Analyses of bromodeoxyuridine-associated sister chromatid exchanges (SCEs) in Bloom syndrome based on cell fusion: single and twin SCEs in endoreduplication.

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Abstract
When [human] Bloom syndrome (BS) cells labeled with bromodeoxyuridine (BrdUrd) for 1 round of DNA replication were fused with nonlabeled normal cells, the hybrid cells had a normal level of sister chromatid exchange (SCE) at the 1st mitosis after fusion. However, when normal cells treated with mitomycin C(MC) were fused with nontreated normal cells, the MC-induced SCE was not affected by fusion with normal cells. Single and twin SCE were analyzed in the Colcemid-induced endoreduplicated normal and BS lymphoid B cells from diplochromosomes. In normal cells, the same number of SCE occurs in each of the 2 cell cycles; the SCE ratio of single (6.30 SCE/cell) to twin (2.92 SCE/cell) was 2:1 on the endoreduplicated-cell basis, showing 1:1 on the diploid-cell basis. In BS cells, the SCE ratio of single (144.8 SCE/cell) to twin (5.9 SCE/cell) was 25:1 on the endoreduplicated-cell basis and was 12:1 on the diploid-cell basis. Most of the BS SCE apparently occur during the 2nd cell cycle when BrdUrd-containing DNA is used as template for replication. The normal level of BS SCE observed at the 1st mitosis of the hybrid cells is the result of SCE inhibition resulting from the fusion with normal cells.