Biomarkers in Research
Technological advances in identifying and quantifying infectious agents in humans have led to rapid progress in diagnosing and controlling infectious diseases. Similarly, biomarkers are likely to change public health practices and revolutionize our understanding of environmental causes for many human diseases and disabilities. Biomonitoring studies have already improved our understanding of adverse health effects related to environmental contaminants, including lead, methyl mercury, polychlorinated biphenyls (PCBs), and tobacco.
Methyl (organic) mercury is almost completely absorbed when ingested. It easily crosses the placenta and is stored by the fetus.123 At high doses, fetal exposure can result in death or widespread developmental damage, such as mental retardation and spastic paralysis.1At lower doses, exposure has been linked with deficits in neuromotor performance, memory, and language.4Canadian Health Measurements Survey (CHMS) measured whole blood mercury levels and found nearly all Canadians sampled were below the Health Canada safe guideline value of 20 μg/L (Figure 1).5 Still, mercury, like other toxic substances, may cause harm at levels previously thought to be safe. Gina Muckle and her colleagues found an elevated risk for ADHD for children who were exposed to mercury concentrations < 20 μg/L.6
Polychlorinated Biphenyls (PCBs)
Extremely high PCB exposure during fetal development can cause low birth weight, dark pigmentation of the skin, acneform rash, and death.7 Fetal exposure to PCBs has been associated with weak reflexes, delayed motor development, and poor memory in children.78 Using cord blood or breast milk, epidemiologists have found that prenatal PCB exposure was associated with deficits in intelligence, memory, and attention.89 Paradoxically, even though PCBs were found in most subjects’ breast milk, PCB-related cognitive decrements were observed only in the children who were not breast fed, suggesting that the benefits of breast milk may have compensated for the toxic effects of PCBs.9
Prenatal tobacco exposure is a risk factor for lowered intellectual ability and behavioural problems, such as ADHD (attention deficit hyperactivity disorder) and conduct disorder—a disorder associated with aggressive and bullying behaviours.1011 In one study, Braun and his coworkers found that mothers who smoked during pregnancy were over 7-times more likely to have a child who had conduct disorder.12
Several conditions have linked genetic polymorphisms with greater risk from tobacco exposure. Robert Kahn and his colleagues found that children who had the high-risk dopamine transporter were only at increased risk for ADHD-related behaviours if they were exposed to tobacco in utero.13 Frank Gilliland and his colleagues found that children who lacked a key enzyme that helps detoxify tobacco were only at higher risk for persistent asthma if they were exposed in utero to tobacco (Figure 2).14
Similarly, Xiaobin Wang and her coworkers found that women who lacked a gene that codes for a detoxifying enzyme only gave birth to low-birth-weight babies if they smoked tobacco.15 These studies illuminate the interplay of genes and environmental influences, like tobacco. They also indicate that we can prevent disease simply by eliminating exposures to environmental hazards.
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- 2. Ramirez GB, Cruz MC, Pagulayan O, et al. The Tagum study I: analysis and clinical correlates of mercury in maternal and cord blood, breast milk, meconium, and Infants' hair. Pediatrics. 2000; 106:774-781.
- 3. Mahaffey KR, Clickner RP, Bodurow CC. Blood organic mercury and dietary mercury intake: National Health and Nutrition Examination Survey, 1999-2000. Environ Health Perspect. 2004;112(5): 562-570.
- 4. Grandjean P, Weihe P, White RF, et al. Cognitive deficit in 7-year-old children with prenatal exposure to methylmercury. Neurotoxicol Teratol. 1997;19:417-428.
- 5. Statistics Canada. Canadian Health Measures Survey: Cycle 1 Data Tables. Catalogue no. 82-623-X. 2010.
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- 7. a. b. Rogan WJ. Environmental poisoning of children: lessons from the past. Environ Health Persp. 1995;103 (Suppl 6):19-23.
- 8. a. b. Jacobson JL, Jacobson SW. Intellectual impairment in children exposed to polychlorinated biphenyls in utero. N Engl J Med. 1996;335:783-789.
- 9. a. b. Schantz SL, Widholm JJ, Rice DC. Effects of PCB exposure on neuropsychological function in children. Environ Health Persp. 2003;111:357-376.
- 10. Wakschlag LS, Pickett KE, Cook E Jr, et al. Maternal smoking during pregnancy and severe antisocial behavior in offspring: a review. Am J Public Health. 2002;92:966-974.
- 11. Weitzman M, Byrd RS, Aligne CA, et al. The effects of tobacco exposure on children's behavioral and cognitive functioning: implications for clinical and public health policy and future research. Neurotoxicol Teratol. 2002;24:397-406.
- 12. Braun JM, Froehlich TE, Daniels JL, et al. Association of environmental toxicants and conduct disorder in U.S. children: NHANES 2001-2004. Environ Health Perspect. 2008;116(7):956-962.
- 13. Kahn RS, Khoury J, Nichols WC, et al. Role of dopamine transporter genotype and maternal prenatal smoking in childhood hyperactive-impulsive, inattentive, and oppositional behaviors. J Pediatr. 2003;143:104-110.
- 14. Gilliland FD, Li YF, Dubeau L, Berhane K, Avol E, McConnell R, Gauderman WJ, Peters JM. Effects of glutathione S-transferase M1, maternal smoking during pregnancy, and environmental tobacco smoke on asthma and wheezing in children. Am J Respir Crit Care Med. 2002 Aug 15;166(4):457-463.
- 15. Wang X, Zuckerman B, Pearson C et al. Maternal cigarette smoking, metabolic gene polymorphism, and infant birth weight. JAMA. 2002 Jan 9;287(2):195-202.