[Improvement of detection of paternally inherited fetal mutant genes for β-globin in maternal plasma by PNA clamp].

Objective: To study improvement of detection of paternally herited fetal mutant genes for β-globin in maternal plasma by PNA clamp to seek a noninvasive prenatal diagnostic method for β-thalassemia.

Methods: A total of 38 maternal blood samples were collected at 7 to 20 weeks of gestation, samples in which the father carried CD41-42 mutation and mother carried normal gene or the other point mutation for β-thalassemia were examined. The results of fetal DNA in amniotic fluid, cord blood or peripheral blood of newborns were used as the gold standard for comparison. In the study group, the total cell-free DNA was extracted from maternal plasma using QIAamp DNA Blood Mini Kit. After extraction, the total cell-free DNA was separated by agarose gel (1%) electrophoresis, and the cell-free DNA with a size of 100-300 bp was retrieved from the gel slice. Then, the retrieved DNA-free cell underwent PCR amplified with a PNA clamp. The genotype was confirmed by the conventional method (reverse dot blot hybridization), and the results were compared to gold standard. Simultaneously, two control groups with different PCR procedures were set up. The PCR procedure of control group A was amplified with the extracted total cell-free DNA and PNA clamp, and the PCR procedure of control group B was amplified with the retrieved size-fractionated DNA-free cell without PNA clamp.

Results: Plasma samples from 38 pregnant women were detected using PCR products for hybridization, the results were compared with the gold standard. Regarding the 21 samples confirmed by gold standard with fetal genotype 41-42M/N, 19, 8, 12 cases were detected as fetal genotype 41-42M in study group, control group A and control group B respectively, the sensitivity was 90.5% (19/21), 38.1% (8/21) and 57.1% (12/21) respectively;Concerning the 17 samples confirmed by gold standard with fetal normal genotype, the amount of false positive cases were 1, 2 and 1 respectively. The respective specificity was 94.1% (16/17), 94.1% (16/17) and 88.2% (15/17) respectively. The respective accuracies were 92.1% (35/38), 63.2% (24/38) and 71.1% (27/38) respectively. The difference in sensitivity and specificity was pairwise compared by means of McNemar's test. There was significant difference between new study group and control group A or control group B (all P﹤0.05).

Conclusion: The method of detection of paternally inherited fetal mutation genes for β-thalassemia using small size of fetal DNA-free cell in maternal plasma with PNA clamp had several advantages of reliable sensitivity, specificity and accuracy, indicating its potential of clinical practicality.