PGD and PGH are procedures that can weed out genetically defective human embryos before
they have a chance initiate a pregnancy. This is usually requested by prospective parents who are
concerned about passing an incurable genetically based disease or disorder to their child.
Typically one or both partners have been genetically screened previously, and found to be a carrier.
This "technically demanding and complex procedure" was developed only recently.
1 Currently, it is only available in a few clinics worldwide. It involves the following steps:
The woman is given drugs to produce "super-ovulation." She normally
produces many eggs, which are collected.
As for a standard in-vitro fertilization (IVF) procedure, the eggs are placed in a dish and are fertilized by donated sperm (usually from the
woman's partner).
About three days after IVF, each successful embryo has divided to about the 8 cell
level.
This photograph shows the start of the procedure. Here, a 7-cell embryo
which is fixed in position with a holding pipette at the left. A
second pipette, on the right is used to drill a hole through the shell
of the embryo. A single cell is dislodged from the embryo with a
gentle suction. * The procedure is typically performed on an embryo at
the 4 to 10 cells stage of development.
One or two cells are removed and is:
"... subjected to a molecular
analysis. This requires the removal of the genetic material— DNA. This
minuscule amount of DNA is amplified, meaning multiple copies are made
through a molecular process known as PCR (polymerase chain reaction).
These copies are then subjected to a molecular analysis that assists in
identifying the sequence (code) that will determine the inheritance of
the gene in question." 2
If a genetic
defect is found, then the embryo from which it was taken is destroyed.
Typically, three of the embryos which are free of abnormalities are implanted in the woman's
womb. The remaining, spare, preembryos are destroyed.
Sometimes, none of the embryos develop into fetuses, and the procedure is repeated.
Often one lives and develop into a fetus which is later born. Less commonly,
multiple births can result.
As of 2005, cells can be checked for dozens of genetically determined diseases.
One site lists:
achondroplasia
adenosine deaminase deficiency
alpha-1-antitrypsin deficiency
Alzheimer disease (AAP gene)
beta thalassemia
cystic fibrosis
epidermolysis bullosa
Fanconi anemia
Gaucher disease
hemophilia A and B
Huntington disease
muscular dystrophy (Duchenne and Becker)
myotonic dystrophy
neurofibromatosis type I
OTC deficiency
p 53 cancers
phenylketonuria
retinoblastoma
retinitis pigmentosa
sickle cell disease
spinal muscular atrophy
Tay Sachs disease 3
Elsewhere on the Internet, websites also list: Fragile
X syndrome, Lesch-Nyhan syndrome - Retinitis
pigmentosa, Charcot-Marie-Tooth disease, Barth's syndrome, Turner syndrome, Down's syndrome and Rett's
syndrome. Female ova can be also be checked for a gene that increases the propensity to develop
breast cancer. By mid-2006, about 200 diseases and disorders could be tested
for. 4
The first "PGD baby" was born in 1989. 5,6 By 1997, over 30 babies had been born
world-wide, following the use of this technique.
Dr. Perry Phillips, an obstetrician and gynecologist, is one of the directors of IVF
Canada. He said:
"This is the beginning of the end of genetic disease…That's the
dream of medicine. It's our dream. This should have the same impact [that]
antibiotics did to bacterial disease."
Sponsored link:
Sex-linked genetic diseases:
Many genetic diseases are sex-linked. For example, some are known to only be passed
only on to male children; they are known as x-linked diseases. Thus, even if a particular sex-linked disease cannot be detected directly, the PGD
method could theoretically be used to eliminate all of the male embryos and implant only female embryos. This would prevent the transmission of the disease to the
next generation. Of course, just because such a technique is theoretically
possible, it may or many not be considered an ethical procedure.
The University College Hospital (UCH) in London, UK, has applied
to the Human Fertilisation and Embryology Authority for permission to
use PGD techniques to help couples with a family history of autism. Boys are
considerably more likely to develop autism than girls. The couple would go
through the usual IVF procedure in which perhaps two dozen embryos are
removed from the woman, all are fertilized and four of the healthiest
embryos are implanted. They would deviate from the standard IVF procedure by
culling out the approximately twelve male embryos from being considered for
implantation. Professor Joy Delhanty of UCH said: "Normally we would not
consider this unless there were at least two boys affected in the immediate
family."
Josephine Quintavalle, of Comment on Reproductive Ethics,
disapproves of this technique. She said:
"It is not about taking an embryo and curing it, but about diagnosing
and then throwing away....The requirements are getting wider and wider
and the science can be more and more hypothetical. This getting rid of
male embryos is shoddy and shocking. We need to see more evidence on the
genetic causes of autism." 7
Simone Aspis, of the British Council of Disabled People, warned:
"Screening out autism would breed a fear that anyone who is different
in any way will not be accepted. It would create a society where only
perfection is valued." 7
We have found a few clinics that provide PGD and which have web sites on
the Internet. None have asked to be included on this list; none have paid to
be on the list:
UK: According to the The Human Fertilisation and Embryology
Authority, four centers in the UK are licensed to carry out PGD. 8
References:
The following information sources were used to prepare and update the above
essay. The hyperlinks are not necessarily still active today.
"Using PGD to prevent sex-linked diseases," at: http://www.healthlibrary.com/ This
web site has a remarkable series of microphotographs, including the one
shown on this page. They show how a single
cell is extracted from a seven-cell embryo. See: http://www.healthlibrary.com/
Ricki Lewis, "Preimplantation Genetic Diagnosis: The next big thing?,"
The Scientist, 14[22]:16, 2000-NOV-13. See:
http://www.the-scientist.com/yr2000/
Ian Sample, "New embryo test to screen for 6,000 diseases," The
Guardian, 2006-JUN-19, at:
http://www.guardian.co.uk/
Page Translator: