Epigenetics And The Evil Twin

Biology concepts – epigenetics, monozygotic twins, mirror image twins

The armadillo is a fascinating animal, and shares a couple of characteristics only with humans. People and armadillos are the only animals susceptible to leprosy, so the armadillos are used for Hansen’s disease research. Also, nine banded armadillos (the above is a six banded armadillo) is the only animal other than humans that can split an embryo twice, allowing for identical quadruplets.

Last week’s topic on parasitic twins was a bit depressing for me. This week, let’s focus on some amazing kinds of monozygotic (MZ) twins. Yes, there are many kinds of monozygotic twins, some we have talked about (conjoined, parasitic, absorbed, vanished) and some we’ll talk about today. Just because they’re monozygotic doesn’t mean that they only come in one type.

The popular idea is that MZ twins are “identical,” but nothing could be further from the truth. It may not even be the case that they share the same genes, but more about that later. When an embryo splits, each new embryo usually gets the same chromosomes. But a lot can happen after that.

We talked recently about the determination of right and left sides in the embryo by a flow of fluid from left to right (see this post). Well, that same flow can make MZ twins look different. One subtle difference can be their fingerprints. MZ twins don’t have the same fingerprints.

Fingerprints do have a genetic component, so the fingerprints of MZ twins will resemble each other more than non-twin siblings’ will. But the environment of the amniotic sac during gestation, especially during the first trimester, will help to determine the details of the fingerprint. Nutrition level, stress, movement within the sac, even a slight difference in umbilical cord length; these will all result in differences between the prints of MZ twins.

Talk about stress, wartime babies have different fingerprinting patterns than those born in peace, at least the seasonal variations in ridge counts disappear if the gestation is during war. All these things we have been talking about go beyond the foundation of genetics. Traits are established and influenced by genetics, but how they turn out (their phenotype, where pheno = visual form) can influenced by the environment. This is epigenetics (epi = beyond).

The top image shows the three general types of fingerprints that humans display. Identical twins will have similar types of prints about 80% of the time, because they are partly controlled by genetics. The bottoms images A and C are monozygotic twins. You can see they are of the same type, but have different details.

Purely environmental factors influencing phenotype might be considered a rather broad interpretation of the term epigenetics. The word does mean beyond genetics, but the strict definition involves factors that an individual encounters that will turn genes on or off. The changing expression then influences the phenotype.

The control of gene expression via chemical reactions is the example most often given. The chemical reaction affects the function of a given stretch of DNA, but doesn’t change the sequence of that DNA. The genetics remains intact to be passed on, but which genes are expressed is what changes.

There are two common examples of chemical reactions that affect DNA are methylation and histone acetylation. Let’s look at an example in nature that uses both systems. And it uses a multiple births in the example too – how convenient.

A honey bee colony has a strict structure. There is one queen (usually - of course there are exceptions), a few hundred male drones, and many thousands of female workers. The queen flies out to mate (with 12-24 drones of another colony) during first first two weeks of her life, but then settles down to lay eggs.

The top cartoon shows methylation of cytosine bases as a way to control gene expression. The enzyme DNMT adds the methyl group from SAM to the C base. CpG motifs (a C followed by a G) are common targets for methylation to silence genes. The bottom image is for histone acetylation. Acetylated histones (yellow added) open DNA to be transcribed (see orange lines in acetylated and deacetylated).

The workers build the honeycomb with different size chambers, small for workers, medium for drones and large for a new queen. It's the size of the chamber that tells the queen whether or not to fertilize the egg she lays in it - she has the choice since she stores the male gametes separate from the eggs.

If she doesn’t fertilize the egg, it will be a worker, a sterile female clone of herself. If she does fertilize the egg, it will become a male drone. But if she lays a fertilized egg in the large chamber, it becomes a female queen, not a male drone. Why the difference? Their diet.

There is a substance the bees produce called royal jelly. All the larvae are fed royal jelly for the first three days, but after that, only the larva that will become the new queen is fed royal jelly. In fact, that is all the queen will eat her entire life.

The royal jelly is a secretion that comes from the hypopharnyx of the worker bees. It's mostly water, with some protein and amino acids. The active ingredient is called royalactin. It ages and becomes less active over time, so the workers keep making it all the time. Get the subtle point here, the queen lays the eggs, but the workers make the queens.

Bees are highly organized and social. Workers are smaller than drones or queens. These workers are older, because they are leaving to forage. Younger workers make more royal jelly and tend the queen and the larvae. Being older might account for the bad eyesight and flying into each other.

What does royal jelly have to do with DNA methylation and histone acetylation? I’m glad you asked. More methyl groups added to C’s of DNA keep the workers from becoming queens. There are enzymes that control methylation and de-methylation of DNA. What happens in the case of queen development is that more de-methylation activity occurs and less methylation. The methyl groups on DNA control whether the gene can be expressed or not; more methylation - less transcription.

