Prenatal lead exposure modifies the effect of shorter gestation on increased blood pressure in children

Abstract

1. Introduction

High blood pressure (BP) in childhood can lead to adult hypertension (Belsha et al., 1998; Berenson et al., 1998; Chen and Wang, 2008; Hanevold et al., 2004; Sorof et al., 2003). Elevated BP in childhood, in the absence of congenital heart disease, is nearly always renal in origin (Boubred et al., 2013). While some studies have examined the effect of environmental nephrotoxic exposures during susceptible windows of renal development on elevated childhood BP, none has examined the effect of environmental exposure as a modifier of the effect of shorter gestation.

Lower gestational age is a well-established risk factor for increased systolic blood pressure (SBP) in early life (Johansson et al., 2005; Keijzer-Veen et al., 2010) and even among adults (Raju et al., 2017). For example, a meta-analysis of 10 studies of former low birth weight or preterm infants compared to term controls found a pooled estimate of 2.5 mm Hg (95% CI: 1.7, 3.3) higher SBP later in life (average age = 17.1 years) (de Jong et al., 2012). Infants born prior to 36 weeks’ gestation, the time by which nephrogenesis (the formation of new nephrons) is complete (Benz and Amann, 2010; Solhaug et al., 2004), represent a particularly susceptible population for altered renal function. Based on the timing of in utero nephron development, we hypothesize that a nonlinear model would more appropriately examine critical windows of renal development. By comparing associations before and after 36 weeks, we hypothesize that there would be a stronger association with increased children’s BP among shorter gestations since this is coupled with renal immaturity.

Moreover, we speculate that nephrotoxic exposures during perinatal life in combination with shorter gestation could alter kidney growth/ developmental trajectories and program adult diseases of renal origin. Gestation and early childhood are potential susceptibility windows for nephrotoxic metals, as these life stages are associated with development of glomerular filtration and maturation of tubular function (e.g. absorption and secretion) (Gubhaju et al., 2014; Luyckx et al., 2013; Sutherland et al., 2014). Relatively minor insults can offset the normal developmental trajectory towards a hypertensive phenotype in later life, as BP will increase with age. Lead is a common nephrotoxicant in adults (Chiu and Yang, 2005; Hellstrom et al., 2001; Loghman-Adham, 1997; Sommar et al., 2013). Specifically, in the US, lead accounts for 5% of the population attributable risk for hypertension in adults participating in National Health and Nutrition Examination Survey (NHANES) from 2009 to 2012 (Shiue and Hristova, 2014). Previously, we have shown sex-specific associations between prenatal lead levels and elevated BP in children in Mexico City (Zhang et al., 2012). Other studies have shown links between lead and children’s elevated SBP or diastolic blood pressure (DBP) (Gump et al., 2005; Gump et al., 2007; Hawkesworth et al., 2013; Sorensen et al., 1999). However no studies to-date have examined the interaction of prenatal lead exposure in combination with shorter gestation on childhood BP.

We sought to understand whether the association between shorter gestation and increased childhood BP was modified by prenatal lead exposure, selected a priori as a prevalent nephrotoxic metal. To assess effects on BP at 4–6 years of age in a prospective cohort of 565 mother-child dyads, we examined the relationship between gestational age and BP using linear and nonlinear piecewise models. To investigate the modification effect of lead exposure, we used a data driven approach to assess 1) a change point in gestational age associated with SBP; and 2) a cutpoint level of ‘high’ or ‘low’ lead that might exacerbate developmental nephrotoxic exposure. This study expands our understanding of the early origins of adult cardiovascular disease, a leading cause of mortality and morbidity in the US (Heron, 2013), and provides suggestive evidence for the role of prenatal lead exposure in the developmental programming childhood BP.

