New molecular methods in prenatal diagnosis of aneuploidy mosaicism

镶嵌现象的发生对染色体异常的产前诊断提出了严峻的挑战。本研究的目的是研究染色体嵌合体在羊水细胞产前诊断中的频率，并评估新的分子方法在嵌合体检测中的应用。研究对象为1102例羊水细胞培养。我们将嵌合体分为1级，在单个细胞中看到染色体异常，2级，异常仅限于单个培养瓶中的多个细胞，3级，异常在2个以上的烧瓶中看到。共77例（7.0%）伴有任何嵌合：3级嵌合5例[21三体2例，18三体各1例，+inv dup（15）和t(14; 15)]， 2级嵌合10例，1级嵌合62例。我们应用荧光原位杂交（FISH）和引物原位杂交（PRINS）方法对3级病例的真嵌合性进行诊断。在第15、18和21号染色体上的FISH - α - DNA探针和在第18和21号染色体上的带α - DNA特异性引物的PRINS探针均显示，这些病例间期细胞核上的信号明显多于正常对照。此外，对人工合成的马赛克18三体的研究表明，FISH方法甚至可以检测出12.5%的马赛克。羊水细胞培养中染色体嵌合体的频率非常高，提示应非常小心，以避免误诊。我们认为FISH和PRINS方法可以在短时间内分析大量间期细胞，对确定真正嵌合的诊断特别有用。

The occurrence of mosaicism has been imposing a serious challenge to the prenatal diagnosis of chromosome abnormalities. The objective of this study is to investigate the frequency of chromosome mosaicism in prenatal diagnosis of amniotic fluid cells and to assess the utility of new molecular methods in the detection of mosaicism. The study subjects were 1,102 cases of amniotic fluid cell cultures. We categorized the mosaicism into a level 1 where a chromosome abnormality is seen in a single cell, level 2 where the abnormality is confined to plural cells in a single culture flask, and level 3 where the abnormality is seen in more than 2 flasks. A total of 77 (7.0%) cases were associated with any mosacism : 5 cases had the level 3 mosaicism [2 cases of trisomy 21, each case of trisomy 18, +inv dup (15) and t (14 ; 15)], 10 cases the level 2 mosaicism, and 62 cases the level 1 mosaicism. We applied the fluorescence in situ hybridization (FISH) and primed in situ hybridization (PRINS) methods to the diagnosis of the true mosaicism in the level 3 cases. Both FISH to alphoid DNA probes for chromosomes 15,18 and 21 and PRINS with primers specific to alphoid DNAs for chromosomes 18 and 21 showed significantly more signals on interphase nuclei in those cases than in normal controls. Furthermore, study in artificially made mosaic trisomy 18 showed that the FISH method can even detect a 12.5% of mosaicism. The very high frequency of chromosome mosaicism in amniotic fluid cell cultures suggest that much care should be taken to avoid misdiagnosis. We believe that FISH and PRINS methods which can analyze a great number of interphase cells in a short time are particularly useful in confirming the diagnosis of the true mosaicism.