A tale of twin sisters

It was the best of times, it was the worst of times, it was the age of peak fertility, it was the age of premature ovarian failure — in short, it was a series of sisters that mystified top gynecologists.

In the early 2000s, Dr. Sherman Silber saw something strange. Several sets of sisters showed up at his fertility clinic, where one sister had premature ovarian failure but the other was fertile. The puzzling thing was that these sisters were identical twins.¹

Usually, premature ovarian failure is the result of a mutation that disrupts egg formation or maintenance. Identical twins share the same DNA and developmental environment, so it was highly surprising that one twin had ovarian failure but the other had normal fertility. It was even more surprising to see this not once, but several times.

So, what happened?

First, you’ll need to know some background about the biology of twins.

Monozygotic (or “identical”) twins arise when a single egg splits into two embryos sometime after fertilization.² Depending on when the split occurs, this can give various biological outcomes.

• Dichorionic twins are basically like fraternal twins (except for being genetically identical). The zygote splits shortly after fertilization and two embryos develop separately. This happens in about 25% of monozygotic twins.

• Monochorionic twins share a placenta and chorionic membrane (the outermost fetal membrane) but are in separate amniotic sacs. The embryo splits after trophectoderm formation but before amnion formation. This is the most common type of monozygotic twins (~75%).

• Monoamniotic twins share a single amniotic sac, in addition to also being monochorionic. This only happens about 0.3% of the time.

• Conjoined twins do not fully separate, and remain attached to each other even after birth. This is quite rare and such twins often do not survive.

  

  Source: Kevin Dufendach via Wikimedia

Later investigation revealed that most of the twins who were discordant for premature ovarian failure were monoamniotic, and none were dichorionic. This was a big clue.

Purloined PGCs

Primordial germ cells (PGCs) are early precursors of eggs and sperm. In humans and other primates, PGCs form around day 11 – 17 post-fertilization. There is actually an ongoing controversy in the field about whether PGCs are formed in the amnion or the posterior epiblast³. But in any case, PGCs are specified before the gonads develop, and need to migrate into the developing gonads.

In males, PGCs develop into spermatogonial stem cells, which self-renew in the testis over the entire life of the man. However, in females, PGCs develop into oogonia, which further differentiate into a finite number of oocytes. Starting out with a deficit of PGCs would therefore lead to premature ovarian failure.

The problem is that in these women, the lack of a barrier between the two embryos allowed the PGCs to be distributed unevenly during migration.⁴ Clearly PGC specification took place correctly, since one woman in the twin pair was fertile. But that woman actually stole the PGCs from her sister!

To perform a more detailed investigation, researchers from Amander Clark’s lab collected cells from monoamniotic twin pairs, one of whom was fertile and the other of whom had premature ovarian failure. The researchers reprogrammed the cells into iPSCs and attempted to differentiate them into PGC-like cells.

If the infertile woman’s premature ovarian failure was due to a de novo mutation or epigenetic abnormality, then PGC-like cell specification from her iPSCs would be expected to be impaired. But, consistent with the hypothesis that the infertility was due to PGC theft, the efficiency of PGC-like cell specification was the same from both sisters, across two different twin pairs. Additionally, the researchers performed whole genome sequencing and ruled out de novo mutations causing ovarian failure.

A surgical solution

Fortunately, the cause of the problem (monoamniotic twinning) also presented a solution. Since the twins were genetically identical, an ovarian tissue transplant from the fertile twin could restore fertility in the infertile one. This is how Dr. Silber treated his patients. As expected, the transplants were not rejected (due to being a perfect immunological match), fertility was restored, and the eggs produced by the ovarian tissue were genetically equivalent to those that would have been produced normally.

But I can only imagine the conversation that must have taken place between the twins: “Hey sis, you stole my eggs, and I want them back!”

1. See: https://www.nejm.org/doi/10.1056/NEJMoa043157 and https://pubmed.ncbi.nlm.nih.gov/18285322/

2. Incidentally, this is a very good counterexample for the religious belief that a human zygote has a unique, indivisible soul.

3. At a conference last month, I participated in a rather stimulating discussion about this that went until 2 am! Basically, there’s good evidence for PGCs being specified in the epiblast in mice and rabbits, but the primate data is ambigous but leaning towards amnion. But in vitro, it’s certainly possible to induce human PGC-like cells from cells corresponding to the epiblast (I’ve personally done it). The problem is that it’s quite difficult to culture primate embryos to definitively resolve this. It’s also possible that PGCs are specified in both locations.

In my opinion, the case of these twins is better explained by PGC specification in the amnion, since in that situation it would be clear that monoamniotic twins would compete for PGCs.

4. It’s also possible that some abnormality resulting from the twinning process blocked PGC specification in one (and only one) of the embryos. But I think this is less likely, since PGCs are known to migrate, and the simplest explanation is that this migration was disrupted by the presence of a twin.