Preimplantation Genetic Diagnosis

Updated: Dec 30, 2015
  • Author: Molina B Dayal, MD, MPH; Chief Editor: Richard Scott Lucidi, MD, FACOG  more...
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Overview

Overview

Preimplantation genetic testing is a technique used to identify genetic defects in embryos created through in vitro fertilization (IVF) before pregnancy. Preimplantation genetic diagnosis (PGD) refers specifically to when one or both genetic parents has a known genetic abnormality and testing is performed on an embryo to determine if it also carries a genetic abnormality. In contrast, preimplantation genetic screening (PGS) refers to techniques where embryos from presumed chromosomally normal genetic parents are screened for aneuploidy.

Because only unaffected embryos are transferred to the uterus for implantation, preimplantation genetic testing provides an alternative to current postconception diagnostic procedures (ie, amniocentesis or chorionic villus sampling), which are frequently followed by the difficult decision of pregnancy termination if results are unfavorable. PGD and PGS are presently the only options available for avoiding a high risk of having a child affected with a genetic disease prior to implantation. It is an attractive means of preventing heritable genetic disease, thereby eliminating the dilemma of pregnancy termination following unfavorable prenatal diagnosis.

History

Edwards and Gardner successfully performed the first known embryo biopsy on rabbit embryos in 1968. In humans, PGD was developed in the United Kingdom in the mid 1980s as an alternative to current prenatal diagnoses. [1] Initially, PGD revolved around determination of gender as an indirect means of avoiding an X-linked disorder. In 1989 in London, Handyside and colleagues reported the first unaffected child born following PGD performed for an X-linked disorder.

As of 2006, more than 15,000 PGD cycles have been reported. [2] PGD is currently available for most known genetic mutations. [3] Although the indications for PGD are well established, PGS is a relatively new, evolving technique and remains controversial.

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Indications and Conditions

Indications for Preimplantation Genetic Diagnosis

Preimplantation genetic diagnosis (PGD) is recommended when couples are at risk of transmitting a known genetic abnormality to their children. Only healthy and normal embryos are transferred into the mother's uterus, thus diminishing the risk of inheriting a genetic abnormality and late pregnancy termination (after positive prenatal diagnosis).

Primary candidates for PGD

These include the following:

  • Couples with a family history of X-linked disorders (Couples with a family history of X-linked disease have a 25% risk of having an affected embryo [half of male embryos].)
  • Couples with chromosome translocations, which can cause implantation failure, recurrent pregnancy loss, or mental or physical problems in offspring [4]
  • Carriers of autosomal recessive diseases (For carriers of autosomal recessive diseases, the risk an embryo may be affected is 25%.)
  • Carriers of autosomal dominant diseases (For carriers of autosomal dominant disease, the risk an embryo may be affected is 50%.)

Conditions diagnosed using PGD

PGD should be offered for 3 major groups of disease: (1) sex-linked disorders, (2) single gene defects, and (3) chromosomal disorders.

Sex-linked disorders

X-linked diseases are passed to the child through a mother who is a carrier. They are passed by an abnormal X chromosome and manifest in sons, who do not inherit the normal X chromosome from the father. Because the X chromosome is transmitted to offspring/embryos through the mother, affected fathers have sons who are not affected, but their daughters have a 50% risk of being carriers if the mother is healthy. Sex-linked recessive disorders include hemophilia, fragile X syndrome, most neuromuscular dystrophies (currently, >900 neuromuscular dystrophies are known), and hundreds of other diseases. Sex-linked dominant disorders include Rett syndrome, incontinentia pigmenti, pseudohyperparathyroidism, and vitamin D–resistant rickets.

Single gene defects

PGD is used to identify single gene defects such as cystic fibrosis, Tay-Sachs disease, sickle cell anemia, and Huntington disease. In such diseases, the abnormality is detectable with molecular techniques using polymerase chain reaction (PCR) amplification of DNA from a single cell. Although progress has been made, some single gene defects, such as cystic fibrosis, have multiple known mutations. In cystic fibrosis, only 25 mutations are currently routinely tested. Because most of these rare mutations are not routinely tested, a parent without any clinical manifestations of cystic fibrosis could still be a carrier. This allows the possibility for a parent carrying a rare mutation gene to be tested as negative but still have the ability to pass on the mutant cystic fibrosis gene.

