Microarray Preimplantation Diagnosis (MA-PGD) created much excitement and interest at three recent meetings attended by Dr. Schriock; Pacific Coast Reproductive Society, The Midwest Reproductive Symposium, and the IVF Comprehensive Update.

PGD is a technique used to diagnosis genetic disorders by performing a biopsy of the embryo on day 3 or 5. PGD can diagnose single gene or chromosomal defects. PFC has been doing embryo biopsy for over 10 years. During this time the major method of diagnosing chromosomal disorders has been fluorescent in situ hybridization, FISH. FISH uses a fluorescent color to label individual chromosomes. This technique lacks accuracy and is now seldom used to screen embryos for the presence of missing or extra chromosomes. (refer to Fertility Flash Vol. 5 Issue 2). This technique, however, is still valid for identifying the gender of the embryo. MA-PGD uses a new technology, Single Nucleotide Polymorphisms (SNPs). SNPs are single bases, the building blocks of DNA, which can be in a different sequence in different individuals. Six to ten million SNPs have been characterized. This is the technology used in DNA fingerprinting in criminal or forensic work. Compared to FISH, where only one color marker identifies the chromosome, SNPs havethousands of markers per chromosome.

FISH can only identify 8-12 of the 24 unique chromosomes; MA-PGD will identify all 24 chromosomes, similar to amniocentesis. Identifying both single gene defects and chromosome abnormalities from one embryo cell was not possible with the older techniques, but can be done with MA-PGD. MA-PGD will identify whether the abnormal chromosome came from the mother or father. If from the mother, it will determine if the error was in meiosis I or II, or mitosis. In other words, it can identify in which stage of early cell division the genetic error occurred. Using MA-PGD, it may be possible to determine which embryo produced the baby when more than one embryo is transferred. The most important advance, however, will be the accuracy of the result. New research using MA-PGD shows that FISH is inaccurate over 40% of the time. MA-PGD appears to be nearly 100% accurate in diagnosing abnormal embryos.

This new technology is also helping to answer scientific questions. 50 – 70% of embryos with one missing or extra chromosome still develop to a healthy-looking day 5 blastocyst. This helps explain why beautiful blastocysts do not always turn into healthy pregnancies. MA-PGD will also raise new questions: Only 55% of chromosomally normal embryos turn into successful pregnancies in 30-year-olds, only 25% in 40-year-olds. Why do these embryos with a normal number of chromosomes fail? There is more to the embryo than chromosomes and more research is needed to determine what factors allow an embryo to develop into a healthy baby. Current areas of investigation include RNA production (transcriptomics), protein production (proteomics), and metabolic by products (metabolomics).

We will continue to update readers on PFC’s experience with MA-PGD in future Fertility Flash issues.

Tags: PGD - Preimplantation Genetic Diagnosis

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