Donovan Drouillard ’18 and Brendan Genaw ’19

In 2010, Canada declared bisphenol-A (BPA) a toxic substance, yet BPA may be lurking right in your own kitchen (Austen, 2010). Today, BPA can be found in canned foods, plastic bottles, tupperware, and receipts (NIH, 2017). Consumption of BPA has been linked to increased rates of cancer (Ho et al. 2006), issues with brain development (Nakamura et al. 2006), and higher rates of reproductive issues (Santangeli et al., 2006). Even though the FDA does not classify BPA as a toxic substance in America, the FDA passed laws in 2012 and 2013 prohibiting the use of BPA in children sippy cups, baby bottles, and infant formula packaging (FDA 2018). Many other countries around the world have also banned the use of BPA in infant feeding bottles and pacifiers (EFSA, 2018). Nevertheless, BPA can be found in the urine of about 95% of the US population (Calafat et al., 2004). With these facts come many questions. What is BPA, where does it come from, and why is it harmful? Should it be banned completely?

BPA is an industrial chemical used to make certain plastics and epoxy resins. These plastics that require BPA are commonly used for storing food and beverages, such as bottles and tupperware containers. The epoxy resins are used to coat metal products, such as canned foods and water lines. You likely have been told before not to microwave plastic or put it in the dishwasher. One reason for this is that heating up plastic breaks it down faster, allowing BPA to leach into your food.  If a plastic container has a recycling code of “3” or “7” on it, there is a chance it could contain BPA (Bauer, 2016). However, not all plastics and resins contain BPA, and many plastic containers today have “BPA Free” proudly printed on them. 

What does BPA do once it is inside the body to cause all these harmful effects? To understand the effects BPA can have on our bodies we must dive into the basics of epigenetics. A rapidly growing field in biology, epigenetics focuses on the intersection of DNA and the environment we live in. If our body is a huge office and each cell is a cubicle, think of DNA as a gigantic instructional manual with each cubicle having a copy. There’s so much DNA in our cells that if you took the DNA in every cell of your body and stretched it out to form a straight line, it would be long enough to wrap around the Earth 2.5 million times (Annunziato, 2008). To fit all this DNA inside each tiny cell of your body, DNA is packaged tightly around proteins called histones. Histones can be imagined as the cell’s file cabinets that serve to neatly condense and organize the instruction manual.  Just like any other office desk with filing cabinets, some documents are frequently used and easy to access while others are not. The DNA that is tightly wrapped around the histone protein is much more difficult for the cell to access. This would be equivalent to instructions that are stored in the depths of the file cabinets. Other parts of the DNA are frequently utilized and loosely wrapped around histone proteins. These easy-to-access instructions would be similar to the documents you frequently visit and leave on the edge of your desk or in the front of the cabinet.

Our DNA is composed of units called genes, and even though each cell has the exact same DNA, the cells access this DNA at varying frequencies . For example, the cubicles in the office’s finance department will keep their instruction file pertaining to financial math in an easily accessible location, such as the top of their desk. Meanwhile, the communication department of the office will keep their instruction file for contacting other departments in an easy-to-reach location. Even though the communications department has the instruction file for financial math, they would rarely access it. Since our office wants each cubicle worker to be efficient, information that is rarely used by a particular cubicle is stored in the back of that cubicle’s cabinet to avoid wasting energy sorting through it.  

When the office needs to accomplish large tasks, it assigns cubicle workers with specific jobs. The cubicle workers then reference their instruction protocols and use these instructions to create products or services that accomplish their assigned task. Similarly to this, the body messages our cells which then access the DNA instructions to build proteins that accomplish the cells’ desired tasks. When proteins are made, the genes in the DNA are said to be expressed.  For example, cells that digest food often express the DNA’s digestion genes to create digestive juices for food breakdown. Think of expression as following the instructions. When a gene is expressed, that file is being read, and the instructions are being followed to create the desired product. When a file is put back into the cabinet and not being read, it is said to be repressed.

The field of epigenetics looks at mechanisms that affect a gene’s level of expression without directly altering the DNA’s genetic code. One way genetic expression can be epigenetically altered is through the addition or removal of epigenetic marks on the DNA. For example, if our office building is in an environment with many earthquakes, like California, then the instruction protocols for earthquake safety procedure should be marked with a tab so it’s an easy-to-access file at every cubicle. This tab would be equivalent to an epigenetic mark because it does not change the cubicle’s instructions but allows for easier access of specific instructions. However, not all epigenetic markers make the DNA easier to access. Methylation, or the addition of a methyl group, results in less genetic expression of the DNA or DNA that is more difficult for the cell to access. This would be like binder clipping documents together so specific pages in the middle of the packet would be a hassle to access.  In recent years, BPA has been found to alter the methylation status of important developmental genes.

