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,