Subject: FAQ Sheet - Prenatal
& Preimplanation Testing in HD
This information was a response to a question I asked Joyce Hammel July 24, 2000, the then
HDSA - Medical & Scientific Manager:
In response to your question on pre-natal testing, I provide the following information:
There are several ways prospective parents can determine if their child will develop HD. The
specific method of determining the embryo's genetic status depends on whether it was conceived naturally or through in vitro
fertilization (IVF).
If
the embryo was conceived naturally,parents can use prenatal testing or non-disclosing
prenatal testing to determine the child's genetic status.
During in vitro fertilization procedures, Preimplantation Genetic Diagnosis (PGD) can be used test
embryos prior to their transfer to the uterus.
Prenatal Testing
The prenatal procedures currently available are amniocentesis and chorionic villus sampling (CVS). Individuals or couples considering prenatal testing are advised to seek genetic counseling prior to becoming pregnant. Prenatal testing is usually requested in one of the two following circumstances:
1. In case I, a prospective parent has been diagnosed with HD or has been found to be a gene-carrier and will eventually develop HD. In this instance, prenatal testing can determine whether the child is at 0 or 100% risk of developing HD.
2. In case II, the prospective parent is at 50% risk of developing HD and is pre-symptomatic.
This means that one of his/her parents had HD but s/he isn't showing symptoms. Prenatal testing will reveal the embryo's and
the parent's genetic status.
The test results in this case can be over-whelmingly traumatic if both are positive for HD.
Prospective parents should be counseled about possible emotional ramifications before undergoing the tests.
Nondisclosing Prenatal Testing
If the parent who has a 50% risk for HD does not wish to know his/her gene status but
the couple wants prenatal testing, then non-disclosing prenatal testing can be performed. In this situation, the embryo is
at a 25% risk of developing HD.
Using linkage analysis, a procedure that compares the chromosomes of family members with that
of the embryo, this test may reduce the fetus' risk from 25% to zero or increase it to 50%
Couples or individuals seeking this testing option are encouraged to seek counseling and
prepare for the test prior to conception.
Preimplantation Diagnosis
Preimplantation Genetic Diagnosis (PGD) can be used during in vitro fertilization (IVF)
procedures to test embryos for HD or other genetic disorders. This allows couples or individuals
that have serious genetic disorders to decrease the risk of having a child who is affected
with the same problem.
At present, PGD is only offered in a few centers, usually under the supervision of an institutional ethics review board, but its use may become more widespread in the near future.
There are only a few prenatal tests for HD done each year. The centers that do these tests, especially the Genetics and In Vitro Fertilization Institute (GIVF)are experienced with preimplantation
diagnosis as well as CVS and amniocentesis testing for HD. GIVF can also offer testing when knowledge of parental carrier
status is not desired.
Please contact the following person at GIVF with additional questions
about testing.
Lee Fallon, MS, CGC
Supervisor, Genetic Counseling
Genetics &
IVF Institute
Fairfax, VA 22031
T: 703-698-3959
E-mail: lee_fallon@givf.com
Website:
http://www.givf.com/
Hope that this information is helpful!
Joyce
================================
Prenatal testing for HD is possible, and should be performed in conjunction with detailed genetic
counseling. Affected or at-risk individuals or couples should be informed of all of their reproductive options (shown in table
1), with the under-standing that different options are appropriate or desirablefor different people.
For those who desire prenatal testing, the best time to make arrangements is prior to the pregnancy.
Chorionic villus sampling can be performed very early, at 8-10 weeks, and a non-disclosing prenatal test, which determines
only whether the fetus received a chromosome from the affected grand-parent or the
unaffected grandparent, without determining whether the fetus or at-risk parent actually carries the HD gene, requires
samples from several individuals.
TABLE 1: REPRODUCTIVE OPTIONS
- Natural reproduction without genetic testing
- Prenatal testing by amniocentesis or chorionic villus sampling
- Non-disclosing prenatal test
- Decision not to reproduce (may include sterilization)
- Artificial insemination
- Adoption
- Surrogate mother
- Pre-implantation genetic testing and embryo selection
================================
|
Pre-natal and Vitro Fertilization
for HD
Testing of a fetus (prenatal testing)
presents special challenges and risks; in fact some centers do not perform genetic testing on fetuses. Because a positive test result using direct genetic testing means the at-risk parent is also
a gene carrier, at-risk individuals who are considering a pregnancy are advised to seek genetic counseling prior to conception.
