PGD is the analysis of a single cell removed from an embryo. The purpose of PGD is to identify and select embryos with the greatest potential for the establishment of a healthy pregnancy. Reproductive Science Institute performs all of the IVF services and embryo biopsy involved in PGD. The genetic testing is performed by two collaborating laboratories recognized worldwide as experts in the field of genetic diagnosis. To enable the analysis a single cell from an embryo, a patient first undergoes an IVF cycle to produce multiple eggs which are fertilized in the laboratory. Approximately two days after fertilization, a biopsy of the embryo is performed by an embryologist at RSI. At this early stage of embryo development, the embryo is still undifferentiated. This means that all the cells in the embryo are identical and each has the potential to develop into any type of tissue. The biopsy involves the removal of one or two cells (blastomeres) from the 6-8 cell embryo through microsurgery. Following the biopsy, the embryo is returned to culture for several days while the biopsied cell is analyzed.

FISH (Fluorescent In-Situ Hybridization)

Human cells contain 46 chromosomes: 22 matched pairs of chromosomes plus 2 sex chromosomes, 2 X chromosomes in females, or an X and a Y in males. This normal chromosomal complement is termed “euploid”. Any other chromosome number is considered abnormal or “aneuploid”.

Occasionally an error occurs in chromosome number when the egg, sperm, or embryo is formed. This results in too many or too few copies of a chromosome or chromosomes in an embryo, or aneuploidy. Most aneuploid embryos fail to make a pregnancy or result in a miscarriage. Some however can result in the birth of a child with birth or developmental defects. While there are many errors in chromosome number, the most common occur in chromosomes 13, 16, 18, 21, X and Y.

To analyze the chromosomes in an embryo, the biopsied cells are fixed onto a glass microscope slide. The slides are sent to RMA Genetics where the FISH is performed. RMA stains the nucleus of the cell with fluorescent stains that bond to sites on specific chromosomes. The nucleus can then be analyzed using a fluorescent microscope to determine the number of each chromosome present. Not all 23 pairs of chromosomes are analyzed, only a select panel of at least 5.

Once the FISH analysis is complete, RMA advises the IVF team of the results. Only the embryos which have been identified as “euploid” or normal for the chromosomes analyzed are considered for transfer to the uterus or for cryopreservation.

PCR (Polymerase Chain Reaction)

PCR is performed on the biopsied blastomere(s) to determine the presence of a single gene. This is done when a couple has a significantly increased risk of conceiving a child with a severe genetic disorder. When PCR is to be performed, the blastomere obtained at biopsy is loaded into a tiny tube of medium and sent to Genesis Genetics Institute. Genesis Genetics amplifies the area of DNA of interest and makes many, many copies of the DNA through repeated cycles of DNA strand separation and replication. The sample can then be analyzed for the presence of a specific sequence
of DNA or gene.

Once the PCR analysis is complete, Genesis Genetics advises the IVF team of the results. Only the embryos which do not carry the disease gene(s) are considered for transfer to the uterus or cryopreservation.

When is Embryo Biopsy and PGD an Option?

FISH, or aneuploidy testing, may be appropriate for some patients. Repeated failure to achieve a pregnancy after IVF, repeated pregnancy loss, advanced maternal age, or previous conceptions with chromosomal anomalies may make aneuploidy testing a useful adjunct to your therapy. In addition RSI performs chromosome testing to determine the gender of an embryo under certain circumstances.

PCR is used when a couple has an increased risk of conceiving a child with a genetic disease. When one or both partners carry the gene for a hereditary disease, such as Sickle Cell Disease, Tay Sachs, Cystic Fibrosis, Huntington Disease, and many others, PCR can help the IVF team select the embryos that are disease-free for transfer.