Mosaic ring chromosome 21, monosomy 21, and isodicentric ring chromosome 21: prenatal diagnosis, molecular cytogenetic characterization, and association with 2-Mb deletion of 21q21.1-q21.2 and 5-Mb deletion of 21q22.3.

[monosomy 21]

To present the perinatal findings and molecular cytogenetic characterization of prenatally detected mosaic r (21 ).A 29 -year -old primigravid woman underwent amniocentesis at 22 weeks ' gestation because of hyperechogenic cardiac foci and intrauterine growth restriction . Amniocentesis revealed a karyotype of 46 ,XY ,r (21 )[15 ]/45 ,XY ,-21 [5 ]. The parental karyotypes were normal . The woman requested repeat amniocentesis . Oligonucleotide-based array comparative genomic hybridization was applied to the uncultured amniocytes , rapidly detecting a 2 .09 -Mb deletion of 21 q 21 .1 -q 21 .2 (21 ,495 ,262 -23 ,580 ,815 bp ) and a 5 .03 -Mb deletion of 21 q 22 .3 -q 22 .3 (41 ,887 ,412 -46 ,914 ,715 bp ). Cytogenetic analysis revealed a karyotype of 46 ,XY ,r (21 )[8 ]/45 ,XY ,-21 [3 ]/46 ,XY ,idic r (21 )[1 ]. The pregnancy was terminated , and a malformed fetus was delivered with clinodactyly , short big toes , separation between the first and second toes , prominent nasal bridge , downward slanting palpebral fissures , protuberant occiput , prominent forehead , broad anteverted nasal tip , long philtrum , thin upper lip , small mouth , and micrognathia . The placenta had a karyotype of 46 ,XY ,r (21 )[83 ]/45 ,XY ,-21 [11 ]/46 ,XY ,idic r (21 )[6 ], and the cord blood lymphocytes had a karyotype of 46 ,XY ,r (21 )[88 ]/45 ,XY ,-21 [9 ]/46 ,XY ,idic r (21 )[3 ]. Polymorphic DNA marker analysis determined a maternal origin for the deletion .An extra interstitial 21 q deletion can be associated with mosaic r (21 ) in addition to a terminal 21 q deletion . aCGH is useful in determining the breakpoints and associated subtle structural abnormalities in cases of prenatally detected ring chromosome in order to facilitate genetic counseling .