PGD can also be used to identify genetic mutations like BRCA -1, which does not cause a specific disease but increases the risk of a set of diseases.

Chromosomal disorders

The last group includes chromosomal disorders in which a variety of chromosomal rearrangements, including translocations, inversions, and deletions, can be detected using fluorescent in situ hybridization (FISH). FISH uses telomeric probes specific to the loci site of interest. Some parents may have never achieved a viable pregnancy without using PGD because previous conceptions resulted in chromosomally unbalanced embryos and were spontaneously miscarried.

Indications for Preimplantation Genetic Screening

Most early pregnancy losses can be attributed to aneuploidy. Because only chromosomally normal embryos are transferred into the uterus, the risk of first and second trimester loss is markedly reduced. At present, no specific list of indications for preimplantation genetic screening (PGS) is available.

Primary candidates for PGS can include the following:

  • Women of advanced maternal age
  • Couples with history of recurrent pregnancy loss
  • Couples with repeated IVF failure
  • Male partner with severe male factor infertility

These patient populations are at risk of failure with IVF because of a high proportion of aneuploid embryos. PGD is believed to decrease this risk by selecting chromosomally normal embryos that have a higher chance of implantation.

Advanced maternal age

The risk of aneuploidy in children increases as women age. The chromosomes in the egg are less likely to divide properly, leading to an extra or missing chromosome in the embryo (see Table 1). The rate of aneuploidy in embryos is greater than 20% in mothers aged 35-39 years and is nearly 40% in mothers aged 40 years or older. The rate of aneuploidy in children is 0.6-1.4% in mothers aged 35-39 years and is 1.6-10% in mothers older than 40 years. The difference in percentages between affected embryos and live births is due to the fact that an embryo with aneuploidy is less likely to be carried to term and will most likely be miscarried, some even before pregnancy is suspected or confirmed. Therefore, using PGD to determine the chromosomal constitution of embryos increases the chance of a healthy pregnancy and reduces the number of pregnancy losses and affected offspring. [5]

One of the most frequent aneuploidies, trisomy (ie, 3 identical chromosomes present in the embryo), is trisomy of chromosome 21, which leads to Down syndrome. This particular abnormality also frequently leads to spontaneous miscarriage, the precise frequency of which is difficult to determine. Thus, the only reliable information is on the frequency of babies born with Down syndrome. An informative article in the Journal of the American Medical Association [6] includes information on estimating the incidence of trisomy 21/Down syndrome in fetuses at 16 weeks of pregnancy (also see Table 2).

Table 1. Chromosomal Abnormalities (Open Table in a new window)

Age, y Embryos (Normal), % Embryos (Aneuploidy), % Other Abnormality, %
25-35 61 8 31
36-37 60 10 30
38-39 47 18 35
40-41 43 26 31
42-44 39 30 31

 

Table 2. Frequency of Down Syndrome Per Maternal Age (Open Table in a new window)

Age, y Frequency of Fetuses With Down Syndrome to



Normal Fetuses at 16 Weeks of Pregnancy



Frequency of Live Births of Babies



With Down Syndrome to Normal Births



15-19 . . . 1/1250
20-24 . . . 1/1400
25-29 . . . 1/1100
30-31 . . . 1/900
32 . . . 1/750
33 1/420 1/625
34 1/325 1/500
35 1/250 1/350
36 1/200 1/275
37 1/150 1/225
38 1/120 1/175
39 1/100 1/140
40 1/75 1/100
41 1/60 1/85
42 1/45 1/65
42 1/35 1/50
44 1/30 1/40
45 and older 1/20 1/25

 

Early studies of PGS in patients with advanced maternal age seemed promising, with a decreased rate of miscarriages and a higher proportion of live births in the PGS group compared with the control group. Although data indicate an increased incidence of aneuploidy in older patients, several randomized controlled trials showed that routine PGS/PGD did not increase pregnancy rates after IVF in patients with advanced maternal age. [7, 8] Although PGS does not appear to statistically improve the chance of pregnancy in women with advanced maternal age, one study detected a trend towards decreasing the risk of miscarriage, thereby increasing the chance of a live birth. [9] Therefore, PGS should be selectively offered because it has outcome benefits in only some patients.

