When You’re Not Just Yourself

Biology concepts – chimeras, twins, immune tolerance, ABO blood groups, random assortment

The latest Wrath of the Titans movie had a chimeraas one of the monsters that had to be defeated. It had two lion heads, breathed fire, wings, and a tail that was the head of a snake. Those aren’t the chimeras we’re talking about today.

A strange tale of child support played out in Washington state in 2002. While she was pregnant with her third child, Lydia Fairchild and her long time boyfriend split up. She sued for child support. As a part of the proceedings, Lydia had to take a blood test to show that she was in fact the mother of the two children she already had. The test showed she wasn’t!

The partner was indeed the father, but something about Lydia was funky. Was she trying to game the system? Had she secretly been a surrogate mother? At the birth of the third child, the judge ordered an immediate genetic test for the newborn and the mother, just as she delivered. Once again, the test determined that she wasn’t the geneticmother. Certainly she was the gestational mother, the baby had just come out of her!

On a tip from another lawyer, her attorney asked that other cells be tested. They looked at several cell types, and those from Lydia’s thyroid and cervical smear demonstrated maternity for all three children. She wasn’t a surrogate, she hadn’t stolen embryos; she was their mother.

Lydia was a chimera; she had two populations of cells in her body, each with a different set of chromosomes. Most likely she had started out as a dizygotic (DZ) twin and there was a fusion or exchange event. In fact, Lydia recalled that she had a twin brother who died in infancy. We’ll talk about that possibility, but we’ll also discuss how it could have come from something much weirder.

There are a couple of ways to end up a chimeric twin; blood group chimerism and tetragametic chimerism. Blood chimerism is slightly simpler so let’s talk about it first.

People who get organ transplants are iatrogenic (medicine- induced) chimeras because they now have two populations of cells with different genetic profiles. If you get a blood transfusion, you will transiently be a chimeric, but you’ll soon be your old self. What if a chimeric donated blood for you?

You have antigens on your red blood cells; type A cells have antigen A and the individual makes an anti-B antibody. Type B cells have B antigen on them and the individual makes antibody to the A antigen. If you are type AB, you have both antigens on your RBCs and you make no anti-blood group antibody. Finally, people with type O blood have neither antibody on their cells, but make both anti-A and anti-B.

When you receive blood in a transfusion, it is important that you receive blood that doesn't contain antibodies to the antigens on your RBCs. For instance, a type A person can’t use type B blood because it has anti-A immunoglobulin in the serum. Type O is the universal donor, while type AB is the universal recipient – can you explain why?

Apparently, it’s much more common than previously thought that DZ twins will mix blood systems, including hematopoietic(blood making) stem cells. This can occur in monochorionic twins (one placenta, see this post) where the placental vessels of each twin have a common portion where the blood can mix. Each twin has their own tissue cells and bone marrow cells, but they also have some bone marrow cells from their twin.  The weird part? A case from 2013 and one from 2014 show that one placenta is possible and not uncommon for dizygotic twins.

It’s estimated that 8% of DZ twins are blood chimeras, and the actual number may be much higher. With the incidence of DZ twins increasing (see last week’s post) so will the number of blood group chimeras.

Science depends on accurate definitions, so we all know exactly what we’re discussing. This makes it hard because so many people use chimera and mosaic incorrectly. A chimera comes from two zygotes (even if from different times) while a mosaic results from genetic changes in a single zygote.

Sharing part of a common placental artery isn’t the only way a twin might end up with blood group chimerism. If two twins are in the same amniotic sac (monoamniotic, see this post), they can exchange cells, including some stem cells, through the amniotic fluid. If the stem cells end up in the bone marrow, then a twin would produce some blood cells of the other twin’s blood group type. However, the 2013 case above was in a pair of monochorionic, diamniotic twins, so the chimerism must have come from the mixed blood in the placental system.

This is exactly what is believed to have happened in the first example of blood group chimerism investigated, in 1953. A woman tested for both blood type O and blood type A. Sixty percent of her cells were O, and her cheek cells didn’t have A antigen, so O was her blood type and A was the type of her twin, who had died in infancy – same as Lydia Fairchild's twin brother.

This chimerism is usually of little importance, but in the 1953 case, a benefit was possible. While her own immune system didn’t attack O cells because they were self, it also didn’t attack A type cells because it had become tolerized to them (saw them as self). So the adult female could use type A blood and even receive transplants from a type A donor. The tolerance of one set of cells for another of different genetic profile is the subject of much study now (we’ll see why next week). Can you think of a case where it would be harmful to be a blood group chimera?

We don’t know how embryos fuse to become a chimera. It must happen early, but wouldn’t they still be surrounded  by the zona pellucida before they hatch (see this  post)? You figure that one out for a Nobel Prize.

