Project Leader: Dr Helen White

Invasive prenatal diagnosis is offered routinely to pregnant women who have been identified as having an increased risk of foetal chromosome abnormalities. Pregnancies at high risk are identified by serum or ultrasound screening, advanced maternal age or because one parent is known to carry a chromosome abnormality. Invasive sampling takes place at either 10-12 weeks (chorionic villus sampling) or 15-20 weeks (amniocentesis) and diagnosis has traditionally been based on karyotype analysis which can detect both numerical and structural chromosome abnormalities. The most commonly detected abnormalities are trisomies for chromosome 21 (Down syndrome), chromosome 18 (Edwards syndrome), chromosome 13 (Patau syndrome) and sex chromosome aneuploidy (leading to syndromes such as Turner (monosomy X) and Klinefelter (XXY)).

In an effort to improve pregnancy management and alleviate maternal anxiety rapid aneuploidy detection techniques are now being implemented into routine prenatal diagnosis e.g. interphase FISH, quantitative fluorescent PCR (QF PCR), multiplex ligation dependent amplification (MLPA). These tests are usually capable of delivering results within 1-3 days and are viewed as a prelude to, rather than a replacement of, full karyotype analysis.

Rapid prenatal aneuploidy tests need to fulfil certain criteria: the assay must be accurate and no false positive results should be obtained as this could result in the termination of a healthy pregnancy. The test should be robust enough to cope with variable sample quality, provide unambiguous results and have a low failure rate. Ambiguous results have the potential to increase maternal anxiety and can cause delays in reporting while additional investigations are carried out. The test should be adaptable to cope with high sample throughput and test costs should be low since rapid tests are often performed in addition to karyotyping. Ideally, the test should be able to detect maternal cell contamination (MCC), mosaicism and triploidy (Mann et al., Strategies for the rapid prenatal diagnosis of chromosomal aneuploidy. Eur J Hum Genet 2004, 12: 907-15).

QF PCR analysis of short tandem repeats (STR) is being used successfully in many UK and European laboratories for the rapid diagnosis of prenatal aneuploidy. Chromosome specific polymorphic repeat sequences, which vary in length between individuals, are amplified using fluorescent primers. The PCR amplicons are analysed using an automated genetic analyser capable of 2bp resolution and the representative amount of each allele is quantified by calculating the ratio of the peak height or area using appropriate software.

Commercially available QF-PCR kits have been launched which are IVD/CE certified under 98/79/EC. We have evaluated two kits: