Because PGD for single gene disorders is based on single-cell genetic analysis, its accuracy depends largely on the limitations of single-cell DNA analysis, which may potentially cause misdiagnosis. One of the key contributors to misdiagnosis is the phenomenon of preferential amplification, also known as allele-specific amplification failure [allele dropout (ADO)], requiring the application of special protocols to ensure the highest ADO detection rate.[1'2] A few previously reported misdiagnoses, involving PGD for cystic fibrosis (CF), myotonic dystrophy (DM), and fragile-X syndrome (XMR1), might have been due to this phenomenon, which has not initially been fully realized.[3,4]
It has been demonstrated that ADO rates in single cells might be different for different types of heterozygous cells. The ADO rate may exceed 20% in blastomeres, compared with ADO rate in single fibroblasts and PB1, which was shown to be under 10%. A high rate of ADO in blastomeres may lead to an obvious misdiagnosis, especially in compound heterozygous embryos. As mentioned, most misdiagnoses, especially those at the initial stage of application of PGD for single gene disorders, were in the cases of blastomere biopsy from apparently compound heterozygous embryos. To avoid a misdiagnosis because of preferential amplification, a simultaneous detection of the mutant gene together with up to three highly polymorphic markers, closely linked to the gene tested, was introduced.1-1'2-1 Each additional linked marker reduces the risk for misdiagnosis significantly, which may practically be eliminated with the application of three markers. So a multiplex nested PCR analysis is performed, with the initial PCR reaction containing all the pairs of outside primers, so that following the first-round PCR, separate aliquots of the resulting PCR product may be amplified using the inside primers specific for each site. Only when the polymorphic sites and the mutation agree are embryos transferred. So the multiplex amplification allows detecting ADO and preventing the transfer of misdiagnosed affected embryos.
Another efficient approach for avoiding misdiagnosis is a sequential genetic analysis of the PB1 and PB2 in PGD for maternally derived mutations. Detection of both mutant and normal alleles in the heterozygous PB1, together with the mutant allele in the corresponding PB2, leaves no doubt that the resulting maternal contribution to the embryo is normal, even without testing for the linked markers as a control. However, it is ideal to test simultaneously at least for one linked marker to confirm the diagnosis. Alternatively, the mutation-free oocytes may also be predicted when corresponding PB1 is homozygous mutant, in which case the corresponding PB2 should be hemizygous normal, similar to the resulting maternal pronucleus. However, the genotype of the resulting maternal contribution may be quite opposite, i.e., mutant, if the corresponding PB1 is in fact heterozygous but not detected because of ADO of the normal allele.
The other method with the proved potential for detecting and avoiding misdiagnosis because of preferential amplification is fluorescence PCR (F-PCR), which may allow detecting of some of the heterozygous PB1 or blastomeres misdiagnosed as homozygous in conventional PCR. In addition, the method also allows a simultaneous gender determination, DNA fingerprinting, and detection of common aneuploidies.
Finally, because of high rate of chromosomal mosai-cism at the cleavage stage, testing for the chromosome, in which the gene in question is mapped, is of an obvious value to exclude the lack of mutant allele because of monosomy of this chromosome in the biopsied blasto-mere. As mentioned, aneuploidy testing is technically feasible and may be performed by adding primers for chromosome-specific microsatellite markers to the multiplex PCR protocols worked out for specific genetic disorder.
Because of need for the development of a custom-made PGD design for each mutation and each couple, a preparatory work has become an integral part of PGD for single gene disorders to ensure avoiding the potential misdiagnosis. For example, in some cases, a particular set of outside primers has to be designed to eliminate false priming to the pseudogene, as described in PGD for long-chain 3-hydroxyacyl-Coa dehydrogenase deficiency. In addition, the preparatory work may frequently involve a single sperm typing needed for establishing paternal haplotypes, so that linked marker analysis could be performed in addition to mutation analysis, especially in cases of paternally derived dominant conditions or PGD combined with preimplantation HLA matching.
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