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Adverse Exposures to the Fetus and Neonate
Exposure to environmental toxicants during pregnancy may have a significant impact on the developing fetus. Fetal adverse exposures are growing in importance as our population expands and the number of women participating in the workforce continues to rise. In 2015, women comprised 47% of the workforce in the United States, and 73.5 million women aged 16 and over were employed. Approximately 75% worked full time. In addition, more pregnant women are staying in the workforce and continuing to work until later into pregnancy. According to the U.S. Census Bureau (2011), 66% percent of women worked during their first pregnancy in 2008, and 88% percent continued into the third trimester.
Environmental exposures increase as the world's population and economic needs expand. In 1900, there were 1.25 billion people on Earth. This number doubled by 1950 and doubled again in 1987 to 5 billion. The world population was estimated at 7.6 billion in 2017; it is projected to increase to 9.8 billion by 2050, and 11.2 billion by 2100. Pressures of population growth are reflected in increasing need for land, water, food, and fuel. With population expansion, economic growth, and technologic advances comes increasing interaction with chemicals in our environment that are developed to support our society. The growth of industry, commerce, and agriculture brings overuse of land; changes to the environment such as global warming; and contamination of air, water, food, and soil. It is inevitable that all humans are exposed to environmental toxicants, including the developing fetus.
The Developmental Origins of Health and Disease (DOHaD) is a field of study that explores the relationship between exposure to environmental stressors during critical periods of fetal development and adverse health outcomes that occur across a lifetime (see also chapter 16 ). This paradigm suggests that the developing fetus exhibits significant genetic plasticity that may be impacted by the intrauterine environment. Thus, genetic “reprogramming” that occurs prenatally can alter phenotypic expression, both following birth and in generations to come. The DOHaD is based on work originally published in 1990 by a British epidemiologist, Dr. David Barker. The Barker hypothesis (or fetal origins hypothesis) describes fetal nutritional and metabolic programming as the mechanism linking intrauterine growth restriction to the development of coronary artery disease as an adult. Fetal exposure to toxicants is a stressor that has also been implicated in the DOHaD and is the subject of this chapter. Toxic environmental exposures may be non-concurrent or concurrent with pregnancy. This chapter describes the role of epigenetics and developmental plasticity, pathways of fetal exposure to environmental toxicants, unique pharmacokinetics of the fetus, and the spectrum of adverse outcomes associated with fetal exposure to specific environmental toxicants ( Fig. 14.1 ).
Maternal concurrent and nonconcurrent exposures and fetal exposure.
Exposures Not Concurrent With Pregnancy
Environmental exposures that affect the fetus may occur long before conception and possibly in an earlier generation than the parents. Commonly, the exposure impacts the ovum or sperm. In addition, there are chemicals that bioaccumulate prior to pregnancy and impact the fetus by enhanced elimination during pregnancy. Lipid-soluble toxins and heavy metals such as lead are stored in adipose and bone and mobilized during pregnancy. Some chemicals, such as organohalogens, can affect the fetus by both non-concurrent and concurrent exposure.
Preconceptual Effects
The Epigenome
Epigenetics is the study of phenotypic changes occurring in the absence of modification of DNA sequence. Environmental epigenetics describes the relationship between endogenous and exogenous factors (such as chemical exposure) and the epigenome. Changes to the epigenome are stable, heritable, and a target for environmental toxicants. The term epimutation refers to a heritable change in gene expression that does not affect DNA base pair structure.
The epigenome is susceptible to dysregulation at any time but is highly vulnerable during fetal life when the rate of DNA synthesis is high. Epigenetic mechanisms include alterations in expression patterns of microRNAs, modifications of histone proteins, and DNA methylation. These epigenetic mechanisms are described in Fig. 14.2 . MicroRNAs are small segments of noncoding RNA that regulate post-transcriptional expression of mRNA. Histones are a group of eight nuclear proteins that package and tightly compact DNA chromatin into smaller, coiled segments called nucleosomes containing 146 base pairs. Histone modifications such as acetylation and phosphorylation alter the histone protein at the N-terminal tail, changing nucleosome structure, and activating or repressing transcription. DNA methylation is a well-described epigenetic modification involving addition of a methyl group to the nucleotide cytosine when it precedes guanine, frequently at the promoter region. Methylated segments of DNA are more tightly coiled, hiding the promoter region and limiting gene expression. In less methylated areas, the promoter is open, allowing for DNA transcription. DNA methylation is the mechanism of X-chromosome inactivation during embryogenesis in females. Aberrant DNA methylation of the X-chromosome is implicated in fragile X syndrome. While DNA methylation is not entirely static across a lifetime, much of the epigenome is established during fetal development.
