On the nature of interactions leading to radiation-induced chromosomal exchange

Induction of DNA Damage by Light Ions Relative to 60Co γ-rays

The specific types and numbers of clusters of DNA lesions, including both DNA double-strand breaks (DSBs) and non-DSB clusters, are widely considered 1 of the most important initiating events underlying the relative biological effectiveness (RBE) of the light ions of interest in the treatment of cancer related to megavoltage x-rays and 60Co γ-rays. This review summarizes the categorization of DNA damage, reviews the underlying mechanisms of action by ionizing radiation, and quantifies the general trends in DSB and non-DSB cluster formation by light ions under normoxic and anoxic conditions, as predicted by Monte Carlo simulations that reflect the accumulated evidence from decades of research on radiation damage to DNA. The significance of the absolute and relative numbers of clusters and the local complexity of DSB and non-DSB clusters are discussed in relation to the formation of chromosome aberrations and the loss of cell reproductive capacity. Clinical implications of the dependence of DSB induction on ionization density is reviewed with an eye towards increasing the therapeutic ratio of proton and carbon ion therapy through the explicit optimization of RBE-weighted dose.

Advances in the biophysical and molecular bases of radiation cytogenetics

PURPOSE

For more than 70 years radiation cytogenetics has continued to be a topic of major concern in relation to the action of radiation on living cells. To date, diverse cytogenetic findings have developed into orderly, quantitative interpretations and have stimulated numerous biophysical models. However, it is generally agreed that any one of the models used alone is still unable to explain all aspects of the observed chromosomal effects. In this review, a large number of radiation-induced chromosome aberration findings from the literature are reassessed with special attention given to the reaction kinetics and the relevant molecular processes.

CONCLUSION

It is now clear that DNA double-strand breaks (DSB) are an integral component of radiation-induced chromosome aberration. At the nexus of the maintenance of genome integrity, cells are equipped with excellent systems to repair DSB, notably non-homologous end-joining (NHEJ) and homologous recombination repair (HRR). These repair mechanisms are strictly regulated along with the DNA turnover cycle. NHEJ functions in all phases of the cell cycle, whereas HRR has a supplementary role specifically in S/G2 phase, where homologous DNA sequences are available in close proximity. The repair pathways are further regulated by a complex nuclear dynamism, where DSB are sensed and large numbers of repair proteins are recruited and assembled to form a repair complex involving multiple DSB. Considering such DSB repair dynamism, radiation-induced chromosome aberrations could be well understood as DSB-DSB pairwise interactions associated with the NHEJ pathway in all phases of the cell cycle and misrepair of a single DSB associated with the complementary HRR pathway in late S/G2 phase.

UV radiation induces delayed hyperrecombination associated with hypermutation in human cells

Ionizing radiation induces delayed genomic instability in human cells, including chromosomal abnormalities and hyperrecombination. Here, we investigate delayed genome instability of cells exposed to UV radiation. We examined homologous recombination-mediated reactivation of a green fluorescent protein (GFP) gene in p53-proficient human cells. We observed an approximately 5-fold enhancement of delayed hyperrecombination (DHR) among cells surviving a low dose of UV-C (5 J/m2), revealed as mixed GFP+/- colonies. UV-B did not induce DHR at an equitoxic (75 J/m2) dose or a higher dose (150 J/m2). UV is known to induce delayed hypermutation associated with increased oxidative stress. We found that hypoxanthine phosphoribosyltransferase (HPRT) mutation frequencies were approximately 5-fold higher in strains derived from GFP+/- (DHR) colonies than in strains in which recombination was directly induced by UV (GFP+ colonies). To determine whether hypermutation was directly caused by hyperrecombination, we analyzed hprt mutation spectra. Large-scale alterations reflecting large deletions and insertions were observed in 25% of GFP+ strains, and most mutants had a single change in HPRT. In striking contrast, all mutations arising in the hypermutable GFP+/- strains were small (1- to 2-base) changes, including substitutions, deletions, and insertions (reminiscent of mutagenesis from oxidative damage), and the majority were compound, with an average of four hprt mutations per mutant. The absence of large hprt deletions in DHR strains indicates that DHR does not cause hypermutation. We propose that UV-induced DHR and hypermutation result from a common source, namely, increased oxidative stress. These two forms of delayed genome instability may collaborate in skin cancer initiation and progression.

