Preimplantation Genetic - Diagnosis in blastomeres
Currently, PGD for single gene disorders can be accomplished
by either polar body removal or blastomere biopsy. Which
method may be used is determined on a case-by-case basis.
This procedure can be used to select for embryos that do not
have a specific genetic disease by testing the polar bodies
or blastomeres for the genetic mutation.
Inside the test tube containing the blastomere, a solution
is added that permits cell lysis and thus the liberation of
DNA from the cell nucleus. Subsequently, by means of in
vitro enymatic ampification known as the Polymerase Chain
Reaction (PCR), the genetic region of interest, involved in
the mutations that is being searched for, is amplified
millions of times. The genic amplification product then
undergoes mutation analysis to search for genic mutions
present in the couple. The analysis of mutations is the most
important and delicate phase of PGD.
To guarantee maximum interpretative reliability, it is
indispensable that it be carried out with methods and
instruments that permit the univocal identification of the
mutations that are being searched for. Automated sequence
analysis is presently the best method of genetic analysis,
inasmuch as it permits the exact determination and direct
visualization of a specific mutation. The application of
this technique to PGD is done by the use of completely
automated state of the art equipment.
PGD centers worldwide are concerned about the possibility of
misdiagnosis, which can occur as a result of failure of
allele specific amplification, or allele dropout (ADO). In
basic terms, this is the failure of one of the genes
(allele) to show up in the analysis (it ?drops out? of the
picture.) ADO is of concern primarily in blastomere biopsy
when each parent carries a different mutation for a
To minimize the potential for diagnostic error by PGD, it is
preferable to offer polar body removal or to perform a
combined analysis of the specific mutation and specific
linked markers that are inherited along with the gene. This
dual amplification allows for higher accuracy and for the
detection of ADO, thus enhancing diagnostic accuracy and
reducing the risk for misdiagnosis.
Embryo transfer and implantation