One of the best-described epigenetic processes is genomic imprinting. Genomic imprinting occurs during early development and involves silencing of one parental allele leading to monoallelic gene expression. Dysregulation of genomic imprinting, potentially due to aberrant methylation of DNA, leads to disorders such as Angelman syndrome, Prader-Willi syndrome, and Beckwith-Wiedemann syndrome, and has been linked to autism and cancer later in life. Imprinted genes are ideal models to study epigenetic modifications. As imprinting occurs early in fetal development, epigenetic adaptations are likely to have a significant effect. For example, adults who were conceived during the Dutch famine of World War II (1944–1945) demonstrate widespread and persistent changes in DNA methylation of imprinted insulin-like growth factor-2 (IGF-2) loci, with diverse biologic functions. These genes are known to be important modulators of growth, and epigenetic modifications are implicated in the development of metabolic and cardiovascular disease in adults. Offspring conceived during the famine have developed these metabolic and cardiovascular conditions more frequently as adults.
Fig. 14.3 illustrates potential periods of fetal development in which environmental toxins may impact the phenotype of the developing fetus. Environmental toxicants have been shown to alter the epigenome during fetal life via modifications in DNA methylation. Measures of global DNA methylation can quantify the methylation state of the epigenome and have been utilized to investigate the relationship between toxicant exposure and epigenetic modifications. Global DNA hypomethylation is correlated with instability of the genome and cancer risk.
An association between prenatal exposure to metals such as arsenic, cadmium, and lead and DNA methylation has been documented in the literature. Buccal samples from children prenatally exposed to maternal cigarette smoking show both global hypomethylation and in specific loci important in modulating cellular proliferation. Polycyclic aromatic hydrocarbons (PAHs) are neurotoxic and carcinogenic chemicals found in fumes from vehicle exhaust, coal, charbroiled foods, and cigarette smoke. PAHs are lipophilic and cross the placenta and fetal blood-brain barrier. Byproducts of PAH metabolism have the capacity to bind DNA, forming PAH-DNA adducts. The fetus is highly susceptible to formation of PAH-DNA adducts and associated genetic mutations, a known cancer risk. In a cohort of 159 children, prenatal airborne PAH exposure was associated with global hypomethylation of umbilical cord WBC DNA, which persisted at 3 years of age. In addition, increased global methylation was associated with the presence of BPH-DNA adducts in cord blood, two independent findings associated with increased cancer risk.
Maternal Exposures
In early fetal life, oogonia are formed by meiotic division. Before birth, oogonia develop into primary oocytes and complete prophase of the first meiotic division. Oocytes remain in this state until puberty. As cells are highly susceptible to environmental toxicants while in active phases of division, fetal life represents a critical period of vulnerability. This hypothesis is supported by the increasing incidence of nondisjunction events with advancing maternal age and prolonged environmental exposures. Exposures impacting regulatory and endocrine functions of the ovum can influence ovarian competence throughout life. Fig. 14.4 represents critical windows of exposures that may impact the male and female reproductive system.
Active smoking during pregnancy is associated with loss of ova in the fetus, which may reduce fertility in women born to mothers who smoke. In animal models, fetal exposure to nicotine results in granulosa cell proliferation, impaired ovarian steroidogenesis and angiogenesis, increased ovarian cell apoptosis, and reduced fertility. Endocrine disrupting chemicals (EDCs) are compounds that act as agonists or antagonists to the endocrine system. Epidemiologic studies demonstrate an association between developmental exposure to EDCs and infertility. Bisphenol A (BPA) is an EDC found in plastic products. An inverse relationship has been found between the number of eggs recovered during in-vitro fertilization and urinary BPA levels.
