Prenatal diagnosis and molecular cytogenetic characterization of mosaic ring chromosome 21 associated with low PAPP-A and low PlGF in the first-trimester maternal serum screening.

Objective: We present diagnosis and molecular cytogenetic characterization of mosaic ring chromosome 21 [r(21)].

Case Report: A 17-year-old, gravida 2, para 1, woman underwent amniocentesis at 17 weeks of gestation because of an abnormal result of the first-trimester maternal serum screening for Down syndrome with a free β-hCG level of 1.736 multiples of the median (MoM), a pregnancy associated plasma protein-A (PAPP-A) level of 0.275 MoM, a placental growth factor (PlGF) level of 0.281 MoM, a Down syndrome risk of 1:222 and a preeclampsia risk of 1:175. Cytogenetic analysis of cultured amniocytes revealed the result of 46,XX,r(21) (p12q22.3)[19]/45,XX,-21[13]. Array comparative genomic hybridization (aCGH) analysis of cultured amniocytes revealed the result of arr [GRCh37] 21q11.2q22.2 (15,485,008-40,625,594) × 1∼2, 21q22.2q22.3 (40,703,792-46,682,184) × 2∼3, 21q22.3 (46,761,631-48,084,156) × 1, consistent with mosaic monosomy 21 and r(21) (p12q22.3). The pregnancy was subsequently terminated, and a malformed fetus was delivered with low-set ears and hypotelorism. Postnatal cytogenetic analysis revealed a karyotype of 46,XX,r(21) (p12q22.3)[30]/45,XX,-21[8]/46,XX,idic r(21) (p12q22.3)[2] in the cord blood, 46,XX,r(21) (p12q22.3)[34]/45,XX,-21[6] in the skin, 46,XX,r(21) (p12q22.3)[37]/45,XX,-21[3] in the umbilical cord and 46,XX,dup(21) (q22.2q22.3)[32]/46,XX,r(21) (p12q22.3)[8] in the placenta. aCGH analysis of cord blood revealed the result of arr 21q11.2q22.2 (15,499,847-40,662,581) × 2.3, arr 21q22.2q22.3 (40,703,792-46,682,184) × 3.6, arr 21q22.3 (46,761,632-48,090,317) × 1, consistent with mosaic duplication of 21q11.2-q22.2 and 21q22.2-q22.3, and a 1.33-Mb 21q22.3 deletion encompassing the genes of COL18A1, SLC19A1, PCBP3, COL6A1, COL6A2, FTCD, LSS, MCM3AP, YBEY, PCNT, DIP2A, S100B and PRMT2.

Conclusion: Mosaic r(21) at amniocentesis may be associated with monosomy 21, idic r(21) and dup(21), and low PAPP-A and low PlGF in the first-trimester maternal serum screening.