In this session, Patricia Muñoz, Embryologist at IVF-Spain, will explain maternal spindle transfer (MST), when it can be indicated, and how the process looks.
Mitochondria are the most common organelles in the cells. They have very specific particularities, like self-replicate, and the fact that they have their own genome.
Another particular thing is that they are under dual control, one from the mitochondria and the other from the nuclear genome. This is going to affect their function.
The main function of mitochondria is to produce the energy that the cells need to perform all processes, providing almost 95% of the energy that the cells need. They also have other important roles, like participating in programmed cell death, calcium homeostasis, and lipid metabolism; however, producing ATP for the cells may be the main function of mitochondria. This is the reason why problems in mitochondria can produce severe illnesses. If you have a lot of these mutations and many affected mitochondria, you could develop several health problems, such as retardation, psychomotor problems, neuropathies, or ataxia.
This is very important to research and to know how to avoid the transmission of these diseases.
How common are mitochondrial diseases?It is important to highlight that mitochondrial diseases could be caused by mutations in both nuclear DNA and mitochondrial DNA. Also, the prevalence in the population ranges from 5 to 15 per 100,000 in childhood.
Here in the slides, you can see the most known mitochondrial diseases. The thing is that, actually, currently there are only effective treatments and no cures.
It’s really important to research these mitochondrial diseases and how to try not to transmit them to the offspring. For this reason, we need to know how they transmit because this is very particular.
The mitochondria have a very particular way of transmitting to the offspring because the babies have only the mitochondria that come from the mother. During fertilization, the one who gives the mitochondria and the cytoplasm to the embryo is the oocyte, the mother. Therefore, the mother is going to transmit this mitochondrial DNA mutation to the offspring.
It’s important to say that the way they transmit is particular because you can find different cell types containing different numbers of mitochondrial DNA. For example, in the tissue that is going to form the brain, you may have a high percentage of this affected mitochondrial DNA, but in other tissues, you may have a low number of these mutated mitochondria.
It can also happen that you have identical mitochondrial DNA in your cells or a mixture of different mitochondrial DNA. The thing is that this can affect the way this illness transmits to the offspring and also how it’s going to affect the offspring. As we can see in the diagram, in the primordial germ cells, which are the ones that are going to form the oocytes, we have a low number of mitochondria, between 10 and 200. Then, these cells are going to give the oocytes.
You could have a heteroplasmy, where you have both normal mitochondrial DNA and affected mitochondrial DNA, and depending on the grade of heteroplasmy, you can produce oocytes without mutations, or maybe oocytes with low-level mutations. However, you can also have oocytes with a higher percentage of these mutations, which is going to affect the offspring. The offspring might have more health problems or severe health issues.
The threshold that we established is between 60 and 80%, but this varies depending on the mitochondrial disease, so it’s specific to each mutation. Generally, it is between 60 and 80%. If you have a higher percentage of this mutated mitochondrial DNA, you will likely have severe symptoms, and a very bad clinical diagnosis, and it could also lead to the death of the children.
Due to this reason, the PGT when you take some cells from the embryo and analyse them is used to look for aneuploidies. However, you can also look for these mitochondrial mutations. Due to these particular inheritance patterns, you may find a low percentage of these mutated mitochondria, but then in other tissues that were not sampled, you have a high percentage. This could lead to having a baby that might be affected in the future, so it’s not the best way to avoid this transmission. Also, if you are affected, you might not be able to produce embryos without these low mutations. This is the reason why new alternatives have emerged, such as mitochondrial replacement techniques.
Today’s topic is maternal spindle transfer, there are also two other techniques: polar body transfer and GV (Germinal Vesicle) nuclear transfer. While these methods are fascinating, they have not yet resulted in any recorded live births and are not as extensively studied as pronuclear and maternal spindle transfer.
This technique involves two oocytes: one from the donor, which contains the healthy mitochondrial DNA, and another from the mother, the patient with the affected mitochondrial DNA.
What you are going to do is remove the spindle, which contains the genetic material, from the donor oocyte, leaving only the cytoplasm. Then, on the other hand, you are going to take out the maternal spindle from the patient and place it inside the donor oocyte, and then you are going to microinject with the spermatozoa. As a result, you are going to have a cell with the maternal spindle (genetic material from the mother), the spermatozoa from the father, and the cytoplasm from the donor, which is not affected.
The main problem of this technique (and this also happens with pronuclear transfer) is when you take out the spindle, you take some part of the cytoplasm, which means you are going to transmit part of this cytoplasm to the oocytes. As a result, the reconstructed oocytes have a small amount of the affected mitochondrial DNA. Studies suggest that this is around 1-5%, but it can vary depending on the tissue and the stage of development, so there is no guarantee that this percentage won’t be higher in the future, potentially affecting the children.
