A group of scientists at the Centre for Preimplantation Genetic Diagnostics at Guy’s and St Thomas’ NHS Foundation Trust in London has successfully refined a technique which can be used by couples who are at risk of passing on serious genetic disorders to their children. This new technique is called Preimplantation Genetic Haplotyping (PGH) and its use will improve reliability of diagnosis and success rates of the required IVF treatment.
The desire to establish a family and to have healthy children is one of the most fundamental human needs, but it can be overshadowed by an existing severe inherited condition which might be passed on to future generations. The knowledge of a genetic condition running in the family usually arises when one partner has a family member with the condition or when the couple already has an affected child. Since the late nineties Preimplantation Genetic Diagnosis (PGD) has in theory been an option for couples who wish to avoid the termination of an already existing pregnancy. However, the techniques available until now have in practice been limited and enabled clinicians only to diagnose relatively common conditions (like Down syndrome) or those where the fault in the gene is the same for all those affected (like cystic fibrosis).
In the case of Duchenne muscular dystrophy, which is an X-chromosome linked disorder, a comparatively simple PGD sexing test was available making sure that only unaffected female embryos were implanted and hence ruling out the option of a couple conceiving healthy boys. In principle, specific tests for the presence of disorders like Duchenne muscular dystrophy could be designed for each individual family, but this was labour-intensive and extremely time consuming, so in practice was not really available.
Although most couples seeking PGD are fertile, they will require “in vitro” fertilisation (IVF) treatment, a technique also used by couples who cannot naturally conceive children. During this process the woman is given hormones to produce a large number of eggs which are then fertilised outside the body. The embryos are then grown for approximately two days after which time a single cell is taken from each embryo and analysed to distinguish between healthy embryos and those that carry the faulty gene. One or two unaffected embryos are then transferred into the woman’s womb in the hope that this will result in a pregnancy unaffected by the genetic condition.
Recently scientists have developed a new method of PGD called Pre-implantation Genetic Haplotyping (PGH). The arrival of PGH will make testing available for more couples, covering a greater number of conditions. The difference is that the genetic material from the single embryonic cell will be amplified a by million times. DNA fingerprinting will then allow the laboratory to determine which of the chromosomes carries the faulty gene and which one carries the healthy gene. The chromosome with the faulty gene must be identified by comparison with the chromosomes of other members of the family, so some families, in which suitable combinations of affected and unaffected individuals are not available for testing, will not be eligible for this technique. Using this technique the inheritance of the affected gene can be tracked through the family without knowledge of the specific mutation. Couples at risk of an X-chromosome linked disorder, like Duchenne or Becker muscular dystrophy, can use this technique to diagnose healthy boys. Compared to simply sexing embryos, which has often been done previously in these conditions, this means that more embryos can be considered for transfer into the woman’s womb, which in turn will increase the chances of successfully establishing a pregnancy.
The centre at Guys and St. Thomas has been using PGH for three months and has already achieved five pregnancies using this new technology. Using this technique requires testing other family members. It is therefore critically important that families who are interested in this technique make early contact with a Medical Genetics Department, well before they are thinking of getting pregnant, so that all the necessary preparatory investigations can be done in good time.
The following diagram illustrates the technique for Duchenne. It is very important to remember that not all mothers of Duchenne boys are carriers, and this only applies if a mother is known, or thought to be a carrier.
In this diagram a couple has a son with Duchenne. The son has inherited the underlined X-chromosome from his mother, which means the underlined X-chromosome carries the faulty gene.
This couple can potentially produce embryos with the following four combinations of the X and the Y-chromosome:
- This boy has inherited the red underlined chromosome and will have Duchenne
- This boy has inherited the healthy chromosome (yellow) and will be healthy.
- This girl has inherited the red underlined X-chromosome from the mother and the X-chromosome from her father. She will be a carrier of the disease, but will be healthy.
This girl has inherited the healthy chromosome (yellow) from her mother and the X-chromosome from her farther. She will be healthy
The article can be found in Reproductive BioMedicine Online, Volume 13, No 1 July 2006 at: www.rbmonline.com
For more information, please contact the Muscular Dystrophy Campaign Research Deparment at email@example.com or call 020 7803 4800