Slide 1 Welcome to chapter 9. The following chapter is called "Preimplantation Genetic Diagnosis (PGD)". The author is Dr. Maria Lalioti.
Slide 2 The learning objectives of this chapter are: To learn the definitions of Preimplantation Genetic Diagnosis (PGD), To understand the clinical indications for PGD, To get familiar with some of its technical aspects and To know what are the future directions of this technique.
Slide 3 The definition of PGD is the testing of embryos at the cleavage stage (generally on day 3 when the embryos have developed between 4 to 8 cells) to select and transfer only the desired embryo (meaning not affected by the particular conditions that has prompted the testing in the first place). The main aims are Limiting the need for prenatal diagnosis later in pregnancy, for example, amniocentesis Avoiding ethical and practical concerns of abortion and Avoiding or decreasing the risk of spontaneous abortion.
Slide 4 The ESHRE (European Society of Human Reproduction and Embryology) separates preimplantation genetic diagnosis in two major categories (high and low risk) according to indications. Preimplantation genetic diagnosis (PGD) for high risk includes: single gene disorders, for example, thalassemia and cystic fibrosis known chromosomal abnormalities, for example, deletions, translocations and inversions Human Leukocyte Antigen (HLA) typing to achieve a bone marrow match for an affected person Preimplantation genetic screening (PGS) for low risk includes: advanced reproductive age repetitive IVF failure recurrent spontaneous abortion, as well as sex selection or family balancing
Slide 5 In order to test the embryos 1-2 blastomeres from an 8-cell embryo are removed and tested for the desired genotype. This is possible because all cells are identical and totipotent at this stage and the embryo is able, most of the time, to replace the cell removed and continue its growth. An alternative and optional approach is to perform a biopsy of first and second polar bodies. It is assumed that ploidy of embryo follows that of polar bodies.
Slide 6 PGD can only be performed following In vitro Fertilization. Oocytes are retrieved after ovarian stimulation and fertilized with sperm. It is possible to use conventional oocyte insemination-ivf for diagnosing aneuploidies, while it is necessary to perform intracytoplasmic sperm injection (ICSI) for diagnosing gene disorders. Fertilized oocyte/embryo can be kept in the lab for 5 days. Biopsy occurs on day 3, and PGD takes 2 days to be completed. The selected normal embryos can be transferred on day 5.
Slide 7 This slide shows the biopsy of one blastomere from an embryo, in order to perform PGD. The pipet on the left is holding and stabilizing the embryo. The one on the right is removing a single blastomere, after the zona pellucida has been pierced.
Slide 8 Depending on the indication, PGD requires different Molecular biology techniques. Single Gene disorder diagnosis and Human Leukocyte Antigen typing is done using Polymerase Chain Reaction (PCR) and enzyme digestion, sequencing, or microsatellite polymorphism typing. Following biopsy, the blastomere is transferred in a PCR tube, and further processed. Aneuploidy and chromosomal rearrangement screening is done using Fluorescence in situ hybridization (FISH). Following biopsy, the blastomere is fixed on a glass slide.
Slide 9 In order to select an embryo without a gene mutation, the embryo is screened for: the actual gene mutation AND a polymorphic marker located near the gene of interest this is done, because the PCR of the affected gene from a single cell may fail, and it will provide a way to follow the chromosome which was inherited. In addition to gene mutation screening, some labs offer also simultaneous aneuploidy screening, albeit only for a limited number of chromosomes.
Slide 10 This is an example of PGD for Marfan syndrome. Marfan syndrome is an autosomal dominant disease caused by mutations on the FBN1 gene located on chromosome 15. Two polymorphic markers are selected to flank the mutation on the gene. The distance between these 2 markers is 200 kb and the chance of recombination between them is 0.2 %. This means that the chance of a recombination event occurring between the 2 markers is 0.2 every100 meiosis (thus strengthening the accuracy of the diagnosis).
Slide 11 In this case the father, symbolized by the blue square, is a carrier of an FBN1 mutation shown as m. The chromosomes of the parents are shown with a line. Two allelic markers flanking the mutation on the father side are indicated as 1 and 5, while markers 2 and 6 are flanking the normal gene. On the mother side the flanking markers are 3 and 7 and 4 and 8. These two sets of alleles were selected because they were different between the parents. This is important because in the event that a phenomenon called Allele Drop Out (ADO) would occur, these two additional markers would clarify if the mutation is really absent or its absence is secondary to the technical loss of the mutated allele. To better understand this, looking at the second affected embryo, the mutation (m) is not present in the embryo, but because the markers 1 and 5 (from the father) that are flanking the abnormal FNB gene have also been inherited, that means that the reason why we could not detect the m (mutation) is because it was lost (for example, there was an allele drop out). Because the Marfan syndrome is a dominant disorder, we need to choose embryos that have inherited the normal FBN1 allele from the father (indicated as capital N in the slide). The first embryo will be affected because it contains the m (mutated) allele from the father. Embryo 3 has inherited the normal allele from the father and one from the mother, and is likely to be unaffected. Embryo 4 has not inherited the chromosome 15 from the father and would have monosomy 15.
