Environmental chemicals and (neuro)development

Due to this complexity and interplay of processes it is not surprising that already small mistakes in one of these processes have great impact on the whole development of the nervous system. It is estimated, that around 22% of the adults in the United States suffer from at least one mental illness and according to the World Health Organization, this percentage is going to increase in the near future (Andersen 2003; Holden 2000). Additionally, 3 – 12%

(depending on the source) of the children under the age of 18 in the US bear at least one neurodevelopmental disorder (Boyle et al. 1994; Hass 2006; Schettler 2001). Many of these mental illnesses can be associated to genetic mutations (Rosenberg et al. 2007), but data from twin studies indicate that the influence of the environment on such disorders should not be regarded as minor (Fishbein 2000; McGuffin et al. 2001).

Environmental chemicals such as mercury or lead are known to disturb neurodevelopment in humans and are therefore - among many other chemicals, which do not necessarily have the strong epidemiological supportive data of these two metals - suspected risk factors for neurodevelopmental disorders (Grandjean and Landrigan 2006).

The following section describes why the developing brain is particularly vulnerable to toxic insults and why neurodevelopmental disorders might be associated to exposure to environmental chemicals.

2.1 Barker Hypothesis

Barker developed the concept that parameters of growth, such as birth weight or head circumference, can be used to predict the risk of adult diseases, such as coronary heart disease, stroke, insulin resistance and diabetes (Osmond and Barker 2000). Initially, the Barker Hypothesis was developed by David Barker in the 1990s (Barker 1997; Zadik 2003).

By using large epidemiological studies as well as reconstruction of birth cohorts in the UK, Barker was able to correlate such adult disease with reduced fetal growth and impaired development during infancy (Barker et al. 1992; Barker et al. 1993). The influence of e.g.

poor nutrition resulting in low birth weight as risk factor for disease later in life is nowadays firmly established in human epidemiology (Calkins and Devaskar 2011). Whether environmental chemicals could also lead to such effects still remains an unproven theory (Silbergeld and Patrick 2005). However, this theory is supported by studies correlating smoking during pregnancy with reduced birth weight and ultimately with cardiovascular disease (Bakker and Jaddoe 2011; Geelhoed et al. 2011). Additionally, it has been shown that effects of chemicals during early development can be transmitted through the germline up to 3 generations (Nilsson et al. 2008; Skinner et al. 2010). All these results suggest that negative events during early development lead to negative outcomes in the adult organism.

As a result of the suspected relation between exposure to environmental chemicals during embryonic or fetal development and adult mental disease, the Barker Hypothesis was expanded to neurodevelopmental disorders during the Mount Sinai Conference on Early Environmental Origins of Neurological Degeneration in 2003 (Landrigan et al. 2005).

2.2 Time of insult vs. time of phenotype onset

Another concept in (neuro)developmental toxicology is in line with the Barker Hypothesis.

This is the long latency period of many (developmental)neurotoxicants. It has been shown, that e.g. signs of toxicity of MeHg emerged several years after the cessation of a 7 year exposure period in nonhuman primates (Rice 1996). Furthermore, it has been shown that

effects of perinatal exposure to MeHg may emerge as late as 9 years after birth (Davidson et al. 2006).

The long latency period of many suspected neurodevelopmental toxicants is often explained by using the two hit theory of neurodevelopmental toxicology (Wang and Slikker 2011). The theory explains the long latency period by the need for a second, not necessarily toxic, event for the symptoms to manifest. The theory also includes the possibility that a toxic event (second hit) in the adult or aging brain is more severe in a brain which was exposed to a toxic agent during development (first hit). This is supported by studies showing that. effects of developmental exposure to e.g. triethyltin only manifest in aged organisms (Barone et al.

1995).

These effects are explained by a decrease in reserve/repair capacity of the brain caused by the first hit, due to which aging or a second hit have more severe outcomes later in life. Thereby the timing of disturbance of neurodevelopment by the first hit might also determine the type of defect or mental illness later in life (Watson et al. 1999).

