Characterization of two extreme variants involving the short arm of chromosome 22: are they identical?

The heteromorphic nature of the short-arms of human acrocentric chromosomes is considered the norm without any dire consequences. We characterized two highly unusual chromosome 22 variants with extremely enlarged short arms by routine and molecular cytogenetic techniques. Routine banding revealed that the two variants were not alike. Therefore, a characterization by fluorescent in situ hybridization (FISH) technique became warranted and revealed their remarkable differences. The first variant apparently had a tandem duplication of bands p11.2-->p13, while the second variant had a loss of the beta-satellite and ribosomal DNA regions with an apparent amplification of the satellite III region. The formation of these extremely enlarged regions can occur by a variety of mechanisms whose clinical significance remains obscure.