Human chromosome-specific changes in a human-hamster hybrid cell line (AL) assessed by fluorescent in situ hybridization (FISH)

PURPOSE

To quantitatively assess all gamma-ray induced chromosomal changes confined to one human chromosome using fluorescence microscopy and in situ hybridization with a fluorescently labeled human chromosome specific nucleic acid probe.

METHODS AND MATERIALS

Synchronized human-hamster hybrid cells containing human chromosome 11 were obtained by a modified mitotic shake-off procedure. G1 phase cells (> 95%) were irradiated with 137Cs gamma rays (0, 0.5, 1.0, 1.5, 2.0, 4.0, 6.0, 8.0, and 10.0 Gy) at a dose rate of 1.1 Gy/min and mitotic cells collected 16-20 h later; chromosomal spreads were prepared, denatured, and hybridized with a fluorescein-tagged nucleic acid probe against total human DNA. Chromosomes were examined by fluorescence microscopy and all categories of change involving the human chromosome 11 as target, recorded.

RESULTS

Overall, of the 3104 human-hamster hybrid cells examined, 82.1% were euploid, of which 88.6% contained one copy of human chromosome 11, 6.2% contained two copies, and 5.2% contained 0 copies. This is compatible with mitotic nondisjunction in a small fraction of cells. Of the remaining 17.9% of cells, 85.2% were tetraploid cells with two copies of human chromosome 11. For all aberrations involving human chromosome 11 there was a linear relationship between yield and absorbed dose of 0.1 aberrations per chromosome per Gy. The yield of dicentrics, translocations, and terminal deletions that involve one lesion on the human chromosome was linear, while the yield of interstitial deletions that arise from two interacting lesions on the human chromosome was curvilinear. The frequencies of dicentrics and translocations were about equal, while there was a high (40-60%) incidence of incomplete exchanges between human and hamster chromosomes.

CONCLUSIONS

Fluorescent in situ hybridization (FISH) procedures allow for the efficient detection of a broad range of induced changes in target chromosomes. Symmetrical exchanges induced in G1 (translocations) were readily scored and found to equate with the complementary asymmetrical exchanges (dicentrics). That is, nonlethal stable changes, which might be of concern in carcinogenic processes, complement lethal, unstable changes. Interstitial deletions that may contribute to the loss of antioncogenes as well as to lethality are also readily detected with enhanced levels detected at higher doses. The high level of induced terminal deletions and of incomplete dicentrics and translocations indicate a partial failure of interaction between lesions induced in human and hamster DNA, and suggest that such interspecies interactions lack the fidelity of intraspecies DNA lesion interactions. This suggests caution in the use of such model systems as indicators of human cell responsiveness.

Frequencies of complex chromosome exchange aberrations induced by 238Pu alpha-particles and detected by fluorescence in situ hybridization using single chromosome-specific probes

We undertook an analysis of chromosome-type exchange aberrations induced by alpha-particles using fluorescence in situ hybridization (FISH) with whole chromosome-specific probes for human chromosomes 1 or 4, together with a pan-centromeric probe. Contact-inhibited primary human fibroblasts (in G1) were irradiated with 0.41-1.00 Gy 238Pu alpha-particles and aberrations were analysed at the next mitosis following a single chromosome paint. Exchange and aberration painting patterns were classified according to Savage and Simpson (1994a). Of exchange aberrations, 38-47% were found to be complex derived, i.e. resulting from three or more breaks in two or more chromosomes, and the variation with dose was minimal. The class of complex aberrations most frequently observed were insertions, derived from a minimum of three breaks in two chromosomes. There was also an elevated frequency of rings. The high level of complex aberrations observed after alpha-particle irradiation indicates that, when chromosome domains are traversed by high linear energy transfer alpha-particle tracks, there is an enhanced probability of production of multiple localized double-strand breaks leading to more complicated interactions.

