Pre-implantation Genetic Diagnosis (PGD) is the diagnosis of genetic and chromosomal alterations in embryos before they are implanted, in order to ensure that children are born free of hereditary diseases. This assisted reproduction technique always requires in-vitro fertilisation treatment (IVF) with sperm microinjection (ICSI), so that the embryos are available in the laboratory.
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In 2006, in a world first, IVI achieved the birth of a baby to a couple in which one partner was a carrier of lymphohistiocytosis, thanks to the technique of assisted reproduction with PGD.
PGD allowed the chain to be broken in chromosomal hereditary diseases carried by 72% of the embryos analysed in 2001. As such, thanks to the study of chromosomal abnormalities through the FISH and arrays techniques, around 50% of the embryos transferred resulted in pregnancy. PCR analysis of monogenic diseases led to a 54% pregnancy rate per transfer.
Although the best-known chromosomal disease is Down’s syndrome, which occurs because the embryo has three copies of chromosome 21 instead of one from the father and one from the mother, this is not the most common disease in PGD performed in Spain. The most common diseases in couples who come to IVI are Fragile X syndrome (mental impairment in men), Huntington’s disease (a motor disorder) and Muscular Dystrophy (a severe disorder of the muscles).
For couples who have been referred due to a monogenic disease, molecular diagnosis can identify whether embryos are genetically normal or whether they will be affected by the disease which has prompted the study. For couples for whom a chromosomal study is recommended, cytogenetic molecular diagnosis allows normal or balanced embryos to be identified in terms of the chromosomes which are included in the study. In order to test for numerical chromosome abnormalities two techniques can be used: the FISH technique (fluorescence in situ hybridisation) and the Array CGH technique.
A large number of patients turn to reproductive medicine, whether because they have had a series of miscarriages or because they suspect that they might be carrying a chromosomal problem. All of these couples, and women over the age of 40 who have not managed to get pregnant naturally, are candidates for PGD with array CGH. With this test it is possible to study all 23 pairs of chromosomes, so that they can become pregnant with a healthy baby.
Until just a few years ago, genetic diagnosis of the embryo only allowed 9 of the 23 pairs of chromosomes to be studied. Thanks to array CGH, all 23 pairs of chromosomes can be examined in order to rule out any aneuploidies before implantation takes place. Aneuploidies are abnormalities in the number of chromosomes that can cause repetition faults in assisted reproduction cycles, spontaneous miscarriage and chromosomal anomalies in new-born babies. This technology makes it possible to identify which pre-embryos are healthy and which are not in the laboratory. CGH is recommended for patients who have suffered from repeated miscarriage, couples who risk presenting chromosomal abnormalities in their offspring, and also for female patients over the age of 40 who are planning to become pregnant using their own ova.
Carrying out a FISH study on spermatozoa prior to an assisted reproduction treatment allows the presence of chromosomal anomalies in the spermatozoa to be assessed and the risk of transmission to offspring to be determined. This technique is used with patients who have an increased risk of presenting chromosomal abnormalities, couples who have suffered repeated miscarriages and couples who have not had any success with assisted reproduction due to a paternal anomaly. In these cases, chromosomes 13, 18, 21, X and Y are usually analysed, which if abnormal could lead to miscarriages or to new-born babies with chromosomal diseases.
Fluorescence in situ hybridisation (FISH) consists of marking specific chromosomes in the nucleus of the spermatozoa with fluorescent DNA probes in order to determine whether or not a chromosomal abnormality is present. The FISH test is extremely useful for advising couples who consult a specialist because of an infertility problem.
The purpose of PGD is to analyse pre-embryos in the laboratory following in vitro fertilisation and before they are transferred to the maternal uterus. A biopsy is performed and the pre-embryos are analysed, allowing us to distinguish between the healthy ones and those which are affected. The doctor can therefore transfer only those which will result in a pregnancy with a completely healthy baby.
The technique of assisted reproduction with PGD is the result of combining in vitro fertilisation with a biopsy of pre-embryo cells by means of micromanipulation and cytogenetic molecular diagnosis techniques.
IVI has a dedicated PGD laboratory in which each case is studied on an individualised basis. Our high success rates, personalised treatment and the high qualification levels of the biologists and embryologists who work in our IVI laboratories have caused the group to become a benchmark in Spain for this technique. This is the case to such an extent that other Spanish health centres get IVI to carry out these types of analyses for them.
Every single cell in the human body has in its nucleus 46 chromosomes (23 from the father and 23 from the mother).
These chromosomes are formed of a substance called DNA which contains our genetic information. This information is distributed over thousands of tiny fragments known as genes. As such, there are two copies of every gene, one of which has come from the mother and the other from the father.
What abnormalities of the chromosomes and genes can cause diseases?