Recurrent pregnancy loss

Recurrent pregnancy loss (RPL) is usually defined as 2 or more consecutive pregnancy losses before 20 weeks' gestation. The cause is frequently unknown but may be secondary to fetal anomalies or uterine abnormalities. Chromosomal abnormalities are noted in 50-80% of abortuses, [10] and couples with RPL have a higher percentage of aneuploid embryos than patients without RPL. [11] However, the use of PGS does not improve the pregnancy rate in this group of patients but increases the likelihood of delivery at term. [12] PGS also benefits the subgroup of patients who have proven abnormal concepti by cytogenetic analysis. [12]

Recurrent IVF failure

Recurrent IVF failure (RIF) is usually defined as 3 or more failed IVF attempts involving high-quality embryos. Evidence suggests that this patient population has a higher number of chromosomally abnormal embryos. [13] However, no study has shown an improvement in pregnancy rate with PGS in patients who have a history of RIF. Although most IVF failures can be accounted for by embryonic aneuploidy, various immunological and uterine factors likely contribute to implantation failure.

Male factor infertility

Gonadal failure in men has been linked to the generation of embryos with an increased incidence of inherited and de novo chromosomal abnormalities. Normal fertile men have approximately 3-8% of sperm that are chromosomally abnormal. This risk increases significantly in men with severe infertility (ie, low sperm count, poor morphology, and poor motility) to approximately 27-74% abnormal spermatozoa. [14] With the introduction of intracytoplasmic sperm injection (ICSI) in assisted reproductive techniques, clinicians have given men with poor sperm quality the opportunity to overcome natural selection and successfully produce a zygote.

Various genetic defects have been found to be associated with male factor infertility. This includes aneuploidy, most commonly Klinefelter syndrome, Robertsonian translocations, Y chromosome microdeletions, androgen receptor mutations, and other autosomal gene mutations (eg, cystic fibrosis transmembrane conductance regulator gene and sex hormone-binding globulin gene mutations). [15] Therefore, a high risk of transmission of genetic mutations to the patient's offspring is associated with IVF involving ICSI.

The use of PGS/PGD in couples with severe male factor infertility may decrease pregnancy rates but also limits the prevalence of chromosome abnormalities. Data are insufficient to recommend routine genetic screening of all embryos in this group of patients. However, if couples do choose to proceed with PGS/PGD, they should be properly counseled on the decreased chance of conception due to the likely reduced number of normal embryos available to transfer. [16]

Sex selection

Because PGS can determine the sex of the embryo, many couples request PGS for sex selection, which can be motivated by cultural, social, ethnic, psychological, and other reasons, such as the desire for family balancing.

The use of PGS for sex selection unrelated to disease is controversial and has elicited moral outrage about not implanting normal embryos when they are found to be of the undesired sex. Frequent objections include the danger of sex discrimination, the perpetuation of oppression against females, the ethics of expanding control over nonessential characteristics (those not required for life) of offspring, and the relative importance of sex selection when weighed against medical and financial burdens to parents. Personal, religious, ethical, and moral norms vary among different populations, and proper respect must be given to these views when discussing the performance of PGS for sex selection. Much discussion is still necessary to achieve a reasonable consensus and acceptance of PGS for sex selection.

Human leukocyte antigen (HLA) matching

Among the new indications of PGD is preimplantation HLA matching. This technique can be applied to exclude the presence of a genetic disorder but also provide a potential donor for stem cell or bone marrow transplantation to an affected child with recessive diseases, including thalassemias or acquired malignancies such as leukemia. This has been previously used to avoid the birth of a child with Fanconi anemia, an autosomal recessive disorder, whose HLA-matched cord blood stem cells were successfully transplanted to cure the affected sibling. [17]

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Process

Approach Considerations

Before requesting preimplantation genetic diagnosis (PGD), candidates should consult a geneticist or a genetic counselor to evaluate the risk of transferring their genetic abnormality to their offspring. Tests should be performed to confirm the diagnosis of the affected parent, to pinpoint the genetic change leading to the condition in question, and to ensure that the currently available technology can identify that genetic change in a polar body, cleavage state, or blastocyst embryo biopsy.

In order to have embryos to biopsy for PGD/PGS, patients must undergo in vitro fertilization (IVF). After fertilization of the egg with sperm, embryos are allowed to develop into cleavage-stage embryos. On day 3 after egg retrieval, a single blastomere is removed from the developing embryo for genetic evaluation of the embryo. Genetic evaluation is performed using PCR, FISH, or comparative genomic hybridization (CGH). Nonaffected or normal embryos are then transferred into the uterus for subsequent implantation/pregnancy.

