De novo mosaic and partial monosomy of chromosome 21 in a case with superior vena cava duplication.

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.