Prenatal diagnosis and molecular cytogenetic characterization of low-level mosaic trisomy 12 at amniocentesis associated with a favorable pregnancy outcome.

OBJECTIVE: We present prenatal diagnosis of low-level mosaic trisomy 12.

CASE REPORT: A 40-year-old woman underwent amniocentesis at 18 weeks of gestation because of advanced maternal age, which revealed a karyotype of 47,XX,+12[5]/46,XX[24] consistent with 17.2% (5/29) mosaicism for trisomy 12. Repeat amniocentesis performed at 21 weeks of gestation revealed a karyotype of 47,XX,+12[4]/46,XX[6] consistent with 40% (4/10) mosaicism for trisomy 12. Interphase fluorescence in situ hybridization (FISH) on 112 uncultured amniocytes detected 23 cells with trisomy 12 consistent with 20.5% (23/112) mosaicism for trisomy 12. Polymorphic DNA marker analysis excluded uniparental disomy 12. Array comparative genomic hybridization (aCGH) on uncultured amniocytes revealed a result of arr 12p13.33q24.33 (230,451-133,773,499) × 2.2, 17p12 (14,191,925-15,442,037) × 1.0 consistent with 10-20% mosaic trisomy 12. The father carried the 17p12 microdeletion. The fetal ultrasound findings were unremarkable. A 3958-g female fetus was delivered at 37 weeks of gestation with no phenotypic abnormality. The cord blood had a karyotype of 46,XX. Postnatal interphase FISH on urinary cells revealed 7.14% (7/98) mosaicism for trisomy 12.

CONCLUSION: Low-level mosaic trisomy 12 at amniocentesis can be associated with a favorable pregnancy outcome. Interphase FISH and aCGH on uncultured amniocytes are useful for confirmation of low-level mosaic trisomy 12 at amniocentesis.