Even those who pay little attention to the vicissitudes of scientific opinion are probably aware of the ongoing discussion about which contributes more to an individual's behavior: genetic inheritance (nature), or personal experience (nurture). The debate started long before we had any understanding of the mechanisms of genetics, but by the turn of the century it had quieted down to a courteous agreement between behaviorists and determinists that recognized the importance of both factors in shaping personality. This agreement is typically respected in polite society. In fact, it has become something of a sign of ignorance to publicly proclaim one element to be solely important to the neglect of the other. But many of us, in the private arena of our minds, and in the company of similarly opined conspirators, still tend to place more significance on either nature or nurture. 

This is due in part to the elusiveness of the mechanism that mediates the interaction between genes and environment. We know how genes work and, although it makes intuitive sense to say that the environment must have an impact on one's personality, arguments to that effect can be less than persuasive when one struggles to illustrate how that effect is achieved. Geneticists, on the other hand, are able to offer up compelling evidence of the contribution of genes with the manifold techniques that can be grouped under the rubric of genetic engineering.

Some of the earliest and simplest—but nonetheless powerful—experiments that demonstrated the influence of genes on behavior were twin studies. Many studies of identical twins raised apart have found remarkable similarities between the adult siblings, despite differences in their early social environments. These findings have led to the formulation of what some refer to as the first law of behavioral genetics: all behavioral traits are heritable. This isn't intended to be quite as dogmatic as it sounds (although that depends a lot on who is citing it), but you can see how it might engender bias on the subject.

Imagine, however, this scenario: monozygotic twins are separated at birth, at which time they are both glabrous, wrinkly, and seemingly identical baby boys. In adulthood they are brought together again as part of a twin study, and the contrasts far outnumber the similarities. One is healthy, active, and fit. The other is obese, lethargic, and plagued with adult-onset diabetes. Also surprising is the stark contrast in hair color. The former has a brown head of hair, while the latter has a straggly mop of blazing red. Your first thought might be that there was some sort of mix-up. These two men clearly cannot be genetically identical. Well, your first thought would probably be right. However, while a disparity this extreme has never been seen in humans, it occurs in a large number of litters born to a strain of mouse known as agouti viable yellow, or A^vy.

Mice of a different color

In wild-type mice, a gene called agouti is responsible for producing the pigment that causes coat color. Agouti expression usually results in black pigment, but for a brief period in development yellow pigment is generated, causing a yellow band on each hair beneath the coat's veneer. This mixture of black with a band of yellow results in a coat that appears gray-brown overall, a hue that is generally adaptive for a mouse in its natural habitat. The color produced by the blend is referred to as agouti, and it is achieved in mice through the agouti gene product (agouti protein) acting as an antagonist at the melanocortin receptor 1.

But, in A^vy mice, this strictly regulated expression of yellow is disrupted. Instead of producing just a yellow band, the agouti gene gets stuck on yellow. The yellow pigment can be produced for a variable period of time, resulting in a spectrum of coat colors ranging from a yellow background mottled with dark brown spots to a covering of pure, unmarked yellow. The aberration, however, doesn't end there. In these mice the agouti gene, normally expressed only in hair follicles, is expressed throughout the entire body. This allows the agouti protein to act as an antagonist at other melanocortin receptors—namely the melanocortin receptor 4—with deleterious effects. The yellow (and mottled) A^vy mice end up being obese, hyperinsulinemic, hyperglycemic, diabetic, and prone to tumors. Some A^vy mice don’t display this anomalous agouti signaling, though. They look like normal, gray-brown mice. They are lean and healthy, and are referred to as pseudoagouti because, although their coat mimics the agouti coloring of the wild-type, they are still genetically A^vy . A litter of A^vy mice contains striking diversity, ranging from yellow to pseudoagouti, with various degrees of mottling in between.

And yet, they are all genetically identical. The difference between a yellow and a pseudoagouti mouse isn't in the DNA sequence itself, but rather in how that sequence is read. The stretch of DNA that encodes the agouti protein in pseudoagouti mice has methyl groups attached to it that block the underlying DNA from being transcribed (at the same time recruiting other proteins to help silence transcription). In yellow mice, the same region contains less of these occluding methyl groups, and thus transcription is allowed. So, the difference between the siblings resides in their levels of DNA methylation, and hence the availability of that DNA for transcription. This makes these mice poster children for epigenetics, a field concentrated on phenotypic variation that isn't due to differences in DNA itself, but rather to how that DNA is packaged and read.

