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IN the fact sheet on birth control that I wrote for
the special report on reproductive health that appeared in the
October 16 issue of this paper, I said that there are ways to tell
before birth if a baby will have Down’s syndrome or not.
A reader wanted to know how this
is done.
Down’s syndrome is a common
congenital disorder that could affect babies born to mothers over 40
years. It’s named after an English physician, J. L.H. Down, who
identified it in the middle of the 19th century. It’s also called
mongolism.
The defect is traced to the
failure of one chromosome of a germ cell to divide in the normal way
to form a healthy ovum with 23 chromosomes. An ovum with 24
chromosomes, if fertilized, will develop into an embryo with an
extra chromosome.
The head of a child with Down’s
syndrome is small with prominent cheekbones, flat nose, slanted eyes
and a fold over the inner part of either eye. Although a child with
Down’s syndrome could live a reasonably normal life, it suffers in
varying degrees a mental handicap.
The diagnostic tests for Down’s
syndrome—both invasive—are amniocentesis and chorionic villus
sampling (CVs). Fetal cells are taken from the embryo and the
chromosomes counted.
The tests, however, carry a risk
of miscarriage, about one in 200, according to the American College
of Obstetricians and Gynecologists.
Two genetic techniques have been
developed recently that are non-invasive (Andrew Pollack, New York
Times, October 9). They require only a blood sample from the
pregnant woman.
The first test, called Sequenom,
was developed by an American biotechnology company in San Diego,
California and had been in use since June this year.
The other test was developed at
Stanford University. A description of it was published on October 6
in The Proceedings of the National Academy of Sciences online.
Sequenom has still to be written
about in a refereed journal.
Both tests, according to the New
York Times’ “have perfect records so far.” However Seque-nom
is gearing up to test its accuracy on 10,000 women and to release
the results late next year. For now, the Sequenom test is used to
see who would have to undergo amniocentesis or CVS.
Dr. Diana Bianchi of Tufts
Medical Center and the president of the International Society for
Prenatal Diagnosis said that there is a need is for a
“non-invasive diagnostic test” but Sequenom’s results are
“preliminary and promising, but not definitive.”
Other prenatalists say that these
genetic tests are an improvement over present screening methods as
amniocentesis and CVS miss 5 percent to 30 percent of Down’s
syndrome cases. A false positive result of 5 percent means that many
pregnant women undergo amniocentesis or CVS unnecessarily. The risk
of Down’s syndrome in a baby born to a mother nearing 30 years is
approximately one in 1000. So if 10,000 women in that age group
underwent screening by amniocentesis or CVS only 10 actual cases
would be detected. For the others, the tests would have been
unnecessary and one of them might miscarry a healthy baby.
Most efforts to develop
non-invasive tests are focused on fetal cells, which are
“extremely rare and hard to detect.” (NYT, October 9).
The Sequenom and Stanford tests
are based on the recent discovery of free-floating DNA in a
mother’s bloodstream. However, the extra chromosome that causes
Down’s syndrome is not easily detected in the woman’s DNA.
The inventor of the Sequenom
test, Y.M. Dennis Lo of the Chinese University of Hong Kong, focused
instead on the RNA.
A gene made of RNA is produced
only when that gene is active. Lo isolated genes on Chromosome 21
that are active in the fetus but not in the mother. This means that
RNA found in a woman’s bloodstream comes from the fetus. The test
then looks for those versions of the genes inherited by the fetus
from the father that differ from those inherited from the mother.
If the fetus has two copies of
Chromosome 21, the RNA in the mother’s and father’s versions
should be the same. If the baby has an extra copy of the Chromosome,
one version would be more abundant than the other.
The Stanford technique involves
determining the sequence of DNA fragments, millions of them, in the
mother’s bloodstream. It does not matter if the fragments come
from the mother or the fetus. Using the map by the Human Genome
Project, the scientists can determine the chromosome each fragment
comes from. If the fetus has an extra copy of Chromosome 21, there
will be more fragments of that Chromosome.
The Stanford test has an
advantage over the Sequenom technique in that it will work for any
woman and for any ethnic group. This is important for Asians whose
genes from the mother and father do not differ in the spots that are
important for the Sequenom test.
Both the Sequenom and Stanford
tests should be available in the Philippines soon if they are
endorsed by the ob-gyn profession and the Department of Health.
opinion@manilatimes.net
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