2. Methods

3. Results

3.2. Association between gestational age and SBP at 4 years

Linear examination of the relationship between gestational age and SBP showed that a one week reduction in gestational age was associated with a 0.5 mm Hg (95%CI: 0.2, 0.8) increase in SBP and a 0.4 mm Hg (95%CI: 0.1, 0.6) increase in DBP, adjusted for child’s sex, age, height, maternal SES, and prenatal exposure to smoke (Table 2). Likewise, a one week reduction in gestational age was associated with a 0.4 mm Hg (95%CI: 0.2, 0.7) increase in MAP (Table 2). The estimates were relatively unchanged when adjusted for covariates, and similar relationships were observed with z-scored SBP and DBP (Table 2). No significant relationship was identified between gestational age and PP in adjusted or unadjusted models.

Table 2

The relationship between shorter gestation and BP using adjusted and unadjusted linear regression models (n=565).

	Effect estimatea		p–Value	Effect estimateb		p–Value
	β	(95% CI)		β	(95% CI)
SBP	0.45	(0.10, 0.81)	0.01	0.48	(0.15, 0.80)	0.004
DBP	0.34	(0.06, 0.63)	0.02	0.37	(0.10, 0.64)	0.007
PP	0.11	(−0.17, 0.39)	0.45	0.10	(−0.17, 0.37)	0.46
MAP	0.41	(0.12, 0.69)	0.005	0.44	(0.17, 0.70)	0.002
SBP z–scorec	0.05	(0.02, 0.08)	0.003	0.04	(0.01, 0.07)	0.005
DBP z–scorec	0.04	(0.01, 0.06)	0.003	0.03	(0.01, 0.06)	0.008

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^aUnadjusted model.

^bAdjusted for child’s age, height, and sex and maternal SES and tobacco smoke in the home.

^cBP Z–score model adjusted only for SES and tobacco smoke.

Examination of the relationship between gestational age and SBP using nonlinear piecewise models suggested evidence of different effect estimate magnitudes, depending on gestational age at delivery (Fig. 1) though these did not reach statistical significance. Our piecewise model demonstrated evidence of a join-point δ at 35.9 weeks’ (95%CI: 28.8, 43.0). Among children born before 35.9 weeks, SBP was 0.9 (95%CI: −0.5, 2.3) mm Hg higher per one week reduction in gestational age; and among children born later than 35.9 weeks, SBP was 0.4 (95%CI: −0.06, 0.9) mm Hg higher. While we are intrigued by the estimated join-point δ and potential indication of a nonlinear association, we cautiously interpret these findings and note the wide confidence interval and that a test comparing the difference in slopes β1 and γ1 had a p-value > 0.05.

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Fig. 1.

Associations of gestational age and child SBP at 4–6 years using nonlinear piecewise regression (n = 565). Plotted SBP values are residuals accounting for child’s age, sex, and height. Red circles are predicted values. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3.3. Prenatal lead modifies the association between shorter gestation and higher SBP

Using the data-driven SSE best-fit model approach, we identified the cutpoint BLL, π, of 2.5 μg/dL. When BLLs were < 2.5 μg/dL, evidence for nonlinearity was not present, therefore a linear model showed that each week reduction in gestational age was associated with a 0.4 mm Hg (95%CI: −0.9, 0.01) decrease in SBP (Fig. 2a). We note that for BLLs below 2.5 μg/dL this relationship was not statistically significant. However, when BLLs were ≥2.5 μg/dL, the data showed evidence of nonlinearity with the join-point δ estimated as 37.0 (95%CI: 32.1, 41.9) (Fig. 2b). Specifically, when BLLs were ≥2.5 μg/dL, SBP was 1.6 (95%CI: 0.3, 2.9) mm Hg higher per each week reduction in gestational age among children born before 37.0 weeks of gestation; and among children born after 37.0 weeks of gestation, this relationship was attenuated yet remained significant [β: 0.9, 95%CI (0.2, 1.6)] (Fig. 2b). However, the slopes β1 and γ1 were not statistically different. The overall p-value for the nonlinear model was p < 0.0001; however, the goodness of fit test comparing the piecewise model to a linear model (with high lead) was not significant.

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Fig. 2.

