Sister chromatid differentiation in 5-bromo-2'-deoxyuridine-substituted chromosomes: a study with DNA-specific ligands and monoclonal antibody to histone H2B

A historical overview of bromo-substituted DNA and sister chromatid differentiation

The thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) has been widely used to make sister chromatid differentiation (SCD) evident in metaphase chromosomes of cells grown for two cycles in BrdU and, thus, containing varying amounts of the thymidine analogue. A direct consequence was the possibility of making sister chromatid exchange (SCE) evident without using autoradiographic procedures. The latter phenomenon was first discovered in 1953, and its frequency is considered a reliable marker of pathological cell situations, as well as an indicator of mutagenic compounds. Several experimental procedures were found which produced SCD, such as the use of fluorochromes like 33258 Hoechst or acridine orange, whose observation under fluorescence microscopy was directly recorded by photos or stained with Giemsa to make chromosome preparations permanent. Other treatments followed by Giemsa staining required the use of saline hot solutions, acid solutions, nuclease attack and specific monoclonal antibodies. Basically two molecular mechanisms were invoked to explain the different affinity of Giemsa stain for differential BrdU-substituted chromatid DNA. The first implied debromination of chromatid DNA, whose occurrence would be greater in chromatids containing an amount of BrdU greater than that present in sister chromatids. The second mechanism, although not denying the importance of DNA debromination, postulated that chromatin structural organization, in terms of DNA-protein and/or protein-protein DNA interaction, is responsible for SCD production.

Identifying different types of chromatin using Giemsa staining

Mixtures of polychrome methylene blue-eosin Y (i.e., Giemsa stain) are widely used in biological staining. They induce a striking purple coloration of chromatin DNA (the Romanowsky-Giemsa effect), which contrasts with the blue-stained RNA-containing cytoplasm and nucleoli. After specific prestaining treatments that induce chromatin disorganization (giving banded or harlequin chromosomes), Giemsa staining produces a differential coloration, with C- and G-bands appearing in purple whereas remaining chromosome regions are blue. Unsubstituted (TT) and bromo-substituted (BT) DNAs also appear purple and blue, respectively. The same occurs in the case of BT and BB chromatids.In addition to discussing the use of Giemsa stain as a suitable method to reveal specific features of chromosome structure, some molecular processes and models are also described to explain Giemsa staining mechanisms of chromatin.

5-Bromo-2-deoxyuridine activates DNA damage signalling responses and induces a senescence-like phenotype in p16-null lung cancer cells

5-Bromo-2-deoxyuridine (BrdU) is a thymidine analogue that is incorporated into replicating DNA. Although originally designed as a chemotherapeutic agent, sublethal concentrations of BrdU have long been known to alter the growth and phenotype of a wide range of cell types. Mechanisms underlying these BrdU-mediated effects remain unknown, however. We have characterized the effects of BrdU on A549 lung cancer cells by examining DNA damage responses, cell cycle effects and phenotypic changes. A549 cells express wild-type p53, but are p16-null. Sublethal concentrations of BrdU evoke a DNA damage response in these cells that involves the activation of Chk1, Chk2 and p53. Increased numbers of enlarged nuclei and multinucleated cells are evident in the treated populations. Cell cycle inhibition occurs, resulting in reduced proliferation and accumulation of cells in the S, G2/M and G0 phases. BrdU induces an early inhibition of p21 expression that coincides with nuclear localization of proliferating cell nuclear antigen. Subsequently, p21 levels increase, whereas proliferating cell nuclear antigen levels decrease compared with control cells. Upregulation of p27 and p57 expression also occurs. By day 7 of exposure to BrdU, treated cells acquire a senescent-like phenotype with an increase in cell size, granularity and beta-galactosidase activity. We conclude that BrdU induces a DNA damage response in A549 cells, which results in reduced proliferation mitotic exit and phenotypic changes that resemble senescence.

Sequence-dependent formation of intrastrand crosslink products from the UVB irradiation of duplex DNA containing a 5-bromo-2'-deoxyuridine or 5-bromo-2'-deoxycytidine

The replacement of thymidine with 5-bromo-2′-deoxyuridine (^BrdU) is well-known to sensitize cells to ionizing radiation and photoirradiation. We reported here the sequence-dependent formation of intrastrand crosslink products from the UVB irradiation of duplex oligodeoxynucleotides harboring a ^BrdU or its closely related 5-bromo-2′-deoxycytidine (^BrdC). Our results showed that two types of crosslink products could be induced from d(^BrCG), d(^BrUG), d(G^BrU), or d(A^BrU); the C(5) of cytosine or uracil could be covalently bonded to the N(2) or C(8) of its neighboring guanine, and the C(5) of uracil could couple with the C(2) or C(8) of its neighboring adenine. By using those crosslink product-bearing dinucleoside monophosphates as standards, we demonstrated, by using liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS), that all the crosslink products described above except d(G[N(2)-5]U) and d(G[N(2)-5]C) could form in duplex DNA. In addition, LC-MS/MS quantification results revealed that both the nature of the halogenated pyrimidine base and its 5′ flanking nucleoside affected markedly the generation of intrastrand crosslink products. The yields of crosslink products were much higher while the 5′ neighboring nucleoside was a dG than while it was a dA, and ^BrdC induced the formation of crosslink products much more efficiently than ^BrdU. The formation of intrastrand crosslink products from these halopyrimidines in duplex DNA may account for the photosensitizing effects of these nucleosides.