Prenatal diagnosis and molecular cytogenetic characterization of low-level true mosaicism for trisomy 21 using uncultured amniocytes.

OBJECTIVE: We present a prenatal diagnosis and molecular cytogenetic characterization of low-level true mosaicism for trisomy 21 using uncultured amniocytes.

CASE REPORT: A 35-year-old woman presented with a borderline-positive result of noninvasive prenatal testing for trisomy 21. She underwent amniocentesis at 18 weeks of gestation, which revealed a karyotype of 47,XY,+21(5)/46,XY(53). Repeat amniocentesis at 22 weeks of gestation revealed a karyotype of 47,XY,+21(6)/46,XY(26). Array comparative genomic hybridization (aCGH) analysis on uncultured amniocytes revealed mosaic levels of 10% to 15% for trisomy 21. Interphase fluorescence in situ hybridization (FISH) analysis on uncultured amniocytes revealed a mosaic level of 21.7% (28/129 cells) for trisomy 21. Following genetic counseling and detailed ultrasound examination, the parents decided to continue the pregnancy. The pregnancy was carried to term, and a normal 3664-g male baby was delivered. The cord blood lymphocytes had a karyotype of 47,XY,+21(2)/46,XY(38). Postnatal interphase FISH analysis of urine detected no trisomy 21 in all 39/39 urinary cells. The neonate was phenotypically normal at age 7 months.

CONCLUSION: Low-level true mosaicism for trisomy 21 can be associated with a favorable fetal outcome. aCGH and interphase FISH analyses on uncultured amniocytes are useful for rapid confirmation of low-level true mosaicism for trisomy 21 at repeated amniocentesis.