The association between lower gestational age and child SBP, when stratified by second trimester blood lead cutpoint π as a) BLLs < 2.5 and b) BLLs ≥ 2.5 μg/dL. Plotted SBP values are residuals accounting for child’s age, sex, and height. The blue line is a linear regression; red circles are predicted values from the nonlinear piecewise model. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4. Discussion

Results from our nonlinear model suggest that lower gestational age combined with higher prenatal lead exposure contributes to a higher risk of elevated BPs at 4–6 years of age among infants born < 37 weeks of gestation. We propose that the most appropriate interpretation of these data is that infants below this gestational age are more susceptible to the effects of in utero lead exposure due to the immaturity of renal development at the time of transition to extrauterine life. We identified a BLL cutpoint in our analyses as 2.5 μg/dL, a relatively low lead level, suggesting even low-level prenatal lead exposure that may have implications for later life renal health with far ranging public health significance. This is particularly important as current practice uses 5 μg/dL as the level of concern. Our statistical approach demonstrates the utility of nonlinear modeling for identifying i) critical windows of susceptibility (e.g. the joint point δ) and ii) threshold exposure levels (e.g. cutpoint BLL π). These data inform issues of temporality and dose related to early life lead exposure and contribute to the limited existing literature on the adverse cardiorenal effects of metals in children (Sanders et al., 2018; Zheng et al., 2017).

To avoid methodological concerns introduced by stratifying on preterm vs. term status or adjusting for gestational age (e.g., collider bias) (Wilcox et al., 2011), we explored the nonlinear piecewise model as a method to more appropriately examine critical windows of in utero renal development. Further, we observed a null relationship between lead and gestational age as previously reported (β: 0.03, 95%CI: −0.02, 0.08) (Renzetti et al., 2017). Similarly, the relationship between lead and SBP in this study demonstrated a null relationship with the inverse direction (β: −1.3, 95%CI: −3.2, 0.6). Our findings are intriguing as we identified an estimate for δ at 35.9 weeks, which closely aligned with our hypothesis of an expected δ near the time of complete nephron formation: 36 weeks. Our model demonstrates a novel statistical approach because methodologically accounting for gestational age as a known risk factor is difficult, and builds upon existing literature that shorter gestations as a risk factor for higher BP. Moreover, due to concern of prenatal lead exposure as a paradigm nephrotoxicant in the study population, we then explored the nonlinear piecewise model combined with lead stratified by ‘high’ and ‘low’ levels derived using a data-adaptive approach. Since a low nephron number alone may not predict increased BP, a kidney with fewer nephrons could be less able to withstand additional toxic injury (Luyckx and Brenner, 2015; Luyckx et al., 2017). Intriguingly, our results from the nonlinear piecewise model with lead exposure ≥ 2.5 μg/dL support this hypothesis. These findings have potential far-ranging public health significance as childhood renal health predicts adult renal health (Gluckman et al., 2008; Huang et al., 2009; Luyckx et al., 2013).

To our knowledge, our study is the only study to-date to examine the joint effects of lower gestational age and lead exposure. However, three previous studies evaluated the linear association between prenatal lead exposure and childhood BP with inconsistent results. Inconsistent associations reported across populations may be due to variations in the timing exposure or outcome assessment. In a prospective study of 690 Bangladeshi children, Skroder et al. (2016) identified no significant relationship with maternal blood erythrocyte lead measured in the second or third trimesters (14 or 30 weeks gestation) and SBP at 4.5 years of age. We note that Skroder et al. observed an inverse relationship between maternal blood lead and child kidney volume measured by ultrasound in a subset of 117 dyads. Zhang et al. (2012) in a prospective birth cohort of 457 mother-child pairs in Mexico, reported that an interquartile range (IQR) increase of prenatal lead measured in maternal tibia 1-month postpartum (reflecting prenatal exposures) resulted in a 2.1 mm Hg higher SBP (95% CI: 0.7, 3.5) and 1.6 mm Hg higher DBP (95% CI: 0.3, 2.9) among girls only at 10 years of age. No associations were observed in boys, or for lead measured in cord blood or patella. And lastly, Gump et al. (2005) showed in a retrospective study of 122 US children that higher cord blood lead levels were associated with significantly higher SBP (β = 12.16, p = 0.02) at 9.5 years of age. Prior studies that observed relationships between lead and BP were conducted later in childhood (9.5 years as compared to 4–6 years of age in our study) and assessed lead exposure via bone or cord blood rather than prenatal BLLs. Critical windows during pregnancy may reflect more sensitive timings during nephron development and subsequent studies in PROGRESS will examine relationships with BP at 7–10 years of age. Our study contributes to the body of literature suggesting that prenatal lead exposure contributes to altered BP in childhood, particularly among infants born preterm. Future studies should consider the potential interaction effect of gestational age and other nephrotoxicant exposures as well. Finally, these data may indicate that lead plays a role in the development of hypertension in adults born prematurely.

