TRISOMY-16 MOSAICISM IN AMNIOTIC-FLUID CELL-CULTURES

Trisomy 16 mosaicism was found in amniotic fluid cells in a patient un dergoing amniocentesis because of elevated second-trimester maternal s erum alpha-fetoprotein (MSAFP) (2.80 MOM), a markedly elevated human c horionic gonadotropin level (hCG) (12.12 MOM), and a Down syndrome ris k of 1:55. Ultrasound evaluation of the fetus indicated the presence o f an atrial septal defect and clinodactyly. Cytogenetic analyses of va rious fetal tissues using fluorescence in situ hybridization (FISH) fa iled to detect substantial numbers of trisomy 16 cells; however, triso my 16 mosaicism was identified in placental tissue. Molecular genetic analysis at five different loci [four analysed by polymerase chain rea ction (PCR) and one by Southern blot analysis] failed to show any evid ence for uniparental disomy. Although trisomy 16 cells could not be cl early demonstrated in the fetus, the presence of a clinically signific ant proportion of aneuploid cells early in development could not be ex cluded and it therefore cannot be assumed that a 'confined placental m osaicism' existed. The markedly elevated hCG and elevated MSAFP levels are consistent with abnormal placental function in trisomy 16 mosaici sm. Serial ultrasound evaluation (to detect any late-onset growth reta rdation) and fetal echocardiography may be indicated for patients with extraordinarily high levels of hCG, especially if MSAFP is also eleva ted.