Converting stem cells into gametes is the most difficult transformation

Cristina Eguizabal came from Cambridge, UK, to the CMRB two and a half years ago to try to get male haploid cells (spermatozoids) from human pluripotent stem cells. She uses both human embryonic cells (hES) and induced pluripotent stem cells (iPS). The first are obtained through fertility clinics from unused fertilised embryos. The hES are isolated from the embryos and derived into cell lines at the Stem Cell Bank of the CMRB, on the 4th floor of the PRBB, where Eguizabal is working under the coordination of Anna Veiga. iPS cells, on the other hand, can be derived in vitro from skin cells or cord blood, as was shown recently, although the protocol is not optimal and these cells are not yet safe for clinical use.

Once she has the pluripotent stem cells in culture, Eguizabal tries to find the best conditions to differentiate them into primordial germ cells (PGC), which can then give rise to the spermatozoids or oocytes. What would be the practical applications of this, if it works?

Giving hope to parents 

The simplest would be to allow people who are sterile to have children with their genetic characteristics. Currently, a sterile man depends upon anonymous semen donations to have a child. With this technique, a simple skin biopsy could lead to the creation of his own iPS cells, which would be differentiated in vitro into PGCs and which would give rise to sperm with his DNA which could be used in “in vitro fertilisation” (IVF) to have his child.

Another related application would be to allow people to have children free of a genetic disease which they themselves suffer. In this case, the procedure would be the same, but once the iPS cells were obtained, their genetic error would be corrected. This would be possible for well-studied monogenic diseases, caused by a single gene, such as cystic fibrosis or Duchene muscular dystrophy.

Many steps to go  

“This won’t be happening for 10 or 15 years”, predicts Eguizabal. “Apart from the step I am working on, the differentiation of the iPS cells into PGCs, of which very little is known, all the other steps in the process need more research. For example, deriving the iPS cells from skin cells needs to become safer, as at the moment one of the genes included in this transformation is an oncogene, which could lead to cancer. The genetic correction of the iPS cells is also something people are working hard on”, continues the researcher. Both of these issues are also studied at the CMRB.

The differentiation of stem cells into sexual gametes, spermatozoids and oocytes, is the most difficult, much more so than getting stem cells to become neurones or cardiovascular cells, according to the Basque biologist. “First of all, the sexual gametes are the only cells that go through meiosis, a type of cell division that implies losing half of your genome. And secondly, their aim is to give rise to a living being, so their genetic and epigenetic information must be 100% correct”, she says. During normal physiology, germ cells suffer a huge epigenetic re-programming: their whole genome is methylated and de-methylated again. Methylation is a reversible modification of the DNA which affects gene expression. And in order to get functional gametes, it is important to reproduce this methylation in vitro. But it is not easy. Eguizabal plans to use molecular techniques such as bisulphite sequencing to check the methylation status of the DNA once she gets the cells she is interested in.

This article was published in the El·lipse publication of the PRBB.


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