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Preimplantation Genetic Diagnosis ( PGD )
Preimplantation genetic diagnosis ( PGD ) allows screening
of embryos for specific genetic diseases / chromosomes before the embryos
are placed in the uterus. PGD is primarily used to evaluate known carriers
of specific single - gene defects, such as cystic fibrosis, or for specific
chromosomal abnormalities such as trisomy 21 / Down's Syndrome, Turner's
Syndrome, and specific unbalanced translocations. Transferring "screened
embryos" lacking the genetic defect makes it extremely unlikely that
the disease could be passed to the child.
PGD is possible because of the advances made in IVF.
The eggs are retrieved and fertilized with the partner's sperm, often
utilizing intracytoplasmic sperm injection ( ICSI
). Once the embryo reaches the six to eight cell stage, one or two
cells are removed ( biopsied ). For single gene defects, the DNA is analyzed
by making multiple copies of the suspected gene by a technique known as
the polymerase chain reaction ( PCR ). Unaffected embryos are selected
to be transferred to the uterus.
PGD also is used to evaluate the embryo cells for abnormal
numbers of specific chromosomes ( aneuploidy ). A normal embryonic cell
has 23 chromosomes from the mother and 23 chromosomes from the father
yielding 46. Sometimes the dividing cells do not equally distribute their
chromosomal complement. This occurs more often as the age of the mother
increases and is one of the reasons why fertility declines with increasing
female age.
Chromosomes most commonly involved in miscarriages or
live birth  abnormalities
( such as chromosomes 13, 21, 18, X and Y ) can be counted. In the photomicrograph
shown, you can see the nucleus of a biopsied cell ( blue "globe"
) with the fluorescent red, green, and yellow spots within the nucleus
demonstrating chromosomes 13 / 21 ( red ), 18
( aqua ), X ( green ) and or & Y ( yellow ).
The technique utilized in this example and process is
fluorescent in situ hybridization or FISH. FISH can be used to screen
up to nine chromosomes in a cell from the developing embryo; this encompasses
approximately 85% of the chromosomal abnormalities seen. Currently, technology
makes it difficult to screen all the chromosomes in a cell removed from
the embryo, but the Jones Institute has and will continue to investigate
other methods that will make complete chromosome screening possible in
the future.
The FISH technique also can be used to evaluate specific
chromosome structural rearrangements known as translocations. When a person
carries a balanced translocation, the offspring are at risk of having
an unbalanced translocation resulting in either extra or missing pieces
of the involved chromosomes. Many times, this could result in multiple
implantation failures, miscarriages, or severe abnormalities at birth.
One of our first PGD successes was Brittany Abshire,
who was the first child in the world born after PGD to rule out Tay -
Sachs disease. This procedure was performed at the Jones Institute.
Patients who might benefit from PGD include:
- Carriers of known genetic diseases,
- Women over the age of 38,
- Women who have had recurrent
miscarriages,
- Couples who have had previous aneuploid conceptions,
or
- Couples who have had more than three IVF failures.
A partial list of the genetic diseases presently
evaluated at the Jones Institute includes:
- Tay - Sachs disease
- Cystic fibrosis
- Thalassemia
- Sickle cell disease
- X - linked diseases ( such as hemophilia, muscular
dystrophy )
- Spinal muscular atrophy
- Common chromosomal abnormalities
- Chromosomal translocations
We test for many additional diseases not listed here.
For more information and to discuss your situation, please contact Sue
Gitlin, PhD, at ( 757 ) 446-7168.
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Phone
800-515-6637 or 757-446-7100