Developmental exposure to environmental toxicants can have an impact on fertility that may be transmitted to future generations. Inheritance of epigenetic information between generations that leads to phenotypic variation in the absence of direct environmental influence is termed epigenetic transgenerational inheritance . Epigenetic transgenerational inheritance requires epimutations to be present in the germline, as it is only sperm and eggs that pass this information to the next generation. Epigenetic transgenerational inheritance has been proposed as a mechanism for phenotypic aberrations that have occurred following fetal exposure to the EDC diethylstilbestrol (DES). DES is an estrogenic drug that was prescribed to 5–10 million women worldwide from the 1940s to the 1970s to prevent miscarriage. Although vaginal clear cell adenocarcinoma has occurred infrequently in daughters of women exposed to DES, infertility and poor pregnancy outcomes are extremely prevalent. In addition, other disorders of the reproductive, cardiovascular, and immune systems have been reported in both male and female offspring and these effects have persisted in the grandchildren of DES-treated women. The incidence of breast cancer is at least twofold higher in the daughters of DES-exposed mothers.
Paternal Exposures
Environmental toxicants that impact spermatogenesis have been identified. Spermatogonia are highly sensitive to apoptosis after exposure to cytotoxic agents. Toxicants may also impact male fertility by limiting sperm production. Unlike the female, cell divisions in the male that produce mature spermatozoa occur after puberty. However, mature spermatozoa have no DNA repair mechanisms and are vulnerable to the effects of mutagens. Transient aneuploidy of autosomal and sex chromosomes has been reported in sperm of men treated for Hodgkin disease with chemotherapy in the preceding 3 months.
A gradual decline in sperm concentration has been reported in reproductive-age males, and human male infertility is currently approaching 10%. Increased exposure to environmental toxicants has been postulated as a contributing factor. For many of these exposures, findings have been reproduced for several generations, and epigenetic transgenerational inheritance is a proposed mechanism. Chronic occupational exposure to the pesticide 1,2-Dibromo-3-chloropropane (DBCP) is associated with cessation or reduction in spermatogenesis. In the rat model, exposure of the pregnant female to two EDCs (vinclozolin, a fungicide used in the wine industry, and the pesticide methoxychlor) led to impaired spermatogenesis and infertility in males in four subsequent generations. Phthalates are plasticizers and known EDCs that are now ubiquitous in our environment. Bis (2-ethylhexyl) phthalate (DEHP) is found in polyvinyl compounds, while dibutyl phthalate (DBP) is used primarily to add flexibility to plastics. Both compounds can adversely affect male reproductive function. In animal studies, prenatal exposure has been linked to spermatogenic cell apoptosis, seminiferous tubule atrophy, and testicular dysfunction. Phenotypic alterations following exposure prenatally included cryptorchidism, hypospadias, ambiguous genitalia, reduced sperm production, and decreased anogenital distance.
The relationship between paternal occupation and cancer in offspring has been extensively studied. One study reported increased risk of central nervous system (CNS) tumors with paternal occupational exposure to pesticides (relative risk [RR] 2.36; 95% CI 1.27-4.39) and work as a painter (RR 2.18; 95% CI 1.26-3.78). Increased incidence of childhood leukemia has been associated with paternal periconceptual occupational exposure to woodworking, solvents, paints, pesticides, and motor vehicles (driving, exhaust fumes, and inhaled particulate hydrocarbons). A case control study evaluating paternal exposure to pesticides reported increased risk of astrocytoma in offspring; combining occupational and home exposures significantly elevated this risk (OR = 1.8, 95% CI 1.1–3.1).
Approximately 60% of congenital malformations of unknown etiology are estimated to be secondary to environmental toxicants. Increased prevalence of birth defects in offspring of fathers employed as janitors, painters, printers, agricultural workers, groundskeepers, welders, electrical industry workers, and firefighters have been reported. In addition, paternal exposure to organic solvents and pesticides was a significant risk factor for congenital anomalies in offspring.