DNA strand breaks and chromosomal aberrations induced by H2O2 and 60Co gamma-radiation

DNA strand breaks and chromosomal aberrations (CAs) were studied in human cells treated with hydrogen peroxide or with ionizing radiation. DNA strand breaks could be produced at dose levels of H2O2 much lower than those which induced CAs. Doses as low as 0.5 mM of H2O2 produced about as many DNA strand breaks as 2 Gy of 60Co gamma-radiation. On the other hand, as much as 20 mM H2O2 produced only half as many CAs as 1 Gy of 60Co gamma-radiation. The different mechanisms involved in the production of human genetic damage by H2O2 and gamma-radiation are discussed.

Analysis of restriction enzyme-induced chromosomal aberrations by fluorescence in situ hybridization

Fluorescence in situ hybridization and Giemsa staining of metaphase chromosomes were used to determine the relative frequencies of symmetric exchange aberrations (translocations) and asymmetric exchange aberrations (rings, dicentrics, and polycentrics) after exposure of human lymphoblastoid cells to restriction enzymes or X-rays. The yield of symmetric exchanges was determined with the use of chromosome-specific probes for human chromosomes 2 or 4, which were hybridized to metaphase chromosomes from cells exposed to the enzymes PvuII, SacI, or XbaI or 3 or 5 Gy of X-rays. The yield of asymmetric exchanges was determined in Giemsa-stained metaphase chromosomes from the same enzyme-treated or irradiated cell population. About 1.5- to 3-fold more symmetric than asymmetric exchanges were induced after restriction enzyme treatment. However, after X-ray treatment the yield of dicentrics relative to the yield of reciprocal translocations was close to the expected 1:1 ratio.

Telomeres and their possible role in chromosome stabilization

The evidence to date generally supports the hypothesis that telomere capping makes chromosome fragments refractory to subsequent rejoining events, but this control may be somewhat relaxed after chromosome breakage. Cell survival requires that the fragments rejoin before metaphase. Unprotected ends such as those produced by DNA damage are subject to degradation, presumably by endogenous cellular exo- and endonucleases. Telomere repeat sequences may be added to broken chromosome ends to protect the ends from further degradation. That telomeric DNA does not always prevent rejoining raises interesting questions as to what constitutes capping, and how rapidly it occurs after DNA damage in relation to chromosome break rejoining. The prevention of degradation and control of rejoining may be mediated by telomere-specific binding proteins, especially the telomere terminal binding protein [Gualberto et al., 1992; Longtine et al., 1989; Price, 1990; Price and Cech, 1989]. Some of these proteins may be involved in scavenging telomeric DNA when the cell senses that chromosomal breaks have occurred. This mechanism is consistent with the observations of Murnane and Yu [1993], who found that a plasmid with telomere sequences was stably integrated in vivo into a chromosome terminal breakpoint lacking telomere repeats. It is also consistent with the high frequency of interstitial telomere sequences observed in normal cells; a history of DNA damage and repair may be recorded by these sequences (Ijdo et al., 1991]. Although chromosome break rejoining is an efficient process in eukaryotic cells, some breaks are never rejoined and can result in terminal deletions and chromatid and isochromatid deletions at metaphase. It is unclear why these breaks are not rejoined, but it may be due to one or more of the following: 1) chance: broken chromosomes are separated, do not approach sufficiently close to one another, and are consequently physically unable to rejoin; 2) a large number of added telomere repeat sequences indicating to the cell that the chromosome has an authentic telomere; 3) some other DNA modification event that protects DNA ends from degradation, e.g., folding back of DNA ends to form a hairpin, as has been implicated in VDJ recombination [Lieber, 1993].