Another mechanism for twins which would include chimerism is called tetragametic chimerism. The name kind of gives away the mechanism, and also hints at how much more rare this type of chimerism is. Tetragametic chimerism isn’t so much twinning as it vanished twin, likewe saw in our monozygotic (MZ) twin posts.

Tetra- means four and -gametic means from gamete cells. To get four of them, imagine two oocytes fertilized by two male gamete cells – DZ twins. But in this case, the two embryos fuse; just the opposite of MZ twins where one embryo splits. The result is one developing fetus made of two genetically different populations of cells.

Why do they have two different chromosomal profile cells if come from just one mom and one dad? It goes back to meiosis and random assortment. When replicated chromosomes (two chromatids each) line up in meiosis I, they arrange themselves as pairs – but mom’s can be on the right orthe left.

Random assortment ensures that all eggs (and all male gametes) will have different genetic profiles – or will they? There is a 1 in 4 million chance that two eggs will have the same random assortment patter, same for male gametes. That means a 1in 16 million chance that two identical eggs could be fertilized by identical male gametes. But they would have to occur in the same cycle – now we’re getting into really large numbers.

They get pulled apart to make the primary oocyte and first polar body (see this post) or primary male gamete cell, so the version (Ma's or Pa's) for each chromosome in each cell is random – ie, random assortment. The second split (meiosis II) then splits the chromatid pairs, so each daughter will be the same, genetically.

The picture (right) shows six possibilities if we only had two pairs of chromosomes, but we have 23! What are the chances of two eggs with exactly the same random assortment being fertilized by two male gametes that also just happened to have the exact same random assortment? Pretty low, so any fusion event is going to produce a chimeric individual.

It’s possible to get a blood group-confined chimera via embryonic fusion; in fact there was a case of it described in China in 2011. The baby was type AB, but formation of antigen-antibody complexes (agglutination) was mixed. Looked at parents blood types – AB and O. They assumed there was a parentage issue – why? Why can’t an AB and an O have an AB baby? Molecular cloning showed that the baby's hematopoietic cells were AO and BO, and other sites showed the same allele patterning, so the baby was a definite tetragametic chimera, but the mixed cells were confined to the blood compartment.

A chimera might show subtle signs, depending on where the two populations of cells land and starting propagating. A shaft of curly hair on a straight haired girl – maybe a chimera. Max Scherzer of the Detroit Tigers might be a chimeric or a mosaic – you can see it in his eyes - but there many other ways to end up with two different colored irises.

Tetragametic chimerics usually show more diverse populations of chimeric tissues – like Lydia. Depending on where the cells of each twin take up residence in the fused embryo, they will become different kinds of tissues in different parts of the fetus. Think about the right and left orientation we discussed a month ago and how it determines what embryonic cells become what tissues (situs solitus versus situs inversus).

It wouldn’t have to be just right versus left; consider the Fairchild case. Blood cells and at least some of her cheek cells were from here twin, while her thyroid cells and her ovarian and cervical cells were her own. You can’t predict where the chimeric cells will end up because you don’t know how many cells fused and what positions they were originally in.

You usually don’t see the effects of tetragametic chimerism phenotypically, the two versions of mom’s chromosomes and the two versions of dad’s are usually close enough that the fused embryo fetus looks like a normal kid. But there could be indications – one ear lobe that hangs down and one that doesn’t - one big thumb (it’s genetic, ask my wife) and one thin thumb - patchy color of skin, eyes, or hair (heterochromia). If the two twins were of different sexes, then where the XX cells and the XY cells end up in the reproductive tissue will determine the sex of the baby - the offspring could be male, female, or intersex (true hermaphrodite).

Lines of Blaschko follow the same pattern in everyone. The bottom right image of dermatitis is following the lines because the cells of each patch are slightly different and may react to an allergen or infection differently. In a chimeric, you might be able to see a difference in the skin cells under UV light, or if the pigmentation expression is very different, with the naked eye (top right).

One last topic for today, people with stripes! Chimeric humans can be sometimes be identified by UV light. Skin cells grow in a very specific pattern. A few cells propagate and spread in a common pattern for everyone. Named for dermatologist Alfred Blaschko, most people have genetically identical skin cells so you can’t see the line pattern. But there are subtle differences between the cells of each area, so an infection will often move along the Blaschko lines.

In tetragametic chimeras, there may be small differences in the pigmentation of the cells of the different areas, so a UV light can illuminate the patterns. If the genetic pigmentation expression is very different, you can see them easily.

Next week – it’s highly possible that we’re all chimeras. Maybe not twins, but it is likely that we all carry cells from our mom and she has some cells from us. You might even have cells from your older sister living inside you – oh yuck!