First report of a healthy birth from an “abnormally fertilized” 4PN embryo

Researchers in a 2023 case report detail the first case of a healthy live birth from a 4PN embryo, thought to arise from abnormal fertilization, but was confirmed normal by PGT-A.

During fertilization, the egg and sperm DNA are packaged in structures called a “pronucleus.” The presence of two pronuclei (2PN), one from the egg and one from the sperm, indicates that the egg has fertilized normally and will develop into an embryo.

Occasionally, abnormal fertilization occurs and creates more than two pronuclei. Often, either one or three pronuclei (1PN/3PN) are seen during abnormal fertilization. This can happen if an egg is inseminated by two sperm (3PN), or if the egg and sperm pronuclei don’t form at the same time so only one pronucleus is seen (1PN).

These types of abnormal fertilization are pretty common, and can result in normal embryos, although many clinics discard them as they’re considered abnormal. It has been recommended that abnormally fertilized eggs be kept and evaluated by PGT-A to ensure they have the right number of chromosomes, and if they do, they can be transferred.

The worry is that these abnormally fertilized embryos might contain the wrong number of chromosomes. Humans are diploid, and contain 2 (di-) copies of each chromosome. A 3PN might contain 3 sets of chromosomes and be a triploid. Normal human embryos are diploid (contain 2 sets of chromosomes) and euploid (have an exact replica of each set of chromosomes, or 2 copies of 23 chromosomes = 46 in total). To be clear, diploid/triploid tells us about the number of copies of chromosomes, while euploid/aneuploid tells us about the number of chromosomes in each set (with 23 being the normal number in each human chromosome set).

I have more information on abnormal fertilization on my post Abnormal fertilization and first cleavage: a summary.

While 1PN and 3PN abnormal fertilization can be common, a 4PN is much less common. Bredbacka et al. (2023) report on the first case of a successful pregnancy from a 4PN.

A couple had presented with infertility due to male factor (azoospermia, where the man doesn’t produce any sperm in his semen). They had undergone 9 ICSI cycles previously using sperm from a testicular biopsy, and 6 IUI cycles using donor sperm. The embryos that were produced were mostly poor quality and they never had a pregnancy.

They decided to use donor eggs and sperm, which resulted in a good quality day 3 embryo that didn’t lead to a pregnancy. Some of the other embryos produced were 3PN and 4PN, which became blastocysts.

It’s not clear why this donor cycle, involving both donated eggs and sperm, had such a high rate of abnormal fertilization. The authors report that the egg donor had been used in multiple cycles and this wasn’t seen before, and attribute it to “suboptimal cytoplasmic maturation.”

A special type of PGT-A was used, utilizing “targeted next-generation sequencing,” to help identify the possible tetraploidy. Tetraploidy is a chromosomal abnormality resulting from 4 copies of each chromosome. Since each pronucleus might contain a set of chromosomes, the 4PN was predicted to have 4 copies of each chromosome and be tetraploid.

PGT-A showed that the 3PNs were aneuploid, triploid or tetraploid, but the 4PN was diploid — it had two copies of each chromosome — and was euploid!

It’s not clear why there were 4 pronuclei and the embryo was diploid, but the authors comment that it may be due to errors in how the pronuclei formed.

The patients were counselled and they decided to go ahead and transfer the 4PN euploid, which led to a pregnancy and normal fetal development. A healthy baby boy was born around 40 weeks without complications. The baby is now 4 years old and has developed normally. The couple refused genetic testing of the fetus and of the baby.

The authors emphasize that although abnormally fertilized eggs are usually discarded, PGT-A testing of 1PN, 3PN and now 4PN embryos has been used to show that these embryos can be normal and can result in live births.

Reference

Bredbacka P, Capalbo A, Kananen K, Picchetta L, Tomás C. Healthy live birth following embryo transfer of a blastocyst of tetrapronuclear (4PN) origin: a case report. Hum Reprod. 2023 Sep 5;38(9):1700-1704. doi: 10.1093/humrep/dead151. PMID: 37528053.

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About Embryoman

Embryoman (Sean Lauber) is a former embryologist and the founder of Remembryo, an IVF research and fertility education website. After working in an IVF lab in the US, he returned to Canada and now focuses on making fertility research more accessible. He holds a Master’s in Immunology and launched Remembryo in 2018 to help patients and professionals make sense of IVF research. Sean shares weekly study updates on Facebook, Instagram, and Reddit regularly. He also answers questions on Reddit or in his private Facebook group.

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