Insights into the mechanisms of sister chromatid exchange formation

Abstract

The DNA lesions responsible for the formation of sister chromatid exchanges (SCEs) have been the object of research for a long time. SCEs can be visualized by growing cells for either two rounds of replication in the presence of 5-bromo-2′-deoxyuridine (BrdU) or for one round with BrdU and the next without. If BrdU is added after cells were treated with a DNA-damaging agent, the effect on SCEs can only be analyzed in the second post-treatment mitosis. If one wishes to analyze the first post-treatment mitosis, cells unifilarily labeled with BrdU must be treated. Due to the highly reactive bromine atom, BrdU interacts with such agents like ionizing and UV radiation enhancing the frequency of SCEs. However, its precise role in this process was difficult to assess for a long time, because no alternative technique existed that allowed differential staining of chromatids. We have recently developed a method to differentially label sister chromatids with biotin-16-2′-deoxyuridine-5′-triphosphate (biotin-dUTP) circumventing the disadvantage of BrdU. This technique was applied to study the SCEs induced by ionizing and UV radiation as well as by mitomycin C, DNaseI and AluI. This article is a review of the results and conclusions of our previous studies.

Sister chromatid exchanges occur in G2-irradiated cells

“…Consistent with the paradigm that HR represents the underlying mechanism of SCE formation, downregulation of the HR core components Brca2 or exponentially growing cell populations do not exhibit SCEs above background level within the first 4 h post irradiation. 22 Here, we present evidence that "true" SCEs are formed in G 2 -irradiated cells, by a mechanism based on HR. We discuss the results of our recent work in the field of DNA double-strand break (DSB) repair and explain pitfalls in the experimental setup that have to be considered when measuring SCEs which arise after irradiation in G 2 .…”

Mechanisms and consequences of methylating agent-induced SCEs and chromosomal aberrations: a long road traveled and still a far way to go

“…This is quite a high conversion frequency compared, for instance, to cyclobutane dimers for which a relation of 1:600 has been estimated (De Weerd- Kastelein et al, 1977). It should be noted, however, that this calculation might need re-estimation in view of the fact that UV-C-induced BrdU photolysis may contribute to SCE formation (Wojcik et al, 2004). The O 6 MeG conversion probabilities for chromosomal aberrations are much lower, estimated in the range of 1:22,000 and 1:147,000 for aberrations induced in M2 and M1, respectively (Kaina et al, 1993).…”

Sister chromatid exchange in selected horse breeds (<i>Equus caballus</i>)

“…After replication, when the condensed sister chromatids are combined in pairs, the swapping occurs between identical DNA sequences that are close to each other. SCE is the result of semiconservative DNA replication from the damaged matrix and can take place only if the changes in the DNA are not removed before the cell enters the S phase (Wójcik et al 2004, Bayani & Squire 2005. SCE can be recognised as sudden breaks in the continuity of staining patterns of two chromatids of the same chromosome.…”