Researchers first discovered the impact of BPA in a landmark study examining the agouti gene found in rats. When the agouti gene is expressed during development, mice exhibit a yellow fur coat and increased levels of obesity, diabetes, and tumor formation. Overall, these yellow agouti mice greatly express the agouti gene and are less healthy than their brown furred, pseudo-agouti counterparts, who exhibit less expression of the agouti gene. Researchers observed that pregnant rats placed on diets high in BPA gave birth to a larger proportion of the unhealthy yellow agouti offspring. Meaning, the mom’s consumption of BPA increased the chances that the offspring would be yellow, less healthy, agouti mice (Dolinoy). The difference in colors and types of offspring can be seen to the right. When researchers examined the agouti gene, they found that the mom’s consumption of BPA lead to demethylation, or removal of methyl marks, from the agouti gene regions in the offspring. Thus, if the agouti file is normally clipped shut and difficult to access, the mom’s consumption of BPA removed these clips and her offspring are reading and using the agouti file more frequently.  Using this knowledge, researchers then chose to feed the maternal rats diets high in methyl donors such as folic acid. Methyl donors, such as leafy green vegetables, citrus fruits, beans, and pasta, repress expression (American Pregnancy Association, 2015). Overall, these methyl donors add methyl markers, which act like clips making certain files more difficult to access.  Researchers observed this had the opposite effect of BPA, and maternal rats that consumed these methyl donors had more methylation of the agouti gene and a higher ratio of brown, pseudo-agouti offspring. In rats, BPA decreases methylation of the agouti gene, so the gene is easier to access and expressed in greater quantities. This increased the ratio of yellow, agouti offspring, which are significantly less healthy than their brown haired counterparts. Overall, this study showed that maternal BPA can have a direct impact on the methylation status of the offspring. In terms of the office, if a company is undergoing expansion, the new branches of the company are modeled off of the pre-existing office headquarters. Meaning, the new branches would employ a similar filing system as that of the headquarter office. There may be some differences, but the majority of the office protocol and filing system is going to be the same as the office headquarters.

BPA has also been found to affect brain development of offspring. In one particular study, pregnant rats were given BPA within the limit set by the National Toxicology Program’s Report (2001). Meaning, the BPA concentration measured in the rats was deemed safe.  However, the developing offspring of these maternal rats were found to have brain cells that were pushed through development at too early of a stage (K. Nakamura 2006). This resulted in brain cells not moving to the correct positions in the brain and an incomplete connection between the brain and thalamus. The thalamus works with the brain and produces thyroid hormone. Thyroid hormone problems in humans result in irreversible developmental issues (K. Nakamura 2007). Similarly to how most business fail in their developmental phase, animals are most vulnerable to BPA and other environmental factors while developing.

In neurons, which are the cells that create our nervous system and brain, chloride levels are initially very high. However, as the development of the nervous system and brain continues, chloride levels plummet. This decrease of neuronal chloride concentration is essential for each cells’ development and the maturation of our nervous system. Additionally, this drop in chloride concentration plays a key role in the neurons’ migration to their proper locations during the development of the brain. Kcc2 is the gene that is activated to cause this drop in neuronal chloride concentration. In fact, many neuropsychiatric disease and issues such as epilepsy and chronic pain are associated with low levels of developmental Kcc2.

A recent study in 2012 examined the effects of BPA on human and rat neurons. The researchers found that BPA caused repression of the Kcc2 gene and also found an epigenetic marker that can repress Kcc2. Earlier, methylation was seen as an epigenetic marker that BPA interacted with on the agouti gene; however, methylation is not the only epigenetic marker BPA can influence. Acetylation is an epigenetic marker that typically works in the opposite way of methylation allowing for more expression of the gene. In addition, the acetylation marker is on the histone protein, the file cabinet holding all the files, rather than directly on the DNA. Where methylation markers acted as paper clips making files more difficult to access, acetylation markers act as tabs that highlight specific files making them easier to access. When a histone is acetylated the DNA does not bind as tightly to the histone as it would if there was no acetylation mark. This loose DNA becomes easier to access resulting in more expression of certain genes. When examining a rat brain, BPA resulted in less binding to these easy-to-access histones around the Kcc2 gene. As a result, the Kcc2 gene is likely going to be bound more tightly to histones. This would lower the expression of the Kcc2 gene. The results of the study indicate that BPA appears to decrease the expression of Kcc2 in the brain through the use of the epigenetic markers. This could result in numerous problems with nervous system development, as insufficient Kcc2 would not be able to cause a drop in chloride levels in the neurons. Researchers decided to also examine this phenomenon in live rats. To look specifically at development, researchers gave pregnant mother rats BPA and examined the offspring. In an attempt to offset the effects of the Kcc2 gene binding tighter to histones, the researchers gave the rats TSA. TSA is a drug that stops the protein that removes the acetylation marks from the histones. By removing the “remover” protein, Kcc2 expression should increase because it will be easier to access. Kcc2 expression did, in fact, increase from treatment with TSA. Although TSA possesses the ability to counter the decrease BPA causes in Kcc2 expression, TSA has numerous side effects in addition to this and cannot serve as a “cure” to these negative effects of BPA.

