While estimates of heritability have often been derived from familial studies, work over the past 5–10 years has increasingly demonstrated that a substantial proportion of heritability is instantiated in common genetic variation captured by whole-genome genotyping arrays across a broad swath of anthropomorphic and neuropsychiatric traits . Thus, a comprehensive understanding of both normative brain and cognitive development and their relation to early SU and abuse requires genetically-informed approaches, including both familial studies of heritability and molecular genetic studies. The ABCD study will take both approaches. Another paper in this issue describes the twin component to ABCD.Human and animal studies document that early life exposures to environmental neurotoxicants including heavy metals , and prenatal exposure to drugs of abuse can negatively impact brain development, leading to maladaptive and persistent alterations in brain structure and function and cognitive and behavioral development. Despite numerous studies describing the neuro developmental toxicity of early life environmental exposures, documenting associations between in utero and early life exposures with adverse health effects is hindered by the absence of direct fetal biomarkers that can be used safely to measure exposure in large study populations. Most associations between prenatal chemical exposure and neuro developmental outcomes are based on the analyses of maternal samples obtained at the time of birth, and perhaps at one to two time points during pregnancy. This timing may be months after the exposure occurred and the pharmacokinetic and tissue distribution of various chemicals may be quite different at different stages of fetal and child development . Documenting prenatal exposure to drugs of abuse is further compounded by social stigma associated with reporting licit and illicit drug use during pregnancy . To address this gap in our understanding of fetal and early life exposures, Dr. Manish Arora and colleagues have recently developed a novel methodology to retrospectively and objectively quantify the dose and timing of environmental exposures throughout pregnancy and early childhood using naturally shed deciduous “baby” teeth .
In the following sections we briefly summarize the literature demonstrating associations between early life exposure to environmental toxicants and drugs of abuse on developmental outcomes,marijuana grow system describe the novel approach to detecting exposures to some of these toxicants in shed deciduous “baby” teeth, and present the protocol for baby tooth processing and archival. By collecting teeth and leveraging the novel tooth biomarker described below, the ABCD Study is building a valuable repository that provides a unique, exciting and valuable opportunity to study the individual, interactive and/or additive effects of early life environmental exposures on childhood neuro developmental outcomes.A growing population of children around the world is exposed to various neurotoxicants present in urban and rural environments, which may damage their developing brains. Within the last several decades, strong evidence suggests that infants and children are uniquely vulnerable to environmental toxicants due to disproportionally higher exposures, immature metabolic pathways, and rapid growth and development . It is now well accepted that low-level chronic exposure to environmental chemicals may contribute to the growing epidemic of childhood neuro developmental disorders worldwide . In adults, exposure to metals has been shown to induce psychotic behaviors or depressive symptoms and emotional instability in adults reviewed in . In children, epidemiologic studies demonstrate associations between early life exposure to metals with poor cognitive, emotional and behavioral functioning in children reviewed in . Notably, current knowledge of the neuro developmental health risks associated with environmental chemical exposure has been derived mainly from the study of single agents; however, no human is exposed to just one chemical at a time. Evidence suggest combined effects of multiple chemicals might occur at levels far below those observable for any one component reviewed in . Notably, an individual’s risk of exposure to neurotoxicants, as well as the risk of adverse outcomes associated with exposure, may vary based on socio-economic status reviewed in . Childhood socioeconomic status is characterized by a combination of factors, including family income, parental educational attainment and occupational status , and is known to be an influential factor for brain development and cognitive function .
Such associations could stem from ongoing disparities in postnatal experience or exposures, such as family stress, cognitive stimulation, environmental toxicants, or nutrition, or from corresponding differences in the prenatal environment. Given SES-related differences in brain development , and, observed relationships between brain structure and function and environmental toxicants , low SES youth may be at increased risk for negative outcomes from a multitude of environmental factors. Interactive and/or additive effects of various neurotoxicants and other environmental factors can be examined in the baby tooth biomarker as part of the ABCD study.Human studies of prenatal exposure to drugs of abuse such as alcohol , tobacco smoke , cocaine , and marijuana have shown brain and cognitive abnormalities among offspring of mothers who reported use during pregnancy. Most human studies on the impact of prenatal drug exposure on brain and cognitive development utilize retrospective samples and rely on mothers’ recollection of drug consumption patterns years after pregnancy , and/or select prospective samples of children with “heavy” exposure vs. low or no exposure. Validity of retrospective reports on maternal life style during pregnancy 10–12 years post-partum, including SU, has been shown to be sub-optimal . The ABCD protocol includes a developmental history where parents/guardians are asked to recall SU patterns both before pregnancy, and after pregnancy recognition. While data collection is on-going in the ABCD study, maternal self-report in a sample of over 2000 participants studied as of the end of May 2017, when questioned about SU prior to pregnancy most report no SU, but, approximately 25% report use of alcohol, 0.6% cocaine, 5% marijuana, and 13.6% tobacco . These percentages substantially decreased when parents/ guardians were asked about their post-pregnancy recognition SU, but some continued after pregnancy recognition. Of course, we do not know how many, if any, parent/guardians of ABCD participants denied SU when there actually was use, but, given social stigma in many communities, it is unlikely that individuals would report drug use during pregnancy if there was none. However, as described below, some of these substances can be measured using novel assays of baby teeth.As discussed above, determining exposure to environmental toxicants during the prenatal period has been hampered by the lack of appropriate biomarkers to measure direct fetal exposure. Further, until recently, no single biomarker could provide continuous, time-resolved documentation of exposure throughout the fetal and early childhood period.
