Couples
at high risk of transmitting an inherited disease to their
offspring, have the option of undergoing prenatal diagnosis
to allow the detection of the genetic disorder in fetus.
However, if the analysis reveals a genetically affected
fetus the only options available to couples are to have a
child with a genetic disease or to terminate the affected
pregnancy. This is a difficult and often traumatic decision,
especially in advanced pregnancies.
Many couples may also experience repeated pregnancy
terminations in attempts to conceive a healthy child and
might feel unable to accept further affected pregnancies. In
some cases this may also not be a viable option for
religious or moral reasons.
Preimplantation genetic diagnosis (PGD) has been introduced
as an alternative to prenatal diagnosis, to increase the
options available for couples who have a known genetically
transmittable disease, providing reassurance and a reduced
anxiety associated with reproduction. PGD can be considered
as a very early form of prenatal diagnosis. Its intended
goal is to diagnose a specific genetic disease on oocytes or
embryos before a clinical pregnancy has been established, by
selecting and transferring to the uterus only embryos
resulted unaffected after mutation analysis. Consequently,
PGD may spare the couple decisions regarding possible
pregnancy termination, ensuring a pregnancy free of the
disease under consideration.
PGD usually requires that the couple undergoes to an in
vitro fertilization (IVF) treatment. This involves hormonal
treatments that allow the collection of multiple eggs from
the mother. The eggs are then fertilized using the father’s
sperm and the resulting embryos are transferred to an
incubator. After three days the embryos usually consist of a
tiny ball of eight cells, known as blastomeres. To test the
blastomere, an opening is made in the covering of the
embryo. One or two blastomeres are then removed (biopsied)
from each embryo and subjected to genetic testing. If a
blastomere is found to be unaffected by the inherited
disease then the embryo that it was removed from will also
be unaffected. Embryos that are revealed to be healthy can
be transferred to the womb, ultimately producing unaffected
babies.
Testing of the biopsied cells destroys them because their
membranes must be broken open to release the DNA. As such,
one cannot use them for any other purpose or return them to
the embryo.