2.3 Susceptibility of the developing brain to chemicals

Another well established concept is, that a developing organism/organ is much more susceptible to toxic insults than an adult organism/organ. It has been shown by many studies that low doses of chemicals not toxic for the mature CNS can cause defects in the developing nervous system (Claudio et al. 2000; Tilson 2000). Many studies support the fact, that the timing of a toxic insult is much more important than the type (e.g. type of chemical) (Fan and Chang 1996; Rice and Barone 2000). A particular sensitive time window seems to be the period of organogenesis (GD 20 – 40). It has been shown that radiation during this period often leads to malformations (De Santis et al. 2007). It is estimated, that 90% of all human embryos that experience a disturbance during early organogenesis are spontaneously aborted (Opitz et al. 1987). Additionally, protection and detoxification mechanisms are not fully developed in the early stages of development. An example would be PON1, the enzyme metabolizing chlorpyrifos and other organophosphate pesticides, which is not fully active in humans until the age of 9 (Huen et al. 2009). Other protection mechanisms, such the blood brain barrier (BBB) or DNA repair systems are either not present or not fully functional during development (Adinolfi 1985; Saunders 1986). Another important aspect is the lack of a liver detoxification system in the early fetus. During development, the fetus relies on the detoxification processes of the mother. Current existing exposure limits, aiming to protect the

nervous system, often do not take these aspects into consideration. They are designed to protect workers, not a developing fetus. Concentrations considered to be safe for adults might not necessarily be safe for a developing organism (Ginsberg et al. 2004; Tilson 2000).

Additionally, many chemicals known to have an effect on the developing nervous system, such as e.g. MeHg, tend to accumulate in fetal blood or lipid rich compartments, such as the mother milk, resulting in much higher exposure concentrations for the fetus/child compared to the mother (Jensen 1983; Landrigan et al. 2002; Sonawane 1995).

2.4 Environmental chemicals and developmental disabilities

As already described earlier, neurodevelopmental disorders and mental illnesses are a real and increasing problem in western countries. It is well established that mental disorders like autism or schizophrenia are the result of the complex interplay between genetic and environmental factors. It is believed that the individual genetic background influences the response to environmental factors. This concept is often referred to as G x E interaction (Gene-Environment interaction) (Tsuang et al. 2004). A famous example would be the alcohol flush syndrome. Due to genetic mutations, the activity of the aldehyde dehydrogenase (ALDH) is decreased, resulting in flushing of the face after alcohol consumption. Such genetic mutations are mainly observed in the Asian population (Takeshita et al. 1996;

Wermter et al. 2010). Whether such G x E interactions also play a role in environmental chemicals leading to mental disorders is still under debate and needs further investigation.

The following paragraphs try to summarize the evidence for the correlation of environmental chemicals with some of the most well-known developmental disabilities.

• Mental retardation

Mental retardation is a disorder appearing before adulthood, characterized by a low IQ (below 70), deficits in adaptive behaviors and impaired cognitive function (Fredericks and Williams 1998). Roughly 3% of the children in the US are affected by some form of mental retardation. Genetic disorders account for only 25-30% of the causes of mental retardation (Daily et al. 2000). Due to the fact that nervous system malformations ultimately lead to mental retardation, the disease should be regarded as a result of impairment to the CNS development in general, and not as an individual well defined disease.

Although the genetic background accounts for almost a third of diagnosed mental retardation, it is nowadays well established that environmental factors significantly contribute to this disorder (Matilainen et al. 1995; Simonoff et al. 1996). Several chemicals, including ethanol, lead, MeHg and cadmium, have been associated with mental retardation and more subtle forms of reduced IQ (Beattie et al. 1975;

Grandjean and Landrigan 2006; Marlowe et al. 1983; Mendola et al. 2002; Sokol et al. 2003).

• Schizophrenia

Schizophrenia is characterized by delusions, sensory hallucinations and impairment of speech organization (Goldner et al. 2002). Schizophrenia and autism may be the only mental disorder for which a possible cause in neurodevelopment, namely a delay in neurodevelopment, is widely accepted and therefore resulted in the neurodevelopmental hypothesis of schizophrenia (Powell 2010).

