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Healthy embryos: Using PGD to screen for disorders

Posted on 23.11.15


Having a healthy baby is what every women wants but for some mothers there is a chance of their child inheriting a genetic disease. The Bridge Centre is at the forefront of a technique called PGD that screens embryos for genetic or chromosome abnormalities before they are fertilised.

When is PGD used?

PGD techniques are mainly applied in cases of infertility in women of an older maternal age or with a history of miscarriage. In these cases there is likely to be an imbalance in the embryo. The risk of most chromosomal disorders in children - such as Down’s syndrome - is known to increase with older maternal age. While an abnormal arrangement of chromosomes may explain the recurrence of miscarriage in some women.

Testing embryos for chromosome abnormality

The techniques for testing embryos for chromosome abnormality are widely used at the Bridge Centre in its specialist programmes for ‘fertility for the over 40s’ and for recurrent miscarriage.  The aim at the clinic is to only transfer embryos that have been screened as being chromosomally normal.

The tests involve removing a single cell from the embryo or egg by a biopsy. It is tested to check for a complete set of chromosomes using a technique known as array comparative genomic hybridization, or CGH for short.

Its objective is to improve pregnancy rates by identifying normal embryos and transferring those to improve the chances of having a healthy baby.

‘Screening embryos at this stage is known as preimplantation genetic screening (PGS) and is the most common technique of genetic testing performed at the Bridge,’ says the Centre’s genetics counsellor, Karen Sage. Its objective is to improve pregnancy rates by identifying normal embryos and transferring those to improve the chances of having a healthy baby. ‘In the past 10 years,’ says Karen, ‘more than 700 procedures have been performed, the majority to screen for aneuploidies.’

Explaining aneuploidy

Aneuploidy refers to the presence of an abnormal number of chromosomes in a cell – having 45 or 47 chromosomes rather than the expected 46. An extra or missing chromosome is the common cause of genetic disorders. It happens during cell division when the chromosomes do not separate properly between the two cells.

PGS: An alternative to prenatal testing

The well-established technique for embryo testing established at the Bridge aims to prevent inherited diseases in the children of single (and usually fertile) couples. PGD is an alternative to prenatal testing because it identifies genetic disease in an embryo and not a foetus, and thereby removes any need for pregnancy terminations.

Geneticist Alan Handyside first successfully described PGD more than 25 years ago and he continues to direct the genetics programme at the Bridge. The basic principles of PGD are much the same today as they were in those first pioneering experiments.

‘Due to advances in genetic medicine, we can now readily and easily detect healthy carriers of genetic conditions.’

‘There are many babies born with a range of inherited genetic conditions,’ says Karen, ‘and these can often have a devastating effect on the lives of the child and the family. Detecting these genetic abnormalities before pregnancy is an increasingly precise science. Due to advances in genetic medicine, we can now readily and easily detect healthy carriers of genetic conditions through the provision of preconception screening. The use of PGD can then identify affected embryos to avoid the risk of having an unhealthy child.’

What can PGD identify?

Most of the disorders that PGD can prevent are those associated with abnormalities of a single gene or a single chromosome. ‘The most common, says Karen, ‘is cystic fibrosis, but cases of beta thalassaemia or sickle cell anaemia are also frequent. This testing is also used for rare genetic conditions and presently, the Human Fertilisation and Embryology Authority (HFEA) lists more than 300 single gene defects for which
PGD is approved.’

The risks for these conditions are usually evident in family history, which is why couples hoping to avoid transmission to a baby are often referred by their family doctors or they refer themselves.

Cystic fibrosis, for example, may be evident in a family. A child subsequently born to two carriers of the cystic fibrosis genetic alteration will have a one-in-four chance of having the disease. PGD can test the embryos for evidence of the genetic alteration and ensure that only unaffected embryos are transferred – removing any risk of the baby inheriting the disease.

‘It’s a long and complex procedure,’ says Karen. And it’s one where her skills in counselling are always necessary, not just to explain the assessment of family pedigrees, but to discuss the complications of the procedure and the chances of its success, and provide support throughout the treatment.

The possible downside of PGD analysis is that all embryos tested are abnormal, and none are found suitable for transfer. After the investment of much time, resources and hope, such news can be devastating to couples.

Faster testing with karyomapping

More recently Professor Handyside has introduced a shorter, more comprehensive PGD test known as ‘karyomapping’ which, says Karen, can test for single gene defects and certain chromosome abnormalities (known as translocations) from single DNA samples in a very short time.

While conventional PGD requires knowledge of the specific genetic alteration (known as a mutation) and a specific test for its identification in an embryo, karyomapping can identify a genetic disorder by comparison of DNA ‘sections’ from different family members. Embryos can then be screened for this section of DNA to find out if they have inherited the mutation or not.

‘Karyomapping can vastly reduce the time needed to test for a single gene disorder.’

The screening involves the assessment of many thousands of gene variants known as single nucleotide polymorphisms (or SNPs), which can also identify chromosome rearrangements and the number of chromosomes in an embryo.

‘Karyomapping can vastly reduce the time needed to test for a single gene disorder,’ says Karen. ‘We only need DNA from the parents and one other family member (usually an affected or unaffected child) to develop an accurate karyomap test.’ The expectation is that with one test, an embryo that is free of the genetic mutation and chromosomally normal can be transferred.

If you’d like to find out more about of screening services at the Bridge, please contact us directly and one of our team will be happy to help.

 

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