In Vitro Fertilization

The IVF procedure consists of ovarian stimulation, egg retrieval, egg fertilization, embryo development, and embryo transfer (see the image below).

General process of steps required for preimplantat General process of steps required for preimplantation genetic diagnosis and preimplantation genetic screening.

The steps can be summarized as follows:

  1. Ovarian stimulation is needed in order to produce multiple eggs. During the 8- to 14-day hormonal stimulation period, frequent ultrasonographic examinations and laboratory tests are performed to monitor the development and maturation of follicles (egg-containing ovarian cysts).
  2. The oocyte retrieval procedure is typically performed under anesthesia. Under sonographic guidance and a transvaginal approach, follicles are punctured and their follicular fluid aspirated. Oocytes are then identified in the embryology laboratory by embryologists. The procedure usually lasts less than 15 minutes.
  3. The eggs are then cultured for a few hours after their retrieval to allow for final maturation to occur. If desired, a polar body can then be removed for PGD/PGS. For the PGD/PGS procedure at a later stage of embryonic development, intracytoplasmic sperm injection (ICSI), where a single sperm is injected into a single egg, is preferred. In this manner, ICSI prevents the chance of polyspermy and the accidental acquisition of “extra” chromosomal material from the sperm, which can then impact the results of the PGD/PGS (ie, give false positive results).
  4. Sperm for purposes of egg fertilization are typically obtained from the male partner by masturbation on the day of egg retrieval.
  5. The morning after ICSI, the eggs are examined for signs of fertilization, which is determined by the presence of 2 pronuclei, representing the male and female contribution to the embryo.
  6. Embryos continue to divide into multicellular entities. Three days after egg retrieval, when the embryo is normally at the 6-10 cell stage, the embryos can be prepared for a cleavage-stage biopsy. Normal development includes progression to the 2-4 cell stage two days after egg retrieval, and, after three days, usually 6-10 cells.

Biopsy Techniques

Most clinics perform a cleavage-stage embryo biopsy. However, one of the following 3 techniques can be used for PGD:

Polar body biopsy

Polar body biopsy works only for female chromosomal disorders. The mature metaphase II egg extrudes a single polar body. This polar body can be removed and tested, providing information on only the chromosomal content of the egg. Importantly, this does not provide any information regarding the chromosomal constitution of the subsequent embryo.

Because only information about the mother can be obtained by analyzing polar bodies, chromosomal abnormalities occurring after fertilization (when the sperm meets the egg) are not detected.

This technique is infrequently used given the limitations listed above.

Cleavage-stage embryo biopsy

The most common approach for PGD/PGS is to biopsy a single blastomere from day 3 embryos; this allows extraction of a single blastomere from a developing embryo. The removal of the blastomere is a technically challenging procedure. The embryologist's goal, accomplished using a special microscope and micromanipulators, is to remove an intact cell with minimal trauma to the remaining embryo (see the image below).

Removal of blastomere from an 8-cell embryo (cleav Removal of blastomere from an 8-cell embryo (cleavage-stage embryo).

Before extracting the single cell from a 6-10 cell embryo, the embryo is incubated in calcium- and magnesium-free medium for approximately 20 minutes in order to reduce blastomere-to-blastomere adherence.

The embryo is then anchored on one side with a holding pipette; simultaneously, a small opening within the zona pellucida is made in order to readily access the blastomeres. This opening procedure is called assisted hatching. Assisted hatching can be performed with either a dilute acidic Tyrode solution, with a laser, or with a sharp curette. After the small opening is made, a pipette is placed through the opening and focused on the blastomere of choice, containing a visible nucleus. The blastomere is subsequently gently aspirated into the pipette and expelled into the surrounding medium.

The embryo, now containing one less blastomere, is returned to the incubator into the appropriate culture medium. The blastomere is then processed for either FISH or PCR, depending on the genetic condition to be studied.

A limitation of the cleavage stage biopsy is that the acquired blastomere may not be completely representative of the entire embryo in that embryos can be mosaic (ie, the embryos may be composed of more than one population of cells).

