Mouse Lymphoma Cells that Undergo Interphase Death Show Markedly Increased Sensitivity to Radiation-induced DNA Double-strand Breakage as Compared with Cells that Undergo Mitotic Death

Summary

The relationship between radiation-induced DNA double-strand breakage (dsb) and reproductive death (clonogenicity) for two mouse lymphoma cell lines was compared with that for the fibroblast-like hamster cell line V79. One of the lymphoma lines (STRij-4-2.2), which undergoes rapid disintegration following cytotoxic insult, showed extreme sensitivity to γ-ray or DNA-associated ¹²⁵I decay-induced DNA dsb (7 ± 1 ¹²⁵I decays per clonogenic lethal event). Surprisingly, the other lymphoma line (WEHI-22.1), which does not undergo rapid disintegration, was also much more sensitive to DNA dsb than were V79 cells (17 ± 1 versus 61 ± 2 ¹²⁵I decays per clonogenic lethal event). Ultrastructure, DNA degradation, and flow cytometric cell cycle data suggested that both lymphoma cell lines may undergo interphase death, but that the induction of this process in WEHI-22.1 may depend upon blockage in the G₂ phase. It is concluded that there are marked differences between the radiation responses of lymphoma and fibroblast lines, that there may be different forms of radiation-induced interphase death, and that the low number of DNA dsb required to produce a clonogenic lethal event in cells undergoing interphase death could explain the radiosensitivity of organs such as ovary, testis and thymus.     

DNA lesion complexity and induction of apoptosis by ionizing radiation

PURPOSE: To determine whether murine lymphoid cell lines can discriminate between high- and low-LET (linear energy transfer) radiation-induced DNA lesions. MATERIALS AND METHODS: Sensitivity to killing by DNA-incorporated 3H and 125I decays, accumulated during storage in the gas phase of a liquid nitrogen tank, was determined by clonogenic survival assay. RESULTS: Induction of a lethal event in the STRij-4-2.2, WEHI-22.1, and L5178Y-R cell lines required approximately 30 times more 3H than 125I decays. Hence, the same ratio of 3H to 125I decays was found irrespective of whether the cell lines contained mutant or wild-type p53 and irrespective of whether they underwent rapid interphase or mitosis-related apoptosis after irradiation. The 18-81 cell line differed in showing a ratio of around 21 and it is argued that this may be a consequence of v-ABL over-expression. The assumption that DNA-incorporated 3H and 125I decays are low- and high-LET-like events respectively was confirmed by the similar sensitivity of L5178Y-R and -S cells to killing by 125I decays in contrast with their difference in sensitivity to 3H decays. CONCLUSIONS: The difference in lethal effectiveness between DNA-incorporated 3H and 125I decays can be explained by the hypothesis that simple DSB (double-strand breaks) are non-lethal and that cell killing is attributable to complex DSB. The low-LET radiation-specific sensitization of L5178Y-S cells may reflect defective repair of a DNA lesion class (presumably simple DSB) that is differentially induced by high- and low-LET radiation and is non-lethal to cells with normal repair capacity.     

DNA lesion complexity and induction of apoptosis by ionizing radiation

Purpose : To determine whether murine lymphoid cell lines can discriminate between high- and low-LET (linear energy transfer) radiation-induced DNA lesions. Materials and methods : Sensitivity to killing by DNA-incorporated 3 H and 125 I decays, accumulated during storage in the gas phase of a liquid nitrogen tank, was determined by clonogenic survival assay. Results : Induction of a lethal event in the STRij-4-2.2, WEHI-22.1, and L5178Y-R cell lines required approximately 30 times more 3 H than 125 I decays. Hence, the same ratio of 3 H to 125 I decays was found irrespective of whether the cell lines contained mutant or wild-type p53 and irrespective of whether they underwent rapid interphase or mitosis-related apoptosis after irradiation. The 18-81 cell line differed in showing a ratio of around 21 and it is argued that this may be a consequence of v-ABL over-expression. The assumption that DNA-incorporated 3 H and 125 I decays are low- and high-LET-like events respectively was confirmed by the similar sensitivity of L5178Y-R and -S cells to killing by 125 I decays in contrast with their difference in sensitivity to 3 H decays. Conclusions : The difference in lethal effectiveness between DNA-incorporated 3 H and 125 I decays can be explained by the hypothesis that simple DSB (double-strand breaks) are non-lethal and that cell killing is attributable to complex DSB. The low-LET radiation-specific sensitization of L5178Y-S cells may reflect defective repair of a DNA lesion class (presumably simple DSB) that is differentially induced by high- and low-LET radiation and is non-lethal to cells with normal repair capacity.     

