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Techniques
One of the major applications of somatic cell genetics to clinical
medicine is in the area of prenatal diagnosis by genetic amniocentesis.
This procedure permits the accurate prediction of some fetal diseases.
The procedure is usually performed under a local anesthetic during
the 15th to 17th week of pregnancy. By means of a needle introduced
into the uterus, some of the amniotic fluid that surrounds the
fetus is withdrawn for analysis; the fluid contains cells shed
by the fetal skin and mucous membranes. These fetal cells can
be cultured by the methods of somatic cell genetics and utilized
either for 
chromosome analysis or for diagnosis of genetic biochemical diseases
in which enzymatic assays for an abnormal protein are possible.
The amniotic
fluid can also be analyzed for the presence of an excessive amount
of a normal fetal serum protein called alpha-fetoprotein; the
concentration of alpha-fetoprotein is increased when the fetus
is afflicted with a serious disease of the spine or brain known
as a neural tube defect. In some biochemical diseases, the particular
enzyme involved is not normally produced by the amniotic fluid
cells (amniocytes), and therefore a diagnosis cannot be made by
enzymatic assays. The mutant gene, however, is present in the
amniocytes, and advances in recombinant DNA research have made
it possible to produce probes for many of these defective genes.
Because of
its invasive nature, genetic amniocentesis is used only to determine
the possibility of the most serious and tragic conditions.
A common
indication is for women who become pregnant after the age of 35,
when the risk of Down
syndrome is increased. In the majority of cases, the
test relieves a tremendous burden of anxiety from the prospective
parents.
One of the
unfortunate situations associated with genetic amniocentesis is
that the diagnosis of a serious fetal disease is usually not made
before the 19th to 21st week of pregnancy. If the parents choose
to interrupt such a pregnancy, the accompanying emotional trauma
is undoubtedly significant. An alternative procedure, which can
be performed during the eighth to 10th week of pregnancy, is the
biopsy of a piece of fetal placenta (chorionic villus sample).
The cells thus obtained can be utilized for enzyme assays, recombinant
DNA
techniques,
or chromosome analysis. The diagnosis of serious fetal disease
by the 10th week of pregnancy may make pregnancy interruption
emotionally and physically less traumatic.
Charles
Siebert
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