Fetal nucleated red blood cells in maternal peripheral blood for non-invasive prenatal diagnosis

Birth defect is the main reason that causes abortion, stillbirth and even the death or disability of babies, which brings great mental and economic burden to the family and the society. Traditional invasive prenatal diagnosis is highly risky and may cause damage to pregnancy women as well as their fetuses. Therefore, it is very urgent to develop safer and more convenient techniques that are able to real-time monitor the fetal health and diagnose the potential fetal diseases in time. Fetal nucleated red blood cells (FNRBCs) are found to circulate in maternal peripheral blood during the entire pregnancy while disappear rapidly after delivery. These cells not only contain the entire genome of the fetus, but also possess organelles, metabolites and biomolecules that are relating to the cellular compositions and behaviors. Therefore, FNRBCs can provide abundant materials and information for assessing the status of the fetus, which makes FNRBCs an excellent biomarker for non-invasive prenatal diagnosis. However, in 1 mL maternal peripheral blood, there are approximately 10(9) mature red blood cells (RBCs) and 10(6) white blood cells (WBCs), while only several to hundreds of FNRBCs can be acquired nowadays. The huge blood background and the rarity of FNRBCs make it a great challenge to isolate and enrich FNRBCs for downstream biomedical analysis. Furthermore, maternal nucleated red blood cells can also confuse with FNRBCs and thus hinder the accuracy and reliability of cell-based prenatal diagnosis. Conventionally, density gradient concentration, fluorescent activated cell sorting (FACS), magnetic activated cell sorting (MACS) or the combination among these three techniques are employed to isolate and enrich FNRBCs. Nevertheless, their isolation efficiency and purity are very limited. Based on newly-developed micromachining technologies, various strategies such as microfluidic chips and micro/nanostructured interfaces show excellent performance for capturing FNRBCs from maternal blood and are able to efficiently eliminate the interference of RBCs and WBCs. These strategies include immuno-affinity labeling methods aiming at antigen-antibody recognition based on specific membrane proteins of FNRBCs and label-free methods utilizing the unique physical properties of FNRBCs. Given the fact that current isolation and enrichment techniques still cannot avoid the contamination of WBCs and maternal nucleated red blood cells, it is essential to validate the fetal origin of the isolated nucleated red blood cells. Cell morphology and constitution, immunostaining of specific proteins, fluorescence in situ hybridization (FISH) and short tandem repeat (STR) analysis of unique genes are the most commonly used means to distinguish FNRBCs from other interfering cells. The identified cells can be applied in clinical non-invasive prenatal diagnosis of various diseases of both the pregnant women and their fetuses. On one hand, the abnormal variation of FNRBC enumeration during the pregnancy is related to the pathological status of either the mother (such as obesity, preeclampsia, gestational hypertension and diabetes) or the fetus (such as hypoxia and intrauterine asphyxia). On the other hand, by using molecule genetic techniques for analyzing the chromosomes and genes inside the nuclei of FNRBCs, various genetic disorders of the fetuses are able to be diagnosed, for example, 13/18/21 trisomy and other aneuploidies, single gene diseases such as sickle-cell disease and thalassemia, and other chromosomal or genetic microdeletions or mutations. With the help of the development of novel and solid isolation technologies and biomedical analytical tools on the cellular/molecular level, it is greatly promising that FNRBCs will become the powerful biomarker for non-invasive prenatal diagnosis of fetal diseases.