The Initial Effects of Ionizing Radiations in Cells

The sensitivity of 10 mouse lymphoid or myeloid cell lines to γ-ray- and DNA-associated ¹²⁵I-decay-induced clonogenic cell killing have been compared with their rate of loss of viability (membrane integrity) and with their putative cell type of origin. The pseudodiploid haematopoietic cell lines showed D₀ values for ¹²⁵I-induced DNA double-strand breakage (dsb) that ranged from 7·7 ± 0·7 to 40·8 ± 2·8 decays. These lines generally appeared to be more sensitive to killing by radiation-induced DNA dsb than are fibroblast-like cell lines. The increased sensitivity of haematopoietic cell lines to killing by DNA dsb may be related to their mode of death (apoptosis versus necrosis). Mode of cell death may thus be an important factor in determining the ‘inherent radiosensitivity’ of normal cells/tissues. Haematopoietic cell lines that undergo rapid interphase apoptotic death showed extreme sensitivity to DNA dsb. The latter cell lines were found to have derived from immature lymphoid cells, and it is speculated that their high radiosensitivity might reflect the action of a mechanism that normally eliminates cells containing illegitimate V(D)J recombinase-induced DNA dsb.     

Response of mammalian cell lines that undergo necrosis after ionizing irradiation to DNA-incorporated tritium and iodine-125 decays: implications for models of radiation action

Purpose : To test the validity of enzymatic DNA repair-based models of ionizing radiation-induced mammalian cell killing. Materials and methods : Sensitivity to ionizing radiation-induced killing was determined by clonogenic survival assay. Results : A panel of seven cell lines that have apparently normal enzymatic DNA repair capability and die predominantly by necrosis were shown to have marked differences in their sensitivity to killing by γ- or X-irradiation. Despite such differences, the pseudo-diploid cell lines CHO, V79, and Vero (when corrected for differences in cell cycle distribution) and normal mouse embryo fibroblasts all had comparable D o values for killing by DNA-incorporated 125 I that were in the range of 57-66 decays; whilst the near-tetraploid or -triploid B16-F1, RUC-2 and SQ-20B cell lines had D o values of around 130 125 I decays. The sensitivity of these cell lines to killing by DNA-incorporated 3 H decays was similarly related to ploidy. Conclusions : Differences in sensitivity to killing by DNA-incorporated 3 H or 125 I decays between the cell lines tested were primarily related to differences in ploidy and, second, to differences in cell cycle distribution or nuclear volume. The data do not support suggestions that generalized variability in an hypothetical aspect of enzymatic DNA repair can explain differences in sensitivity to killing by external-beam, low-linear energy transfer ionizing radiation.     

Response of mammalian cell lines that undergo necrosis after ionizing irradiation to DNA-incorporated tritium and iodine-125 decays: implications for models of radiation action

PURPOSE: To test the validity of enzymatic DNA repair-based models of ionizing radiation-induced mammalian cell killing. MATERIALS AND METHODS: Sensitivity to ionizing radiation-induced killing was determined by clonogenic survival assay. RESULTS: A panel of seven cell lines that have apparently normal enzymatic DNA repair capability and die predominantly by necrosis were shown to have marked differences in their sensitivity to killing by gamma- or X-irradiation. Despite such differences, the pseudo-diploid cell lines CHO, V79, and Vero (when corrected for differences in cell cycle distribution) and normal mouse embryo fibroblasts all had comparable D(o) values for killing by DNA-incorporated (125)I that were in the range of 57-66 decays; whilst the near-tetraploid or-triploid B16-F1, RUC-2 and SQ-20B cell lines had D(o) values of around 130 (125)I decays. The sensitivity of these cell lines to killing by DNA-incorporated (3)H decays was similarly related to ploidy. CONCLUSIONS: Differences in sensitivity to killing by DNA-incorporated (3)H or (125)I decays between the cell lines tested were primarily related to differences in ploidy and, second, to differences in cell cycle distribution or nuclear volume. The data do not support suggestions that generalized variability in an hypothetical aspect of enzymatic DNA repair can explain differences in sensitivity to killing by external-beam, low-linear energy transfer ionizing radiation.     

