Prenatal diagnosis: blood test instead of invasive procedures
Amniocentesis (puncture of the fetal bladder) and biopsy of chorionic villi (the outer shell surrounding the embryo) are currently the gold standard for detecting genetic abnormalities of the developing embryo, such as Down syndrome. These invasive procedures, which increase the risk of miscarriage, are usually prescribed only to women at risk, which does not completely exclude the birth of children with chromosomal abnormalities. However, advances in the development of new non-invasive tests indicate that in the near future, genetic defects of the fetus can be detected using a blood test.
The placenta, on the one hand, connects the circulatory systems of the fetus and the mother, providing nutrition and blood supply to the embryo, and on the other – prevents the direct mixing of their blood. However, a very small amount of free nucleic acids and fetal cells penetrates through this barrier and circulates in the mother's bloodstream. For several decades, experts have been trying to find a way to isolate them, which would make it possible to diagnose the condition of the developing fetus, including possible hereditary diseases.
It is quite simple to isolate DNA and RNA fragments circulating in the blood and decipher their sequences, however, it is very difficult to isolate fragments belonging to the fetus from the entire pool of nucleic acids. In addition, in diseases such as Down syndrome, the criterion of which is the presence of an additional copy of the 21st chromosome, diagnosis is impossible without studying an intact fetal cell.
To solve this problem, scientists at the Chinese University of Hong Kong, working under the guidance of Professor Dennis Lo, proposed using single nucleotide polymorphisms, or "snips" (from the English single nucleotide polimorphysm, SNP) – the differences that normally occur in the genes of single nucleotides. A normal fetus with two copies of chromosome 21 is very likely to have two different variants of a certain gene located on this chromosome, expressed in a ratio of 1:1. A fetus with Down syndrome will have an additional copy of one of the gene variants, which will lead to a change in the ratio to 2:1.
Analysis of RNA ratios synthesized on genes containing snips allows us to indirectly estimate the number of copies of chromosome 21 in fetal cells. At the same time, the greater the number of snips taken into account during the analysis, the higher its quality. Sequenom, a San Diego-based company specializing in molecular diagnostics, is already working on creating a clinical test based on this principle for the prenatal diagnosis of Down syndrome.
It is much easier to diagnose the Rh incompatibility syndrome by analyzing the content of fetal nucleic acids in the mother's blood. Rh factor (Rh-factor) is a protein that is part of the membrane of Rh-positive erythrocytes. If the mother is Rh-negative (the membrane of her red blood cells does not contain Rh factor), her immune system, especially during repeated pregnancy with a Rh-positive child, may react aggressively to the fetus. In this case, a mother's blood test for the presence of a Rh factor-coding gene or the corresponding RNA allows you to determine the need for preventive measures.
In Europe, such a test has been used for many years, while in the USA the first version of the test, developed by the Lo group together with Sequenom and implemented by Lenetix Medical Screening Laboratory, appeared on the market only in December last year.
Experts believe that this test is good enough, but not perfect, because when using it, there is a possibility of obtaining false negative results.
A similar test developed by Biocept, also based in San Diego, will go on sale in early May this year. This test excludes false negative results due to the analysis of a large number of snips, which allow to distinguish maternal DNA from fetal DNA with a greater degree of accuracy.
An additional approach to noninvasive prenatal diagnosis is the isolation of fetal cells from the mother's blood. This strategy is particularly suitable for the diagnosis of Down syndrome and other abnormalities of the number of copies of chromosomes. Having fetal cells, it is not difficult to diagnose Down syndrome: to do this, you only need to mark the 21st chromosome with a fluorescent probe and count the number of copies of it in the cell under a microscope.
The problem is that a milliliter of mother's blood contains no more than five fetal cells, the isolation of which is an exceptionally difficult task. Biocept company proposes to solve this problem by using antibodies specific to immature nucleated red blood cells. Such cells are more likely to belong to the fetus than to the mother's body. Immature erythrocytes isolated with the help of specific antibodies to surface proteins, due to their unusual morphology, are easily differentiated under a microscope.
The proposed Biocept platform is based on the use of a microfluidic system, when passing through the channels of which the mother's blood interacts with antibodies attached to the surface of the channels that bind immature red blood cells. The microscopic structure of the surface maximizes the degree of interaction of blood and antibodies, significantly increasing the chances of isolation of fetal cells.
The fetal cell depicted in the upper part of the drawing penetrated through the placenta into the mother's blood. A complex microfluidic system studded with antibodies made it possible to isolate this single fetal cell from the millions of cells contained in the mother's blood sample. After that, fluorescent molecular tags were used to label the most important chromosomes: two copies of the X chromosome (bright blue), which means the female sex of the fetus, and three copies of the 21st chromosome (pink), which makes it possible to diagnose Down syndrome.
Next month, the company plans to begin clinical trials of a non-invasive test for prenatal diagnosis of Down syndrome, which may already appear on the market by the end of 2008.
The company Ikonisys, based in New Haven (Connecticut), is also engaged in the development of non-invasive methods of prenatal diagnostics. Instead of physical sorting of cells, the company's specialists suggest using a computational method. The automated microscope developed by the company scans numerous smears of maternal blood and identifies fetal cells among the nucleated cells of the mother by their shape, which differs from similar "adult" cells. Software is attached to the microscope that records the location of cells corresponding to the desired parameters, and the final decision on their status is made by a person.
Ikonisys plans to launch production of its Down syndrome detection test at the end of 2008. The company is also considering the possibility of adapting this approach for use in oncology, since the identification of single tumor cells among numerous healthy cells of the body will improve the diagnosis of certain types of cancer and their response to treatment.
Evgeniya Ryabtseva
Portal "Eternal youth" www.vechnayamolodost.ru based on the materials of ScienceDaily
30.04.2008