Pre-Implantation Genetic Diagnosis (PGD)

Pre-implantation genetic diagnosis (PGD) or pre-implantation genetic screening (PGS) aim to eliminate risk for couples who have an increased chance of transmitting hereditary diseases to their offspring, as well as to avoid the significant psychological burden from an induced termination of pregnancy, when a serious issue is diagnosed with the traditional prenatal screening method (during the pregnancy). Additionally, pre-implantation genetic diagnosis helps eliminate multiple unsuccessful IVF efforts and repeated miscarriages.

Indications

Parental Genetic diseases

• When both parents are carriers of a recessive disease
• When one parent is a carrier of a dominant or sex-linked disease

Detection of chromosomal anomalies

• Couples with offspring or pregnancies of embryos with chromosomal disorders
• Women over 36 years of age
• Couples with a history of miscarriages
• Couples with a history of multiple failed IVF attempts

Giving birth to HLA-compatible offspring

• For the treatment of ill children with stem cell transplant

Pre-implantation genetic diagnosis is used for the identification of single gene disorders in embryos, such as:

Single gene disorders

• Cystic fibrosis
• Thalassemia
• Huntington’s Chorea

Sex-linked diseases

• Hemophilia
• Duchenne’s muscular dystrophy
• Sex-linked mental retardation

Obviously, pre-implantation diagnosis can only be performed in the context of IVF. The couple’s embryos are submitted to a biopsy and the cells retrieved from each embryo are submitted to genetic screening. Only normal embryos are implanted in the woman’s uterus.

PGD is a prerequisite for IVF. Before attempting it, the couple should discuss with the geneticist so they can decide which technique is appropriate based on it’s history. Then a blood sample is recieved to determine the type of mutation.

The pair’s embryos are submitted to biopsy on the third day after the egg retrieval, cells that are removed from each embryo is subjected to genetic analysis. Normal embryos that were tested are placed in the uterus on the fifth day after oocyte collection.

PGD Diagnosis Technique

α) Single gene deaseases

The technique being used is PCR(Polymerase Chain Reaction). With it, the DNA is being isolated from the cell after a biopsy replicating it to multiple copies that it’s ammount is detectable

β) X-Sex Related Deseases

Sex selection is being made by FISH technique(see bellow), Only female emmbryos are transferred

Technique Limitations

The PCR technique used in PGD, is very sensitive to contaminations with foreign DNA. This would lead to misdiagnosis. The possibility is of course very small. Also there is an equally slight chance of DNA amplification failure consistently to be wrong diagnosis. Therefore a prenatal diagnosis is recommended after pregnancy to certify the diagnosis made by PGD.

The PGD and PGS does not appeal to all couples. Scientific staff is needed as this is not a routine technique. It is expensive but may relieve couples from the painful, mentally at least, experience of interrupting a pregnancy in case of abnormal fetus with classical prenatal testing.