The future of PGD: Should we be screening all of the chromosomes?

Key Points

The ranks of patients seeking preimplantation genetic diagnosis (PGD) to identify embryos with monogenic disorders like cystic fibrosis or thalassemia are growing rapidly. Even so, the most common indication for preimplantation embryo testing remains the risk of chromosomal imbalance (aneuploidy; i.e., too many or too few chromosomes). In most cases, the PGD strategy employed for chromosomal testing involves biopsying a single cell (blastomere) from each embryo at the 6 to 10-cell stage, 3 days after fertilization. The cell is placed on a microscope slide, fixed, and then subjected to cytogenetic analysis. While the biopsied cell is being analyzed, the rest of the embryo is maintained in culture. Most infertility clinics prefer to transfer the embryos no later than day-5 post fertilization. Consequently, PGD methods need to be extremely rapid, providing a result in less than 48 hours.

Unfortunately, classical cytogenetic techniques (e.g., G-banding) are unreliable at the single-cell level, due to the high frequency of artifacts generated during fixation and spreading. Furthermore, the vast majority of biopsied blastomeres are found to be in the interphase stage of the cell cycle, while traditional cytogenetic methods depend on the presence of metaphase chromosomes. To overcome these difficulties, most chromosomal PGD protocols employ fluorescence in situ hybridization (FISH). This approach involves the hybridization of chromosome-specific DNA probes, labeled with different colors, to nuclei or chromosomes spread on a microscope slide. The method is rapid, performs equally well whether applied to metaphase chromosomes or interphase nuclei, and permits enumeration of up to 10 chromosomes per cell.¹

PGD for aneuploidy

WHAT MOTIVATES NORMAL IVF patients to ask for PGD? Their desire to have PGD to help identify viable embryos, thereby improving the chances of a successful IVF cycle. Aneuploidy is almost always lethal, leading to embryonic arrest, implantation failure, or miscarriage, and yet embryos with a chromosomal imbalance are morphologically indistinguishable from those that are euploid. PGD is the only approach capable of identifying chromosomally normal embryos and avoiding the transfer of embryos carrying lethal genetic flaws.

Several studies have confirmed a statistically significant improvement in the outcome of IVF treatment following screening of embryos with a limited number of FISH probes. In these studies the proportion of embryos that successfully implanted in the uterus was doubled, rates of syndromes such as Down were reduced fourfold, and for patients over age 40, miscarriage rates were cut in half. ^2-5

While most scientists agree that PGD reduces the risk of Down syndrome and miscarriage, the ability of PGD to improve implantation and pregnancy rates is still debated. A recent study aimed at assessing PGD concluded that rather than improving IVF success rates, PGD actually reduced the chances of pregnancy.⁶ How ever, there are serious concerns about the validity of this research. Approximately 20% of the PGD tests undertaken produced no result, a failure rate about four times higher than reported by more experienced laboratories. The inclusion in the study group of relatively young patients (<37 years old) and large numbers of patients with few embryos (<five) is also questionable, as previous publications had already suggested that these patients do not have increased pregnancy rates after PGD.³ Most worryingly of all was the low implantation rate for biopsied embryos that received no diagnosis (only 6%). This suggests a serious problem with biopsy technique, compromising embryo viability and essentially eliminating any possible advantage provided by PGD. The methods used for PGD are sensitive and require extensive experience if their potential is to be realized. Appropriate patient selection is also essential.