Prenatal diagnosis of trisomy 18p and distal 21q22.3 deletion

OBJECTIVES: To present the perinatal findings and molecular cytogenetic analysis of concomitant trisomy 18p (18p11.2-->pter) and distal 21q22.3 deletion. CASE AND METHODS: A 29-year-old woman, gravida 2 para 1, underwent amniocentesis at 17 weeks' gestation because she was a carrier of a balanced reciprocal translocation, 46,XX,t(18;21)(p11.2;q22.3). Cytogenetic analysis of the cultured amniocytes revealed a karyotype of 46,XX,der(21)t(18;21)(p11.2;q22.3). The fetus had a derivative chromosome 21 with an extra short arm of chromosome 18 attached to the terminal region of the long arm of chromosome 21. Level II sonograms did not find prominent structural anomalies. The pregnancy was terminated subsequently. At autopsy, the proband displayed a mild phenotype of hypertelorism, a small mouth, micrognathia, a narrowly arched palate, low-set ears, and clinodactyly. The brain and other organs were unremarkable. Genetic marker analysis showed a distal deletion at 21q22.3 and a breakpoint between D21S53 (present) and D21S212 (absent), centromeric to the known holoprosencephaly (HPE) minimal critical region D21S113-21qter. CONCLUSION: Genetic marker analysis helps in delineating the region of deletion in prenatally detected unbalanced cryptic translocation. Fetuses with concomitant trisomy 18p and distal 21q22.3 deletion may manifest inapparent phenotypic abnormalities in utero. Haploinsufficiency of the HPE critical region at 21q22.3 may not cause an HPE phenotype.