Genetic diagnosis before implantationBMJ 1997; 315 doi: https://doi.org/10.1136/bmj.315.7112.828 (Published 04 October 1997) Cite this as: BMJ 1997;315:828
Applications of the technique are growing
- Joy D A Delhanty, Professor of human geneticsa,
- Dagan Wells, Research fellowa,
- Joyce C Harper, Lecturer in human genetics and embryologyb
- a Galton Laboratory, University College London, London NW1 2HE
- b Department of Obstetrics and Gynaecology, University College London Medical School, London WC1E 6HX
Research on the feasibility of preimplantation genetic diagnosis began in the 1980s as a result of pressure from patients. The couples concerned had experienced repeated termination of pregnancy, had moral objections to abortion, or were at risk of transmitting an X linked disorder for which the only available option was termination of all male pregnancies (of which half would be unaffected). These couples wanted to start a pregnancy with reasonable certainty that their child would be free of the familial inherited disorder. The first couples were treated in 1990 at Hammersmith Hospital and 55 couples have now been treated in Britain. This slow rate of application is about to change with the recent granting of two further treatment licences to University College London and Guy's/St Thomas's Hospital. Over the next year this will open up the opportunity for treatment to a far greater number of patients. So what can we offer in the way of genetic testing before implantation?
Early research showed that it was possible at three days after fertilisation to remove one or two cells from an 8–10 celled embryo without detriment to its further development.1 Embryos were sexed on the basis of the presence or absence of a DNA fragment specific for the Y chromosome; in 1990 two sets of twin girls were born to five couples at risk of passing on an X linked disorder.2 Alarmed at the rapid progress of embryo research, the British government permitted research only up to 14 days after in vitro fertilisation. Centres performing research (or diagnosis) on preimplantation embryos had to be licensed, and until recently the Hammersmith Hospital was the only licensed centre in Britain.
Sexing the embryo to avoid X linked disease remains the commonest reason for preimplantation diagnosis, now optimally carried out by the molecular cytogenetic technique of FISH (fluorescent in situ hybridisation) with DNA probes derived from the X and Y chromosomes.3 4 This approach allows chromosome copy number to be determined and avoids the transfer of embryos with a single X chromosome (potential Turner syndrome and at high risk of the X linked disorder). Of the autosomal recessive disorders, cystic fibrosis was the first to be successfully managed by preimplantation diagnosis5 and remains the most common application worldwide.6 Success has also been achieved with Tay Sachs disease, Rh D blood typing, and X linked disorders such as Duchenne muscular dystrophy and Lesch Nyhan syndrome. Worldwide, almost 100 babies have been born after preimplantation genetic diagnosis, with no reported increase in congenital anomalies.
Protocols have been developed for the various mutations causing ß thalassemia and for sickle cell disease, and both the new centres will offer treatment for these disorders. In principle, providing the molecular basis of a disorder is known, mutation detection is possible at the single cell level which can then be applied to the one or two blastomeres available from the cleavage stage embryos—for example, chronic granulomatous disease (currently under research at University College).
The growing number of diseases caused by the expansion of triplet repeats in the DNA within or close to the gene, such as fragile X syndrome and Huntington's disease, are best diagnosed embryologically by making use of closely linked genetic markers. Technical problems involving the necessary amplification of DNA from single cells also make the preimplantation diagnosis of dominant disorders difficult. Nevertheless, diagnosis has been achieved for Marfan's syndrome7 and recently for the inherited cancer syndrome familial adenomatous polyposis coli.8 Diagnosis before implantation seems particularly appropriate for the inherited cancer syndromes. Despite the fact that the molecular basis has been understood for several years, families at risk of familial adenomatous polyposis coli have shown little interest in prenatal diagnosis. Couples who are reluctant to terminate an established and otherwise normal pregnancy in the case of a late onset disorder are, however, showing an interest in preimplantation diagnosis as a means of reducing the risk of passing on the disease.
Rather unexpectedly, one of the commonest reasons for requesting preimplantation diagnosis is because one partner is at high risk of transmitting a chromosome anomaly. This is usually due to chromosomal translocation but can be caused by gonadal mosaicism for trisomy 21 for example. Such couples have suffered repeated spontaneous or induced abortions and often also periods of infertility requiring assisted conception.9 The fact that routine prenatal diagnosis has not been effective in helping these couples achieve a normal pregnancy suggests that particular adverse factors are operating, and research on the untransferred embryos after preimplantation diagnosis is beginning to reveal the nature of these factors.9
In fact, it is the application of the FISH technique to spare, untransferred embryos after in vitro fertilisation cycles that has led to the most interesting finding: that 30% of normally developing cleavage stage embryos are chromosomally mosaic.10 11 This may provide one explanation for the low success rate of in vitro fertilisation, the poor fecundity of humans, and the origin of confined placental mosaicism that plagues chromosomal prenatal diagnosis by chorionic villus sampling.