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PGD in Italy: 1PB Testing
Technical description of the procedure
Because 1PB contains a genotype opposite to that of its
corresponding oocyte, it may be removed before
fertilization, and tested to assess the genetic status of
the oocyte.
In women who are carriers for a genetic disease, genetic
analysis of 1PB allows the identification of oocytes that
contain the maternal unaffected gene. In fact, if the 1PB
analysis reveals the presence of the maternal mutation, as a
consequence both copies of the mutated gene have been
extruded and the oocyte can be predicted to be free of
mutation
. Conversely, if the 1PB does not show any mutation, the
oocyte presumptively contains the mutant allele (Figure 1).
Only those oocytes diagnosed to contain the normal gene will
be fertilized and transferred to the mother without risk of
establishing a pregnancy affected by the genetic disease.
Genetic analysis of 1PB may be complicated by recombination
events which occur regularly between homologous chromosomes.
If crossing-over involves the region containing the gene
under investigation, both the mutated and the normal allele
segregate to the 1PB while the oocyte retains one copy of
both the normal and the mutated gene (Figure 1C).
Figure 1. First polar bodies (1PBs) (smaller circles)
are extruded during maturation of oocytes, after the first
meiotic division. Because 1PB is the mirror image of the
oocyte, the genotype of the latter can be indirectly derived
from the opposite diagnosis of its 1PB. If 1PB analysis does
not show any mutation (A), the oocyte presumptively contains
the mutant allele. Conversely, if 1PB analysis reveals the
presence of the maternal mutation (B), as a consequence both
copies of the mutated gene have been extruded and the oocyte
can be predicted to be free of mutation.
First polar body diagnosis may be complicated by
recombination events. If a recombination occurs, involving
the region containing the gene under investigation, both the
mutated and the normal allele segregate to the 1PB while the
oocyte retains one copy of both the normal and the mutated
gene (C). In this case, 2PB testing or analysis of the
resulting embryos is necessary to predicted genetic status
of the oocyte.
The black/filled rectangle represents the gene with maternal
mutation. Numbers in bold represent alleles of polymorphic
STR markers linked to the gene. 1-2-3-4 is the haplotype
associated with the maternal mutation; 5-6-7-8 is the
haplotype associated with the normal gene. If crossing over
does occur, the 1PB will contain both the normal and the
mutated haplotype, showing a heterozygous genotype for the
STR markers investigated. (1-5; 2-6; 3-7; 4-8).
Next :
Mutation analysis
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