Male fertility diminishes and sperm DNA mutations increase with advancing paternal age. Advanced paternal age is associated with pregnancy loss, birth defects, and autosomal dominant genetic disorders such as Marfan syndrome and achondroplasia. Congenital malformations seen more frequently in fetuses of older fathers include cleft lip and palate, hydrocephalus, neural tube defects, limb reduction defects, tracheoesophageal fistula, congenital cataracts, and congenital heart disease. A possible mechanism for paternally mediated effects is the impairment of a paternal gene necessary for the normal growth and development of the fetus. Replacement of the father's genetic material with a second copy of the mother's genetic material (uniparental disomy), or vice versa, results in a nonviable conceptus. In Prader-Willi syndrome, there is a functional mutation in paternal 15q, resulting in inactivation of the genes in that region of the chromosome. Environmental factors may play a role in uniparental disomy and lead to paternally mediated effects on the fetus. Studies have shown association between paternal exposure to hydrocarbons and Prader-Willi syndrome. In one study, approximately 50% of fathers of children with Prader-Willi syndrome were occupationally exposed to hydrocarbons.
Secondary Fetal Exposure: Maternal Body Burden
The developing fetus may be exposed to xenobiotics (chemicals not naturally produced by the organism) after increased mobilization from maternal storage compartments during pregnancy. Adipose tissue is a storage site for hydrophobic chemicals, while both lead and fluoride are stored in bone.
Polychlorinated Biphenyls
Polychlorinated biphenyls (PCBs) are endocrine disruptors (EDCs) that are mobilized from adipose tissue during pregnancy and readily cross the placenta. They are lipophilic organochlorine chemicals that bioaccumulate in the environment, enter the food chain, and are stored in adipose tissue. Exposure occurs primarily through ingestion of dairy, animal fat, and fish. PCBs were used as liquid insulators for transformers and capacitors in the 1970s. Because PCBs remain stable in the environment and are resistant to biologic breakdown, they are classified as persistent organic pollutants (POPs). Although use has been banned or limited in many countries, exposure remains a public health concern. Two recent studies (2011, 2014) document the presence of PCBs in blood samples from pregnant women and in breast milk.
Prenatal exposure to PCBs has been correlated with neurodevelopmental impairment and autism. Maternal occupational exposure is linked to low birth weight and prematurity. Human poisonings have occurred through dietary consumption of PCBs. In 1979, an epidemic of PCB poisoning from contaminated rice oil occurred in Taiwan termed yu-cheng disease . Adults who were exposed developed hyperpigmentation, acne, and peripheral neuropathy. Of the first 39 hyperpigmented children born to poisoned women, 8 died. Children born up to 6 years after the outbreak of yu-cheng disease had ectodermal defects and developmental delay. Children born 6 years after maternal exposure were as developmentally impaired as those born within 1 year of the epidemic, indicating significant maternal body burden. Cognitive defects seen in Taiwanese children with yu-cheng disease are comparable to those observed in American children prenatally exposed to PCBs. Studies in Michigan and North Carolina showed neurodevelopmental impairment in children prenatally exposed to PCBs through maternal body burden. In Michigan, cognitive deficits were seen in children who had elevated cord blood levels of PCBs and whose mothers had regularly consumed contaminated sport fish.
Lead
The major repository for lead is bone, and chronic exposure results in significant accumulation of lead in the skeleton. Lead stores are mobilized from bone during pregnancy, potentially exposing the fetus during critical stages of brain development. It is postulated that enhanced calcium turnover during pregnancy increases lead mobilization. Maternal tibial and patellar bone lead levels are correlated with low birth weight and decreased head circumference. One study established maternal trabecular bone lead level as an independent risk factor for cognitive delay at 24 months of age. Lead is readily transported across the placenta and blood-brain barrier. Lead is a potent neurotoxin. Developmental exposure is linked to learning disability, cognitive and language deficits, and attention deficit hyperactivity disorder (ADHD). Treatment of pregnant women with calcium supplementation may limit lead mobilization from bone and could decrease circulating maternal lead levels and placental transfer to the fetus.
Maternal Exposures Concurrent With Pregnancy
Biomarkers of fetal exposure have been developed using cord blood and meconium. In 2007, The Environmental Working Group measured cord blood levels of 413 chemicals from 10 US newborns. They found 287 toxicants, with an average of 200 chemicals in cord blood of each infant. Chemicals included pesticides, PCBs, and heavy metals such as mercury. Meconium analysis is a sensitive tool utilized to determine antenatal exposure to environmental toxicants. In a study of 426 infants born in the Philippines, exposure rate was 26.5% for lead, 83.9% for mercury, and 53% for the organophosphate pesticide malathion.