Blastocyst biopsy

Blastocyst formation begins on day 5 post-egg retrieval and is defined by the presence of an inner cell mass and the outer cell mass or trophectoderm. At this stage of development, the embryo is formed of more than 100 cells. A hole is breached in the zona pellucida in a similar manner as described for a cleavage-stage embryo biopsy, and cells are removed from the trophectoderm using a fine biopsy pipette. The inner cell mass is left undisturbed. Genetic analysis is performed via FISH or PCR analysis as described below.

A limitation of this procedure is the potential acquisition of cells from the trophectoderm that are not representative of the developing embryo (inner cell mass) due to mosaicism (having multiple different types of cell lines). In addition, genetic/aneuploidy testing is completed approximately 24-48 hours of the embryo biopsy; due to the limited viability of embryos in the laboratory (≤6 d after egg retrieval), many embryos do not survive until the time of embryo transfer. Therefore, biopsied blastocysts must be frozen.

Genetic Testing

In most instances, the genetic/aneuploidy testing can be completed within 24 hours of the embryo biopsy, allowing for a day 4 or day 5 embryo transfer. Due to the limited viability of embryos in the laboratory, fewer than half of all chromosomes can be evaluated for aneuploidy in a timely fashion. However, many laboratories are developing rapid means for evaluating all 24 chromosomes (22 autosomes plus 2 sex chromosomes).

Polymerase chain reaction

PCR is used for the diagnosis of single gene defects, including dominant and recessive disorders. PCR, sometimes called DNA amplification, is a technique in which a particular DNA sequence is copied many times in order to facilitate its analysis. PCR rapidly multiplies a single DNA molecule into billions of molecules.

The DNA is immersed in a solution containing the DNA polymerase enzyme, unattached nucleotide bases, and primers. The solution is heated to break the bonds between the strands of the DNA. When the solution cools, the primers bind to the separated strands, and the DNA polymerase quickly builds new strands by joining the free nucleotide bases to the primers. By repeating this process, a strand that was formed with one primer binds to the other primer, resulting in a new strand that is specific solely to the desired segment. Further repetitions of the process can produce billions of copies of a small piece of DNA in several hours.

PCR is a relatively fast and convenient way to test DNA. The method has been used in a variety of preimplantation genetic testing protocols. However, it requires sufficient amounts of a pure, high-quality sample of DNA, which is sometimes difficult to obtain from a single cell such as a polar body or blastomere. In addition, laboratory contamination and allele dropout are possible complications.

Only one cell should be amplified; however, if another cell or piece of DNA enters the tube, it is also amplified. ICSI must be used to minimize this problem and to ensure that no excess sperm are present (paternal contamination) and that all the cumulus cells have been removed (maternal contamination).

The laboratory environment must be strictly controlled to avoid the introduction of contaminants to the tested material. The laboratory technicians must be trained extremely well to avoid all types of outside interferences.

Errors in PCR can result in misdiagnoses leading to an affected embryo being transferred or the discarding of a normal embryo. One error is caused by a phenomenon known as allele dropout. This refers to the preferential amplification of one allele over another during the PCR process and is mainly a problem for PGD of dominant disorders or when 2 different mutations are carried for a recessive disorder and only one mutation is being analyzed. In autosomal dominant diseases, the risk of transferring an affected embryo is 11% and 2% for recessive disorders.

Fluorescence in situ hybridization

FISH is used for the determination of sex for X-linked diseases, chromosomal abnormalities, and aneuploidy screening. FISH is used more commonly in PGS secondary due to its utility as an aneuploidy screen. Probes (ie, small pieces of DNA that are a match for the chromosomes being analyzed) bind to a particular chromosome. Each probe is labeled with a different fluorescent dye. These fluorescent probes are applied to the cell biopsy sample and are expected to attach to the specific chromosomes. They can be visualized under a fluorescent microscope. The number of chromosomes of each type (color) present in that cell is counted. The geneticist can thus distinguish normal cells from abnormal cells, such as those with aneuploidy (see the images below).

Abnormalities of chromosomes 13, 18, and 21. Abnormalities of chromosomes 13, 18, and 21.
Multiple fluorescence in situ hybridization demons Multiple fluorescence in situ hybridization demonstrating chromosomes 13 (triploid), 18, 21, and Y.

Chromosomes that can be analyzed with FISH probes include X, Y, 1, 13, 16, 18, and 21.

