Researchers told the 22nd annual conference of the European Society of Human Reproduction and Embryology today (Monday) that preliminary work investigating genetic imprinting in mouse embryos had shown that certain culture media and concentrations of oxygen altered the expression of several imprinted genes.
Imprinting is the process by which some genes are activated or inactivated depending on whether they have been inherited in chromosomes from the mother or the father.
Professor Paolo F Rinaudo, a scientist at the Center for Reproductive Sciences, University of California, San Francisco, USA, said: “We found that the culture of mouse embryos in the laboratory is sufficient to alter the expression of several imprinted genes and that this effect can be modified by the composition of the culture medium and oxygen concentration.
“Interestingly, the expression of one gene, H19, was reduced regardless of the culture conditions, and as H19 is associated with Beckwith Wiedeman syndrome, this finding needs to be investigated further.”
However, he cautioned against reading too much into his results at this stage. “These studies are important for better directing future resources and studies in humans. But we must remember that these are preliminary results in a mouse model, and they need to be repeated and confirmed in other strains of mice before translating to studies in humans.”
Other research has suggested that culturing embryos in the laboratory during IVF could be affecting embryos adversely. “Emerging new evidence shows that some neurological and behavioural abnormalities are associated with assisted reproductive techniques. Cases of Angelman and Beckwith Wiedeman syndromes in humans, which are due to aberrant genomic imprinting, and other abnormalities in growth and development in mice have been described after culture in vitro,” said Professor Rinaudo. Angelman syndrome is characterised by severe mental retardation, speech impairment, balance disorder and a happy, excitable demeanour; it occurs in about one in 10,000 to 30,000 of the population. Beckwith Wiedeman syndrome is characterised by overgrowth, with an abnormally large tongue, umbilical hernia, neonatal hypoglycaemia and a predisposition to certain tumours, in particular, Wilms tumour and hepatoblastoma; it is rare, occurring in one in 36,000 of the population.
Professor Rinaudo and his colleagues cultured mouse embryos in four media with 5% oxygen. Two of the four media were also used with 20% oxygen, making six different cultures in total. As a control, some embryos were left to mature in the mice (in vivo).
“We identified 38 imprinted genes, most of which showed no difference in expression after in vitro culture compared to the in vivo embryos. Five genes showed a statistical difference compared to the in vivo control, depending on the culture conditions,” he said.
The five genes were:
- Cd81– expresses a membrane protein with unknown function during embryogenesis. It is involved in T cell development (part of the immune system) and the hepatitis C virus binds to it to gain entry to cells.
- H19 – a developmentally regulated gene, thought to be a tumour suppressor and associated with Beckwith Wiedeman syndrome.
- Slc38a4 – plays a role in amino acid transport to cells.
- Copg2 – plays a role in transport of proteins within cells and is associated with Silver-Russell syndrome, a growth disorder occurring in 1 in 75,000 births (involves intrauterine growth restriction and subsequent poor growth).
- Gnas – plays a role in cell signalling and is associated with McCune-Albright syndrome (involves bone, hormone and skin abnormalities), pseudohypoparathyroidism Ib (involves inadequate levels of calcium and excessive levels of phosphates in the blood), and GH-secreting adenomas (type of tumour).
Of these, the greatest changes in gene expression were seen in Cd81 (consistently over-expressed in all media) and H19 (consistently under-expressed in all media), but Professor Rinaudo said the significance of these findings was unclear at present.
“The next stage of our research is to examine further the effects of IVF and ICSI on the genetic make-up and the form and function of cells from mouse embryos. We want to understand why these techniques appear to be affecting the imprinting of these genes. We hope this understanding will enable the scientific community to make better culture media for IVF in humans.”
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