Characterization of a small supernumerary marker chromosome as r(8) at prenatal diagnosis by MFISH

To identify the mosaic marker chromosome detected in amniotic fluid cells of a 26-year-old woman, with raised triple test values and an ultrasound scan, which showed a fetus with echogenic bowels.

Routine karyotyping with G- and C-banding was carried out for both, amniotic fluid at 18 weeks of gestation as well as fetal blood at 22(+6) weeks. Peripheral blood of both parents was karytoyped. MFISH and the all centromeric human probe were used on fetal lymphocytes to identify the marker chromosome.

Both parents had a normal karyotype. Amniotic fluid culture showed a de novo supernumerary marker chromosome (SMC) in 14 of the 30 colonies from four different cover slip cultures. The marker was confirmed in 50% of the fetal lymphocytes. G- and C-banding provided little information except that the marker had some heterochromatic material. The all centromeric human probe also showed the presence of a centromere along with a rim of euchromatic material. MFISH identified this ring marker to be belonging to chromosome 8.

SMCs with chromosome 8 have been shown to be variable phenotypes. Presence of only heterochromatic material seems to have no discernable phenotypic effects, but, with the presence of euchromatic material, mental and physical developmental delay has been reported. The parents opted to go ahead with the pregnancy and an apparently normal female baby was born at 40 weeks with no complications.