NOR activity and centromere suppression related in a de novo fusion tdic(9;13)(p22;p13) chromosome in a child with del(9p) syndrome.

A female infant with del(9p) syndrome was found to have the karyotype 45,XX,tdic(9;13)(p22;p13) de novo. In the translocation chromosome, various combinations of AgNOR activity and inactivity were found with suppression of either the 9 or neither centromere. These phenomena of discontinuous centromeric suppression and variation in NOR activity in the one chromosome were scored on AgNOR, GTG, and a combination of AgNOR- and GTG-banded preparations. With AgNOR staining, 15.8% tdic chromosomes were AgNOR-positive, and this coincided (in preparations with GTG banding only) with 16% tdic chromosomes with a nonstaining gap present at the NOR site. This suggested that when the NOR-nonstaining gap was present the AgNOR staining would be positive; this was confirmed by the absence of gaps in combination AgNOR-GTG-banded preparations. In cells with tdic-NOR-negative chromosomes, equal proportions of cells with the 13 or both centromeres constricted were observed, but in cells with tdic-AgNOR-positive chromosomes there was only half the proportion of tdic chromosomes with both centromeres constricted; ie, there was a (significant) tendency towards inactivity of the NOR when both centromeres were constricted in the tdic chromosome. Therefore, the 2 phenomena, variation in NOR activity and centromeric suppression, are interrelated in this case.