Bloom's syndrome and EM9 cells in BrdU-containing medium exhibit similarly elevated frequencies of sister chromatid exchange but dissimilar amounts of cellular proliferation and chromosome disruption

Abstract

Bloom's syndrome (BS) and EM9 cells both display elevated frequencies of sister chromatid exchange (SCE) following growth for two rounds of DNA replication in bromodeoxyuridine (BrdU)-containing medium. To learn whether hyperresponsiveness to BrdU itself might play a role in causing the SCE elevation, the effects of BrdU on two other parameters, cellular proliferation and chromosome disruption, were examined, comparing the responses of BS and normal lymphoblastoid cells and of EM9 and CHO cells. BS and normal cells responded similarly with respect to growth for 4 days in BrdU-containing medium (0, 1, 3, and 5 micrograms/ml). Chromosome aberrations were increased only slightly in the BS and normal cells after 2 days in BrdU. CHO cells responded to growth in BrdU-containing medium like BS and normal cells; however, little growth of EM9 was detected at any of the BrdU concentrations employed. CHO and EM9 cells also exhibited strikingly different amounts of chromosome damage following growth in BrdU. After 2 days in 1, 3, and 5 micrograms/ml BrdU 21%, 46%, and 50%, respectively, of the CHO cells had chromosome aberrations in contrast to 92%, 96%, and 98% of the EM9 cells. Most of the aberrations in the BrdU-treated CHO cells consisted of what appeared to be polycentric and ring chromosomes or chromosomes exhibiting telemere association. Acentric fragments were absent from most cells with polycentric and ring chromosomes, indicating either that the abnormal chromosomes were formed during an earlier cell cycle or that the abnormal chromosomes represent a form of association in which the telomeres are apposed so tightly that the juncture between chromosomes cannot be identified microscopically.(ABSTRACT TRUNCATED AT 250 WORDS)

Different mutations are responsible for the elevated sister-chromatid exchange frequencies characteristic of Bloom's syndrome and hamster EM9 cells.

“…Three low-SCE and three high-SCE hybrid lines, each chosen at random, were analyzed as indicated in Table 2. Only 3% as many EM9 cells were present after 5-day growth in BrdUrd-containing medium as were present in medium lacking BrdUrd, in agreement with previous reports (16,17). The three high-SCE hybrid lines resembled the parental EM9 cells with respect to their abilities to proliferate in BrdUrd-containing medium; growth of these hybrid cells in BrdUrd was only 8-17% of their growth in medium lacking BrdUrd.…”

“…The three high-SCE hybrid lines resembled the parental EM9 cells with respect to their abilities to proliferate in BrdUrd-containing medium; growth of these hybrid cells in BrdUrd was only 8-17% of their growth in medium lacking BrdUrd. In contrast, the response of the low-SCE hybrids to BrdUrd resembled the responses of CHO, normal human, and parental BS cells to BrdUrd (16,17); there were 57-71% as many cells after incubation in BrdUrd-containing medium as in BrdUrd-free medium. Thus, those BS/EM9 lines examined that had lost their high-SCE frequency also had lost their hypersensitivity with respect to growth in BrdUrd.…”

“…Shortly before the experiments described here were performed, BS and EM9 cells were discovered to differ from one another in their proliferative responses to BrdUrd. BS cells grow at a slightly reduced rate in BrdUrd-containing medium, as do normal human and CHO cells whereas EM9 cells are extremely sensitive, manifesting extensive chromosome damage (16) and the prompt cessation of proliferation (16,17). {One report that contradicts these findings had appeared, showing that an LCL derived from a Japanese BS patient was exquisitely sensitive to BrdUrd (18 (22,23).…”

Rqh1 blocks recombination between sister chromatids during double strand break repair, independent of its helicase activity

“…This is supported by studies showing that sister-chromatid exchanges, which are a visualization of crossovers, increase dramatically in BLM Ϫ/Ϫ cells (49,50). Also, biochemical data demonstrated that the BLM protein can catalyze the branch migration of HJs as well as participate in the resolution of a double-HJ-like four-way junction through a process called dissolution (51,52).…”

Unique and important consequences of RECQ1 deficiency in mammalian cells

“…Increased SCEs are a hallmark of BS cells. 35,36 These apparently reciprocal exchanges arise primarily due to HR events that occur during repair of DNA damage arising in the S or G 2 phases of the cell cycle. The elevated SCE observed in RECQ1-deficient mouse and human cells may represent unsuccessful attempts to "repair" damaged replication forks by HR at DSBs.…”