Mutagen-induced sister chromatid exchange rate in Bloom syndrome remains unaltered in the presence of Bloom corrective factor

Cytogenetics of Bloom's syndrome

The quantitative aspects of Bloom's syndrome cytogenetics are reviewed. The most characteristic feature is an increased rate of homologous chromatid exchange, both sister chromatid exchange and mitotic crossing-over. Other phenomena are a tendency of somatic cells to fuse, an increased rate of chromosome breaks, often with sister chromatid reunion, formation of nonhomologous quadriradials, and occurrence of allocyclic and triradial chromosomes. Mitotic chiasmata are situated highly nonrandomly, preferably in Q-dark regions. Chromosomes containing chiasma "hot-spots" appear to contain more active genes than similarly sized control chromosomes. They also contain a high proportion of localized oncogenes. Bloom's syndrome homozygotes show a high incidence of cancer (1/4). This may depend on a) the high rate of homozygosity resulting from mitotic crossing-over, which would allow the expression of recessive cancer genes; b) unequal crossing-over would amplify these genes; c) chromosome structural changes that might transfer oncogenes to new locations and, thus, activate them; and d) immunodeficiency, which would promote malignant growth.

Radiation sensitivity of Bloom's syndrome lymphocytes during S and G2 phases

In order to study chromosome sensitivity of Bloom's syndrome (BS) cells in relation to the replication stage, gamma-ray irradiation was performed immediately before adding bromodeoxyuridine (BrdU) to lymphocyte cultures of one BS patient and of one control. It was found that BS cells are much more sensitive to the irradiation than control cells at the end of S and at G2 phases. The rate of induction of chromosome breaks is significantly increased and that of chromatid breaks and exchanges is also increased, though to a lesser degree. Our results also favor the existence of a cell subpopulation in BS characterized by a slow cycle, a high spontaneous chromosome aberration rate, and a high radiation sensitivity.

Heterozygous carriers for Bloom syndrome exhibit a spontaneously increased micronucleus formation in cultured fibroblasts

Cultured fibroblasts of homozygotes and heterozygotes for Bloom syndrome exhibit an enhanced formation of micronuclei. The number of spontaneously occurring micronuclei permit clear separation of heterozygotes from either normal controls or homozygous patients without overlap between these groups. The observed differences could not be enhanced further by the addition of various mutagens. We conclude from the increased chromosomal damage that heterozygotes for Bloom syndrome may have a higher risk for malignant diseases.