This is something we need to continue studying. We need to develop more and gather more data because it’s essential to understand this information. We also need to consider who will benefit from this technology, with the primary beneficiaries being people with mitochondrial diseases, but this is only useful when mitochondrial diseases are due to mitochondrial DNA mutations. If these mitochondrial diseases are due to nuclear mutations, this technique is not useful for them.
Some clinics are also carrying out this technology for repeated IVF treatment failures due to poor quality eggs or for patients who have undergone many IVF treatments and the embryos still arrest. This is due to some cytoplasmic defects. You can use this technique to try to improve these results because mitochondria are very important for oocyte maturation, meiosis, fertilization, and embryonic development.
If you have the cytoplasm from a donor with good fertility and combine it with the genetic material from a mother who has experienced many fertility treatment failures, you might be able to have a baby with your genetic material. However, the primary indication remains the prevention of mitochondrial diseases.
The third one is diabetes. Some research has shown that diabetes in the mother can be due to mitochondrial DNA mutations, which can be passed on to the offspring. Therefore, it’s important to consider how these technologies are currently being used and the need for further research on their long-term safety, efficacy, and the potential presence of mutated mitochondrial DNA in the offspring.
These are the most important points. Of course, any technique has its own advantages and disadvantages that make them different. But I think in all techniques if we are going to use them in the clinic, we need to have these three things clear: long-term safety, efficacy, and reduced or non-mutated mitochondrial DNA that could affect the offspring. This is the most important aspect.
Currently, the only country with specific regulations for these techniques is the UK. You can perform PNT, pronuclear transfer, and maternal spindle transfer, but only to prevent the transmission of mitochondrial diseases.
The regulations in the UK are very strict and strong, and they have not yet reported any live births because they check all cases that seek these treatments and decide whether to accept them or not.
On the other hand, and in contrast, in Mexico and Ukraine, there is no explicit regulation regarding these technologies. In Mexico, they reported the first baby after maternal spindle transfer in 2016 by Dr Zhang. In Ukraine, they also reported the first baby after pronuclear transfer. In Ukraine, they use this technique to treat infertility and to help patients with poor-quality eggs or arrested oocytes that do not get to the blastocyst stage.
Similarly, in Greece, they are conducting clinical trials to use this technology for infertility treatment, in collaboration with a Spanish company, as there are no specific laws regulating this there either. As a result, Ukraine has reported more than seven live births using this technique, treating it as a relatively normal process in clinics. Greece has reported about five or more babies at this moment.
For the future and the clinical application of these technologies, it is crucial that these newborns and live births are reported, and that we have access to this information. We need to gather and recover all this information because it’s very important to know if these technologies are safe for future offspring. This is why further research is necessary to use these technologies as a common technique in the clinic, but this is my opinion.
Some key ideas, highlighting important points discussed:
Please, remember that in Spain, this procedure is not allowed. What I know is that they are doing this in Greece, as I said, in a clinical trial in collaboration with Embryotools, a Spanish company. I know they have reported at least 5 live births, and they are going to have more because it’s a clinical trial, and they use this technique for infertility treatments.
In Ukraine, you can visit the web because several clinics have permission to do these techniques. I know that more than seven children have been born in the clinics I have seen, but I suppose there are more than seven using this technology.
In Spain, it is not possible, and I think we have a long way to go to get this permission.
There are two main reasons. One is that you have a mitochondrial disease and want to avoid transmitting it to your offspring, so maybe you have good-quality eggs or a high ovarian reserve. The reason would be this, you have a mitochondrial disease, and you have to avoid transmitting it to your offspring.
The other reason is when you have poor quality eggs, that sometimes is due to this mitochondrial. The thing is that, for example, this clinic in Ukraine needs you to be less than 40 or 37 years old because they need the nuclear genetic material to be good. Also, I think you need a not-bad AMH because you need a higher quantity of eggs. After all, this process, at this moment, is a bit demanding.
There are papers or research that report that the success is the same as with untreated oocytes, but of course, you are producing all this process to the oocytes, so you might have more possibilities of unknown fertilization issues, and also you have more possibilities that maybe PGT will not be good. I think you need a high number of oocytes, but also what you can do, for the future, although it’s not possible in Spain now, some papers have said that you can freeze the oocytes from the patient to accumulate oocytes. Then, you can perform this process with the donor in fresh. This way, even if you have a low ovarian reserve, you can do this process because you continue freezing your oocytes until you have a high enough quantity to be successful.
I know from my reading that in Ukraine, they are doing this pronuclear transfer, not the maternal spindle transfer, but it’s very similar. The clinic I said is called Nadiya, which collaborates with Dr. Zhang, who performed this treatment in Mexico.
In Mexico, the first baby born after maternal spindle transfer was born with the collaboration of a doctor from the United States, from some hospital.
