A French team[1] of fertility experts has revealed for the first time that maternal genetic imprinting errors can occur at the very earliest stages of egg follicle development when the follicles are matured in-vitro.
The research was done in mouse oocytes, and Dr. Catherine Poirot from Groupe Hospitalier Pitié Salpêtrière in Paris, who presented the team's findings today (Monday 30 June) at the annual meeting of the European Society of Human Reproduction and Embryology, said it was not possible yet to say whether the same would be true of human eggs.
However, the team are concerned that these very early errors could be involved in subsequent abnormal development.
Normally, genes act in the same way whether they are transmitted by the mother or the father. But, a few genes break this genetic rule. Whether they are switched on (expressed) or off depends on whether they are inherited from the mother or the father. The process of inheriting specifically from the mother or the father is called imprinting.
Imprinting genes can be found on chromosomes 7 (H19), 6 (Mest/Peg1) and 17 (Igf2R genes) and these were three key genes – all involved in growth – analysed by Dr Poirot and her team. From the ovaries of 11 and 12-day-old mice they isolated pre-antral follicles. These are follicles that have begun to grow but which are still a solid mass of cells containing the nucleus of immature oocytes. They cultured the follicles for 11 days until the antral stage when the central cavity filled with follicular fluid develops inside the follicle. As controls, the team collected immature oocytes from antral follicles visible on the surface of adult mouse ovaries. They then studied the methylation status of the three genes in the immature in-vitro oocytes and in the controls.
Methylation is one of the key processes controlling genes, determining whether some genes are switched on or off. If a particular gene or group of genes is not needed in a particular type of cell, that cell will be 'tagged' by tiny specialised bits of DNA. The number and placement of these tags provides a signal to specific proteins with the cell. These proteins recognise the tags as a 'no' command, so they bind the tags together and this stops the unnecessary gene being switched on. Methylation is essential for normal development and organ function. If the process goes wrong it results in abnormalities and developmental diseases.
When the researchers analysed the control oocytes they found that the H19 alleles (variants of the gene) remained unmethylated, whereas Mest/Peg1 alleles and Igf2R alleles were almost all methylated. These methylation profiles are characteristic of the maternal imprint and provided evidence that maternal methylation imprint was already established in the in-vivo full-grown oocyte.
However, the in-vitro full-grown oocytes showed a different pattern. H19 had gained DNA methylation, but Mest/Peg1 and Igf2R had a loss of DNA methylation.
"What we've shown, for the first time, is that disruption – loss, absence or delay – of methylation imprint establishment is occurring during in-vitro folliculogenesis at the place on the chromosome where these imprinting genes are located. What we don't know yet is what consequences that may have for the oocytes to be able develop properly in-vitro. That is the next stage in our research. However, it is tempting to speculate that such early genetic disruption could be involved in subsequent abnormal development."
Dr. Poirot concluded: "It is too early to say what relevance this may have to ovarian tissue cryopreservation in humans. But, we think it is important now to find the best culture medium and/or physical culture conditions possible that will enable us to develop as good oocytes in the test-tube as exist naturally. This study could also be a marker for researchers to identify optimum culture conditions for in-vitro follicular growth."
Abstract no: O-012 (Monday 10.45hrs CET, Auditorium B América)
[1] The team was from: Groupe Hospitalier Pitié Salpêtrière, Paris; Hôpital Cochin and Inserm GDPM, Institut Cochin, Paris; Institut Jacques Monod, Paris.
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