Most of these experiments were performed on rats, yet similar results were observed in humans.  In humans, the chloride drop in neurons caused by Kcc2 is still essential for proper development. In fact, researchers found that in human neurons, there is a molecule called MECP2 that is produced inside the body that will lower levels of Kcc2. In other words, MECP2 locks the file cabinet that the Kcc2 file is in. BPA has been shown to increase levels of MECP2 in the brain. The Kcc2 gene is typically locked away after the brain has developed. However, as seen in the rats, locking away this Kcc2 file too early will result in a lack of chloride drop and the brain not forming correctly. Overproduction of the MECP2 molecule, which can happen in humans exposed to BPA, can lock away the Kcc2 gene too early. This has been linked to many neurodevelopmental disorders in humans such as Rett syndrome, which is the leading cause of mental retardation in females.

BPA is also shown to impact genders differently. Expression of receptors for the hormone estrogen in the womb is critical for changing the brain of the fetus into a female or male brain. A hormone is a chemical signal that travels through the body. It needs to bind to a receptor for the signal to have an effect. The differences between the male and female brains are primarily in the areas controlling specific behaviors and controlling release of hormones, such as estrogen. When pregnant rats were given BPA, it affected males and females in the womb differently. BPA caused more expression of estrogen receptors in males and less expression in females at low to medium doses. However, at high doses, the effects were reversed. Males had less expression and females had more at higher doses. This is important because the more receptors there are for the hormone, the greater effect the chemical signal can have. In addition, BPA affected levels of DNA methyltransferase (the protein that adds the epigenetic methyl marks to DNA) in a similar pattern. Think of DNA methyltransferase as the poorly paid intern in the office who clips all the paperwork together for everyone. DNA methyltransferase was increased in males at low to medium doses of BPA, but decreased at high doses. The opposite was observed in females. These results are concerning because BPA can directly affect behavior and certain populations may be more sensitive to the effects of BPA. Overall, these results demonstrate that more BPA does not mean more of an effect. This makes it hard to determine the dose of BPA at which humans suffer the most negative consequences.

As we now know, BPA’s effects can be wide reaching. It can negatively affect brain development and behavior and it does so in a sex-specific way. BPA is primarily dangerous to young, developing children and pregnant mothers. Many of these studies were performed in rats because it would be unethical to give humans large doses of a possible toxin when the negative side effects are known. However, additional studies have found that men who have higher BPA concentrations in their urine have lower sperm counts. In women, higher BPA concentrations in urine was associated with lower number of oocytes, the precursor cells to eggs. Additionally, mothers with higher levels of BPA in their urine during pregnancy had daughters that scored lower on the Behavioral Assessment System for Children, which is an indicator of behavioral problems (Braun). These results mirrored those seen in rats where BPA altered behavior during developmental exposure. Currently, the Food and Drug Administration (FDA) estimates that humans are exposed to 0.2-0.5 micrograms of BPA per kilogram of body weight per day (FDA, 2014). This dose of BPA is around 10 times lower than the “low dose” given in studies examining BPA in rats. The FDA states this is a safe level of BPA exposure, but many health and environmental groups remain unhappy with this ruling. Even though the FDA banned BPA from children sippy cups, baby bottles, and infant formula packaging, it refuses to label BPA as a toxin. Science is often termed unbiased, yet the spheres of government and business exert influence on research. Companies that rely on BPA for the production of their products would suffer if BPA were labeled as toxic. However, in cases were BPA is absolutely needed, other chemical alternatives have also been shown to be toxic. Now that you have a better understanding of BPA, you can decide whether it is worth it to make the lifestyle changes to cut down on your BPA exposure. If you would like reduce your level of BPA exposure, look for plastics containing the “BPA-free” label, cut back on canned foods, avoid exposing plastic to repeated heat exposure (including the dishwasher), or simply avoid using plastic food and water containers all together.

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