Their unique variations in color allow A^vy mice to be used as epigenetic biomarkers. Just by looking at their yellow to black ratio, one can speculate on the level of DNA methylation that is present at their agouti locus, and by extension throughout their epigenome. This is intriguing because DNA methylation is not just relegated to the control of an obscure mouse gene. It is a pervasive epigenetic mechanism in mammals (although by no means confined to the mammalian class). It has been linked to a plethora of functions and diseases, and is widely considered to be involved in the development of cancer. It also plays a crucial role in normal embryonic development, prompting researchers to utilize the A^vy biomarker to explore the influence of the prenatal environment on a growing embryo.

A hypothetical confluence

Substances like folic acid, vitamin B12, betaine, and choline are known as methyl-donors because they readily donate their extra methyl groups to enzymes that then carry them to DNA for use in methylation. George Wolff and colleagues (1) found that by feeding pregnant A^vy dams diets supplemented with methyl-donors, they caused them to have a larger number of pseudoagouti pups. That is, methyl-donors increase methylation, which suppresses abnormal agouti transcription, reducing the number of yellow pups. This demonstrates the importance of maternal diet during pregnancy with extraordinary directness. Even more exciting, the epigenetic changes appear to be transmitted to the pups' offspring (the dams' grandchildren). The result is transgenerational inheritance that doesn't involve changes in DNA sequence!

Researchers have also used these mice to illustrate the potential negative effects of the prenatal environment. Dolinoy et al. (2) exposed pregnant A^vy mothers to bisphenol A (BPA), a compound sometimes used in the production of plastics. BPA has been linked (primarily in animal studies) to a number of health problems, causing some states to enact bans against using it in children's cups and bottles. BPA ingestion caused A^vy pups' coats to shift in the opposite direction, resulting in more yellows. Dolinoy et al., through the use of a specific technique called bisulfite sequencing, verified that this shift was due to decreased methylation levels. They were able to negate the shift by providing the dams with methyl-supplemented diets (like the one mentioned above). 

Another study used A^vy mice to investigate the effects of alcohol on the epigenome (3). They found that giving pregnant dams even very moderate amounts of alcohol (not enough to produce intoxication) influenced coat color, this time shifting coats toward pseudoagouti. The effect was observed not just when the dams consumed alcohol during gestation, but also when they were given it only prior to conception. Along with the changes in coat color, the mice displayed varying degrees of craniofacial abnormalities and a decrease in mean body weight, both signs of fetal alcohol syndrome in humans. Again, since these are genetically identical mice, this suggests that alcohol is having an epigenetic effect on the developing embryo.

Epigenetic influences on development, however, are not just limited to the prenatal environment (or to A^vy mice, for that matter). The attention a rat mother pays to her pups in the form of licking and grooming has been found to affect how the pups will respond to stress later in life. A higher frequency of licking and grooming results in a calmer and less fearful rat in adulthood (4). Weaver et al. (5) demonstrated that this influence of maternal care is directly linked to levels of DNA methylation at the glucocorticoid receptor in the hippocampus. Thus, a rat's social environment growing up can cause epigenetic changes that persist through to adulthood, altering behavior in the process. This very well may be the elusive mechanism of the nature/nurture interaction laid bare.

He ain’t heavy

The title of this essay may be a bit deceiving, for, as the previous paragraph demonstrates, A^vy mice are representative of just one way epigenetics is changing the nature/nurture debate. Epigenetic mechanisms (DNA methylation is only one of several) may potentially be the missing link that connects genes and environment. So, while A^vy mice provide researchers with a novel way to investigate the effect a mother's interaction with her surroundings has on her offspring, they are only a small part of what is a nascent—but burgeoning—field. It is a field that may eventually help to settle the nature/nurture dispute by appealing to both sides, and by providing an incontrovertible explanation of how the two converge. Unbeknownst to these sluggish, overweight, and unhealthy mice, they may be influencing one of the great scientific debates of our time. And, while I risk giving them too much credit with the title of this essay, there are worse offenses than shining a favorable light on these otherwise hopeless critters.

References

1. G.L. Wolff, R.L. Kodell, S.R. Moore, C.A. Cooney, FASEB J. 12, 949-57 (1998).
2. D.C. Dolinoy, D. Huang, R.L. Jirtle, PNAS 104, 13056-61 (2007).
3. N. Kaminen-Ahola et al., PLoS Genetics 6, e1000811 (2010).
4. D. Liu et al., Science 277, 1659-1662 (1997).
5. I.C.G. Weaver et al., Nature Neuroscience 7, 847-854 (2004).