Cytotoxicity of [125I]iodoHoechst 33342: contribution of scavengeable effects

PURPOSE: The incubation of the DNA minor-groove binder [125I]iodoHoechst 33342 (125IH) with plasmid DNA leads to the production of one double-strand break (dsb) per decay, both in the presence and absence of dimethylsulfoxide (DMSO). In contrast, when 125I is incorporated into mammalian cell DNA as an iodinated pyrimidine base, DMSO decreases the dsb yield and enhances survival. Because these variations in radioprotective effects may be due either to the location of 125I vis-à-vis the DNA helix or to differences in DNA architecture, the toxicity of 125IH and its modification by DMSO were examined in mammalian cells. METHODS: Uptake and retention of 125IH in V79 cells were measured, and survival was determined after accumulation of 125I decays at 0.3 degrees C +/-10% DMSO. RESULTS: A linear-quadratic survival curve was obtained both in the absence [D37 = 114+/-36 decays/cell, alpha = (5.39 1.17) x10(-3) cell/decay] and presence [D37 = 211+/-65 decays/cell, alpha = (1.27+/-0.52) x10(-3) cell/decay] of DMSO. The dose modification factor for the linear component of the survival curve was 4.25+/-1.97, indicating the predominance of indirect mechanisms. This value is similar to that obtained with DNA-incorporated 125I (4.05+/-1.72) and for the initial slope (alpha) of 137Cs gamma-rays (4.43+/- 1.41). CONCLUSIONS: Cytotoxicity resulting from the decay of the Auger electron emitter 125I in the mammalian cell nucleus is caused mainly by indirect mechanisms.     

Effect of Ploidy on the Response of V79 Cells to Ionizing Radiation

Summary

The responses of diploid, tetraploid and near-hexaploid V79 cells to X-irradiation or DNA-associated ¹²⁵I-decay were compared. When cell killing, following X-irradiation, was plotted against the induced level of DNA double-strand breakage (dsb) per unit length of DNA, there was no significant difference between the relationships for each cell line. This suggested that the number of X-ray-induced DNA dsb per cell required to produce a lethal lesion was proportional to ploidy. Consistent with the X-ray results, tetraploid cells required 121 ± 4 and diploid cells 60 ± 1 ¹²⁵I-decays to produce a lethal lesion. However, the hexaploid cells deviated from this relationship and required 137 ± 5 decays. The relationship between relative elution and ¹²⁵I decays/cell reflected cellular DNA content. It is concluded that current models of radiation action are unable to explain these findings satisfactorily.     

Importance of DNA damage in the induction of apoptosis by ionizing radiation: effect of the scid mutation and DNA ploidy on the radiosensitivity of murine lymphoid cell lines

PURPOSE: To study the effects of the murine scid mutation and DNA ploidy on the susceptibility of lymphoid cell lines to induction of apoptosis by ionizing radiation and thereby to determine whether DNA lesions are critical initiators of apoptosis. MATERIALS AND METHODS: Sensitivity to killing and rapidity of induction of apoptosis following y-irradiation or DNA-associated 125I decays were compared in pre-B and pre-T cell lines derived from wild-type mice and from mice homozygous or heterozygous for the scid mutation. Effects of differences in DNA ploidy on the same endpoints were studied using pseudo-diploid and -tetraploid clones of a murine pre-T cell line. RESULTS: Pairs of pre-B- and pre-T cell-derived lines that expressed wild-type p53 and underwent rapid interphase apoptosis after irradiation were identified. In both cases, the scid homozygous cell lines were more sensitive to killing, suggesting that DNA repair capability influences susceptibility to induction of apoptosis. Increasing DNA ploidy in a cell line that undergoes rapid interphase apoptosis produced a corresponding increase in the number of DNA lesions required to produce a lethal event; again consistent with DNA being the target for radiation action. CONCLUSION: DNA damage is an important, if not the sole, initiator of external beam ionizing radiation-induced apoptosis.     

Measurement of DNA double-strand breaks in CHO cells at various stages of the cell cycle using pulsed field gel electrophoresis: calibration by means of 125I decay.

