Comparative genomic hybridisation (CGH) screening
What is CGH?
CGH is an emerging form of preimplantation genetic screening (PGS) that is used to detect abnormalities in the number of chromosomes in an egg or an embryo.
Because this is a new form of screening and has only been used in a small number of cases, it is not yet possible to say how successful it is.
Abnormalities in chromosomal number (aneuploidy) are a major cause of the failure of embryos to implant or miscarry and can also cause conditions such as Down’s syndrome. Most aneuploidy in embryos can be attributed to maternal age, though paternal age also has some effect.
With CGH, a sample taken from an egg (the oocyte polar body) or a single cell of a blastocyst stage embryo is compared to a normal control sample. Variations in the chromosomal content are then detected using fluorescent dyes.
CGH takes advantage of new ‘microarray’ technology that provides much more detail than ‘standard’ PGS. The full set of 23 pairs of chromosomes can be compared so that abnormalities in the numbers of multiple chromosomes can be detected.
With ‘standard’ PGS, which uses a technique called fluorescence in situ hybridisation (FiSH), even the most advanced labs can only test up to 12 chromosome pairs, not the full 23 pairs tested using CGH.
One of the limitations of FiSH PGS is that only a maximum of 5 coloured dyes can be used on each slide, one coloured dye being used to investigate one chromosome pair. If any more colours are used it becomes too difficult to see differences through a microscope and identify which chromosome pair they arise from.
How is CGH licensed by the HFEA?
Clinics wishing to use CGH must be licensed to carry out preimplantation genetic screening (PGS).
Up until 1st October 2009 clinics must apply to the HFEA for a variation to their licence to use CGH as their method of carrying out PGS. After that date (when the HFEA’s 8th Code of Practice comes into force) clinics can use whichever technique they prefer, but must determine whether it is suitable for the patients they intend to treat.
There are some specific requirements for clinics that carry out PGS. The HFEA’s 8th Code of Practice says that clinicians must use their professional judgement as to whether PGS (whichever technique is used) is appropriate in each case and that they must ensure that patients are fully informed about the efficacy of the test.
The HFEA’s Scientific and Clinical Advances Committee (SCAAC) has been monitoring the development of microarrays, including their use for CGH. As with any emerging technology, the HFEA will continue to monitor this new technique.
Read the latest papers considered by SCAAC on:
Which clinics offer CGH?
CGH is a new screening technique and is currently offered by only a couple of clinics, but clinics regularly apply to the HFEA to add new techniques to their licence. To find a clinic that offers CGH, you will need to search for those clinics that offer PGS.
Page last updated: 02 September 2009