According to new research conducted by scientists at John Hopkins University in Maryland, mosaicism – differences in the chromosomes between cells during early embryonic development – appears to be more common and compatible with healthy embryos than previous studies have suggested.
'What we found is that low-level mosaicism is common,' said Margaret Starostik, a graduate student in biology and lead author of the study. 'It may be a normal phenotype.'
Published in the journal Genome Research, these findings suggest this type of genetic imbalance in some cells of an embryo are also less of a risk to healthy embryonic development, and in some cases even beneficial in promoting cell defence mechanisms.
There is wide debate in the field of IVF around the efficacy of implanting embryos featuring higher rates of chromosomal mosaicism. Viable cells with abnormal numbers of chromosomes – a state described as 'aneuploidy' – normally have 47 or 45 chromosomes rather than the standard 46 for human cells. When aneuploidy is present in a sperm or egg the resulting embryo subsequently has this error in all cells, leading to the occurrence of a syndrome - the most common example being the trisomy of chromosome 21 causing Down’s syndrome.
The researchers focused on detecting embryos where the aneuploidy was only found in some cells. They found that eight out of every ten potentially healthy embryos they studied contained those abnormalities.
The study used an alternative to the standard preimplantation genetic testing (PGT), where just five cells are biopsied from the outer layer of an embryo – the cells that develop into the placenta. This test is used in IVF and other studies to identify abnormal embryos so they can be discarded. The paper highlights the controversy over this technique, as it is assumed that the biopsy is representative of the embryo as a whole.
Dr Rajiv McCoy, senior author of the research, said 'Clinicians have wrestled with the decision to transfer embryos featuring mosaic aneuploidy when no other embryos are available. In recent years some have implanted such embryos and reported healthy births, indicating embryos may have resilience or self-correction of mosaicism.'