There are several methods of genetic analysis that are used to perform PGD. Those methods are FISH, PCR, CGH, SNP and also P-STR and the most recent method Karyomapping.
FISH – Fluorescent In Situ Hybridization
This method is used to determine the sex for X-linked diseases (recessive inheritance), abnormalities in the chromosomes and aneuploidy screening. The downside to this method is that it allows to accurately estimate only 10 to 12 chromosomes pairs in each biopsied cell out of the whole 23 pairs of chromosomes.
So in the end the chromosomes are only partly screened for disorders. Furthermore in this method the chromosomes do not undergo screening for translocations and inversions which makes it a less effective test.
PCR – Polymerase Chain Reaction
This method is sometimes referred to as DNA amplification. In most of the cases it is used to diagnose a single gene defect which includes the dominant and recessive disorders.
CGH – Comparative Genomic Hybridization
The CGH method is more effective than FISH because it allows the examination of all 23 chromosomes an in this way provides a significantly more detailed view of the whole length of the chromosome. It is able to detect imbalances in the segments of the chromosomes.
There is also a new technique in the field of CGH called array CGH or microarray CGH. This is an enhanced CGH test which can provide the results for all chromosomes in just 24 hours.
SNP – Single Nucleotide Polymorphism Analysis
This testing technique also provides you with the ability to examine all 23 pairs of chromosomes. Furthermore it allows you to register and copy all the number changes, translocations and inversions in the entire length of the chromosome.
P-STR – polymorphic short tandem repeats
The STR method is a simple alternative to FISH used to detect unbalanced chromosomal states in preimplantation embryos. It helps in discriminating between the normal and the unbalanced chromosome states in each of the embryos.
Next generation sequencing
NGS method allows to sequence whole genomes, zoom into target regions and utilize RNA sequencing. It also allows to analyzing the genome-wide methylation. It does a full chromosome screening and translocations and for monogenic diseases.
It is the newest method in PGD testing. The father, mother and another relative which is carries or is affected with the disorder give their blood samples. What Karyomapping does is that it examines the chromosomes of the mother, father and the relative from 300.000 different points. This way it is searching for the characteristic features of the defective chromosome. Eventually the karyomapping finds the “mark” which is unique for the specific chromosome that carries the gene with the defect. Afterwards it is possible to check the embryos for the presence of the “mark” and any embryo carrying it means it has inherited the chromosome that carries the gene with the defect. If the mark isn’t detected it means that the embryo is free of the genetic disorder and that it is a proper embryo to be transferred to the uterus.
Karyomapping’s advantage is that waiting time before the treatment is just a couple of weeks and with other methods it can be sometimes even months because they have to be prepared individually for each patient. Karyomapping is a universal test for PGD that can find almost all gene defects.