Experiments were performed to calibrate a recently developed pulsed field gel electrophoresis assay, the asymmetric field inversion gel electrophoresis (AFIGE), for the measurement of double-strand breaks (dsb) in the DNA of mammalian cells. Calibration was carried out by means of 125I decay accumulation under conditions preventing repair, and is based on the observation that each 125I decay in the DNA produces approximately one dsb. Iodine was incorporated into DNA in the form of 5'-iododeoxyuridine and decay accumulation was allowed in cells kept frozen at -80 degrees C. Since widely different DNA damage dose-response curves were obtained in cells exposed to X-rays in various phases of the cell cycle, calibration was performed using synchronized populations of cells that were allowed to accumulate DNA damage in G1, G1/S, mid-S, and G2 + M. For this purpose the fraction of activity (in DNA) released from the plug (FAR) was measured and correlated to the number of 125I decays accumulated during the elapsed period of time. Fluctuations in the FAR per 125I decay were observed throughout the cell cycle that were similar to those previously reported for the FAR per Gy of X-rays. Comparison of the FAR per 125I decay with the FAR per Gy gave an induction of 21 +/- 3, 31 +/- 3, 21 +/- 3 and 26 +/- 8 dsb per Gy per diploid DNA complement for G1, G1/S, S, and G2 + M cells, respectively. The results suggest that the observed fluctuations in the FAR per Gy throughout the cycle reflect cell-cycle-associated differences in the physicochemical properties of the DNA molecules that alter their electrophoretic mobility, rather than variations in the induction of dsb per Gy, i.e. the sensitivity of the assay fluctuates throughout the cycle. We propose that similar phenomena underlie the observed fluctuations throughout the cell cycle in the fraction of activity eluted per Gy in the non-unwinding filter elution assay. 125I decays accumulated at 4 degrees C in partly purified DNA from cells embedded in agarose plugs and lysed immediately, gave FAR identical to those obtained with cells kept frozen. This finding suggests that indirect effects do not significantly contribute to DNA damage induction by 125I decay, and indicate that calibration of electrophoresis techniques for dsb measurements can be carried out using this simplified protocol.     

DNA as an important target in radiation-induced apoptosis of MYC and MYC plus RAS transfected rat embryo fibroblasts

Purpose : This study uses a radiation chemistry approach to determine if DNA is an important target for radiation-induced apoptosis of myc (MR4) and myc plus ras (3.7) transfected rat embryo fibroblast cell lines. Materials and methods : The radiation protection efficiency of four thiols was compared with net molecular charge ranging from -1 to +2: mercaptopropionic acid (Z = -1), mercaptoethanol (Z = 0), cysteamine (Z = +1), N(2-mercaptoethyl)-1,3-diaminopropane (Z = +2). Protection factors were determined for these thiols against radiation-induced apoptosis (Apoalert assay), mitotic cell death (clonogenic assay) and double-strand break (dsb) induction (pulse field gel electrophoresis) in MR4 and 3.7 cells. Theoretical protection factors for these thiols against dsb induction were also calculated from second-order chemical repair constants for single-strand breaks (ssb) and the concentration of added thiols in MR4 and 3.7 cell lines. Results : The charge-dependent increases observed for measured protection factors against radiation-induced apoptosis did not differ significantly between the two cell lines, nor did they differ significantly from the corresponding increases observed for radiation-induced mitotic cell killing and for induction of dsb. The calculated protection factor for dsb also showed a thiol charge-dependent increase similar to the measured protection factors for all of the other parameters studied. Conclusions : These results are consistent with the hypothesis that DNA is an important target for radiation-induced apoptosis.     

DNA as an important target in radiation-induced apoptosis of MYC and MYC plus RAS transfected rat embryo fibroblasts

PURPOSE: This study uses a radiation chemistry approach to determine if DNA is an important target for radiation-induced apoptosis of myc (MR4) and myc plus ras (3.7) transfected rat embryo fibroblast cell lines. MATERIALS AND METHODS: The radiation protection efficiency of four thiols was compared with net molecular charge ranging from -1 to +2: mercaptopropionic acid (Z= -1), mercaptoethanol (Z=0), cysteamine (Z= +1), N(2-mercaptoethyl)-1,3-diaminopropane (Z= +2). Protection factors were determined for these thiols against radiation-induced apoptosis (Apoalert assay), mitotic cell death (clonogenic assay) and double-strand break (dsb) induction (pulse field gel electrophoresis) in MR4 and 3.7 cells. Theoretical protection factors for these thiols against dsb induction were also calculated from second-order chemical repair constants for single-strand breaks (ssb) and the concentration of added thiols in MR4 and 3.7 cell lines. RESULTS: The charge-dependent increases observed for measured protection factors against radiation-induced apoptosis did not differ significantly between the two cell lines, nor did they differ significantly from the corresponding increases observed for radiation-induced mitotic cell killing and for induction of dsb. The calculated protection factor for dsb also showed a thiol charge-dependent increase similar to the measured protection factors for all of the other parameters studied. CONCLUSIONS: These results are consistent with the hypothesis that DNA is an important target for radiation-induced apoptosis.     

