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Cell-free fetal DNA (cffDNA) is fetal DNA circulating freely in the maternal blood stream. It can be sampled by venipuncture on the mother. Analysis of cffDNA provides a method of non-invasive prenatal diagnosis. cffDNA originates from the trophoblasts making up the placenta.〔Alberry, M., et al. (2007). "Free fetal DNA in maternal plasma in anembryonic pregnancies: confirmation that the origin is the trophoblast". ''Prenatal Diagnosis'' 27 (5): 415-418.〕〔Gupta, A. K., et al. (2004). "Detection of fetal DNA and RNA in placenta-derived syncytiotrophoblast microparticles generated in vitro". ''Clinical Chemistry'' 50 (11): 2187-2190.〕 It is estimated that 2-6% of the DNA in the maternal blood is fetal in origin.〔Lo, E. S.; Lo, Y. M.; Hjelm, N. M.; Thilaganathan, B. (1998). "Transfer of nucleated maternal cells into fetal circulation during the second trimester of pregnancy". ''British Journal of Haematology'' 100 (3): 605-606.〕 The fetal DNA is fragmented and makes its way into the maternal bloodstream via shedding of the placental microparticles into the maternal bloodstream (figure 1).〔Smets, E. M.; Visser, A.; Go, A. T.; van Vugt, J. M.; Oudejans, C. B. (2006). "Novel biomarkers in preeclampsia". ''Clinica Chimica Acta: International Journal of Clinical Chemistry'' 364 (1-2): 22-32.〕 Studies have shown that cffDNA can first be observed as early as 7 weeks gestation, and the amount of cffDNA increases as the pregnancy progresses.〔Lo, Y. M., et al. (1998). "Quantitative analysis of fetal DNA in maternal plasma and serum: implications for noninvasive prenatal diagnosis". ''American Journal of Human Genetics''. 62 (4): 768-775.〕 cffDNA diminishes quickly after the birth of the baby, so that it is no longer detectable in the maternal blood approximately 2 hours after birth.〔Lo, Y. M., et al. (1999). "Rapid clearance of fetal DNA from maternal plasma". ''American Journal of Human Genetics'' 64 (1): 218-224.〕 cffDNA is significantly smaller than the maternal DNA in the bloodstream, with fragments approximately 200bp in size.〔Chan, K. C., et al. (2004). "Size distributions of maternal and fetal DNA in maternal plasma". ''Clinical Chemistry'' 50 (1): 88-92.〕 Many protocols to extract the fetal DNA from the maternal plasma use its size to distinguish it from the maternal DNA.〔Li, Y., et al. (2004). "Size separation of circulatory DNA in maternal plasma permits ready detection of fetal DNA polymorphisms". ''Clinical Chemistry'' 50 (6): 1002-1011.〕〔Li, Y., et al. (2005). "Detection of paternally inherited fetal point mutations for beta-thalassemia using size-fractionated cell-free DNA in maternal plasma". ''The Journal of the American Medical Association'' 293 (7): 843-849.〕 Studies have looked at, and some even optimized, protocols for testing non-compatible RhD factors, sex determination for X-linked genetic disorders and testing for single gene disorders.〔Hahn, S.; Chitty, L. S. (2008). "Noninvasive prenatal diagnosis: current practice and future perspectives". ''Current Opinion in Obstetrics & Gynecology'' 20 (2): 146-151.〕 Current studies are now looking at determining aneuploidies in the developing fetus.〔Fan, H. C.; Blumenfeld, Y. J.; Chitkara, U.; Hudgins, L.; Quake, S. R. (2008). "Noninvasive diagnosis of fetal aneuploidy by shotgun sequencing DNA from maternal blood". ''Proceedings of the National Academy of Sciences of the United States of America'' 105 (42): 16266-16271.〕〔Lo, Y. M., et al. (2007). "Digital PCR for the molecular detection of fetal chromosomal aneuploidy". ''Proceedings of the National Academy of Sciences of the United States of America'' 104 (32): 13116-13121.〕 These protocols can be done earlier than the current prenatal testing methods, and have no risk of spontaneous abortion, unlike current prenatal testing methods.〔Lo, Y. M., et al. (1998). "Prenatal diagnosis of fetal RhD status by molecular analysis of maternal plasma". ''The New England Journal of Medicine'' 339 (24): 1734-1738.〕 Non-invasive prenatal diagnosis (NIPD) has been implemented in the UK and parts of the US; it has clear benefits above the standard tests of chorionic villi sample (CVS) and amniocentesis 〔Allyse, M.; Sayres, L. C.; King, J. S.; Norton, M. E.; Cho, M. K. (2012). "Cell-free fetal DNA testing for fetal aneuploidy and beyond: clinical integration challenges in the US context". ''Human Reproduction'' 27 (11): 3123-3131.〕 which have procedure-related miscarriage risks of about 1 in 100 pregnancies and 1 in 200 pregnancies, respectively.〔Mujezinovic, F.; Alfirevic, Z. (2007). "Procedure-related complications of amniocentesis and chorionic villous sampling: a systematic review". ''Obstetrics and Gynecology'' 110 (3): 687-694.〕〔Lo, Y. M. (2008). "Fetal nucleic acids in maternal plasma". ''Annals of the New York Academy of Sciences'' 1137: 140-143.〕 As a method of prenatal diagnosis, cell-free fetal DNA techniques share the same ethical and practical issues, such as the possibility of prenatal sex discernment and sex selection.〔 ==Methods: From Extraction to Diagnosis== The general procedure is that a sample of the woman’s blood is taken after 10 weeks of pregnancy, although fetal blood can be detected as early as the fifth week.〔Guibert, J., et al. (2003). "Kinetics of SRY gene appearance in maternal serum: detection by real time PCR in early pregnancy after assisted reproductive technique". ''Human Reproduction'' 18 (8): 1733-1736.〕 The test measures the relative amount of free fetal DNA in the mother’s blood which consists of approximately 2-6% of the total.〔 Limitations include the concentration of all cell-free DNA in maternal blood is low, the total amount of cell-free DNA varies between individuals, cell free fetal DNA molecules are out-numbered by maternal cell-free DNA molecules, the fetus inherits half the genome from the mother. However, there are ways around these limitations.〔Wright, C. F.; Burton, H. (2009). "The use of cell-free fetal nucleic acids in maternal blood for non-invasive prenatal diagnosis". ''Human Reproduction Update'' 15 (1): 139-151.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cell-free fetal DNA」の詳細全文を読む スポンサード リンク
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