PGD - Providing Couples the Best Chance of Having Healthy Babies

Many couples turn to in vitro fertilization (IVF) when their dreams of becoming parents have not been fulfilled. IVF alone can help a portion of these couples, but for others, preimplantation genetic diagnosis (PGD) may be recommended. PGD is laboratory test performed during an IVF cycle to evaluate one cell of an embryo for specific genetic diseases or chromosome abnormalities. Preimplantation genetic screening (PGS) is a specific type of PGD designed to detect an abnormal number of chromosomes, a condition called aneuploidy. This article will introduce you to a new PGS technology called Parental Support by covering the following topics:

? New PGS Technique - Parental Support
? How Parental Support Differs from FISH Technology
? How Parental Support Differs from CGH Technology
? Limitations to PGS

New PGS Technique - Parental Support
Parental Support uses microarray technology to screen all 24 chromosomes. Humans have a total of 46 chromosomes in their cells; 22 pairs of the numbered chromosomes and one pair of sex chromosomes (XX for females and XY for males.) Having an abnormal number of chromosomes (aneuploidy) causes an embryo to develop improperly or stop its growth. Most embryos with aneuploidy are miscarried before the end of the first trimester. However, there are certain types of aneuploidy that allow a pregnancy to continue and a baby can be born with a chromosome finding such as Down syndrome. By screening all of the chromosome pairs, Parental Support helps doctors identify the healthiest embryos for transfer. Testing is typically performed on Day 3 after fertilization. At this stage of development, embryos have around 8 cells. One cell, called a blastomere, is removed from each embryo and sent for screening. The embryos remain at the IVF center. When the lab receives these blastomeres, it isolates and amplifies the DNA within the cells. These prepared DNA samples are added to the microarray. The microarray contains hundreds of thousands of tiny DNA probes, and each DNA probe binds to a specific chromosome area. The probes are so small that they cannot be seen under a microscope and are instead read by a machine called a scanner.

Once the scanning is complete, the resulting data is analyzed. This data can be "noisy" due to the DNA amplification step. Parental SupportTM overcomes this problem by using genetic data from the parents and a sophisticated bioinformatic algorithm to clean the data. The final results are ready within 24 hours and are a highly accurate representation of the chromosomal make-up of each blastomere screened. This quick turnaround time allows couples to have a fresh transfer on Day 5, so no embryo freezing is required.

How Parental Support Differs from FISH Technology
Until recently, PGS has utilized FISH technology. FISH stands for fluorescent in situ hybridization. This technique involves using DNA probes that have been treated with a fluorescent dye so they light up with different colors for each chromosome tested. A microscope is then used to count the colored dots. One of the main limitations of this technology is the fact that only a few probes can be used simultaneously, resulting in only being able to examine a portion (usually 9 or 12) of the total number of chromosomes. The accuracy of FISH is considered to be approximately 90% for the tested chromosomes; by comparison Parental Support results typically exceed 99% reliability. In addition, 12 or more chromosome pairs remain untested with FISH technology, leading to the possibility of transferring embryos with aneuploidy for one of these untested chromosomes.

How Parental Support Differs from CGH Technology
A technology called Comparative Genomic Hybridization (CGH) also screens all 24 chromosomes. CGH uses control DNA and fluorescent dyes to show imbalances in chromosome number. There are two types of CGH, standard CGH and array CGH, or aCGH. The processing time for standard CGH takes much longer than Parental SupportTM, requiring all embryos being screened to be frozen and transferred during a later cycle. The processing time for aCGH is faster and results can be delivered in time for fresh embryo transfer. Unlike Parental Support, neither CGH method can detect when a whole set of chromosomes is missing or extra, a condition known as haploidy or polyploidy, which can occur in up to 10% of embryos. In addition, there is no published data on the accuracy of CGH or aCGH in single cells.

Couples who may benefit from IVF with Parental Support include the following situations:
? Women aged 35 or greater
? Couples who have experienced multiple early miscarriages
? Couples who have had a fetus or child diagnosed with a chromosome abnormality
? Couples who have had previous unsuccessful IVF cycles

Limitations to PGS
Although Parental Support can be helpful for many couples, it is not right for everyone. There are many factors that affect the chance of having a successful IVF cycle and no test can guarantee the birth of a healthy baby. PGS cannot rule out mosaicism (when the single cell analyzed from the embryo differs chromosomally from the other cells in the embryo,) therefore, prenatal testing in the form of CVS or amniocentesis is recommended.

Parental Support is a proprietary technology developed by Gene Security Network. Talk to your doctor regarding the benefits and limitations of testing.

Jennifer Saucier comes to Gene Security Network, PGD Labs, from Genzyme Genetics where she practiced prenatal and cancer genetic counseling. Previous to her role at Genzyme, she was employed as the sole genetic counselor at Scott and White Memorial Hospital in Temple, TX. In addition to providing genetic counseling services in prenatal, cancer and pediatric clinics at Scott and White, she gained valuable experience as an alternate on the Institutional

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