Twins That Share More Than Clothes

Biology concepts – mosaicism, mosaic twins, chimera, anaphase lag, non-disjunction, polar body twins, oogenesis, preimplantation genetic screening

Card stunts are a modern form of mosaic art. North Korea is the master of the card stunt, followed by the University of Iowa in a distant second place. Each individual block is a child with a book of colored pages. They changes pages in eerie synchrony, but it’s not like they have a lot else to do.

Mosaics were a popular art form in ancient Rome and Greece. Individual tiles might be one color, but put them next to one another in the right pattern and a picture would emerge. Nowadays people can do the same thing with photographs. Manipulate the photos or use photographs of the right overall darkness and you can create a large picture from hundreds or thousands of smaller ones.

In organisms with chromosomes, this isn’t the normal case. Each individual is made up of many cells, but each cell has exactly the same complement of chromosomes. But not always. Mosaicismin genetics refers to an individual where not every cell has the same chromosomal makeup. Some cells have genetic makeup "A" and some have genetic makeup "B." But it doesn’t have to be limited to two genetically different cell populations, there could be more.

We have talked about mosaics before (this post and this post). They can come from a chromosome loss in an early cell that then divides and becomes a whole population of cells with that different chromosome number. It can also come from an endoreplication event (see this post) where a cell ends up with too many of a certain chromosome. There could be a non-disjunction event as well; this is how Down syndrome or Klinefelter’s syndrome come about.

Anaphase lag is yet another mechanism to create a cell with different chromosome number. In this case, the spindle apparatus (see this post) in a mitotic or meiotic cell division doesn’t connect completely with a chromosome or doesn’t pull it efficiently. As the spindle pulls the chromatid pairs apart and the two daughter cells start to form nuclei, the tardy chromatid is left out.

Here is an example of anaphase log in meiosis I. The chromosome pairs line up and are pulled by spindles to two new nuclei. If on pair lags, it doesn’t make it into the new nuclear envelope and is lost. Now the two daughters have different chromosome profiles. The phenomenon could occur in meiosis II or mitosis as well.

Now you have a nucleus with one too few chromosomes, and one with the original number (or perhaps one too many depending on if the nucleus forms around the lagger). Either way, you now have daughter cells that are not genetically equal to each other. In an embryo, time and cell divisions will give altered cells in greater number and type, but the normal cells expand as well, so you end up with tissues or organs systems that have two different chromosomal profiles.

Now consider what would occur if the early embryo undergoes a mosaic producing event, and then splits into two embryos – mosaic monozygotic (MZ) twins. A mosaic embryo, no matter what mechanism leads to the mosaic could become mosaic twins, as long as the chromosome change in some cell occurs before the split of the embryo into two embryos. Each twin will end up with some cells of one chromosomal profile, and some of the other. Given all that must happen to create them, is it any wonder that mosaic MZ twins are rare?

But there is another way for a mosaic individual, or MZ twins to form as well. Consider the case of two male gamete cells fertilizing the same egg (called polyspermy). When the zygote cell divides, one cell may get some of male gamete #1 chromosomes and some of male gamete #2 chromosomes. Male gametes carry either an X or a Y, so it is conceivable that some cells will be XX and some will be XY. If the embryo split, could you get MZ twins that are one boy and one girl?

Invertebrate animals have a fast and slow block to prevent more than one male gamete from reaching the egg. The fast one occurs within 10 seconds. But mammals have a couple of slow methods only, including a growth in the thickness of the zona pellucida by the degranulation of cortical granules. In mammals, all the mechanisms are slow (about 1.5 hr), so it is possible for more two male gametes to get in. This is especially true in IVF where one in ten embryos undergo polyspermy.

Yes!!! A case in 2007 is purported to have occurred in just this way, except that the split didn’t leave all the cells of one embryo XX and all the cells of the embryo XY, they were a mix. In this case, one embryo’s cells that formed sexual organs that were male, while the other embryo had a mixture of XX and XY cells in the places where reproductive organs would develop – this embryo formed a true hermaphrodite.

Some people might want to call these twins chimera twins, but chimeras come from the fusion of two embryos into one (two complements of mom’s DNA). Mosaics form from one zygote only. For instance, another mosaic twin case formed after an XXY Klinefelter syndrome embryo lost the Y chromosome in some cell(s) (maybe from an anaphase lag). Then the embryo split and you had an XXY male and an XX female.

Can you think of another way to end up with mosaic twins? What would the result be if a mosaic zygote split so that all the cells of one type ended up in one embryo, while the other embryo ended up with all the cells of the second genotype? Would those be mosaic twins? Would they be MZ twins; would they be genetically identical? How could you tell them apart from dizygotic twins? And yes, there would be a way to tell if they were DZ or started out MZ, can you figure it out?

