Cytogenetic demonstration of a corrective factor in Bloom's syndrome.

Patients with Bloom's syndrome, an autosomal recessively inherited disorder, have a highly increased risk of developing early malignancies of various types. The chromosomes of such patients exhibit an enhanced rate of spontaneous sister chromatid exchange. Cocultivation of patients' fibroblasts (as responder cells) with fibroblasts from healthy donors or from patients with xeroderma pigmentosum or Fanconi's anaemia (as effector cells) produces a dose-dependent reduction in Bloom-specific sister chromatid exchange; however, Bloom heterozygotes exhibit a reduced corrective capacity. The reduction in Bloom-specific sister chromatid exchange is related to the presence of a soluble factor (m.w. 10 000-20 000) which is produced in culture by normal proliferating fibroblasts. This corrective factor might represent a cancer-protective principle that is defective in patients with Bloom's syndrome.