We report the case of a pregnant woman who underwent prenatal diagnosis by chorionic villi sampling for increased risk of trisomy 21 due to advanced age and abnormal results of the first trimester combined screening test.
Karyotype analysis of the chorionic villi sampling showed a normal male karyotype (46,XY) in16 metaphases derived from the trophoblast culture and a mosaic in the mesenchymal culture for the presence of a supernumerary marker chromosome (SMC) in 6 metaphases (47,XY,+mar/46,XY).
To evaluate the presence of a real mosaicism, karyotype analysis was repeated on amniocytes derived from a single primary culture, confirming the presence of an abnormal cell line with a mosaicism of 27% (47,XY+mar/46,XY).
The distribution and the extent of the mosaicism were better characterized by the analysis of fetal blood, which allowed the definition of the SMC as an X derivative with a ring structure present at mosaic in 24% of the peripheral lymphocytes (47,XY,r(X)/46XY).
CGH-array on fetal blood-derived DNA defined the extent for 43 Mb of the X chromosome duplication from Xp21.1 to Xq21.1.
FISH analysis, using X centromeric and XIST probes, confirmed the X derivation of the marker and the inclusion of the XIST gene within the duplicated fragment.
Ultrasound fetal evaluation was unremarkable and the woman, counseled positively for the conservation of the XIST gene, decided to continue the pregnancy, proceeding to term.
The woman delivered an apparently normal male baby who, at the follow-up to 13 months, appears morphologically and developmentally normal.