Determining prenatal, early childhood and cumulative long-term lead exposure using micro-spatial deciduous dentine levels

Abstract

The aim of this study was to assess the validity of micro-spatial dentine lead (Pb) levels as a biomarker for accurately estimating exposure timing over the prenatal and early childhood periods and long-term cumulative exposure to Pb. In a prospective pregnancy cohort sub-sample of 85 subjects, we compared dentine Pb levels measured using laser ablation-inductively coupled plasma mass spectrometry with Pb concentrations in maternal blood collected in the second and third trimesters, maternal bone, umbilical cord blood, and childhood serial blood samples collected from the ages of 3 months to ≥6 years. We found that Pb levels (as 208Pb:43Ca) in dentine formed at birth were significantly associated with cord blood Pb (Spearman ρ = 0.69; n = 27; p<0.0001). The association of prenatal dentine Pb with maternal patella Pb (Spearman ρ = 0.48; n = 59; p<0.0001) was stronger than that observed for tibia Pb levels (Spearman ρ = 0.35; n = 41; p<0.03). When assessing postnatal exposure, we found that Pb levels in dentine formed at 3 months were significantly associated with Pb concentrations in children's blood collected concurrently (Spearman ρ = 0.64; n = 55; p<0.0001). We also found that mean Pb concentrations in secondary dentine (that is formed from root completion to tooth shedding) correlated positively with cumulative blood lead index (Spearman ρ = 0.38; n = 75; p<0.0007). Overall, our results support that micro-spatial measurements of Pb in dentine can be reliably used to reconstruct Pb exposure timing over the prenatal and early childhood periods, and secondary dentine holds the potential to estimate long-term exposure up to the time the tooth is shed.

Figure 1. Overview of the dentine Pb biomarker.

(a) Schematic of a deciduous human incisor. During tooth formation the deposition of the enamel (E) and primary dentine (PD) matrix commences at the enamel-dentine junction (dashed line). At birth the neonatal line (red line) is formed in enamel and dentine providing a landmark to distinguish prenatally formed parts of teeth from those formed postnatally. In our analysis, prenatally formed primary dentine adjacent to the enamel-dentine junction is sampled to obtain prenatal Pb exposure information. Sampling points in dentine at the neonatal line measure fetal Pb exposure at the time of birth. Secondary dentine (SD) formation starts after the completion of the tooth root and proceeds at a slower rate as long at the tooth remains vital. Measurements in this region are used to estimate cumulative long-term Pb exposure from root completion to the time tooth was shed. Pulp chamber (P) and cervical margin (C) between the tooth crown and root are also indicated. (b) Lead distribution and developmental timing of primary dentine sampled in a deciduous molar. It is our hypothesis that Pb levels at each dentine sampling point represent Pb exposure experienced when that part of dentine was being mineralized. In this individual dentine Pb levels showed a marked rise prior to birth. Key time points for change in dentine Pb levels are shown; other points on the graph can be similarly dated.

Figure 2. Association of Pb levels in dentine formed at birth with blood Pb concentrations at birth and childhood.

(a) Significant positive association of Pb levels (as log_e [²⁰⁸Pb:⁴³Ca10⁴]) in dentine at the neonatal line (representing fetal Pb exposure at birth) with umbilical cord blood Pb concentrations (as log_e [Pb×10 µg/dL]). (b) Spearman’s ρ (Y-axis; vertical lines show 95% CI) of Pb levels in dentine formed at birth with Pb concentrations in blood collected at different ages (X-axis; in months). The association of birth dentine Pb is strongest with umbilical cord blood Pb and is progressively weaker with blood sampled at later ages. Sample size at different time points: birth = 27, 3 m = 55; 6 m = 43; 12 m = 61; 18 m = 63; 24 m = 68; 30 m = 64; 36 m = 63; 48 m = 67; 60 m = 48.

Figure 3. Association of Pb levels in prenatal dentine with maternal bone and blood Pb concentrations.

Prenatal dentine Pb (as log_e [AUC ²⁰⁸Pb:⁴³Ca×10⁴]) was positively associated with Pb concentrations (as µg Pb/g bone mineral) in a) tibia (Spearman ρ = 0.35; n = 41; p<0.03) and b) patella (Spearman ρ = 0.48; n = 59; p<0.0001). Relevance of negative bone Pb concentrations is discussed in detail by Hu et al. Significant positive associations were observed between Pb concentrations in maternal blood and prenatal dentine formed during the c) 2^nd trimester (Spearman ρ = 0.60; n = 36; p<0.0001) and d) 3^rd trimester (Spearman ρ = 0.64; n = 44; p<0.0001).

Figure 4. Timing of postnatal Pb exposure estimated from dentine.

Spearman’s correlation coefficients (and 95% CI) for Pb levels in dentine formed at 3 months postnatally and child blood Pb also measured at 3 months. When compared to Pb levels in blood collected at older ages in childhood, the association was progressively weaker supporting the hypothesis that dentine retains the timing of postnatal Pb exposure.