Substances such as lead, amphetamine, ketamine, phencyclidine or cigarette smoke have also been associated with schizophrenia (Keilhoff et al. 2004; Mouri et al. 2007; Opler et al. 2008; Zammit et al. 2003).

• Attention deficit hyperactivity disorder (ADHD)

About 3-5% of the children worldwide suffer from the psychiatric disorder ADHD (Nair et al. 2006; Polanczyk et al. 2007). It usually starts before the age of 7 and often continues into adulthood (Azmitia and Whitaker-Azmitia 1991; Elia et al.

1999) and is more commonly diagnosed in boys than in girls (Dreyer 2006; Malhi and Singhi 2001). Environmental chemicals are suspected to account for the increase in children diagnosed with ADHD over the last years. Although many scientists believe that this increase is rather due to better diagnostic tools, awareness, or even faking of the disease (Cormier 2008; Sansone and Sansone 2011; Simpson et al. 2011), exposure to environmental factors and chemicals such as smoking, manganese, lead, ethanol and PCBs during pregnancy has been shown to be associated with ADHD (Aguiar et al. 2010; Bouchard et al. 2007; Braun et al. 2006; Eubig et al. 2010; Ha et al. 2009; Kukla et al. 2008).

• Autism spectrum disorders (ASD)

Autism spectrum disorders, including autism itself, asperger syndrome or pervasive developmental disorder, are neurodevelopmental disorders characterized by impairment of social interactions and communication, restricted and repetitive pattern of behavior and/or interest (Levy et al. 2009). These symptoms all reliably manifest before the age of 3 (Filipek et al. 1999; Nash and Coury 2003). As already discussed for ADHD boys bare a higher risk of developing ASD (Brugha et al. 2011; Newschaffer et al. 2007). It is estimated that 60 – 70/10 000 births are affected by this lifelong disorder (Fombonne 2009).

What autism has in common with most neurodevelopmental disorders is that the causes are not really known or understood. Many possible causes have been proposed including genetics such as mutations in Mecp2 or Fmr1 (de Leon-Guerrero et al. 2011; Moy and Nadler 2008) and teratogenic agents (Arndt et al.

2005; Trottier et al. 1999). Teratogenic compounds suspected to be a possible cause for autism include, amongst others, thalidomide, heavy metals such as mercury, PCBs or pesticides (Bernard et al. 2001; Jolous-Jamshidi et al. 2010;

McGovern 2007; Stromland et al. 1994).

2.5 Phenotype vs. biological process

All these mental disorders and their characteristic phenotypes are caused by a complex interplay of different biological processes. For psychiatric disorders, endophenotypes have been established to divide behavioral symptoms into stable phenotypes with a clear genetic background. Although the existing definition of endophenotypes is very strict and based on genetic criteria, requiring heritability, illness state independence and illness co-segregation within families (Berti et al. 2011; Gottesman and Gould 2003; Gould and Gottesman 2006;

Hasler and Northoff 2011), the principle behind the concept could be very useful for developmental neurotoxicity testing in vitro (Kadereit et al. 2011).

It will be very hard, most of the time impossible, to model a complex disorder such as e.g.

schizophrenia with all its behavioral aspects 1:1 in vitro. Therefore, the concept of endophenotypes, adapted to DNT testing, could facilitate modeling such disease in vitro.

Instead of correlating a phenotype to a genetic connection, phenotypes such as neuroanatomical or neurobehavioral changes could be associated to basic biological processes which can be modeled in an in vitro system. It is important to bear in mind that the biological

process might be disturbed long before the phenotype manifests (Collman 2011). The effects of many known DNT chemicals have already been linked to such basic biological processes.

A prominent example would be schizophrenia, which has been linked to impaired neurogenesis and neuronal migration. Lead, an environmental chemical suspected to be a possible cause for schizophrenia (Opler et al. 2008) has been shown to impair neurogenesis and alter migration of neuronal progenitors (Dou and Zhang 2011; Jakob and Beckmann 1986). Therefore, modeling such processes in vitro, based on data how these processes are involved in impaired neural development or mental disorders, would facilitate the identification of chemicals causing such impairments.