Preimplantation Genetic Diagnosis (PGD) for Aneuploidy Chromosome Aneuploidy
Chromosomes are string-like structures found in the center of the cell, the nucleus. Chromosomes contain genes that are made of DNA. Therefore, our inherited information is housed on the chromosomes. Normal human cells (embryo, fetus, baby or adult) contain 46 chromosomes or 23 pairs. We receive 23 chromosomes from each parent.
The first 22 pairs of chromosomes are the same for men and women and labeled largest to smallest 1 through 22. The 23rd pair determines our sex. A female has 2 X chromosomes whereas a male has an X and a Y. As such, the woman can only pass an X to her child in her egg. The man passes either the X or the Y in the sperm therefore determining the sex of the child. If an error occurs leading to the egg or sperm having an extra or missing chromosome, the embryo created by that egg or sperm would have an extra or missing chromosome. This situation is called aneuploidy.
Having an extra chromosome is known as trisomy (tri = three of the chromosome) and having a chromosome missing is known as monosomy (mono = one of the chromosome). If the aneuploidy involves the larger chromosomes, the embryo may not attach to the wall of the uterus or may stop developing soon after attaching and miscarry. However, if the aneuploidy involves chromosomes such as the 13, 18, 21, X or Y, the pregnancy may still carry on until birth, even though the pregnancy has a chromosomal disorder. The most common of these is an extra number 21, known as Down syndrome or trisomy 21 (three 21 chromosomes). Other common aneuploidies are Klinefelter syndrome (XXY), trisomy 13 and trisomy 18. The features of the chromosome condition depend upon which chromosome is extra or missing, but can include physical differences and mental retardation.
Risk of Aneuploidy and Maternal Age
As a woman advances in age, the chance of aneuploidy in her pregnancies increases. This association is because a womans eggs are as old as she. Females have all of their eggs in the fetal stage therefore they are born with all the eggs they will have in their lifetime. In males, sperm is made every 65-75 days therefore the sperm is not as old as the man.
Therefore, the theory regarding aneuploidy risk and advancing maternal age is that over time the chromosomes in the egg are less likely to divide properly leading to the egg having an extra or missing chromosome. The risk of aneuploidy increases with maternal age. The chances to deliver an affected child are 1/385 at 30, 1/179 at 35, 1/63 at 40 and 1/19 at the age of 45.
However, the frequency of aneuploidy in embryos is much higher than what would be expected looking only at affected live borns. More than 20% of embryos from women in the age range from 35 to 39 are affected. Almost 40% of embryos from women 40 or older are affected. This difference in percentages in embryos versus live borns is due to the fact that a pregnancy with aneuploidy is less likely to attach to the uterus or go to term. Most will be miscarried. As such, the percentage of affected pregnancies is reduced over the course of the pregnancy due to the affected pregnancies that are lost.
Any embryo with a missing chromosome (monosomy) will cease to grow before implantation (except monosomy X and 21), and only few of those carrying an extra chromosome (trisomy) will go to term. The lack of implantation and loss rate of aneuploid embryos are believed to be the main reasons why the pregnancy rate in women over 40 is so low. The purpose of preimplantation genetic diagnosis for aneuploidy therefore is to select for transfer only chromosomally normal embryos so as to achieve more pregnancies, reduce the number of pregnancy losses, and reduce the number of affected offspring.
PGD – The Procedure
To analyze an egg or embryo, we first have to biopsy it. Two procedures allow this to be done. The PGD team of doctors, geneticists and embryologists will decide which procedure to use.
Biopsy of Polar Bodies
The ripening egg produces two small cells called polar bodies that degenerate after fertilization. The chromosomal content of these cells allows us to infer the chromosomal content of the egg. If one is testing the polar body, an opening is made in the covering of the egg and the polar body is removed with a pipette. The polar body is then analyzed while the egg is placed in an incubator. By analyzing polar bodies, we obtain information from only the mother. Chromosome abnormalities that may occur after fertilization, when the sperm meets the egg, will not be detected.
Biopsy of Blastomeres
A blastomere is a cell from an embryo. To test the blastomere, an opening is made in the covering of the embryo during its third day of development when the embryo has 8-10 cells. A blastomere is removed via aspiration with a pipette. The embryo is placed in an incubator while the cell is analyzed.