A summary of PGD applications categorized by PCR or FISH

Polymerase chain reaction categorizes the following:

  • Single gene defects in autosomal disease
  • Single gene defects in male infertility
  • Identification of sex in X-linked diseases [18]

Fluorescence in situ hybridization (preferred because PCR bears the risk of misdiagnosis caused by contamination) categorizes the following:

  • Aneuploidy screening in women of advanced maternal age
  • Aneuploidy screening for male infertility
  • Identification of sex in X-linked diseases
  • Recurrent miscarriages caused by parental translocations

Comparative genomic hybridization

A human cell contains 23 pairs of chromosomes; however, FISH analysis allows accurate assessment of only 7-9 chromosomes in each biopsied cell. Consequently, many abnormal embryos, incapable of forming a successful pregnancy, remain undetected and may be transferred.

Using CGH, the embryo nucleus is labeled with a fluorescent dye and a control cell is labeled using another color (ie, red or green). The two cells are then cohybridized onto a control metaphase spread, and the ratio between the 2 colors is compared. If the chromosomal analysis shows an excess of red, the embryo nucleus contains an extra chromosome. If an excess of green is apparent, then the embryo nucleus is missing one of these chromosomes. CGH enables not only enumeration of all 23 chromosomes but provides a more detailed picture of the entire length of the chromosome which may detect imbalance of chromosomal segments.

Currently, this technique takes 72 hours, and, given the limited duration of embryo viability in culture, embryo cryopreservation is necessary to provide the time necessary to obtain a diagnosis. Even with a high cell survival rate, cryopreservation can lead to a 30% loss of viable embryos. [19]

Studies have shown CGH protocols that avoid cryopreservation and are compatible with embryo transfer on day 4-5 after fertilization. This is achieved by either using array CGH, an accelerated CGH protocol providing results in 24 hours for all chromosomes, or by using polar body biopsy. Few laboratories currently offer this technology.

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Considerations and Controversies

Considerations and Challenges of Preimplantation Genetic Diagnosis

Fertile patients must undergo IVF to produce suitable embryos and be counseled on the risks associated with IVF treatment (ie, for those undergoing PGD for single gene disorders or for those undergoing PGS for recurrent pregnancy loss).

Patients must have proper genetic counseling on not choosing IVF with PGD and the relevant patterns of inheritance and the impact of disease on the affected child and the family.

Given the inherent limitations of current PGD/PGS technology as well as the potential for misdiagnosis due to embryonic mosaicism, it is recommended that patients undertake prenatal diagnosis (a chorionic villus sampling/CVS or amniocentesis) even if PGD/PGS is performed.

Alternative treatment strategies, such as using donor gametes, must also be discussed.

Even with a successful IVF and PGD procedure, pregnancy is not guaranteed after transfer, and a term or near-term delivery is also not guaranteed.

Removal of a single cell without breaking it or causing serious damage is technically difficult and requires skill and experience. Damage to the embryo (projected to be 0.1%) may accidentally occur during removal of the cell.

The diagnostic methodology for a new disease is a time-consuming and expensive process.

A relatively large number of eggs or embryos may be found to be abnormal, thus leaving only a few or no healthy embryos for transfer.

For aneuploidy screening, not all chromosomal or genetic abnormalities can be diagnosed with PGD because only a restricted number of chromosomes can be examined at one time during the course of a single procedure.

Currently, only a specific examination of a single biopsied cell is available. A single cell cannot be screened for multiple genetic conditions.

Some single gene disorders may not be amenable to PGD due to a variety of mutations that can result in a specific genetic disease (ie, cystic fibrosis). Couples must have specific testing performed in order to generate "probes" for testing. The process of generating molecular probes can take several weeks.

Considerations and Challenges of Preimplantation Genetic Screening

PGS remains a controversial technique. Although initially heralded as a method to identify and avoid aneuploidy in embryos of women at increased risk, recent studies suggest that success might be limited to specific patient populations. Most considerations and challenges listed in the PGD section apply to PGS.

Technical limitations

Currently, FISH offers evaluation of less than half of the 23 chromosomes; usually 9-11 are analyzed. Many laboratories are increasing the efficiency of analyzing all chromosomes (22 autosomes and 2 sex chromosomes). Studies using comparative genetic hybridization (CGH) and FISH demonstrate that as many as 25% of aneuploid embryos are characterized as normal because the abnormal chromosomes were not analyzed. In addition, approximately 10% of cells removed for screening yield no or inconclusive results.

