Reciprocal or nonreciprocal human chromosome translocations?

Summary

Two carriers of a balanced translocation, one paternal t(9q^-;13p⁺) and one maternal t(3q^-;15p⁺), were studied cytogenetically with selective silver staining to localize more precisely the break points on the short arms of the two acrocentric chromosomes involved. The distal ends of both chromosomes 3 and 9 reacted positively to the selective silver staining. This observation allows the conclusion that the NOR (nucleolar organizer region), or parts of it, is in fact active in organizing nucleoli and by implication produces rRNA after the translocation onto chromosomes 3 and 9.

Both translocations are reciprocal. However, the break points on 13p and 15p are different. On 13p the break occurred between the silver-stained region and the centromere, whereas on 15p breakage was within the silver-stained NOR. Both chromosomes involved in this translocation, i.e., 3q and a proximal part of 15p, were stained by AGNO₃.

We propose a model of at least three areas for breakage on the short arms of acrocentric chromosomes within and around the silver-staining regions: (1) distally at the very end of the short arm, (2) within the NOR, (3) proximally between the NOR and the centromere.

This model might be useful for explaining translocations not only between acrocentric and nonacrocentric chromsomes, but also between two acrocentric chromosomes. Furthermore our model might add some information relevant to the controversy concerning the reciprocity of translocations and hence also to the dogma of the integrity of telomeres.