Novel strategies for genetic screening of embryos

Alan H Handyside, Mark D Robinson and Francesco Fiorentino

London Bridge Fertility, Gynaecology and Genetics Centre, London, UK and Laboratorio Genoma, Rome Italy

Genetic screening of the early human embryo following assisted conception, typically involves removal of single or small numbers of cells for analysis using either multicolour fluorescence in situ hybridisation (FISH) with chromosome specific probes to interphase nuclei or highly sensitive PCR amplification methods. Recently, whole genome amplification has been achieved from small quantities of genomic DNA by isothermal multiple displacement amplification (MDA) with bacteriophage 29 DNA polymerase and exonuclease resistant random hexamer primers. MDA performed on single and small numbers of lymphocytes and blastomeres isolated from cleavage stage embryos, yields microgram quantities of amplified DNA which is highly representative of the genotype of the sampled cell(s) (Handyside, Robinson et al, 2004; Handyside, Robinson and Fiorentino, 2005). The use of MDA as a universal first step marks a new era for PGD since, unlike previous PCR based methods, sufficient DNA is amplified for analysis by conventional methods or for genome wide analysis using high throughput sequencing and microarray methods.

One important application will be screening for gross genetic defects, including chromosomal aneuploidy, which is being used increasingly as a method for embryo selection in women of advanced maternal age and other at risk indications. Analysis of interphase nuclei by multicolour FISH is limited by the number of probes which can be used simultaneously and the time required for sequential hybridisation of different probe sets. MDA and array comparative genomic hybridisation may allow molecular karyotyping for all chromosomes within 24 hours.

In an age when it is becoming possible to envisage sequencing the entire 6 billion base pairs in the genome of an individual adult, quickly and relatively cheaply, whole genome amplification raises the possibility of obtaining essentially unlimited genetic information from individual embryos. As the goal of assisted reproduction moves increasingly from overcoming infertility and simply getting patients pregnant towards helping these couples have healthy children, routine genetic testing of embryos, which has already begun for prenatal diagnosis in natural pregnancies, may become the norm as we understand more about the consequences of genetic variation between individuals.

Handyside, AH, Robinson, MD, Simpson, RJ, Omar, MB, Shaw, MA, Grudzinskas, JG, and Rutherford, A (2004). Isothermal whole genome amplification from single and small numbers of cells: a new era for preimplantation genetic diagnosis of inherited disease. Mol Hum Reprod 10, 767-72.

Handyside, AH, Robinson, MD and Fiorentino, F (2005) Pre-implantation genetic diagnosis using whole genome amplification. In ‘Whole Genome Amplification: Methods Express (eds Hughes, S and Lasken, R) Scion Publishing Ltd,