I think that in Mexico they’re not doing a normal process, but in Ukraine, clinics appear to use a normal process in the IVF laboratory. I think there are more than these techniques, but you need a lot of experience and a lot of training to perform these techniques because I have never done this, but I was reading, and it looks complicated to do these techniques well. Not all clinics can do it, at least for the moment, we will see in the future, but we need a lot of study and research.
This maternal spindle transfer is done at the beginning when people perform it on the matured oocyte. The embryo is something that comes after this fertilization. The transfer of the maternal spindle has to be done in the first step when you have the matured oocyte. You have to remove the one from the donor and put the one from the mother.
I know, you cannot do this on a mosaic embryo to make it normal because in an embryo you have a lot of cells; for example, in a blastocyst, you have hundreds of cells, so if you fix one cell is not important to the other hundreds of cells.
This technique has to be performed first during oocyte maturation. Maybe it will happen when you have a poor quality of the oocyte, sometimes it is due to the mitochondria. The mitochondria are very significant for processes like meiosis and maturation, so potentially if you have problems with the mitochondria, you could have more possibilities to have more abnormal embryos. So, possibly in this case, if you do this maternal spindle transfer at the first moment to improve fertility results and the quality of the egg, you can have fewer aneuploid embryos.
However, when you have the embryo formed, you cannot fix it with this technology, which I know for the moment.
Not me, but people due to the spindle have these microtubules. With the polarized microscope, you can detect them from the microtubules, so you can identify the maternal spindle.
With the polarized microscope, you are detecting here in the pipette and also here inside the spindle. So, it’s easier if you have the technology, I suppose.
I don’t perform this technology. I was only explaining how it works and sharing some ideas. They use a polarized microscope, which I suppose is the Polescope. This is a method that I’ve seen they used to localize the maternal spindle because it’s important to know where exactly. This way you can minimize damage to the oocyte, genetic material, and organization. You need to try to get a low quantity of cytoplasm, so you need to be very careful and check out.
For example, spindle transfer has the peculiarity that in pronuclear, you have the pronucleus with the membrane protecting them. So, when you are transferring this pronucleus, there are lower possibilities of losing some part of the genetic material, but in spindle transfer, this doesn’t happen. You’re using all these technologies, but during the process, they are not covered by any membrane or something to protect them.
Some research has shown that sometimes you might have more abnormal embryos or maybe an abnormal fertilization because you lose part of the genetic material or part of the chromosomes. This could be one of the problems.
Also, people are concerned about what we have said before, when you’re doing this technique, you’re going to transfer the affected mitochondrial DNA. If you perform this technique to avoid mitochondrial disease, you can transmit a low quantity of these mitochondrial diseases to the embryo.
Current research shows that you have a low quantity of these mitochondria, but the mitochondria have a very particular way of transmitting, so some research has shown that offspring obtained with these techniques sometimes might have a higher percentage of these mutated mitochondria. So, maybe you could transfer an embryo that looks healthy, but in the future, this child could develop this illness. This is the reason why more research is needed because we need to be sure that the offspring is going to be safe.
There are also some concerns about potential problems when the DNA from the nucleus is from one person and the DNA from the mitochondria is from another person, or if two kinds of mitochondria are in the same oocytes. Researchers are also concerned about this, so more information is needed.
Yes, of course, I want to say that we don’t perform this technique, and I’m not an expert in it. But from what I have been reading, I would recommend due to this reason because I’ve just said, during this process of the maternal spindle, there is a possibility of losing part of this genetic material, so then you may have more possibilities of having an abnormal embryo and transferring an abnormal embryo, also more research is needed. This is also an important thing, and also long-term data about the life births, and the children born after this method.
I would recommend, due to this concern, that the spindle is not recovered, and you might lose some chromosomal material, which could lead to an abnormal embryo.
Also, to check whether the carrier of mitochondrial mutations is low or higher. This is the case that you’re using it due to mitochondrial disease.
I’m very happy for this. It was very frustrating to have this mitochondrial disease and know that you can transmit it to your offspring. Of course, there are other options like adoption or egg donation, but I will be very happy, and I hope that in the future, we can offer this technology to people without any problems and be sure that we are doing as much as we can.
When we start with microinjection, some people will think about what would happen to the baby. After years, we have seen no problems, and we can use this technology. I hope the same happens with these new techniques.
There’s a paper related to different countries about how these technologies, these techniques are there, and here in Spain, it’s not a law, as you said, but for example, the company who is working with Greece to do this treatment, this maternal spindle transfer in Greece, they hope they have said that they hope that with this data they can maybe make pressure in the Spanish government to maybe be more relaxed with these techniques, but I think that Europe is difficult. We need to do more research. Maybe Canada or other countries will be earlier than in Europe. That’s actually in Europe, there are some countries where egg donation is not allowed. PGT is not allowed, so, I think that these countries, if they don’t allow PGT or egg donation, are not going to allow this technique.