Occupation and Paraoccupation
The strongest associations between maternal exposures and adverse pregnancy outcome (spontaneous abortion, miscarriage, and congenital anomalies) have been found for lead, mercury, pesticides, organic solvents, and ionizing radiation. Occupations linked with adverse pregnancy outcome secondary to exposures include anesthesiologists, hair dressers, laboratory technicians, dry cleaners, agricultural workers, and those working in chemical, electronic, or shoe factories.
Specific congenital anomalies have been linked to maternal occupations. For example, cleft lip and palate occurs more frequently in offspring of leather workers, hairdressers, housekeepers, and transport and communication workers. Limb anomalies have been reported in children of agricultural workers, and neural tube defects in children of cleaners and health care providers working with anesthesia and radiation. In a large population-based case control study that examined the relationship between various maternal occupations and 45 birth defects, jobs with the highest number of associated anomalies were janitors/cleaners, scientists, and electronic equipment operators. Teachers and health care workers had the lowest risk of delivering a child with congenital anomalies. However, in health care providers, adverse pregnancy outcomes (miscarriage and congenital anomalies) have been reported with chronic occupational exposure to antineoplastic medications, especially when this exposure occurs during the first trimester.
Paraoccupational exposures occur when others living in the home come in contact with an occupationally exposed individual, or occupational chemicals are brought into the home on clothing or other materials. Paraoccupational exposure to pesticides remains an occupational hazard today. An important route of exposure to neurotoxic organophosphate pesticides (OPs) occurs in families of farm workers. A systematic review of 10 studies assessing prenatal exposure to OPs reported neurocognitive impairment involving working memory and attention in toddlers and children at school age.
Air Pollution
Many chemicals found in outdoor air pollution are neurotoxic. Exposure is associated with disruption of the blood-brain barrier, chronic CNS inflammation, microglia activation, and white matter injury. The United States Environmental Protection Agency (EPA) established air quality standards in 1971. Although some progress has been achieved, outdoor air pollution continues to carry significant health risks. Particulate matter, ground-level ozone, sulfur dioxide, nitrogen dioxide, carbon monoxide, and airborne lead are examples of outdoor air pollutants with known adverse health outcomes. These air pollutants are the focus of air quality standards, such as the EPA's Clean Air Act. The air quality index (AQI) is an indicator of daily air quality calculated by the EPA for these six major air pollutants that are regulated by the Clean Air Act. Both carbon monoxide and airborne lead are typically meeting safety standards.
Particulate matter pollution consists of fine inhalable particles comprised of many different chemicals found in outdoor air (PM 2.5 and PM 10 , 2.5 micrometers and 10 micrometers in diameter, respectively). Particles less than 10 micrometers are readily inhaled into the lungs, interact with the immune system, and are absorbed systemically. Ground-level ozone (the primary component of smog) is formed in the atmosphere by photochemical reactions between sunlight, nitrogen oxide species, and volatile organic compounds (VOCs), which are photochemically reactive hydrocarbons. Sulfur dioxide and nitrogen dioxide are also components of smog. Emissions from industrial facilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are common sources of these toxicants.
Exposure to outdoor air pollution is difficult to avoid during pregnancy, especially in highly polluted areas. There is strong evidence concerning exposure to air pollution to both increased infant mortality and respiratory morbidities such as asthma in early childhood. In addition, there is evidence from epidemiologic studies linking prenatal exposure to air pollution and prematurity, low birth weight, impaired lung and immune function, autism spectrum disorders, and neuromotor impairment in early childhood.
Drinking Water
The drinking water supply is obtained from a variety of sources that include ground water and surface water from rivers, lakes, reservoirs, and aquifers. Environmental toxicants that contaminate drinking water are associated with adverse health outcomes. Common drinking water contaminants include the heavy metals lead and arsenic, inorganic compounds such as nitrates and nitrites found in fertilizers and livestock manure, pesticides such as atrazine and glyphosate, disinfection byproducts such as chloroform, and organic solvents such as trichloroethylene. Congenital anomalies, specifically cardiac defects, and low birth weight were found in studies evaluating trichloroethylene-contaminated drinking water. The EPA and the Food and Drug Administration (FDA) monitor the safety of drinking water; the EPA is responsible for drinking water standards for public water systems.