Our study builds on existing findings by highlighting the multifactorial effects of adverse birth outcome and prenatal lead exposure, as possible mechanisms of developmental BP programming. Nephrogenesis, the precise and complex programming of nephron formation in the metanephric kidney, generally begins at ~5 weeks of gestation and is complete by 36 weeks in the human infant (Benz and Amann, 2010; Solhaug et al., 2004). After birth, both renal blood flow and glomerular filtration, major determinants of renal function, continue to mature until approximately 2 years of age (Calcagno and Rubin, 1963; Veille et al., 1998). Infants born with low birthweight or born preterm have interruptions in normal development and significantly reduced kidney volume and final nephron number compared to normal weight or full term babies (Kandasamy et al., 2013; Luyckx et al., 2013). Mechanistic studies in mice implicate the mammalian target of rapamycin (Mtor) signaling pathway and the inhibitory protein Hamartin play a role in the duration of nephrogenesis and risk of subsequent kidney disease (Volovelsky et al., 2018). Indeed, the Barker hypothesis sprung from the observation that lower birthweight infants were more likely to develop later life hypertension (Barker et al., 1989). While several mechanisms of developmentally programmed hypertension and renal disease have been proposed (Baum, 2010; Block et al., 2015; Schreuder and Nauta, 2007), a consensus document by the Low Birth Weight and Nephron Number working group highlighted “the need to act early to prevent CKD and other related noncommunicable diseases later in life by reducing low birth weight, small for gestational age, prematurity, and low nephron numbers at birth through coordinated interventions” (Low Birth and Nephron Number Working, 2017).

There are a number of potential mechanisms that might explain our results. While speculative, metals including lead are well known to induce oxidative stress through a number of pathways. One mechanism by which preterm infants may be particularly susceptible to the effects of lead on the later development of higher BP may be the added impact of lead induced oxidative stress and the reactive oxygen species that arise from birth on the developing kidney. Indeed, several recent studies and reviews of lead toxicity proposed that the many health effects of lead on different tissues could be explained by oxidative stress, and that free radical damage could be a unifying mechanism to explain these pleiotropic effects (Corsetti et al., 2017; Mitra et al., 2017; Rehman et al., 2018). Future research should not exclude other mechanisms of action, but given the well-known relationship between lead and oxidative stress, this particular mechanism merits future study. Furthermore, when infants are born, at any gestational age, the partial pressure of oxygen rises from in utero levels of 40–50 mm Hg just prior to birth to 60–90 mm Hg within hours (Fanaroff et al., 2006). This increase, especially when combined with any additional supplemental oxygen, which is more commonly administered to preterm infants, can result in an excess of reactive oxygen species (Auten and Davis, 2009). In rats, lead exposure can reduce the ability to excrete reactive oxygen species (Vaziri et al., 1999) suggesting that the combined extra-uterine exposure to oxygen and higher lead concentrations in the setting of incomplete renal development may lead to direct tissue damage and subsequent susceptibility to hypertension. While epidemiological studies cannot determine mechanism, future investigations may aim to address redox and non-redox mechanisms underlying the observations.

