Noninvasive prenatal diagnosis: from dream to reality.

I became interested in the area of prenatal diagnosis when I was a medical student at Oxford. I had been following the impressive advances occurring in the prenatal diagnosis of the hemoglobinopathies through the pioneering work of Prof. Yuet Wai Kan at the University of California San Francisco (1 ) and Prof. David Weatherall at Oxford (2 ). I also had the good fortune of getting firsthand experience in a research laboratory by working under a pathologist, Dr. Kenneth Fleming (Fig. 1), in the afterhours of the medical school curriculum. One day, I heard a lecture from Prof. John Bell, currently Regius Professor of Medicine at Oxford, who at that time had just returned to Oxford from Stanford. Prof. Bell talked about a then-new method, polymerase chain reaction (PCR), that captured my attention. After his lecture, I was able to persuade Prof. Bell to teach me the method. I was soon able to appreciate the versatility (3 ) as well as the potential shortcomings (e.g., susceptibility to contamination) (4 ) of this powerful method. I soon developed an interest in ways to further improve methods for prenatal diagnosis. One important disadvantage of the methods for DNA-based prenatal diagnosis was that invasive sampling of fetal tissues, e.g., by amniocentesis, was necessary to obtain fetal cells for further analysis. Such invasive sampling carries a small, but definite, risk to the fetus. To avoid such risks, numerous scientists had for many years dreamt about the possibility of performing noninvasive prenatal diagnosis. Toward this goal, there had been attempts since the late 1960s to isolate fetal nucleated cells that had entered into the maternal circulation. However, such early attempts were based on conventional cytogenetic (5 ) or staining (e.g., quinacrine staining for male cells) (6, 7 ) methods. I thought that one might be able to use the sensitivity of the PCR to detect such fetal cells that have entered into the maternal circulation. I further postulated that to maximize the chance of success, one could couple two sequential PCRs together. I referred to this as a dual-amplification system, which is now more commonly known as nested PCR. Using such a system, I was indeed able to demonstrate the detection of fetal DNA among maternal blood cells (8 ). Other workers soon also reported detecting circulating fetal cells using PCR (9, 10 ) and molecular cytogenetic methods (11 ), in combination with various fetal cell– enrichment techniques. However, all of these efforts, including my own, were hampered by problems such as false positivity and false negativity. The main reason for such difficulties is the extreme rarity of fetal nucleated cells in the maternal blood (12 ). Another reason is the persistence of certain fetal cell populations from one pregnancy into the next one (13 ). Indeed, I would spend the next 8 years working in this area without significant success. By 2002, it had become clear that the rarity of fetal nucleated cells in the maternal blood would make it very difficult to develop practical and robust methods for noninvasive prenatal testing using this approach (14 ). Hence, interest in such cell-based approaches has waned. 1997 was a very special year for Hong Kong, the city where I was born. It was the year that Hong Kong would return to Chinese rule after being a British colony since 1841. Although many had left Hong Kong before 1997 because of the uncertain future, my wife and I decided that it would be a good time to return home. Knowing that I would need to build a new career after I had moved back to Hong Kong, I thought that it would be a good time to try something new and even potentially risky. In September 1996, just 4 months before I would return to Hong Kong, 2 papers were published in Nature Medicine regarding the presence of tumor DNA in the cell-free fraction of blood obtained from cancer patients (15, 16 ). I thought that a cancer growing inside a cancer patient was somewhat analogous to a fetus developing inside its mother’s womb, and in my clinical practice, I had yet to see a cancer that was as big as an 8-pound baby. Hence, I thought that one should be able to detect fetal DNA in the cell-free portion, i.e., plasma or serum, of the mother’s blood. In collaboration