17 May 2005
Non-invasive prenatal tests such as serum screening and ultrasound are useful in the identification of potential foetal abnormalities, but do not permit definitive diagnoses to be made. Reliable diagnostic tests currently require the use of invasive techniques such as amniocentesis and chorionic villus sampling, procedures that carry an associated risk of miscarriage. For some years now there has been interest in sampling foetal DNA from maternal blood samples as a safe alternative to these invasive procedures; this approach has already been successfully used to determine the gender of male foetuses, by identification of sequences from the foetal Y chromosome in the maternal circulation. However, previous attempts to develop methods for screening foetal DNA from maternal blood for genetic abnormalities have been limited by the very low levels of foetal DNA that can typically be recovered.
Research published this month in the Journal of the American Medical Association (JAMA) by scientists from the US company Ravgen Inc. presents a method to increase the percentage of free foetal DNA that can be obtained from maternal blood samples [Dhallan R et al. (2004) JAMA 291, 1114-1119]. The key to their technique was limiting the degree of lysis (bursting) of maternal cells in the blood samples, since this releases free maternal DNA into the sample and effectively decreases the proportion of foetal DNA. This was achieved by careful processing of blood samples, and by the addition of formaldehyde (a chemical that stabilises cells and reduces levels of lysis) to the blood samples. The authors suggest that formaldehyde may also stabilise DNA and inhibit its breakdown by enzymes, increasing the total levels of free foetal DNA that can be recovered.
In the study, blood was collected and analysed from a total of 79 pregnant women, 75 of whom were carrying a male foetus. Two tubes of blood were collected from each woman: formaldehyde was added to one whilst the other was left untreated. The percentage of foetal DNA present in each sample was determined using PCR amplification of a Y-chromosome specific sequence (representative of foetal DNA), and a second sequence present in both the foetal and maternal DNA; the relative quantities of the foetal-specific and general sequence markers allowed the proportion of foetal DNA to be calculated.
In the first phase of the study, samples from ten women were analysed; the mean proportion of foetal DNA detected in the treated samples was 20.2%, whereas that in the untreated samples was only 7.7%. Because this difference was statistically significant, the study was then extended to a further 69 pregnant women, this time to determine the percentage of free foetal DNA in formaldehyde treated blood samples from a larger population; the mean figure was found to be 25%. However, a considerable degree of variability was observed between individual samples. The authors note that a larger study is desirable to investigate this phenomenon, whilst proposing several potential explanations, but conclude that their methods effectively increase the percentage of foetal DNA present in the maternal blood samples, making its detection and analysis for genetic abnormalities much more feasible.
Comment: The authors