Imagine the case where a zygote splits into MZ twins and then a cell of one embryo (or both embryos) undergoes an endoreplication or anaphase lag. Now one (or both) embryo is/are mosaic, but they’re different from one another. Would they still be considered mosaic twins?

As was the case with MZ twins in general, the incidence of mosaic twins may be increasing because of assisted reproductive technologies (ART, see this post). There are several different kinds of ART methodologies, but the one we have talked about most is in vitro fertilization (IVF). Where IVF and mosaic may twins cross paths is in something called preimplantation genetic screening (PGS).

In one type of preimplantation genetic screening, a single cell of an embryo is harvested and tested for it’s chromosomal profile. Individual genes can be probed too as a way of identifying recessive or dominant mutations that might lead to disease in the baby. In an oocyte, a polar body would be assessed instead of the oocyte itself.

It is the hope of every potential parent that their child/children will be born healthy. And more and more couples are using ART to try and become parents. In cases where there is a family history of disease or of the mother is older and therefore more prone to non-disjunction events, it may be suggested that the oocytes or embryos undergo PGS before they are used for IVF or delivered to the uterus.

To look at the genetic make up of an oocyte, you can’t analyze the nucleus, it’s just one cell and you destroy the nucleus as you do PGS. You have to use something called a polar body. During oogenesis (formation of the egg by mitosis and meiosis) there is the production of a smaller cell that usually isn’t capable of being fertilized; this is the polar body.

The first round of meiosis of the primary oocyte results in the polar body #1 and the secondary oocyte. In meiosis II, the secondary oocyte divides unequally and produces the ovum and the polar body #2. Each division is unequal in terms of cytoplasm, so the polar bodies are smaller than the oocyte.

Sometimes the first polar body will divide again in meiosis II; therefore, you can get three polar bodies for every egg produced. The first polar body (and any daughters of it) will have a different genetic makeup from the egg, but the second polar body will have the same chromosomes as the oocyte.

In PGS of an oocyte, the first or second polar body is harvested for genetic testing before the embryo is implanted in the uterus (not every follicle will retain each polar body). The goal is to identify genetic diseases that the individual willhave, or might be predisposed to. If they find a problem, they don’t use that egg for IVF.

In the case of embryos that have undergone IVF already, they will harvest a single cell of the growing embryo to do PGS analysis on that. If that embryo shows chromosomal anomalies, they won’t deliver it to the uterus.

You can also check for trisomy 13 (Down syndrome) in a fetus by amniocentesis (upper cartoon) or by a new maternal blood test (bottom image) that assesses child DNA fragments in the mother’s blood. But of course, this is after the pregnancy is well along. PGS, on the other hand, assesses the possibility before the embryo is implanted and the pregnancy proceeds.

PGS in oocytes might help for non-disjunction syndromes like Down Syndrome or Klinefelter’s. In embryos PGS can also be used to identify individuals that have acquired two copies of a recessive gene that will guarantee a genetic disease. This is especially sad in cases where the disease doesn’t manifest until the victim has already had children and passed on a possible death sentence – something like Huntington’s disease (see this post) or fatal familial insomnia (see this post).

Unfortunately, most studies indicate that PGS of the oocyte isn’t very helpful in predicting that the embryo will be OK; too many things can go wrong after PGS is done. In one case, two PGS-approved embryos were implanted, but they ended up with triplets. Two babies were mosaic MZ twins and one of them had Down Syndrome.

There is also the suggestion that some oocytes harvested from ovaries for ART are conjoined, made within one follicle. This may increase the chances of mosaicism from fusion. And several instances suggest that PGS can induce twinning, since pulling the cell from the embryo is a lot like induced hatching – and we saw that messing with the zona pellucida is associated with increased MZ twinning.

Here is how the polar bodies are formed. After the haploid egg is formed, the first polar body may divide and leave three polar bodies that might be fertilized. It would be rare, but it might happen. The second polar body is genetically identical to the ovum. The two daughters of the first polar body are identical to each other.

Our discussion of PGS highlights one last type of MZ twinning – potentially. It is theoretically possible that a daughter of the first polar body (if it undergoes meiosis II) could be fertilized. The polar body and the oocyte have different chromosomal makeup because of random assortment in meiosis I, so these might be considered dizygotic twins if the polar body and the oocyte were both fertilized.

But if the secondpolar body and the oocyte were both fertilized, each embryo would have the same maternal DNA but different paternal chromosomes.  Would they be MZ twins? Dizygotic twins? The theoretical names are polar body twins or ½ twins, but science hasn’t proven it can happen yet.

One case of half or semi-identical twins is known, the 2007 case of sex discordant MZ twins with hermaphroditism we talked about above. But since they are mosaics, we know they didn’t come from two separate fertilizations of a polar body and an oocyte. The search continues.

Next week – given the nine types of MZ twins we have talked about, wanna bet that there’s more than one type of dizygotic twins?