We also know that queen development is promoted by more histone acetylation. Acetyl groups added to the histone proteins that help coil DNA make it loose and available for transcription (reading the genes that are there). If the histones are deacetylated, the chromatin becomes tight and the machinery can’t get access the genes.

It turns out that royal jelly has a histone deactylase (HDAC) inhibitor. Therefore, more DNA stays acetylated and open to be transcribed. The part that is transcribed might include the demethylase enzyme genes. This leads to less methylation and activation of genes that turn the larva into a queen. So her diet doesn’t change her genes, it just determines which ones will be expressed. And that makes all the difference.

Ask your friends if monozygotic twins are identical and you’ll many more yeses than maybes or nos. But given our discussion above of epigenetics and the power of environment to alter gene expression, do you have any doubt that there are changes that occur afterfertilization and after splitting of the embryo into twins? Epigenetic factors can produce monozygotic twins with different malformations, different lateral asymmetries and even different sexes! (reviewed here)

These are pairs of chromosome 3 from a three year (top) and 50 year (bottom) monozygotic twins. They used red tags for one twin's epigenetic tags (methyls or histone acetylations) and green tags for the other twin. If they are at the same place in both twins in the overlaid image, the color will be yellow. If they don’t overlap (tagged on different genes), you see the red and green. Notice how many more differences there are in the older twins. Image credit University of Utah.

In MZ twins, the environment in one amniotic sac will be slightly different than in the other. Flow of fluid will be different and even the position of the fetus can make a difference. This is why even twins inside one amniotic sac will still have differences. The differences grow after delivery, so more expression will be different at an older age than in utero (see picture on right). And this is just epigenetic, we aren’t even discussing the genetic changes that can take place via mutation once the embryo splits to become MZ twins.

We talked a few weeks ago about how the embryo distinguishes right from left so that the organs develop in their normal locations. Sometimes they don’t, and this is when we get situs inversus or situs ambiguous. If the split of the embryo that forms MZ twins occurs after the decision has been made as to right-left, then mirror image twins could be a result.

Mirror twins may have moles on their cheeks – one on the right and the other in the same spot, but on the left cheek. They may have mirror hairlines or defects like cleft lip and/or cleft palate. They might even manifest equal but opposite sleep patterns. In most cases, the twins will have some internal organs in mirror locations, but rarely will there be a situs inversus twin and a situs solitus twin. Here’s why.

Every television show gets to an evil twin episode sooner or later. The point is that twins studies can look at physical traits, but also at how genetics and epigenetics affects personality…. and facial hair choice.  Top images are Spock from Star Trek: The Original Series. The bottom image is Michael Knight from the 1980’s version of Knight Rider.

The discrimination between right and left comes about after the blastocyst implants in the wall of the uterus and after the totipotent stem cells of the embryo start to differentiate into layers. The node cells have the monocilia that gyrate to develop the leftward current and the lateral cells have the immotile cilia that detect the current and respond by differentiating differently.

Most embryonic splits for MZ twins occur in the first 2-10 days, but if the split occurs after day 12 or 13, then some degree of mirror imaging is possible. This is the time when right-left decisions have already been made. Of course, the plane in which the split occurs will matter too, it would have to be directly along the node for a true right-left mirror to occur -there's epigenetics again. This is extremely rare, so most mirror image twins, have some mirror traits, not total mirror bodies.

Even in situs ambiguous (see this post) of MZ twins, there can be mirror imaging of some external and/or internal phenotypes in twins with heterotaxy. Yes, phenotypes – remember that these differences are affected by many things, but not directly genetics. They change the features of the twins while they still have identical genes. It’s epigenetic.

Twins studies are good for teasing out the influences of nature and nurture in physical form and behaviors.  There are many different ways to run the studies to find out different things – type of twins, raised together or apart, degree of relatedness, nature of environment, raised by birth parents or adopted. The studies can involve social testing and/or physical testing. The problem – how to control for unknown variables. These are people lives, not laboratory mice.

This can be useful in medicine and science. It would be good to know which phenotypes are under strict genetic control and which can be influenced by environment. So while early twins studies concentrated on what was the same between MZ twins, new research is concentrating on what’s discordant (different) between them.

This research will go a long way to showing just how much of our health and life is actually passed down from parents and is inescapable. Even the differences between mirror twins can help to tease out the mechanics of how twinning occurs and the paths and patterns of normal embryology.

Now that we have a handle on epigenetics and its effects on MZ twins, let's talk next week about a couple of twin types that result from genetic differences. Yes, you read that right, some MZ twins are genetically different.