Our study has several strengths. First, PROGRESS is a prospective study, with carefully collected covariate data, birth outcomes, and simultaneous exposures. Lead exposure was assessed years prior to BP and cannot be biased with respect to BP. We enrolled women during pregnancy and followed mother-child dyads to 4–6 years of age. Given our cohort design, we adjusted for potential confounding variables and analyzed gestational age as a continuous outcome. In linear models, we analyzed SBP and DBP as the primary outcomes collected using automated state of the art equipment. While residual confounding may limit our study, in general the effect estimates were markedly unchanged after covariate adjustment indicating little confounding. Moreover, similar results were observed with BP measures adjusted for key covariates (height, age, and sex) as observed with z-scored DBP and SBP outcomes that account for height, age, and sex using the new American Academy of Pediatrics guidelines (Flynn et al., 2017). We note that while the observed effect estimates (ranging from 0.4 to 1.6 mm Hg increased BP per each week shorter gestation) were not clinically relevant, even small changes in early life BP have been shown to impact an individual’s trajectory for later life hypertension (de Jong et al., 2012). Furthermore, the Prevention Paradox suggests that shifting the blood pressure of a whole population, even slightly, can affect the incidence or prevalence of a common disease such as hypertension (Rose, 2001).

This study had several limitations. We assessed BP at one time point, although the assessment was blind to prenatal BLLs. Cross-sectional examination of childhood lead as well as longitudinal follow-up is ongoing and future studies (including assessment of renal function and risk factors including sodium intake) will assess whether the effects of lead exposure on BP persist into adolescence or ultimately lead to hypertension. The PROGRESS cohort is a racially/ethnically homogeneous population, which may limit external generalizability. Although we observed five cases of high BP in this study (< 1%) in children aged 4–6 years, the estimated prevalence of childhood hypertension in Mexico is roughly 10% in adolescents (Cervantes et al., 2000; Juarez-Rojas et al., 2008), about 3-fold higher than the estimates of 3–4% in the US among children age 8–17 (George et al., 2014), albeit these estimates do not reflect the recently updated definition for childhood hypertension. It is possible that while our results suggest some children may be on the trajectory to high BP, assessment at 4–6 years of age is still very early for the development of clinical hypertension. Moreover, casual BP was obtained in this study and averaged from two oscillometric measures; we recommend future studies apply a minimum of 3 measures. The observed lead levels are generally higher among Mexican than US populations, making this an ideal cohort in which to test our hypotheses. There are still many pockets of lead poisoning in both Mexico and the U.S. and while levels are decreasing there are still thousands of adults with BLLs higher than 2.5 μg/dL (Tsoi et al., 2016). We reiterate that our nonlinear results between gestational age and SBP should be interpreted with caution due to limited data at lower gestational ages and the findings should be replicated. The prevalence of preterm birth in this cohort was 10% (59 cases), which while comparable to national estimates, limited sample size contributing to some models. Our intriguing findings suggesting a lead-modification effect of increased BP for children born prior to 37 weeks should be replicated in other larger cohorts. We acknowledge that sensitivity analyses adjusting for weight, and excluding the 19 Capurro-reassigned gestations resulted in unstable models. This is a limitation of our dataset and these results require replication in additional cohorts.

5. Conclusion

We found that higher maternal BLLs in pregnancy modified the association between lower gestational age and increased BP in children, whereby children with combined prematurity and maternal BLL in pregnancy ≥2.5 μg/dL had higher BP at age 4–6, than children who did not have both characteristics. This study contributes to our understanding of the early origins of adult renal or cardiovascular disease, and suggests the role of prenatal lead exposure in the multi-factorial sequela of programmed childhood BP. If replicated in other cohorts and studies, the findings provide further support that prenatal lead exposure, particularly for children born preterm, is a preventable risk factor for altered early life BP with widespread implications for future health and disease. Future studies will assess renal function and molecular changes and indicate whether the observed association with BP persists into adolescence and adulthood. Our results underscore the importance of preventing prenatal lead exposure, preterm birth, and higher BP in childhood to reduce risk of hypertension and cardiovascular disease later in life.

Supplementary Material

Acknowledgments

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Footnotes

References