Preimplantation Genetic Diagnosis: Limitation of Single Cell Analysis

Embryonic testing for IVF embryos in the form of preimplantation genetic diagnosis, or PGD testing, is recommended when either the mother or father or both of the IVF partners are known to be affected by genetic abnormalities. This includes scenarios wherein the partners seeking IVF treatment are merely carriers of a genetic condition or have a family history of genetic diseases. One the most common forms of PGD testing is testing for Single Gene Defects—genetic defects induced by an irregularity in only one of the genes.

Limitations of Single Cell Analysis

To test the embryo for single gene defects, the IVF clinic needs to extract a cell from the embryo and analyze it. Despite the advances in PGD testing methods, single cell testing methods present some limitations.

Cleavage-Stage Biopsy

This is the most common single cell extraction method. It is used upon three-day-old embryos. The cell biopsy used here is a very demanding procedure, and damage to the embryo is likely. Further, the tools needed for this method include micro-manipulators and special microscopes that may not be available at all IVF clinics. The embryonic cell extracted in this method may not be fully representative of the genetic traits carried in the remaining embryonic cells.

Polar Body Biopsy

Polar bodies are a cellular structure present only in the female egg cell. Testing polar body cells means that single gene defects from the mother's genetic material can be decoded, but those caused by the father's genetic material cannot be detected.

Blastocyst-Stage Biopsy

This biopsy presents a slight advantage since the embryo is a bit more developed, ensuring sizeable number of cells—usually more than 100. However, this procedure has a limitation in the form of cells it can procure, i.e., cells of the inner cell mass are not biopsied. This is particularly applicable to embryos exhibiting mosaicism. Such embryos have different groups of cells, each group pertaining to a particular chromosomal combination.

Single Cell Analysis and Cystic Fibrosis

Some single gene disorders are typically challenging, such as cystic fibrosis. This condition can arise from a vast range of gene mutations. However, single cell analysis used in PGD testing methods can test for 25 mutations only.