PGDlab : For Physicians : What is PGD : Q & A

Non-Disclosing PGD for Huntington disease (HD) confirms that the embryos transferred to achieve a pregnancy do not have a gene change for HD, but, at their request, the disease status of the parents is not disclosed to them.

HD is caused by a genetic alteration in the HD gene on chromosome 4. What are chromosomes and genes? This alteration involves expansion of a region of the HD gene. Normally, individuals have 9 to 30 copies or repeats of a CAG DNA sequence. An individual with over 40 repeats in one of the pair of HD genes causes the production of an abnormal protein which aggregates and leads to cell death, causing HD.

In a PGD cycle, embryos created by IVF are cultured in the laboratory for 3 days, at which time they contain approximately 8 cells (The embryo at this point is called a blastomere). Embryos with normal development on Day 3 are biopsied using micromanipulation techniques. This requires the use of very fine glass needles and tools under microscopic observation and control to remove 1or 2 cells from each blastomere for analysis.

At this early stage of embryological development, the blastomeres are totipotent; they are still capable of forming cells of any organ or tissue. Removal of a few of the cells of the early embryo does not alter the ability of that embryo to develop into a complete, normal pregnancy and child. Data from many years of PGD in animals and approximately 1200 live births in humans indicate that PGD does not lead to an increase in birth defects or chromosomal disorders.

When a PGD biopsy is done, 1-2 cells are removed from the embryo. In order to obtain results of the biopsy, the cell removed must contain a nucleus, as the nucleus contains the genetic information necessary for testing. If the cell removed has no nucleus or if the nucleus ruptures during the extraction or fixation process, that cell cannot be tested. The cells are tested to determine if the embryo inherited the HD gene alteration. Prior to initiation of an IVF cycle, the CAG copy number is identified in each pair of the couples’ HD genes. If a member of the couple has HD, it is necessary to perform PGD for HD. To confirm the status of each embryo, each HD gene must have a different copy number to be able to differentiate between each HD gene. Sometimes a couple may share the same copy number in one of their HD genes, or the copy numbers may be very close and the test may not be able to differentiate the differences. Therefore, an additional test called linkage analysis may be necessary. What is linkage analysis?

If all HD genes have less than 30 copies of the CAG repeat, neither member of the couple will develop HD. It is, therefore, not necessary for this couple to do PGD for HD, however, because the HD program is Non-Disclosing, the couple will not be informed of their test results. Based on the couples’ status, Genetics & IVF will determine for the couple if PGD for HD will be performed prior to IVF and embryo transfer. To maintain strict confidentiality, couples are not informed of their HD status, whether or not PGD for HD was performed, or number of embryos transferred.

For more information about Genetics & IVF Institute's PGD for Hunting Disease,
please call (800) 654-4363 or (703) 698-7355
and ask to speak with the PGD counselor OR email PGD@givf.com.

Please click on the another question for more information.

• Is your patient a woman over 38?
• Has your patient experienced several miscarriages?
• Has your patient had a prior pregnancy with a chromosome abnormality?
• Has your patient experienced several failed IVF cycles?
• Has conceiving a pregnancy been difficult for your patient due to a low sperm count?
• Does your patient or patient's partner carry a balanced structural chromosome rearrangement?
• Does your patient have a family history of Huntington disease?
• Does your patient or patient's partner carry a recessive genetic disease such as cystic fibrosis?
• Does your patient or patient's partner carry an X-linked genetic disease such as hemophilia or Duchenne Muscular Dystrophy?
• Does your patient want to balance the gender in your family?

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