Ring chromosome 21 in a boy and a derivative chromosome 21 in the mother: implication for ring chromosome formation

We report on r(21) chromosome in a boy and a der(21) chromosome in his mother. Cytogenetic studies revealed a mosaic 45,XY[4]/46,XY,r(21)[96] karyotype in the boy and a 46,XX,der(21)[100] karyotype in the mother. Fluorescence in situ hybridization analysis for D21Z1 at the centromere, AML1 at 21q22.1, LSI21 at 21q22.2, and 21qtel at the telomere region showed that the r(21) chromosome retained single copies of D21Z1, AML1, and LSI21 and lacked the 21qtel, whereas the der(21) chromosome had two copies of the 21qtel on both of its ends and single copies of D21Z1, AML1, and LSI21, with a paracentric inversion of AML1 and LSI21 (21qtel-D21Z1-LSI21-AML1-21qtel). Microsatellite analysis for nine loci on 21q22.3 indicated that the r(21) chromosome was missing a distal 21q22.3 region involving D21S1890, D21S1411, and D21S1903 with no maternally derived alleles, and that the der(21) chromosome was associated with duplication of a distal 21q22.3 region encompassing D21S1890 and D21S1446. The results suggest that a U-type exchange occurred between the homologous distal 21q regions duplicated on the der(21) chromosome, leading to the r(21) formation. This is a novel mechanism put forward for the formation of a monocentric ring chromosome.