SNP microarray genetic analyses to determine 23-chromosome ploidy, structural chromosome aberrations and genome-wide scans to identify disease risks from a single embryonic cell

The use of arrays in preimplantation genetic diagnosis and screening

BACKGROUND

In preimplantation genetic diagnosis (PGD), polymerase chain reaction has been used to detect monogenic disorders, and in PGD/preimplantation genetic screening (PGS), fluorescence in situ hybridization (FISH) has been used to analyze chromosomes. Ten randomized controlled trials (RCTs) using FISH-based PGS on cleavage-stage embryos and one on blastocyst-stage embryos have shown that PGS does not increase delivery rates. Is the failure of PGS due to a fundamental flaw in the idea, or are the techniques that are being used unable to overcome their own, inherent flaws? Array-based technology allows for analysis of all of the chromosomes. Two types of arrays are being developed for use in PGD; array comparative genomic hybridization (aCGH) and single nucleotide polymorphism-based (SNP) arrays. Each array can determine the number of chromosomes, however, SNP-based arrays can also be used to haplotype the sample.

OBJECTIVE(S)

To describe aCGH and SNP array technology and make suggestions for the future use of arrays in PGD and PGS.

CONCLUSION(S)

If array-based testing is going to prove useful, three steps need to be taken: [1] Validation of the array platform on appropriate cell and tissue samples to allow for reliable testing, even at the single-cell level; [2] deciding which embryo stage is the best for biopsy: polar body, cleavage, or blastocyst stage; [3] performing RCTs to show improvement in delivery rates. If RCTs are able to show that array-based testing at the optimal stage for embryo biopsy increases delivery rates, this will be a major step forward for assisted reproductive technology patients around the world.

The role of assisted reproductive technology in the management of recurrent pregnancy loss

PURPOSE OF REVIEW

Description of genetic screening of preimplantation embryos as a means of reducing miscarriages in patients with recurrent pregnancy loss.

RECENT FINDINGS

That the promise of preimplantation genetic screening (PGS) for ameliorating recurrent pregnancy loss has been fulfilled is controversial. An array of comparative studies has suggested a positive effect of PGS on implantation rate, but these have been balanced by studies showing no effect or a negative effect, highlighting the need for more rigorously designed studies and randomized controlled trials. Emerging technologies may provide more information from the embryo biopsies even as the mosaicism of the embryo and its implications for interpreting PGS data are recognized.

SUMMARY

Through the screening of embryos for abnormality in chromosome number or structure and selecting only normal embryos for transfer, PGS was envisioned and applied as a therapeutic tool for improving implantation and live birth rates from in-vitro fertilization and providing a means of attenuating pregnancy loss in recurrent pregnancy loss patients. An array of reports on the effects of PGS on embryo implantation and live birth rates has been made since its introduction, showing, variously, increases, decreases or no changes in these parameters. Various factors may influence the efficacy of PGS, including the patient population to which it is applied, technical aspects such as embryo biopsy, the genetic analysis and embryo culture environment, the current limitation of the genetic analysis (a subset of, rather than all, the 24 chromosomes) and the mosaicism of the embryo and blastocyst. Collectively, these contribute to the challenge of optimizing PGS and understanding how the screening result reflects the ultimate genetic constitution of the conceptus. Emerging cytogenetic and molecular technologies such as comparative genomic hybridization and microarray analysis may provide a broader appraisal of the embryo for a more comprehensive evaluation of developmental potential and prognosis for live birth.