Time-lapse microscopy and DNA double-strand breakage of Chinese hamster cells under conditions promoting or preventing PLD repair after irradiation with 60Co gamma rays

We have confirmed previous time-lapse microscopic observations (Suzuki 1985) using Chinese hamster hai and V79 cells. The proportion of non-dividing to dividing cells was the same under conditions of potentially lethal damage (PLD) repair and non-PLD repair after irradiation with 60Co gamma-rays. This finding suggested that the radiation-induced damage to cellular DNA was similarly repaired so that cells undergo a first division to the same extent under both sets of conditions. In fact, direct measurement of double-strand breaks (dsb) in DNA from the two cell lines by the neutral elution technique showed no differences either in the initial amount of damage or in the time-course under conditions promoting or preventing PLD repair. These results indicate that PLD repair (i.e. an increase in cell survival) cannot be simply explained by a difference in the repair of dsb, but it can perhaps be explained by assuming that DNA damage is repaired with either fewer or more errors in the presence or absence of PLD repair respectively.     

Doxorubicin decreases the repair of radiation-induced DNA damage

The mechanism that underlies doxorubicin-induced radiosensitization was investigated in a cell culture system. When V79 hamster cells were treated with doxorubicin, radiosensitization occurred as illustrated by an increase in alpha and a reduction in the mean inactivation dose (D) of the radiation survival curve. Under control conditions the radiation survival curve showed an alpha of 0.010 +/- 0.02 and a D of 4.4 +/- 0.2. After exposure to doxorubicin (1.5 micrograms/ml for 1 h prior to irradiation), alpha increased to 0.28 +/- 0.04 and D decreased to 3.0 +/- 0.3. Similar mean inactivation doses were found for doxorubicin doses of 0.75, 1.5 and 3.0 micrograms/ml, indicating that enhancement was not dose-dependent in the dose range studied. The influence of doxorubicin on radiation-induced DNA double-strand breaks (dsb) was then assessed under the same conditions as were used in the cell survival studies. Pretreatment with doxorubicin minimally affected radiation-induced dsb. However, doxorubicin increased dsb by 1.25 +/- 0.07 under conditions in which repair was allowed to proceed for 45 min. These results show that doxorubicin inhibits the repair of radiation-induced dsb. Doxorubicin is known to change DNA conformation by the stabilization of DNA topoisomerase II complexes. It is possible that these changes alter the ability of repair enzymes to recognize and correct radiation-induced damage.     

Time-lapse Microscopy and DNA Double-strand Breakage of Chinese Hamster Cells Under Conditions Promoting or Preventing PLD Repair after Irradiation with 60Co γ Rays

Summary

We have confirmed previous time-lapse microscopic observations (Suzuki 1985) using Chinese hamster hai and V79 cells. The proportion of non-dividing to dividing cells was the same under conditions of potentially lethal damage (PLD) repair and non-PLD repair after irradiation with ⁶⁰Co γ-rays. This finding suggested that the radiation-induced damage to cellular DNA was similarly repaired so that cells undergo a first division to the same extent under both sets of conditions. In fact, direct measurement of double-strand breaks (dsb) in DNA from the two cell lines by the neutral elution technique showed no differences either in the initial amount of damage or in the time-course under conditions promoting or preventing PLD repair. These results indicate that PLD repair (i.e. an increase in cell survival) cannot be simply explained by a difference in the repair of dsb, but it can perhaps be explained by assuming that DNA damage is repaired with either fewer or more errors in the presence or absence of PLD repair respectively.     

