Centric fission, centromere‐telomere fusion and isochromosome formation: a possible origin of a de novo 12p trisomy

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Abstract
A 5-month-old girl had a typical 12p trisomy syndrome due to a monocentric i(12p) present in a 46-chromosome complement that also included the translocation of all 12q onto the 8p telomere, i.e., her complex karyotype could be written as 46,XX,-8, -12, + der(8),t(8;12)(p23,3;cen),+i(12p). The present concurrence of a whole-arm q translocation and an i(p) for a single chromocome, along with six previous similar instances involving chromosomes 4, 5 and 9, suggests the following origin for such a special rearrangement: a centric fission in G1 initially yielding two telocentrics; at the next replication, the tel(q) translocates onto a nonhomologous telomere (centromere-telomere fusion), whereas the tel(p) becomes an i(p). This mechanism can be either meiotic or postzygotic and surmises that the translocated long arm retains a partial centromere, which subsequently is inactivated and loses its staining properties.