Traditionally, diagnosis of the genetic causes for mental retardation was made through fluorescence in situ hybridization (FISH). However, nowadays, technological advances have proven that this technique is unable to detect the presence of several syndromes caused by genetic alterations. This is the reason why comparative genomic hybridisation (CGH) has been developed.
It consists of an exhaustive study of an individual’s genome based on the microarrays technology. This study allows indentifying and analysing possible genetic alterations in a chromosomal segment, such as gene amplifications (chromosome gain) or gene deletions (chromosome deletion) a in a single trial. The whole process takes normally less than two weeks. It allows detecting new syndromes effectiveness ten times more effectiveness than a conventional karyotype study. CHG is widely used for pre-implantation and pre-natal genetic studies by means of a PGD.
How do CGH arrays work?
This method consists of using control or reference DNA and a sample of DNA for analysis. Both DNAs are marked with a different fluorescent DNA probe in order to be able to identify potential genetic deletions or amplifications between them.
In spite of this technique is far more effective than conventional techniques for genetic study, they actually perform quantitative comparisons, that is to say, if there is too much or a lack of genetic material. Thus, it will be impossible to detect if there are chromosome fragments which are placed in the wrong place, such as mutations, inversions, translocations…
PGD with CGH arrays
Main indications for the use of CGH arrays are cases of mental retardation, autism, dimorphism, birth defects, recurrent miscarriage, advanced maternal age, pre-natal diagnosis in cases of high-risk pregnancy or after ultrasonographic findings, confirmation of results in karyotypes, etc. This is the reason why it is very helpful when it comes to perform a pre-implantation genetic diagnosis (PGD).
This is so, because not all the couples that turn to assisted reproduction clinics do it because they can’t achieve pregnancy, some of them do it because they have the suspicion that thay are carries of some genetic alteration or are suffering from repeated miscarriages.
Thus, every couple with risk of producing altered embryos would be candidates for using PGD with CGH arrays, since it is a technique for diagnosis that allows detecting any alteration related to the set of chromosomes by performing a complete evaluation of the karyotype, which consists of counting and examining each single chromosome (23) in a single cell of an embryo on day 3 or 5 of embryonic growth, and not only the amount of 9 chromosomes that FISH was able to evaluate. This improves the pregnancy rate in patients with risk of developing numerical chromosomal alterations, since it allows choosing only the embryos that are normal in terms of chromosomes.