Slide 12 The challenges of single-cell PCR are the following: the quality of the embryo the limited amount of DNA from a single cell requires nested PCR or high number of amplification cycles (more than 45), the use of a robust and high-fidelity polymerase and hot-start PCR. Another difficulty is Allele dropout (ADO), or failed amplification of all or some PCRs The use of linked polymorphic markers provides some backup in case of PCR failure. Since meiotic recombination (cross-over) can occur between the gene of interest and the nearby polymorphic markers, it is better to use 2 markers (instead of only one) flanking the gene of interest.
Slide 13 Fluorescence in situ hybridization (FISH) is used in order to test embryos for chromosomal inbalances. After biopsy, the nucleus of a blastomere is fixed on a glass slide as shown on the next slide. The selection of the fluorescent probe depends on the chromosomes that need to be tested: For chromosomal rearrangements, telomere and centromere probes of the affected chromosomes are used For aneuploidy, the probes commonly used are on chromosomes 13, 15, 16, 17, 18, 21, 22, X, Y (nine chromosomes screening).
Slide 14 This slide shows the steps of the nucleus fixation on a glass slide. The blastomere is placed on the glass and a mix of Methanol-acetic acid is dropped on top of the cell. The cytoplasm dissolves and the DNA sticks to the slide.
Slide 15 PGS for aneuploidy is used mainly in the following indications: Advanced reproductive age Repetitive IVF failure Recurrent spontaneous abortion Sex selection or family balancing According to data collected from spontaneous miscarriages, the percentage of fetal aneuploidy dramatically increases with age, being more than 80% when the maternal age is >35 years and above.
Slide 16 This slide is an example of an embryo with trisomy 22 (three (abnormal) green signals for chromosome 22 and two signals from each of the other chromosomes tested). The top panel shows the FISH results from a blastomere obtained from an embryo on cycle day 3. The bottom panel shows one cell from the blastocyst of the same embryos obtained on cycle day 5. The key message from this slide is that even embryos with aneuploidy can continue their growth in vitro and develop to the blastocyst stage. Therefore, to prevent the transfer of an embryo with aneuploidy it is not sufficient to hope that abnormal embryos will not make it to blastocysts.
Slide 17 This slide shows the cumulative results published by the ESHRE PGD consortium. The main point here is that different PGD-indications have different success rates. For example, 49% of the embryos screened for monogenic disorders are suitable for transfer (free of the disease and with good embryo morphology), while only 24 % are suitable for transfer in the translocation category. The delivery rate per oocyte retrieval is 16%, 19% and 15% for translocations, monogenic and aneuploidy, respectively. The delivery rate per embryo transfer is similar: 25%, 23% and 22%.
Slide 18 Aneuploidy screening for the indications shown above has been proved very inefficient. The only indications where patients can benefit from preimplantation embryo testing are: carriers of balanced chromosomal rearrangements patients with normal karyotype that previously miscarried due to aneuploidy Therefore, knowing the karyotype of a miscarriage is a very important information for the management of the subsequent pregnancy.
Slide 19 The advantages of FISH are that: it is a fairly rapid technique and does not require chromosomes in metaphase The Disadvantages are that: it can screen only for up to 9 chromosomes since there is limited availability of distinct fluorophores cell fixation can lead to loss or inappropriate spreading of the nucleus. The Error rate is 5-10 %, attributed mainly to probe hybridization failure, loss of a signal or split signal (due to DNA replication). In 5 % of cases it is impossible to have a diagnosis due to absent nucleus, or multinucleated cell.
Slide 20 As with any technological advancement, some questions arise. Is it possible that the biopsy can damage the embryos? Can someone create designer babies that have selected characteristics? Are embryos being wasted, by discarding embryos of the non-desired gender? All these questions are still in need of definitive answers.
Slide 21 This slide illustrates the future directions for PGD: Aneuploidy screening will soon be carried out by DNA arrays technology instead of FISH, so to enable whole genome screening with good resolution. DNA arrays will be also uncovering microdeletions or duplications or rearrangements, which cannot be detected with the current 9-chromosome FISH. Combined screening, for example, for both monogenic disorder and aneuploidy at the same time Other technical issues that need to be tackled include: To shorten the time for results (less than the 48 hours currently required). To overcome the limited amount of DNA with new amplification methods that must generate an accurate representation of the DNA from the original cell. Type of arrays: they need to be readily available and cost-effective. Examples include, cdna-arrays cover many genes but no regulatory regions; BAC-arrays; and SNP-arrays.