Some at-risk parents may wish to know the risk to their fetus
but not their own. In this situation, parents may opt for prenatal testing using linked DNA
markers rather than direct gene testing. In this case, testing does not look for the HD gene itself but instead indicates
whether or not the fetus has inherited a chromosome 4 from the affected grandparent or from the unaffected grandparent on
the side of the family with HD.
If the test shows that the fetus has inherited a chromosome 4 from the affected grandparent,
the parents then learn that the fetus's risk is the same as the parent (50-50), but they learn nothing new about the parent's
risk. If the test shows that the fetus has inherited a chromosome 4 from the unaffected grandparent,
the risk to the fetus is very low (less than 1%) in most cases.
Another option open to parents is in vitro fertilization with
preimplantation screening. In this procedure, embryos are screened to determine which ones carry the HD mutation. Embryos determined not to have
the HD gene mutation are then implanted in the woman's uterus.
==============================
New prenatal test offers quicker results
Sept. 2001
LONDON, Sept. 27 (UPI) -- Scientists in England have developed a new prenatal test that can more quickly determine whether a woman is carrying a baby with potentially
life-threatening genetic abnormalities.
Researchers at Guy's and St. Thomas' Hospital National Health Service Trust
in London report a test called quantitative fluorescence poly-merase chain reaction significantly cuts down
on the time patients and doctors wait for results. Although the extraction of amniotic fluid to study DNA, the genetic code
for all cells, remains the same, advanced laboratory technology can produce answers to anxious parents in one or two days,
compared to the current one week to two weeks it takes with standard testing.
The
technique was tried for one year in which 1,148 amniotic fluid samples, 188 chorionic villus or fetal tissue samples and 37
other fetal tissue samples were examined.
Only 2 percent of the prenatal samples offered no information because the
mother's cells contaminated the sample. But 98 percent did yield data on whether chromosomal abnormalities existed. Researchers
in the United Kingdom could not be reached for comment. These results are reported in the Sept. 29 issue of The Lancet.
Separating the mother's cells from the fetus' can be painstaking. "It's like looking for a needle in a
haystack," Dr. Paul G. McDonough, professor of obstetrics, gynecology, pediatrics and physiology at the Medical College of
Georgia in Augusta, told United Press International. "And once you have those cells, you've got to be sure those are of the
baby" leaving scientists with very small DNA with which to work.
It is crucial to
examine the fetus' DNA because chromosomal abnormalities could indicate disease, such as Down syndrome, and if genetic tests
are performed early enough in the pregnancy, results could affect whether a pregnancy is continued or not.
"Because
the (standard) testing takes so long and because physicians and patients want results as soon as possible...there are a lot
of investigators who are looking at ways for more rapid testing," Dr. Michael Mennuti, professor of obstetrics and gynecology
at the University of Pennsylvania in Philadelphia and secretary of the American College of Obstetricians and Gynecologists,
told UPI.
Mennuti cautioned, however, "if the rapid test doesn't look good,
are you prepared to act on the basis of that?"
Mennuti said this kind of quick testing would make a useful screening
tool for pregnant women, but if there are problems, it should be followed up with standard
prenatal tests before any decisions are made about the future of the pregnancy.
Still, Mennuti added, "We would be
very interested in this...everybody has wanted more rapid testing. This is a step in the
right direction."
(Reported by Katrina Woznicki in Washington)
=============================
There was excellent information on Preimplantation Genetic Diagnosis for Huntington
provided by Mark Hughes, MD, PhD on the old MGH HD forum for anyone interested:
MGH HD Discussion
Dr. Hughes answered some questions with:
1) The oldest baby now, who was born through PGD to avoid genetic disease is 11
years old. The oldest one born to avoid HD is now almost 6 years old.
2) We work with clinics all over the nation.
Yes - we still do this for free. I don't know for how much longer since the donations we have received are not keeping up with the demand. But we will hold out as long as possible.
3) I do not think I should post the actual names
of clinics on a public web site. But we work with 4 large clinics in the NYC area as well as programs all up and down the coast (and across the country).
Our goal is to help desperate couples who seek to have a family, by
providing them the opportunity to begin their pregnancy knowing that HD is not present in their baby.
Rather
than post clinic info here, if you are interested in hearing more about all of this, you should call our genetics program
at the Universtiy at 313-993-1353 and one of the genetic counselors will talk with you about your options
in the NY area.
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Other Web Sites
The American Society for Reproductive Medicine page has lots of good links and consumer information. Atlanta Reproductive
Health Centres great Info
Genetic Diagnosis http://www.asrm.org/
This fact sheet covers how and why clinics do preimplantation diagnosis and lists the
diseases that the procedure can screen out.
To look up any unfamiliar terms described above go to PersonalMD Medical Dictionary |
