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Certain illnesses are carried as traits by parents. When both traits are passed on to a child it can be affected. These are referred to as genetic illnesses. Screening for a genetic illness can be done by performing PGD for a single gene mutation. In this situation, the embryos are screened not for a chromosomal problem, but for a single gene abnormality. The tests require the same specimen, an embryonic cell, but are technically different and run in different labs.
PGD screening for single gene defects has been carried out for scores of genetic illnesses including, cystic fibrosis, Huntington’s Chorea, Mafan Syndrome, Tay Sachs and Sickle Cell Anemia.
This test provides no information on the chromosomal complement of the embryo nor does it offer any information on the potential of a birth defect.
Further, it is possible that the sampled cell has a different genetic complement than the rest of the embryo. This is a condition known as mosaicism and it limits the predictive power of the test. This test is conservatively structured so that it reduces the likelihood of transferring an effected embryo. In addition, it is possible that the procedure can damage an embryo or that an embryo may not be assessed owing to technical issues with the test. Finally, it is possible that none of the embryos screened are normal and therefore no embryo transfer performed.
The risks of PGD remain unclear. the main concern is the microsurgical procedure used to remove the sampled cell from the embryo. While animal and human data suggest that this is safe, the data are limited.
Procedure:
On day 3 post retrieval, a small opening is made in the protein coat surrounding the embryo. A biopsy pipette is introduced into the embryo and a single cell is extracted from the embryo. This cell is then transferred into a small tube. The cells are then screened for the gene defect using a process call PCR.
In order to carry out the PCR, the exact nature of the gene defect must be known and small genetic labels complementary to the defect called primers must be custom made. Therefore, in order to perform PGD for a single gene mutation a sample of blood from a carrier or affected individual must be available. Time must then be allowed for the creation of the primers.
In general it takes 24-48 hours to get the results of the test. Most embryo transfers following PGD for single gene mutation screening will take place on post retrieval day 5. On occasion, the results may be available early and the transfer performed on post retrieval day 4.
If a pregnancy results following PGD, we strongly recommend close monitoring for growth via ultrasound (our routine on all IVF pregnancies) and a genetic analysis of the fetus during the second trimester.
PGD does add cost to the cycle and is not covered by most insurances.