Limitation of single cell analysis

If nondisjunction occurs during meiosis, then all the cells in an embryo are aneuploid. However, if nondisjunction occurs after fertilization during mitosis, then two or more cell lines may be present in the embryo. Thus, a mosaic embryo with normal and abnormal cells may be misdiagnosed with the present single cell biopsy technique.

"Self-correction"

Self-correction refers to evidence that mosaic embryos are able to halt the proliferation of abnormal cells and that many embryos identified as aneuploid will survive and be reidentified as normal.

Advanced Maternal Age

Results for PGS for advanced maternal age are mixed. Couples with recurrent pregnancy loss and established balanced translocation may benefit from PGS.

Current recommendations from the Society for Assisted Reproductive Technology (SART) and American Society for Reproductive Medicine (ASRM) state that available evidence does not support the use of PGS to improve live-birth rates for advanced maternal age, recurrent pregnancy loss, or implantation failure and recommends that patients be counseled about the limitations of the technique and should not make future treatment decisions based solely on PGS results. [20]

Other concerns

To date, there are no reports of increased fetal malformation rates or other identifiable problems in babies born from IVF with PGD/PGS. However, the presentation of other abnormalities later in life as a consequence of the PGD/PGS procedure (biopsy) is possible.

Due to a reduction in the number of chromosomally normal embryos available for embryo transfer, patients should be counseled that IVF with PGD/PGS may result in a lower pregnancy rate than if IVF is performed without PGD/PGS. This is particularly true in patients of advanced maternal age who usually produce few embryos and often of marginal quality that are vulnerable to damage from embryo biopsy.

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Advantages

Currently available technology can help eliminate some genetic diseases in the future (eg, Tay-Sachs disease, cystic fibrosis, Huntington disease, X-linked dystrophies). Complete cures for many genetic diseases are not likely to be found soon; therefore, preventing the disease is preferable to waiting for a possible cure to eventually become available. Furthermore, available treatments often have multiple adverse effects. Prolonging the lifespan of affected patients could cause them to develop diseases not previously known to be associated with the particular genetic condition (eg, diabetes, osteoporosis). For instance, as improved treatment prolongs life for individuals with cystic fibrosis, other manifestations of the pancreatic insufficiency and nutritional malabsorption associated with the disease, such as diabetes and osteoporosis, begin to emerge.

Prenatal testing for genetic diseases is currently performed through amniocentesis or chorionic villus testing (CVS) when the fetus is aged 10-16 weeks. If the examination findings reveal a genetically defective fetus, the options available to parents are to have a child with a genetic disease or to undergo a pregnancy termination. This is a difficult and often traumatic decision, especially in advanced pregnancy. However, PGD is performed before pregnancy begins, thus eliminating this difficult decision.

In the past, persons with a genetic disease or those who know that they are carriers frequently choose not to have children in order to avoid the risk of passing on the disease to future generations. Now, PGD allows these couples the opportunity to have a child free of their particular disease.

In addition, hundreds of infants have been born following PGD/PGS worldwide. To date, there are no reports of increased fetal malformation rates or other identifiable problems.

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Future

The first successful cases of preimplantation genetic diagnosis (PGD) in humans were performed in 1988. However, the development and acceptance of PGD since then has been slow, mainly due to the time necessary to develop and learn single-cell diagnostic techniques and to the costs involved. PGD is a relatively new procedure, and much ongoing research is being performed to expand and improve it. However, much more work also must be completed before PGD becomes a more comprehensive, accepted, and widely available procedure. Given the technical considerations associated with PGD/PGS, these procedures should be limited to centers experienced with micromanipulation.

Almost weekly reports on identification of genetic changes tied to various diseases are published in scientific and lay literature. In the future, genetic links to common diseases (eg, diabetes, hypertension, cardiovascular diseases, endometriosis, cancers) may be identified, and PGD will become available to control the transmission of these diseases to future generations. [21]

Although PGS has been incorporated into the care of patients undergoing IVF treatment, its indications, utility, and outcomes remain an active area of research in reproductive medicine. As preimplantation screening for medical disorders at the embryonic level optimizes, its place in medicine and society will continue to generate controversy and ethical debate.

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