Spinal Muscular Atrophy (SMA)
Spinal Muscular Atrophy (SMA) Type I is a lethal disorder characterized
by progressive muscle weakness. The gene responsible for SMA is called
SMN1. Most children with SMA are missing, or have a deletion, in both
copies of the SMN1 gene. Thus, they have no working copy of the SMN1 gene.
SMA is inherited in an autosomal recessive manner. In approximately 95-98%
of cases of SMA, both parents are identified through genetic testing as
being a carrier for this disease. Testing can determine if parents of
an affected child have one working copy and one deleted copy of the SMN1
gene. Since carriers have the one working copy of the gene, they are unaffected.
In families in which both parents have been identified as a carrier for
SMA, there is a 1 in 4 (25%) chance in each pregnancy to have a child
with the disease. Some couples opt for prenatal diagnosis during pregnancy
to determine if a pregnancy is affected. Others wish to reduce the risk
of conceiving an affected pregnancy through the use of PGD.
In a PGD cycle, embryos created by IVF are cultured in the laboratory
for 3 days at which time they contain approximately 8 cells (Embryos at
this point are called blastomeres). Embryos with normal development on
Day 3 are biopsied using micromanipulation techniques. This involves the
use of very fine glass needles and tools under microscopic observation
and control to remove 1or 2 cells from each blastomere for analysis.
At this early stage of embryological development, the blastomeres are
totipotent; they are still capable of forming cells of any organ or tissue.
Removal of a few of the cells of the early embryo does not alter the ability
of that embryo to develop into a complete, normal pregnancy and child.
Data from many years of PGD in animals and approximately 1200 live births
in humans indicate that PGD does not lead to an increase in birth defects
or chromosomal disorders.
When a PGD biopsy is done, 1-2 cells are removed from the embryo. In
order to obtain results of the biopsy, the cell removed must contain a
nucleus, as the nucleus contains the genetic information necessary for
testing. By testing cells of early embryos for deletions in the SMN1 gene,
only those embryos determined to be unaffected (having two working copies
of the gene) are selected for transfer back into a woman’s uterus.
Thus, the chance to have a healthy unaffected pregnancy is greatly increased.
For more information about Genetics & IVF Institute's PGD for aneuploidy,
please call (800) 654-4363 or (703) 698-7355
and ask to speak with the PGD counselor OR email PGD@givf.com.
Please click on the another question for more information.
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