PGD for Chromosome Translocation

What is a chromosome translocation?

It is estimated that 1 in 625 individuals in the general population carry a balanced chromosomal translocation. Other chromosomal translocations may occur more frequently in the general population. Individuals that carry a balanced chromosomal translocation contain all of their genetic information; however, pieces of some chromosomes have switched places. Chromosomes are structures in our cells that carry our genetic information or genes. Normally, we have 46 chromosomes in our cells. The chromosomes come in pairs and are numbered based on their size. The largest chromosome pair is #1 and the smallest is #22. The sex chromosomes are X and Y. Women have two X chromosomes and men have one X and one Y. We inherit half of our chromosomes (23) from each parent in the egg and sperm. Individuals that carry chromosomal translocations are typically healthy; however they are at an increased risk for infertility, miscarriage, stillbirth, and/or having a child with birth defects.

How can PGD help me and my family?

Preimplantation genetic testing for chromosomal translocations is able to distinguish between chromosomally normal and abnormal embryos. Currently, this is the only way to determine whether the embryo is chromosomally normal or “balanced”, prior to pregnancy. PGD can allow the laboratory to select those embryos that are known to be either “balanced” or normal, thus avoiding achieving a pregnancy with an unbalanced set of chromosomes. Many couples in which one member is a translocation carrier have experienced miscarriages or have had to face difficult decisions when learning about a pregnancy with an “unbalanced” result. PGD can reduce the likelihood of couples having to deal with these particular circumstances. We have found that PGD greatly reduces the risks for miscarriage and the chance for the couple to have a child with mental retardation and birth defects related to an unbalanced chromosome translocation.

How do you test for chromosome translocations?

PGD for translocations can be formed by:

• Polar Body Analysis
• Embryo Biopsy

Polar body testing focuses on the maternal contribution, and is an earlier method of testing. Generally, polar body testing is performed only in cases where the female partner of the couple carries a chromosome rearrangement. However, it may be necessary to do confirmatory testing on the embryos (by embryo biopsy) in the case of a maternal chromosome rearrangement. Embryo biopsy and analysis tests both the female and male partner’s genetic contributions, but this testing is performed after the embryo has formed. Therefore, this testing is performed in cases of the male partner having a chromosome rearrangement. In some instances, testing for a female translocation may be limited to embryo testing as well (usually to accommodate the couple).

Once we have the cell to be analyzed (the polar body and/or embryo cell), the cell goes through the process of nuclear conversion. Nuclear conversion is a new technology that allows us to better visualize the chromosomes in the 2nd polar body and in the blastomere cells (embryo cells). This process allows us to distinguish between eggs or embryos that have a balanced translocation, like their parent, from embryos that have an unbalanced chromosomal translocation. Therefore, by nuclear conversion, we can distinguish between chromosomally normal, balanced and unbalanced embryos.

After the cell(s) have gone through nuclear conversion, FISH analysis is performed for the specific chromosomes involved in the translocation. FISH or Fluorescent In-Situ Hybridization is technology that allows the laboratory to determine the presence or absence of particular chromosomes or chromosome segments. Probes labeled with fluorescent signals are used to identify certain chromosome segments and the probes light up, or fluoresce, in the presence of that chromosome region. Different colors are used to identify different chromosomes. The laboratory will require a blood sample from the individual who carries the chromosome rearrangement so that a specific system of probes can be designed for that specific rearrangement. Once a system of probes has been created for the family, the laboratory tests it on the blood sample to assure that the signals are clear and all pieces can be identified in a single cell setting.

What is the accuracy of the testing?

FISH technology is widely available in modern medicine and has a published accuracy rate of 95%. It is important to keep in mind that the testing focuses only on the chromosomes involved in the rearrangement, and it is not possible to check for all chromosome problems at this early stage of embryo development. However, in many cases, we are also able to test for some common aneuploidy conditions, such as Down syndrome. Please ask our genetic counselors if this might be possible, given your circumstances.

What is the cost?

Please ask one of our genetic counselors for our updated pricing information.

Does PGD replace prenatal testing?

No, PGD does not replace prenatal testing. PGD is a research-based test allowing for a similar diagnosis to those available by prenatal testing. However, prenatal testing, such as chorionic villus sampling or amniocentesis, is recommended to confirm the PGD test results, as prenatal diagnosis is the standard-of-care. Our genetic counselors can discuss what prenatal testing options are available to you.

Next Steps:

Please review our PGD information packet and pamphets and contact our
genetic counselors at 773-472-4900 or contact us with any questions or inquiries
regarding our PGD program.

• Translocation Pamphlet
• Translocation Packet