Common biomarkers used for environmental assessment including maternal blood and urine are often not optimal matrixes for determining prenatal and early life exposure due to the timing and invasiveness of collection. Further, maternal biomarkers are not always a reliable measure for fetal exposure . For prenatal exposure to drugs of abuse, there are additional complications with parent self-report of licit and illicit drug use during pregnancy due to social stigma and length of time since pregnancy in remembering use patterns ∼9 to 10 years prior as will be the case in the ABCD cohort. Establishing timing of exposures, especially over the prenatal period, is a major challenge in environmental epidemiologic studies. Teeth have long been used to estimate long-term cumulative exposure, including prenatal exposure, to environmental and other substances . Notably, much of our knowledge of the impact of early life lead exposure on cognition was gained by examining associations between higher lead levels in children’s teeth and reduced IQ . However, previous tooth biomarker methods examined lead in the whole tooth providing a cumulative exposure of lifetime measure. Dr. Arora’s method incorporates laser ablation and micro-dissection techniques that leverages the physiology of tooth development to provide finely time-resolved assessments of exposure from the beginning of the 2nd trimester through the time the tooth is lost. Deciduous “baby” teeth growth proceeds in an incremental pattern,cannabis vertical farming forming rings and layers similar to the rings of a tree. Toxicants circulating in the fetal blood stream are captured in the layers and measuring the amount of toxicant in the layers provides information about exposures dose and timing. These newly developed high-dimensional analytical methods combine sophisticated histological and chemical analyses to precisely sample tooth layers and have the potential to reconstruct a timeline of exposures during early development . These methods have been tested extensively in prior research , and hold promise for establishing timelines of exposures to environmental toxicants and drugs of abuse in the ABCD sample. In addition to fine-grained timelines of exposure spanning the pre- to postnatal periods, advantages to using baby teeth as biomarkers of toxic exposures is that shed teeth can be stored relatively easily at room temperature, and does not require any invasive procedures such as blood draw.During development, enamel and dentine deposition occurs in a rhythmic manner, forming incremental lines akin to growth rings in both enamel and dentine . At birth, an accentuated incremental line, the neonatal line, is formed due to disturbances in the secretory cells during protein matrix deposition . This line forms a clear histological landmark that demarcates pre- and postnatally formed parts of teeth. Beyond the neonatal line teeth manifest daily growth lines, which allow chronological ages to be determined at various positions within tooth crowns and roots. The analytical approach involves sampling the growth rings of teeth using laser and other microdissection methods. Analyzing the sampled layers using mass spectrometers can yield time resolved information on organic and inorganic environmental compounds and their metabolites . Dr. Arora and colleagues have previously validated this biomarker for certain metals and validation for a range of organic targets is also underway in that laboratory. By collecting multiple baby teeth from individuals enrolled in the ABCD cohort, we are building a repository that may be leveraged in the future to measure not only the validated metals but also early life exposure to organics, maternal stress, and licit and illicit substances. Many other substances should be possible to measure on a detailed timeline during pre and post-natal development using baby teeth.
Previous research has shown that metabolites of licit substances, such as alcohol and tobacco, and illicit substances, such as cocaine and marijuana have been measured in the teeth of adults, though, most of these studies have been done with ground adult teeth using material from dental extractions, and do not allow for timeline of exposure in earlier development . To our knowledge, these biomarkers have not yet been validated using shed deciduous teeth, which would require contemporary measurements of more conventional biomarkers, such as maternal, newborn, and childhood urine/blood samples at various points during pregnancy and childhood. Nonetheless, there is potential for measurements of these substances in deciduous teeth by adapting existing assays, but using methods which allow timelines of exposure during development reviewed in .Participants’ parents are asked for 5 baby teeth shed between the ages of 6–13 years old. Parents either bring the teeth in during a lab visit, or mail shed teeth into the lab with provided kits . Teeth may become brittle in very cold orhot temperatures, thus shipping and storage of shed teeth occurs at room temperature. Data collection sheets completed by the parents include information about how each tooth was shed , and how it was stored .Tobacco use is the leading preventable cause of death in the US, killing over 480,000 people each year. Most of these deaths occur among cigarette smokers, 80% of whom begin smoking by age 18 and 99% of whom begin by age 25. The transition from experimentation to established smoking generally occurs in the late teens and early 20s. Tobacco use patterns vary within the population, with some people never smoking, some remaining occasional users, and others progressing to daily use or quitting. Existing research has identified 4–6 trajectories of smoking, typically classified as never-smokers, experimenters/occasional users, reducers or quitters, those who start smoking young and quickly become daily smokers, and those who start smoking as young adults and become daily smokers. Studies of smoking trajectories have primarily focused on identifying risk factors at the individual or family level, determining associations between trajectory type and health outcomes, and assessing links between trajectories and use of other products.Understanding the factors that influence smoking initiation and the transition to regular smoking is critical to developing tobacco control interventions that improve public health.