Full or partial monosomy of chromosome (chr) 21 is a very rare abnormal cytogenetic finding. It is characterized by variable sizes and deletion breakpoints on the long arm (q) of chr 21 that lead to a broad spectrum of phenotypes that include an increased risk of birth defects, developmental delay and intellectual deficit. We report a 37-year-old G1P0 woman initially screened by non-invasive prenatal testing with no positive findings that was followed by an 18-week anatomy scan with a fetal finding of duplication of the superior vena cava (SVC). The medical and family history was otherwise uneventful. After appropriate genetic counseling, amniocentesis was performed to evaluate suspected chromosomal anomalies. Interphase fluorescent in situ hybridization revealed loss of one chr 21 signal that was further delineated by chromosomal microarray analysis on uncultured amniocytes as a terminal 10 Mb deletion on chr 21q. Karyotype and microarrays on cultured amniocytes showed two cell lines for a mosaic 21q terminal deletion and monosomy 21. The combined molecular cytogenetics results reported following the ISCN 2016 guideline as mos 46,XX,del(21)(q22)dn[20]/45,XX,-21dn[10].nuc ish(D21S342/D21S341/D21S259x1)[100].arr[GRCh37] 21q11.2q22.12(15412676_36272993)x1~2,21q22.12q22.3(36431283_47612400)x1. Parental chromosomal analysis revealed normal karyotypes. Thus, this was a de novo mosaic full and partial monosomy of chr 21 in a case with SVC duplication. Despite the association of congenital heart disease with monsomy 21 we could not find any published literature or online databases for this cytogenetic abnormality. The patient terminated the pregnancy following the abnormal molecular cytogenetic results due to the possible challenges the baby would face if carried to term.

中文翻译：

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上腔静脉重复病例中21号染色体的从头镶嵌和部分单体性。

21 号染色体 (chr) 的全部或部分单体性是非常罕见的异常细胞遗传学发现。它的特点是 chr 21 长臂 (q) 上的可变大小和缺失断点导致广泛的表型，包括出生缺陷、发育迟缓和智力缺陷的风险增加。我们报告了一名 37 岁的 G1P0 女性，最初通过无创产前检测进行筛查，未发现阳性结果，随后进行了 18 周的解剖扫描，胎儿发现上腔静脉 (SVC) 重复。其他方面的医疗和家族史均无大碍。在适当的遗传咨询后，进行羊膜穿刺术以评估疑似染色体异常。间期荧光原位杂交揭示了一个 chr 21 信号的丢失，该信号通过对未培养的羊水细胞的染色体微阵列分析进一步描述为 chr 21q 上的末端 10 Mb 缺失。培养的羊水细胞上的核型和微阵列显示两个细胞系为马赛克 21q 末端缺失和单体 21。根据 ISCN 2016 指南报告的组合分子细胞遗传学结果为 mos 46,XX,del(21)(q22)dn[20]/45 ,XX,-21dn[10].nuc ish(D21S342/D21S341/D21S259x1)[100].arr[GRCh37] 21q11.2q22.12(15412676_36272993)x1~2,21q22.12q22.3(36431283_4761240) 亲本染色体分析显示正常核型。因此，在 SVC 重复的情况下，这是 chr 21 的从头镶嵌完整和部分单体。尽管先天性心脏病与 21 号单体有关，但我们找不到任何已发表的文献或在线数据库来说明这种细胞遗传学异常。由于婴儿在足月时可能面临的挑战，患者在分子细胞遗传学结果异常后终止妊娠。