Transcript An Introduction to Pre-implantation Genetic Diagnosis (PGD)

An Introduction to:
Pre-implantation Genetic Diagnosis (PGD)
Mike Dougherty, Ph.D.
Director of Education, ASHG
Pre-implantation Genetic Diagnosis (PGD)
What is it?
Genetic analysis of a single cell from an eight-cell embryo
done in conjunction with in vitro fertilization (IVF) to
improve the chances of a “normal” pregnancy.
The General Method of IVF
1. Monitor egg maturation in the ovary
-
Ultrasound
-
Hormone levels
2. Collect eggs (mother’s own or from donor)
- Injection of human chorionic gonadotropin (hCG) and follicle
stimulating hormone (FSH) to time egg ripening
- Transvaginal aspiration using hollow needle
Egg retrieval
3. Obtain sperm from father (or donor) and assess quality
4. Combine eggs and sperm in vitro, using intracytoplasmic
sperm injection (ICSI), if sperm is low quality
5. Nurture embryo growth by incubating in medium
containing various nutrients and hormones
- If doing PGD, remove one cell after 2-3 days (6-8 cell stage)
for testing
An 8-cell embryo: 2-3 days
zona
pellucida
Image adapted from and additional information can be found at:
http://www.infertile.com/treatmnt/treats/pgd.htm
5. Nurture embryo growth by incubating in medium containing
various nutrients and hormones
- If not doing PDG, incubate until embryo is 5-6 days old (blastocyst)
6. Transfer embryos (usually 3-6) to uterus, artifically removing
zona pellucida if necessary (“hatching”)
6d embryo at time of implantation (blastocyst)
How well does IVF work?
Success depends on many factors, and reliable statistics are
hard to interpret because of the uniqueness of each situation
and the ways in which private clinics define success.
A Sampling of Success Rates
(from: Genetics and IVF Institute)
The percentages shown in parentheses reference the current
number divided by the number above it:
The "pregnancy rate" statistics look like this:
From: Center for Reproductive Medicine and Fertility
How does this compare to natural pregnanies?
It is generally accepted that a single “exposure” (copulation between
fertile individuals during a period of receptivity) yields a stable
pregnancy only 20-30% of the time.
Failures are due to lack of fertilization, failure of the embryo to
develop (often due to aneuploidy), failure to implant, menstruation.
Where would a naturally conceived embryo be during
the phases of IVF development?
Fertilization normally occurs in the Fallopian tubes, and the
developing embryo moves through the tubes toward the uterus over
a period of several days.
At roughly 4-6 days, the embryo will hatch from the zona pellucida,
a necessary condition for implantation in the uterine wall. Over the
next week, the embryo will interact with the uterine lining in ways
that stimulate placental blood flow, and development will proceed.
Why consider PGD in addition to IVF?
1. recurrent miscarriages
2. one child already affected with a genetic disease
3. family history of inherited disease
4. maternal age older than 38
5. prior failure with IVF
6. family “balancing” for sex
The Methods of Preimplantation Genetic Diagnosis
1. Remove a single cell from the 6-8-cell embryo using a fine
glass needle to puncture the zona pellucida and aspirate
the cell
-
In skilled hands, this generally does not harm the
developing embryo.
-
Each cell is called a blastomere.
Blastomere removal for PGD testing
2. Prepare a metaphase spread of chromosomes to assess
karyotype (number and integrity of each chromosome)
3. Two types of assessment techniques are common:
a. chromosome “painting” (or FISH) using fluorescent probes
specific for each chromosome. These allow number and size of
each chromosome to be checked.
• useful for identifying aneuploidies (incorrect chromosome
numbers) and translocations
• procedure destroys the tested cell
• limited number of chromosomes can be checked
simultaneously; some abnormalites undetectable
Fluorescence in situ hybridization (FISH)
Chromosome 1 in a
normal cell, “painted”
with red fluorophore
Screening aneuploids with multiple probes
Aneuploidy is the most frequent cause of spontaneous abortions.
Detecting a chromosome translocation
Translocations may be:
- “Balanced” if chromosome material merely switches locations
with no net loss or gain; or
- “Unbalanced” if switch is accompanied by a net loss or net
gain of genetic material
Balanced translocations may reduce fertility but otherwise are
generally less likely to cause serious health problems.
Unbalanced translocations often are harmful or lethal.
3. Two types of assessment techniques are common:
a. chromosome “painting” (or FISH)
b. genetic testing for specific disease loci (PCR or gene chips)
Polymerase chain reaction (PCR)
- amplification of DNA specific to a gene of interest (family history
guides choice of genes)
Examples of genetic disorders detectable via PCR-based tests:
- Tay Sachs (autosomal recessive; ~98% accuracy)
- Cystic fibrosis (autosomal recessive; ~85% for common allele mutation)
- Huntington’s disease (autosomal dominant)
- Thalassemias (autosomal recessive blood disorder)
- Duchenne muscular dystrophy (X-linked recessive)
- Spinal muscular atrophy
As more genetic tests are developed as diagnostic tools, more will
be used for predictive purposes in PDG.
Gene chip array showing expression results
- Such chips allow cystic fibrosis accuracy of nearly 100% because all
possible mutations can be screened.
Limitations of PCR-based tests:
• Both alleles may not amplify equally, leading to misdiagnosis or
inconclusive results
• PCR-based tests only detect disorders at target loci; other
mutations may exist elsewhere
• To accommodate these limitations, prenatal amniocentesis or
chorionic villus sampling is usually recommended as a
supplement to PGD.
Risks of IVF to the mother/couple:
• Multiple pregnancies (20-30%)
• Ectopic pregnancy
• Cancellation (over- or under-response to ovarian induction)
• Ovarian hyperstimulation syndrome (fluid build-up in pelvic cavity due to
ovarian enlargement; clotting problems)
• Mechanical injury to bowel, bladder, ureters, or blood vessels during
egg retrieval
• Greater risk of premature delivery and delivery by Caesarean section
Risks to the child conceived via IVF/PGD:
• Low birth weight; premature birth
• Developmental delays
• Cognitive problems (ADHD)
• Urogenital problems
• Cerebral pals
• Certain cancers (e.g., Beckwith-Weidemann syndrome, which may be
related to ICSI)
(Note: The vast majority of children born of IVF appear normal. Low birth
weight, cognitive delays, and cerebral palsy are more common for any
multiple-birth situation. Very few controlled, longitudinal studies have
compare IVF to natural.)
What is the biological status of the embryo?
At the stage of PGD, the embryo consists of 6-8 viable cells. Cells are
the smallest units of life, as defined by biologists. It is generally believed
that, at this stage, a single cell does not possess the potential to
develop into a normal fetus.
At the stage of implantation, the embryo consists of roughly 200 cells
arranged in a hollow ball. Inside is an inner cell mass, the cells used to
generate embryonic stem cell lines, which are cells that have the
potential to develop into any tissue but not into a complete organism.
Excess embryos that are not implanted are generally frozen for a time
and eventually may be discarded.