Contamination

Conventional genetic diagnosis is based upon the amplification of about 100 ng of genomic DNA (corresponding to DNA contained in approximately 20 000 cells). PGD relies on the study of a single cell, containing about 7 pg of DNA (20 000 times less than standard conditions).

Because of the minute amounts of DNA template contained in a single cell, a large number of PCR amplification cycles are required in order to easily analyse the PCR products. The extreme level of DNA amplification may result in contamination, becoming a major problem for single cell PCR that may be absolutely avoided.

Contaminants can enter the PCR from many sources : cumulus cells surrounding the oocyte (maternal origin), skin cells from technicians and scientists carrying out the test (in IVF and PGD labs), or DNA carried over from previously amplified PCR products.

Therefore, a set of measures to prevent contamination should been taken by all laboratories performing PGD. These include separate work areas -each having dedicated special equipment- for PCR preparations and PCR products analysis, sterilizing and aliquoting reagents, working in laminar flow hoods, using protective clothing (coats, haircaps, masks and gloves) and regularly cleaning all working surfaces with DNA degrading detergents.