Translocation of unstable heterochromatin as the mechanism of sister chromatid exchange formation: a proposed hypothesis

The phenomenon of sister chromatid exchange has remained an enigma in that the actual mechanism for its formation has never been elucidated. It has long been suspected that the process involves some form of breakage and rejoining of DNA, but that hypothesis has never been proved. Recent work in this laboratory using cells from a premature aging disorder (Werner's syndrome) has promulgated the hypothesis that heterochromatin may not be an integral structure of chromosomes, but rather serves as a surface feature or covering. Furthermore, heterochromatin in Werner's syndrome chromosomes was found to be unstable and easily sloughed-off the chromosome surface. In this investigation evidence is presented which shows that incorporation of bromodeoxyuridine into DNA causes instability in the purported heterochromatin covering, resulting in translocation of segments of heterochromatin from the unifilarly-substituted chromatid to the bifilarly-substituted sister chromatid. Such translocation may represent the long-elusive mechanism of sister chromatid exchange formation.

'Sloughing-off' of heterochromatin in Werner's syndrome cells during high-temperature phosphate incubation

Previous investigations of cells undergoing rapid division revealed the presence of heterochromatic 'dots' in chromosomes as well as numerous chromocentres in interphase nuclei. Such structures were seen in human embryonic cells, as well as cells from organisms capable of regeneration, and cells from various malignancies. Cells with a reduced capacity for reproduction were found to be virtually devoid of nuclear chromocentres and chromosome dots after incubation in phosphate buffer at high temperature. The lack of heterochromatin in such cells (Werner's syndrome) thereby explained their reduced capacity for cell division and the resultant rapid rate of aging in individuals afflicted. Re-examination of such slides containing these cells revealed that chromocentres and chromosome dots were present initially, but the incubation process resulted in a 'sloughing-off' of such structures. The incubation process left these heterochromatic structures intact in malignant and control cells, inferring a link between cell proliferation and stable intact heterochromatin. These findings implicate heterochromatin as the object of the purported chromosomal instability factor characteristic of Werner's syndrome. The loss of heterochromatin did not result in chromosome breakage, suggesting that heterochromatin may not be an integral part of chromosome structure, but rather a surface feature or covering.

Further studies on chromocentres and their implications in regeneration

Previous investigations using a number of invertebrates, as well as regenerative tissues/organs of various vertebrates, have promulgated the hypothesis that heterochromatin, in the form of nuclear chromocentres, is correlated with the ability to regenerate. In order to test the universality of this hypothesis, cells from a variety of additional animals were examined for the presence of nuclear chromocentres. In accordance with the hypothesis, cells from these organisms contained numerous chromocentres. Evidence indicates that chromocentres, double minute chromosomes, chromosome 'dots', and telomeres may be different forms of the same heterochromatin entity.

'Glowing' chromosomes in cells undergoing rapid division

Previous investigations in which metaphase plates of cells in rapid division were incubated in phosphate buffer at high temperature revealed numerous heterochromatic dots in chromosomes after Giemsa staining. In contrast, chromosomes from cells with a reduced capacity for reproduction were devoid of such dots, or the dots were sloughed-off into rings and patches of heterochromatin. In two types of cells which were rapidly dividing, namely HeLa cells (cervical cancer) and cells from regenerating planaria, phosphate incubation followed by Giemsa staining revealed an 'aura' or 'glowing' effect on the chromosomes, consisting of a densely staining core surrounded by a lightly stained periphery. This finding might be developed into a diagnostic test for certain malignancies, for cells undergoing dedifferentiation, or for tissues undergoing regeneration.

Heterochromatin (chromocentres) and regeneration of invertebrates and vertebrates: a correlation

Previous investigations using several species of invertebrates have indicated a correlation between nuclear chromocentre content and the ability to regenerate. Such findings have resulted in the formulation of the hypothesis that the ability to regenerate may be dependent on cellular heterochromatin content. In order to test this concept further, a number of additional invertebrates were stained for heterochromatin (nuclear chromocentres). In accordance with the hypothesis, each species tested was found to contain numerous chromocentres. Many chromocentres were also found in cells of several vertebrates capable of partial regeneration, thereby extending the hypothesis to vertebrate regeneration. These findings provide additional supporting evidence for a correlation between cellular heterochromatin and the ability to regenerate.

Heterochromatin (chromosome dots and chromocentres): key to planarian regeneration?

Regenerating planaria were treated with colchicine dissolved in planarian saline solution in order to harvest metaphase chromosomes. Slides containing such chromosomes were incubated in phosphate buffer at high temperature for 2h and stained with Giemsa solution in order to view heterochromatic dots in chromosomes. The chromosomes were found to contain massive regions of dense heterochromatin. In addition, most interphase cells exhibited numerous densely staining chromocentres. Heterochromatin has been postulated to reduce the duration and enhance the speed of the cell cycle, and previous work has implicated heterochromatin in cells undergoing rapid division. These findings imply that the profound regenerative potential of planarians may be due to their high cellular levels of heterochromatin.