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.