Chromosomes & Translocations
Chromosomes are string-like structures found in the center of the cell, the nucleus. They 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. They labeled largest to smallest 1 through 22. The 23rd pair determines our sex. A female has two 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.
A translocation is a change in chromosome structure in which chromosomes are attached to each other or pieces of different chromosomes have been interchanged. An individual with a translocation is unaffected if there is no extra or missing chromosome material and if the break in the chromosome did not disrupt gene function. If there is no additional or missing chromosome material, the translocation is considered to be balanced. A translocation is unbalanced if there is extra or missing material.
Individuals with balanced translocations typically have no medical issues though some do have fertility concerns, such as reduced fertility. The concern regarding having a balanced translocation is that, though the individual is healthy, the egg or sperm of that individual can have an unbalanced chromosome make-up that leads to the resultant embryo or pregnancy being unbalanced. The presence of an unbalanced translocation can lead to an embryo not implanting, a pregnancy being lost or a child being born with mental and physical problems. Individuals with a translocation may, therefore, experience multiple pregnancy losses or have a child affected with physical and mental problems that may be lethal.
Approximately one in 625 individuals has a reciprocal translocation. These translocations involve any of the chromosomes. Reciprocal defines the translocation as one in which chromosomes have swapped material. Breaks occur anywhere in the chromosomes allowing for pieces to be interchanged between them.
Approximately one in 900 individuals has a Robertsonian translocation. These translocations involve chromosomes 13, 14, 15, 21 or 22. These chromosomes have a unique structure in that they are primarily made of a bottom half. This translocation results from fusion of two of these chromosomes such that the two bottoms are attached.
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 embryologist will decide which procedure to use depending on the type of translocation and other considerations.
Biopsy of Polar Bodies
When the person with the translocation is female, we may be able to analyze the polar body. 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
We analyze blastomeres when the male has the translocation and, in certain cases, when the female has the translocation. 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 study 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. Probes attach to specific areas of the chromosome or can be used to color the whole chromosome. Under a fluorescent microscope, balanced and unbalanced chromosomal make-up can be identified in that cell. The geneticist, therefore, can distinguish normal cells from cells with an unbalanced translocation.
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 PGD
Reduction in the Chance of Having a Child with the Translocation
Our personnel have performed PGD of translocations in over 300 cycles. Normal or balanced embryos were available to be transferred to the patient in almost all of the cycle attempts. Pregnancy occurred in approximately 40% of the cycles with transfer. None of the delivered babies has been found to have an unbalanced translocation.
Reduction in Pregnancy Losses
The PGD procedure significantly reduces the chance of pregnancy loss. The patients who achieved pregnancy after PGD had experienced miscarriage in the majority (~85%) of their previous pregnancies. When these same patients underwent PGD, just fewer than 10% of pregnancies were miscarried. This is a significant reduction in pregnancy loss.
Issues of Preimplantion Genetic Diagnosis – PGD
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 translocation 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 other chromosome conditions for which we do not test, we recommend prenatal testing via chorionic villious sampling or amniocentesis.
For reciprocal translocations, we request blood from the individual with the translocation. This allows for preliminary analyses to be performed with probes to confirm that PGD can be done. The patient must provide us with a deposit that covers the cost of the probes and the preliminary work. Once the blood and deposit are received, probes are ordered and the preliminary work can be performed.
If the individual with the translocation is a male, we recommend FISH analyses of sperm prior to undergoing PGD. Determination of the percentage of unbalanced sperm will allow for estimates of the percentage of embryos that will be unbalanced, and therefore, determination of whether PGD is the best option.
Cost of PGD
Please inquire as to the current fee for PGD. The PGD fees are in addition to the cost of in vitro feritilization (IVF) and embryo transfer. They include the cost of the DNA probes, FISH analysis and the Biopsy procedure. Insurance companies do not cover the cost of PGD.