Molecular diagnostics in preimplantation genetic diagnosis.

Preimplantation genetic diagnosis (PGD) is a procedure that allows embryos to be tested for genetic disorders before they enter the uterus and before pregnancy has begun. Embryos obtained by in vitro fertilization undergo a biopsy procedure in which one or two cells are removed and tested for a specific disorder. If the cell is unaffected, the embryo from which it was taken is judged to be free of the disorder. The embryo can then be transferred to the uterus to initiate pregnancy. Couples whose children are at increased risk for a specific genetic disorder can benefit from PGD. Some of these couples may have affected family members or family ancestry that puts them at high risk for transmitting a particular disorder to their offspring. PGD is an alternative to prenatal tests such as amniocentesis or chorionic villus sampling and since it is performed before a pregnancy has begun, it may be more acceptable to couples who have either had an affected child, previous termination of pregnancy, or who have objections to termination of pregnancy. PGD tests have largely focused on two methodologies: fluorescent in situ hybridization (FISH) and polymerase chain reaction (PCR). This review will focus on the use of PCR-based methodologies to diagnose single gene disorders in single cells; specifically describing the characteristics and limitations of single cell PCR and mutation detection strategies which have been developed for use in clinical PGD. The hundreds of cycles of preimplantation diagnosis performed to date have resulted in the birth of several hundred healthy children. 1 As shown in Table 1​1 , the genetic conditions for which PGD has been applied are numerous and the various methods used for diagnosis reflect the heterogeneity of causative mutations. Table 1. Strategies for PCR-Based Tests Used for Clinical Preimplantation Genetic Diagnosis The first clinical application of PGD used a generic PCR protocol for gender determination to avoid the transfer of male embryos which have a 50% probability of being affected by an X-linked recessive disorder. Gender was determined in a single blastomere by a single round of PCR using primers for Y-chromosome specific repetitive DNA sequences. The presence of Y-specific PCR amplification products was indicative of a male embryo and the absence of products was scored as female. 2 Although this approach had some success, a misdiagnosis, presumably due to amplification failure, did occur and emphasized the challenges inherent in single cell analysis and, more specifically, the danger in relying on the absence of amplification to diagnose genotype. 3 Subsequently, PCR protocols for preimplantation gender determination were refined to include primer sets which simultaneously amplify sequences common to both sex chromosomes (for example single copy genes such as ZFX/ZFY, 4 AMELX/AMELY, 5 ) and repetitive sequences such as DXZ1 and DYZ1. 6, 7 Sequences common to the sex chromosomes are identical at the site of primer annealing but differ internally in terms of size or include minor polymorphisms. Despite these technical improvements, the majority of embryo sexing is now accomplished using fluorescent in situ hybridization (FISH) which is less prone to contamination and can also provide the copy number for each chromosome tested thereby potentially avoiding the transfer of common chromosome abnormalities such as triploidy or X-monosomy. 8, 9 Although FISH has largely superseded PCR for sex determination, the specific diagnosis of single-gene defects remains dependent on DNA amplification with PCR. In the case of X-linked disorders, testing of the specific gene has the added advantage of ensuring that all embryos free of the mutant gene can be selected for transfer, irrespective of gender. 10, 11, 12 The list of disorders and the particular mutation detection strategies used for PCR-based clinical PGD application are given in Table 1​1 .