Non-Invasive Epigenetic Detection of Fetal Trisomy 21 in First Trimester Maternal Plasma

BackgroundDown syndrome (DS) is the most common known aneuploidy, caused by an extra copy of all or part of chromosome 21. Fetal-specific epigenetic markers have been investigated for non-invasive prenatal detection of fetal DS. The phosphodiesterases gene, PDE9A, located on chromosome 21q22.3, is completely methylated in blood (M-PDE9A) and unmethylated in the placenta (U-PDE9A). Therefore, we estimated the accuracy of non-invasive fetal DS detection during the first trimester of pregnancy using this tissue-specific epigenetic characteristic of PDE9A. Methodology/Principal FindingsA nested, case-control study was conducted using maternal plasma samples collected from 108 pregnant women carrying 18 DS and 90 normal fetuses (each case was matched with 5 controls according to gestational weeks at blood sampling). All pregnancies were singletons at or before 12 weeks of gestation between October 2008 and May 2009. The maternal plasma levels of M-PDE9A and U-PDE9A were measured by quantitative methylation-specific polymerase chain reaction. M-PDE9A and U-PDE9A levels were obtained in all samples and did not differ between male and female fetuses. M-PDE9A levels did not differ between the DS cases and controls (1854.3 vs 2004.5 copies/mL; P = 0.928). U-PDE9A levels were significantly elevated in women with DS fetuses compared with controls (356.8 vs 194.7 copies/mL, PU-PDE9A level and the unmethylation index of PDE9A for non-invasive fetal DS detection were 77.8% and 83.3%, respectively, with a 5% false-positive rate. In the risk assessment for fetal DS, the adjusted odds ratios of U-PDE9A level and UI were 46.2 [95% confidence interval: 7.8–151.6] and 63.7 [95% confidence interval: 23.2–206.7], respectively. ConclusionsOur findings suggest that U-PDE9A level and the unmethylation index of PDE9A may be useful biomarkers for non-invasive fetal DS detection during the first trimester of pregnancy, regardless of fetal gender.

背景 唐氏综合征（Down syndrome, DS）是目前已知最常见的非整倍体遗传病，由21号染色体全部或部分片段额外拷贝所导致。既往已有研究围绕胎儿特异性表观遗传标记开展无创产前胎儿唐氏综合征检测的相关探索。定位于21号染色体q22.3区域的磷酸二酯酶基因PDE9A（phosphodiesterases gene, PDE9A）在血液中呈完全甲基化状态（M-PDE9A），而在胎盘中呈未甲基化状态（U-PDE9A）。基于该组织特异性表观遗传特征，本研究评估了孕早期无创胎儿唐氏综合征检测的准确性。

方法/主要结果 本研究采用嵌套病例对照设计，纳入2008年10月至2009年5月期间孕周≤12周的单胎妊娠孕妇共108名，其中携带唐氏综合征胎儿的病例组18名，正常胎儿对照组90名（每例病例按采血时孕周匹配5例对照）。采用定量甲基化特异性聚合酶链反应（quantitative methylation-specific polymerase chain reaction）检测孕妇血浆中M-PDE9A与U-PDE9A的水平。所有样本均成功获取两类靶点的检测数据，且胎儿性别对二者水平无显著影响。病例组与对照组的M-PDE9A水平无统计学差异（1854.3 vs 2004.5 拷贝/毫升；P=0.928）。携带唐氏综合征胎儿的孕妇，其血浆U-PDE9A水平显著高于对照组（356.8 vs 194.7 拷贝/毫升）。当假阳性率设定为5%时，U-PDE9A水平与PDE9A未甲基化指数用于无创胎儿唐氏综合征检测的准确率分别为77.8%与83.3%。在胎儿唐氏综合征风险评估中，U-PDE9A水平与未甲基化指数的校正比值比分别为46.2[95%置信区间（95% confidence interval）：7.8–151.6]与63.7[95%置信区间（95% confidence interval）：23.2–206.7]。

结论 本研究结果表明，无论胎儿性别如何，孕妇血浆U-PDE9A水平与PDE9A未甲基化指数均可作为孕早期无创胎儿唐氏综合征检测的有效生物标志物。