Thallium is a heavy metal and is widely employed in industry and manufacture.At present,the environmental pollution posed by thallium has become a public health problem.The neurotoxicity of thallium has been observed in animal models.However,accessible epidemiological studies concerning early-life thallium exposure and offspring neurodevelopment are limited,and the underlying mechanism are poorly defined.Objectives To reveal the distribution of maternal serum thallium levels,evaluate preschoolers' cognitive development,and explore the association of serum thallium exposure during pregnancy with cognitive development among preschool children and its potential sensitive period and sex-specific,as well as further elucidate the mediating role of placental inflammation in the association between serum thallium exposure during pregnancy and cognitive development among preschool children.Methods Based on the Ma'anshan Birth Cohort,we assembled maternal basic demographic information through self-administered questionnaires during periodic prenatal visits and collected maternal peripheral venous blood samples during each trimester of pregnancy.After delivery,umbilical cord blood and placenta samples were gathered immediately.At age 5.5 years,blood samples were obtained from the children.We used an inductively coupled plasma mass spectrometry to determine blood thallium levels in maternal and children.Besides,we performed a quantitative real-time polymerase chain reaction to examine placental cytokines m RNA expression levels.Furthermore,we evaluated cognitive development with children aged 4.5 years old using the Chinese version of the Wechsler Preschool and Primary Scale of Intelligence-Fourth Edition.A cohort of 2102 mother-child pairs was finally selected for the study.First,restricted cubic spline(RCS)models were used to flexibly model potential nonlinear relations between thallium concentrations during pregnancy and preschoolers' cognitive development test scores.Second,we performed linear mixed models(LMMs)to analyze the association of thallium exposure during the whole pregnancy with cognitive development among preschool-aged children.Third,multiple informants generalized estimating equations(GEEs)models were fit to jointly estimate the association between the four repeated measurements of thallium concentrations and preschool-aged children's cognitive test scores.In addition,all the above models were stratified by child sex to explore sex-specific of thallium exposure and cognitive development.Based on the analysis described above,participants with no placental samples(n =481)were excluded,and the remaining 1621 mother-child pairs were included.The association between m RNA expression levels of cytokines in placenta samples and preschoolers' cognitive development scores was performed by multiple linear regression(MLR).Then,LMMs and MLR were used to analyze the association between thallium concentration during pregnancy or at different exposure periods and m RNA expression levels of cytokines,respectively;and finally,causal mediation models were fit to explore the mediating role of placental cytokines in the association of serum thallium exposure with preschoolers' cognitive development and sex stratification was performed based on the above models.All data analyses and graphs were produced using SPSS(version 23.0;SPSS Inc),R(version 4.1.2;R Development Core Team),and Graph Pad Prism software(version9.0;Graph Pad Software Inc).Results The detection rate of thallium in maternal and child blood samples was 100%.The serum thallium concentrations at each trimester of pregnancy were 1.80 ± 0.13,1.79 ±0.13,and 1.68 ± 0.19 ng/L,respectively;umbilical cord thallium levels was 1.58 ±0.10 ng/L;plasma thallium concentration in children was 1.62 ± 0.14 ng/L.VCI,VSI,FRI,WMI,PSI,and FSIQ scores in preschool children were 108.9 ± 12.4,106.4 ±12.8,104.8 ± 11.5,103.2 ± 11.7,101.7 ± 10.8,and 107.6 ± 11.1 scores,respectively.After adjusting for covariates and confounders,the RCS identified thallium exposure during whole pregnancy as well as different exposure periods was related to linear change with cognitive development among preschoolers(non-linear P-value greater than 0.05).LMMs showed that per Lg concentration increase in serum thallium levels during pregnancy were correlated with the 3.30,2.58,3.01,and 2.62 decreases in VSI,FRI,PSI,and FSIQ scores among preschool children,respectively.Multiple informants GEEs found that serum thallium concentration in the first trimester was significantly negatively correlated with the VSI and PSI scores;the association between the second-trimester thallium concentrations and cognitive development test scores was not statistically significant;the third-trimester thallium level increased by per Lg concentration,VSI,PSI,and FSIQ decreased by 4.12(95%CI:-7.51,-0.73),3.21(95%CI:-6.17,-0.24)and 3.10(95%CI:-7.51,-0.73)scores,respectively;cord blood thallium concentration was only statistically associated with VCI scores.Sex-stratified analysis showed that thallium exposure during the whole pregnancy or at different exposure periods was negatively correlated with VCI,VSI,FRI,WMI,PSI,and FSIQ in boys.Besides,thallium exposure during the whole pregnancy or at different exposure periods was negatively correlated with VSI,WMI,and PSI in girls.Sensitivity analysis results were essentially robust.Placental HIF-1?,CD68,TNF-?,IFN-?,IL-6,and IL-8 m RNA expression levels were negatively correlated with VCI,VSI,FRI,WMI,and FSIQ scores in preschoolers.It is pertinent to note that serum thallium levels during pregnancy elevated MCP-1,IL-10,HO-1,HIF-1?,GRP78,IL-6,TNF-?,and CD68 m RNA expression levels;the association between thallium concentrations during the third trimester and IL-10,MCP-1,HO-1,HIF-1?,GRP78,and CD68 m RNA expression levels was also statistically significant.In addition,causal mediation analysis showed that HIF-1?,TNF-?,IL-6,and CD68 m RNA expression levels play a negative mediating role in the association between thallium exposure during pregnancy and VCI,VSI,FRI,WMI,and FSIQ in preschoolers,with the mediating effect ranging from 1.7% to16.7%;HIF-1? and CD68 m RNA expression levels play a negative mediating role in the association between thallium exposure in the third trimester and VCI,VSI,and FSIQ in preschoolers,with the mediating effect ranging from 8.5% to 12.8%.After sex stratification,HIF-1?,TNF-?,IL-6,and CD68 m RNA expression levels play a negative mediating role in the association of thallium exposure during pregnancy or the third trimester with VCI,VSI,FRI,WMI,and FSIQ in boys,while HIF-1? and CD68 m RNA expression levels play a negative mediating role in the association of thallium exposure during pregnancy or the third trimester with VCI,VSI,and FSIQ in girls.Conclusions Serum thallium exposure during pregnancy negatively impacts cognitive development among preschoolers.In addition,the association between serum thallium exposure and preschoolers' cognitive development changes significantly with the thallium exposure period,and the third trimester may be a sensitive period.Moreover,sex can modify the negative association between serum thallium concentrations during pregnancy and preschoolers' cognitive development scores,and these negative associations are more prominent in boys.Serum thallium concentrations can significantly increase placental cytokines m RNA expression levels,and the association was more significant in the third trimester;serum thallium exposure during pregnancy can adversely affect cognitive development among preschool-aged children by increasing placental cytokines levels and inducing placental inflammation,especially in boys.