Similar lethal effect in mammalian cells for two radioisotopes of copper with different decay schemes, 64Cu and 67Cu

The decays of 64Cu incorporated in human malignant (A549) or monkey nonmalignant (CVI) cells lead to cell death. When plotted as a function of the radioactivity introduced in the growth medium (microCi/ml at t = 0), the residual colony-forming capability decreases exponentially. The slope of the corresponding curve is steeper for A549 than for CV1 cells. Different data show that the cellular lethal event is a consequence of 64Cu transmutation and not of the irradiation by the simultaneously emitted beta- and beta+ particles. Liquid holding results show that the lethal event is irreparable. The decays of 67Cu, another radioisotope of copper, lead to cell death with the same exponential survival curve and the same lethal efficiency as for 64Cu, in spite of their different decay schemes. The lethal efficiency of both copper isotopes is close to that of 125I utilized in the form of iododeoxyuridine under the same experimental conditions as 64Cu and 67Cu. The high lethal efficiency of radioactive copper transmutations raises questions about the role in DNA functioning of copper atoms known to be present in trace amounts in this macromolecule. The lethal consequence of radioactive copper transmutations suggests that the copper atoms bound to DNA are essential for cellular functioning.     

The irradiation of V79 mammalian cells by protons with energies below 2 MeV. Part II. Measurement of oxygen enhancement ratios and DNA damage

The effectiveness of low-energy (below 2 MeV) protons at inducing DNA damage in the form of single- and double-strand breaks has been determined. Protons with mean energies of 1.90 MeV, 1.15 MeV and 0.76 MeV corresponding to track average LETs of 17 keV/microns, 24 keV/microns and 32 keV/microns, respectively, have been used and compared with 250 kVp X-rays and 3.8 MeV 238Pu alpha-particles. Although there was variation in the RBE for DNA ssb induction with LET, the RBEs for dsb induction at all three proton energies and for 3.8 MeV alpha-particles were all around 1.0. This suggests that, if DNA dsb are important in radiation-induced cell lethality, the probability of an induced dsb leading to a lethal event increases with increasing LET of radiation. Oxygen enhancement ratios were measured for both cell survival and DNA dsb induction, and in both cases a decrease in OER with increasing LET was observed.     

The radiation hypersensitivity of cells at mitosis

PURPOSE: Mitotic cells are hypersensitive to ionizing radiation, exhibiting single-hit inactivation coefficients near to those of repair deficient cell lines and lymphocytes. To elucidate possible mechanisms for this hypersensitivity, the kinetics of oxygen radiosensitization, the proportion of indirect effect by OH radicals and the kinetics of radiation-induced DNA strand breakage in the chromatin of mitotic cells were investigated. MATERIALS AND METHODS: Synchronized populations of >90% mitotic HT-29 cells were obtained by the mitotic shake-off method. Cells were irradiated at < or =4 degrees C with (137)Cs gamma-rays. Cellular oxygen concentration was varied by gassing cell suspensions prior to and during irradiation with mixtures of pure N(2) that contained 5% CO(2) and measured quantities of O(2). The indirect effect of OH radicals was investigated with the radical scavenger, DMSO. DNA strand breakage was measured by the comet assay. RESULTS: Mitotic HT-29 cell inactivation is well described by a single-hit inactivation coefficient (alpha) of 1.14 +/- 0.06 Gy(-1). The oxygen enhancement ratio of mitotic cells (at 10% survival) was found to be approximately 2.0, significantly lower than the value of 2.8 measured for interphase (asynchronous) cells. More than 60% of mitotic cell killing was eliminated when the media contained 2 M DMSO, indicating that indirect effect is as important in the killing of mitotic cells as it is for interphase cells. The chromatin in mitotic cells was found to be ~2.8 times more sensitive to radiation-induced DNA single-strand breakage than the chromatin of interphase cells. CONCLUSIONS: The alpha-inactivation coefficient of mitotic HT-29 cells was ~30 times larger than that of interphase cells. Mitotic cell chromatin appears to contain intrinsic DNA breaks that are not lethal. In addition, chromatin in mitotic cells was found to be more susceptible to radiation-induced DNA strand-breakage than the dispersed chromatin of interphase cells. How the enhanced production of these simple DNA lesions (that are usually reparable) translates into the lethal (non-reparable) events associated with alpha-inactivation is not known. The compaction/dispersion status of DNA throughout the cell cycle appears to be an important factor for determining intrinsic cell radiosensitivity and might be manipulated for radiotherapeutic advantage.