PRE-IMPLANT GENETIC DIAGNOSIS (PGD) OF MONOGENIC DISEASES

POLAR CELLS ANALYSIS

The pre-implant genetic diagnosis (PGD)

The pre-implant genetic diagnosis (PGD) on polar cells allows for the exclusion of the presence of a serious genetic disease, carried by the mother, in the oocyte that will generate the embryo. This analysis is suggested if a serious genetic disease is present in the family so as to allow the transfer, into the mother's body, only of embryos that do not have this genetic disease. PGD has been offered by large centres since 1992 and has allowed many couples carrying genetic diseases to have healthy children.  

WHY AN ANALYSIS ON POLAR CELLS?

In Switzerland, analyses carried out on embryos are prohibited by law. However, this does not mean that it is not possible to do something for couples with a family history of genetic disease or those who discover, under different circumstances, to be carrying a genetic disease.  The polar cell enables the analysis of genetic materials coming from the maternal line of the family and it is therefore indicated in the following cases: 

  • Autosomal dominant diseases of maternal origin (e.g. Neurofibromatosis)
  • Diseases that are linked to the X chromosome (X-linked) of which the mother is a carrier (for instance Duchenne Dystrophy or Haemophilia)
  • Recessive diseases of which both parents are healthy carriers (e.g. Thalassaemia or Cystic fibrosis) 

However, it is not yet possible to test dominant diseases of paternal origin. 

WHAT IS A POLAR CELL?

The first polar cell is produced during the division of the oocyte and contains half of the chromosomal asset of the oocyte. This cell does not have any function and will not be part of the embryo, it is a so-called “by-product” and can be removed for genetic analysis. This way, the embryo is not touched and/or damaged. After the second division of the oocyte, the second polar cell is expelled and it will also be subject to a genetic analysis. Also in this case, it is a by-product that is not part of the embryo.  

PGD PROCEDURE

The pre-implant genetic diagnosis (PGD) requires that the couple undergoes an assisted reproduction procedure. The purpose is to recover the oocytes, analyse them and transfer only the embryos produced from healthy oocytes. The molecular technique used is very sophisticated and requires an analysis preparation (set up) time for all patients. This is to allow for an optimisation of the analysis which is adjusted to the genome and the genetic mutation specific to the patient. The polar cells are extracted after making a small hole in the zona pellucida, using a laser beam. The genetic material is then analysed using a procedure that allows for minimising the possibilities of an incorrect diagnosis. 

After removing the polar cell the Intra-cytoplasmic sperm injection (ICSI) is performed. A few hours after fertilisation, the second polar cell is expelled from the oocyte. This is extracted and analysed with the same procedure. The analysis of these two cells allows for obtaining a very accurate result about the genetic content of the oocyte and therefore itallows the transfer to the uterus, from the third to the fifth day after insemination, only of those embryos that are not affected by the family genetic disease.The error rate is estimated to be <1%.  

FOR WHOM IS THIS TEST INTENDED?

The test is intended for patients who are: 

  • Carriers of or affected by autosomal dominant diseases
  • Carriers of X-linked diseases
  • Carriers of recessive diseases, if also the partner is a carrier

PREGNANCY PERCENTAGES

The couples who turn to an assisted reproduction centre to be able to access the PGD technique, are normally fertile couples with good probabilities of a successful pregnancy. As always, pregnancy probabilities depend partially on the maternal age and vary from 65% to 35% of the pregnancies carried to term. Pregnancies, considering all maternal ages, occur in around 50% of the cases, on average. 

THE PROCESS TO BE FOLLOWED BY COUPLES INTERESTED IN PGD  

  • You should go to your centre and ask for a visit with a gynaecologist specialised in medically assisted reproduction, and with a geneticist. 
  • The geneticist will advise you about the genetic tests necessary to identify mutations present in the family history (if you do not yet know them). 
  • The geneticist will inform you about the feasibility of a pre-implant test (in some cases, in fact, it may not be possible to do a PGD). 
  • The lab will design and validate a customised testing suitable for your family This set up period can be fairly long (2-3 months). 
  • You will then meet with the medically assisted procreation clinic staff and will start the treatment together with the gynaecologist you have previously met, until several oocytes are obtained.  

After the pick-up, the two polar cells will be extracted at different times (the first a few hours after the pick-up, the second about 12-18 hours after the insemination) and they will be genetically analysed in order to exclude the oocytes with the genetic disease.

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