Frequency of sister chromatid exchanges in Bloom syndrome fibroblasts reduced by cocultivation with normal cells

Cocultivation of fibroblast cells from a male patient with Bloom syndrome (BS) and a female control reduced the rate of sister chromatid exchanges in the BS cells from a mean of 54 SCE per metaphase (range 42--65) to 41 (range 24--59). Medium used to culture control cells for 48 h also reduced the rate of SCE (from 40--65 to 33-54), whereas medium used for only 24 h altered the SCE rate only slightly (to 39--61). Dialyzed medium concentrate with molecular cutoff at 15,000 did not alter the SCE rate. These initial studies suggest that normal cells produce an agent, presumably lacking in BS cells, that is capable of mitigating the chromosomal manifestation of the BS mutation (bl) in bl/bl cells.