Preimplantation genetic
diagnosis enables us to identify genetic diseases prior to embryonic
attachment to the uterus (before pregnancy is established) and
eliminates the need for possible pregnancy termination after prenatal
diagnosis of a genetically affected fetus. At
CRH PGD is available to couples who have a known genetically
transmittable disease, to those who have had a child with a genetic
disease in the past and to other couples at risk (i.e., advanced maternal
age). Preimplantation genetic diagnosis should reduce the
psychological trauma experienced by couples who carry a genetic
disease, as it allows intervention before pregnancy is established.
The basic procedure
underlying PGD involves removing a cell from an embryo resulting from
in vitro fertilization, analyzing the genetic material, and then
transferring the embryo into the uterus if the analysis does not
indicate a the presence of a disease. Recent
knowledge indicates that 60% of embryos have abnormalities in women
older than 35. Preimplantation genetic diagnosis offers the potential
to screen for embryos with aneuploidy (unusual
number of chromosomes).
Once perfected, the net result should be a decreased miscarriage rate,
increased pregnancy rates and lower incidence chromosomal anomalies.
At CRH we offer screening
for inherited autosomal recessive disorders, where the specific
gene defect is identified¶
(i.e., Tay Sachs disease, cystic fibrosis). We also offer PGD & sex
selection to avoid X-linked disorders§ (i.e., Duchenne's,
muscular dystrophy, Lesch Nyhan syndrome). With some X-linked
disorders (e.g. Duchenne’s muscular dystrophy) the specific defect can
now be identified in a proportion of families, which means that male
embryos free of the disorder can now be implanted along with female
embryos. PGD has also been used to test for autosomal dominant
conditions£, the gene predisposing to polyposis coli
(and inherited cancer of the colon) and Marfans syndrome). As new
tests become available this list will continue to grow.
A
further reason for requesting PGD at CRH is when one partner is at
high risk of transmitting a chromosome anomaly, such as a
translocation. Such couples have often experienced repeated
miscarriages and periods of infertility and are already receiving
assisted conception treatment. For such patients, PGD may be a way to
achieve a successful pregnancy where they would otherwise have
difficulty. In some instances, PGD may also help individuals at risk
of having a child with severe developmental problems because of a
chromosomal imbalance. Sexing embryos to avoid X-linked disorders
and testing for age related aneuploidy are indications for PGD in this
center.
The
future promise of preimplantation genetic diagnosis is immense. The
number of diseases that can be detected will dramatically increase
with the aid of the data now being collected for the Human Genome
Project.
The first step to PGD is creating
embryos outside the body by IVF. A biopsy is then carried out to
remove a cell from the developing embryo, which can be used to test
whether the embryo carries a genetic disorder. The first and/or second
polar body can be removed and used to determine the genetic status of
the mothers' chromosomes. The genetic diagnosis of the biopsy cells is
attempted using one of two diagnostic techniques: fluorescent in situ
hybridization (FISH), for chromosome disorders, and polymerase chain
reaction (PCR), for single gene defects. Tags are used for the
particular chromosomes involved in the translocation (transfer
of part of a chromosome to a new position on the same or on a
different chromosome with resultant rearrangement of the genes)
so that only unaffected embryos are transferred.
The DNA from a single cell is
incubated with primers which bind close to the gene sequence of
interest, and with the addition of an enzyme, thousands of copies are
made of the relevant section of DNA. The resulting DNA fragments can
be analyzed by a number of different techniques to determine whether
the single cell was at risk of the genetic disease.
To make an
appointment, please call us TODAY at
615-321-8899.
