PGD Preimplantation Genetic Diagnosis
Screening Genetic Diseases and Sex Selection
PGD, preimplantation genetic diagnosis,
is a wonderful option for couples at risk of transmitting
a genetic disease to their children. PGD allows
the physician and embryologist to determine if an embryo
carries a specific disease, identifiable by PGD, based on its genetic composition.
HRC was one of the first (and very few centers to
date) in January 2003 to purchase a highly specialized
laser for use when performing PGD. This laser affords
the embryos less exposure to handling and "outside conditions".
The greatest advantage
to the laser is the precision, control and accuracy
provided during the biopsy stage of PGD. Embryos
are created using IVF
and a sample of DNA is taken from a cell in the embryo for testing. If the genetic disorder is present, the
embryo will not be transferred to the mother's uterus.
PGD for gender selection is also used to
help couples who wish to have a child of a particular
sex. Many couples want to experience the joy of raising
a boy and a girl and preimplantation genetic diagnosis is the most effective effective means
of gender
selection.
PGD is used to screen for many genetic abnormalities including Down Syndrome, Trisomy 21, Tay Sachs
Disease, hemophilia A and B, Gaucher's Disease, Sickle
Cell Anemia, and others.
Since February 2002 Huntington Reproductive Center has
conducted over 399 PGD cases with a 45% pregnancy rate.
HRC is a pioneer PGD center in Southern California offering comprehensive PGD options to interested
couples.
There are various types
of procedures available depending on the needs of the individual
couple:
- PGD
is used for gender selection when couples seek family
balancing options. The embryos are biopsied and it
is determined if they are XX female or XY male. If
the couple desires a boy, only the XY embryos will
be transferred to the mother. Some genetic diseases
are also sex linked meaning they only occur in a particular
gender. For example, hemophilia A and B is generally
X linked recessive and occurs in males. If the mother
has the gene for hemophilia, only female embryos or
embryos free of hemophilia will be transferred to
the uterus.
- A common type of PGD
is used to diagnosis genetic chromosomal abnormalities
due to advancing maternal age. These disorders are
more likely to occur in women 35 years of age and
older and lead to problems such as Down Syndrome or
early miscarriage. The chance of these chromosomal
problems increases with maternal age, regardless of
family history. Testing performed on a woman's embryos
for the most common chromosomal problems enables the
physician and couple to determine which embryos will
most likely to result in a healthy, ongoing pregnancy.
Additionally, for couples who have failed IVF cycles for unexplained reasons, PGD may provide important
information in the determination of the cause(s) of
the failures.
- A third type is offered
to couples at known risk for genetic disorders such
as Tay Sachs disease or cystic fibrosis. These couples
are most likely aware of a family history of such
genetic disorders, or are known to be at risk based
on carrier testing. They may choose to test their
embryos prior to transfer in order to avoid having a child born with the specified genetic condition.
- A fourth type of PGD is offered
to couples in which one of the members has a rearrangement
of their chromosomes, known as a balanced translocation.
Individuals with a balanced translocation may elect
to test their embryos for the translocation prior
to embryo transfer in order to select the embryos
most likely to result in a healthy pregnancy.
PGD- Terms and Processes
Blastomere Biopsy
Single cells from a preimplantation
embryo can be removed and genetically tested in a procedure
called blastomere biopsy. Typically embryos are biopsied
on Day 3 following the first
three cleavage divisions (containing from 6 to 8 cells
or blastomeres), although some researchers have performed
biopsies of blastocysts containing 120 cells. At both
of these stages, the cells of the embryo are not differentiated
into particular body tissues yet and it is assumed that
there is no damage to the resulting embryo. Biopsy of
embryos or blastocysts may be analyzed in a variety
of ways and can detect genetic abnormalities arising
from the maternal or paternal chromosomes.
Polymerase Chain Reaction
In the PGD process, the polymerase chain reaction
(PCR) can be used to amplify the harvested DNA with
subsequent analysis for single gene defects. This technique
is most commonly used when the embryo is at risk for
a specific genetic defect, such as cystic fibrosis,
Thalassemia's and others. Also, this technique can be
used to screen for aneuploidy and gender determination.
FISH
Fluorescent in situ hybridization
(FISH) is a technique that allows direct labeling with
particular color and visualization of specific chromosomes
to determine the number or absence of chromosomes. This
technique is most commonly used to screen for aneuploidy,
gender selection, or to identify chromosomal translocations.
FISH cannot be used to diagnose single genetic defect
disorders.
PGD Results
Results of many genetic tests
are usually ready within 48 hours after
blastomere biopsy which corresponds to Day 5 following
egg retrieval. Depending on their original quality,
embryos may or may not reach the blastocyst stage which
is the final stage of in
vitro development. Usually on Day 5, according to
the results of the specific genetic test, only embryos
free of tested genetic defects will be transferred to
the patient.
HRC remains committed to keeping
pace with the rapid advances in the fields of genetics
and human reproduction and making them available to
couples as soon as is practically possible. HRC also
continues to work on and refine the philosophical basis
for a complex system of medical ethics in order to apply these incredible
advances in reproductive medicine to their fullest and
best use for patients.
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