The biopsied cells are analyzed using a technique called fluorescence_in-situ_hybridization or FISH. This technique uses probes, small pieces of DNA that are a match for the chromosomes we want to analyze, to count the chromosomes present. Each probe is labeled with a different fluorescent dye. These fluorescent probes are applied to the biopsied cell and attach to the chromosomes. Under a fluorescent microscope, we then count the number of chromosomes of each type (color) there are in that cell. The geneticist therefore can distinguish normal cells from cells with aneuploidy.
Testing of the cells destroys them because they must be glued to a glass slide and repeatedly heated and cooled. As such, one cannot use them for another purpose or return them to the embryo. The slides are kept for future reference. This analysis causes no extra inconvenience to the patient as it is accomplished in one day.
Advantages of Preimplantation Genetic Diagnosis
Increased Implantation Rate
It is well known that the pregnancy rate after in-vitro fertilization decreases dramatically with maternal age. Even in IVF centers with the highest pregnancy rates, there is a decrease from greater than 30% per embryo transferred in women 20-33 years old to less than 10% in women over 39.
Aneuploid embryos have much lower survival rates than normal embryos, and half of them (the ones missing a chromosome) seldom implant. It appears likely that the decrease in pregnancy rates with maternal age is mostly caused by a corresponding increase in aneuploid embryos.
By performing PGD for aneuploidy and transferring only chromosomally normal embryos, we may be able to increase the pregnancy rates noticeably. In two studies, we have demonstrated an increase in implantation rates after PGD (Munné et al. 1999, Gianaroli et al. 1999). We have also found that the implantation rate doubled from 12% in controls to 24% in PGD patients when we analyzed 8 chromosomes.
Reduction in Pregnancy Losses
In women 35 and older, approximately 35% of pregnancies are miscarried. Aneuploidy is the cause in 50% or more of these losses. By transferring only chromosomally normal embryos, the number of pregnancies going to term should increase. In one of our recently published studies, we detected a significant reduction in pregnancy losses after PGD in women over 40 years old, from 41% to 22%. The increase in implantation rate and the significant decrease in pregnancy loss rate resulted in a significant increase in ongoing pregnancies and delivered babies.
We have also studied the benefit of PGD for patients who have experienced recurrent pregnancy loss. We recently reported that the loss rate was reduced from an expected 36% to 12% in this difficult-to treat class of patients.
Issues of Preimplantation Genetic Diagnosis
Removal of Cells from the Embryo
The risk of accidental damage to an embryo during removal of the cell(s) is very low -0.6%. No part of the future fetus will be lacking because one or two cells are removed from the embryo approximately two days after fertilization. All the cells of the embryo remain totipotent until about the fourth day. Totipotent means having “all potential”. These cells have not differentiated yet meaning that each cell by itself can grow into a whole and perfect fetus. The procedure merely delays continued cell division for a few hours, after which the embryo reaches the same number of cells as before and continues its normal development.
Normal development has been seen many times in humans and other mammals after cell loss due to embryo freezing. One or more cells may fail to survive thawing, yet the embryos from such develop into normal offspring. An unanswered question is whether biopsied embryos implant less than untouched ones. Data regarding such is incomplete. Embryo biopsy may lower implantation rates slightly while selection of chromosomally normal embryos via PGD may increase it. The balance between potential biopsy damage and beneficial effects of PGD seems to be positive.
The accuracy of PGD for aneuploidy is approximately 95%. This means that the error rate is 5%. Within this chance of misdiagnosis, there is a false negative rate, a false positive rate, the chance for no result and the chance for mosaicism.
A mosaicism is defined as the embryo having cells with different chromosome make-up. Typically, all cells of the embryo have the same chromosomal make-up as they originate from the same fertilized egg. However, it is possible for cells of the same embryo to have differing numbers of chromosomes.
If we analyze a cell that has normal chromosomal content, but another cell has an extra chromosome, we erroneously diagnosed that embryo as being chromosomally normal. Due to the chance of misdiagnosis as well as the presence of anueploidy for which we do not test, we recommend prenatal testing as stated earlier.
Cost of the Procedure
Please inquire as to the current fees for PGD. The PGD fees are in addition to the cost of in-vitro fertilization (IVF) and embryo transfer. The PGD fees include the cost of the DNA probes, FISH analysis and the biopsy procedure. Insurance companies typically do not cover the cost of PGD; therefore, it is usually an out of pocket expense.