Arsenic enters the water supply naturally and is also found in agricultural and industrial chemicals. Arsenic readily crosses the placenta; prenatal exposure is associated with fetal growth restriction and increased infant mortality in animal models. Population studies of health outcomes associated with arsenic exposure have been conducted primarily in countries such as Bangladesh, Taiwan, and Chile, where naturally occurring arsenic levels in groundwater are elevated. A systematic review and meta-analysis of these studies notes an association between prenatal arsenic exposure and spontaneous abortion, stillbirth, low birth weight, and neonatal mortality. While studies on dose response were limited, the strongest correlation was between prenatal arsenic exposure and spontaneous abortion.
Diet
Maternal diet during pregnancy appears to impact the fetal epigenome. In animal models, fetal growth restriction has been linked to cardiovascular disease, including adult-onset hypertension and central adiposity. Diets low in protein or iron and high in saturated fat appear to have the greatest impact on blood pressure. Conversely, several studies have documented the relationship between maternal obesity, gestational diabetes mellitus, and altered epigenome methylation in the placenta and cord blood; findings that may potentially correlate with risk for metabolic conditions in offspring.
The effects of nutritional modification can be selective. Genetically identical agouti mice contain a coat color gene controlled by DNA methylation. If pregnant mice are fed normal rat chow, offspring have either a yellow coat or brown coat. Those with a brown coat have normal weight, while those with a yellow coat are prone to obesity, diabetes, and cancer. Mice fed a diet supplemented with methyl donors such as folic acid tend to develop brown color coat and are not obese. Maternal exposure to bisphenol A (BPA), an endocrine disruptor, shifts distribution of offspring's coat color to yellow. This effect can be reversed by dietary supplementation with methyl donors such as choline or folate.
Similarly, B12 and folate deficiency during pregnancy have been proposed to impact the fetal methionine-homocysteine cycle via epigenetic alterations in DNA methylation. The methionine-homocysteine pathway is an important regulator of methylation and is dependent on Vitamin B12 and folate as cofactors. Deficiency in B12 or folate leads to high levels of homocysteine, a well-established risk factor for adult cardiovascular disease. Pregnant sheep exposed to diets restricted in B12, folate, and methionine deliver offspring who develop increased body mass, impaired immune function, insulin resistance, and hypertension as adults.
Inorganic mercury (methylmercury) is an example of a xenobiotic that contaminates our food supply. Methylmercury is a lipid-soluble and potent neurotoxin found in fish and seafood. Methylmercury readily crosses both the placenta and blood-brain barrier. The US EPA has set an oral reference dose of mercury at 0.1 µg/kg/day. Approximately 8% of women have body burdens exceeding this reference dose. The fetus is very susceptible to the neurotoxic effects of methylmercury. Prenatal exposure has been associated with microcephaly, seizures, and cognitive and sensory impairment. In 1956, methylmercury contaminated the food chain in Minamata Bay, Japan. Offspring of pregnant women from a local fishing village exposed to methylmercury developed severe neurodevelopmental impairment.
Pathways of Fetal Exposure
Placenta-Dependent Pathways
Two routes of fetal exposure to environmental toxins are placenta-dependent pathways and placenta-independent pathways. Placenta-dependent pathways require chemicals to enter the mother's bloodstream and cross the placenta in significant amounts. Not all environmental toxins meet these criteria; asbestos and radon gas do not, unless they have been ingested. Most xenobiotics cross the placenta by passive diffusion. Therefore, the concentration gradient between maternal and fetal circulation and placental blood flow are the primary determinants of exposure.
Properties enabling chemicals to cross the placenta more readily are low molecular weight, protein binding, lipid solubility, ionization, and resemblance to nutrients with specific transport mechanisms. An example of a low-molecular-weight compound is carbon monoxide, a constituent of environmental tobacco smoke (ETS). Carbon monoxide has a very high affinity for hemoglobin and displaces oxygen. Accumulation of carboxyhemoglobin leads to hypoxia and impaired cellular metabolism. Examples of lipid-soluble chemicals that readily cross the placenta are PCBs, ethanol, and polycyclic hydrocarbons such as benzo[a]pyrene, a carcinogen in ETS. Lead is actively transported across the placenta via calcium-specific channels; calcium supplementation may reduce the transfer of lead to the fetus.
Placenta-Independent Pathways
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