Lack of a correlation between micronucleus formation and radiosensitivity in established and primary cultures of human tumours

Clinical Relevance of Tumor Ploidy and Micronucleus Formation for Oral Cavity Cancer

Aims and Background

To study the clinical relevance of tumor ploidy and micronucleus formation as prognostic factors.

Methods and Study Design

Twenty-eight patients with squamous cell carcinoma of the oral cavity were treated with primary radiochemotherapy consisting of irradiation up to 70 Gy in combination with cisplatin. Cell cycle distribution, micronucleus formation and ploidy were evaluated by flow cytometry of biopsies taken before treatment and after irradiation to 10 Gy (5 × 2 Gy). Sexteen out of 28 patients relapsed after a minimum follow-up period of two years.

Results

Flow cytometry of the recurrence biopsy showed hyperpentaploid (5c exceeding) cells in 13/16 (81%) of the relapsed patients. In 7 patients the hyperploid clone was not present in the flow cytometry of the primary tumors. Ploidy could retrospectively be determined also by image cytometry in archival tumor material of the pretreatment specimens. Patients with a level below 100 5c cells per 10,000 cell nuclei were shown to have a significantly better prognosis than patients with more than 100 hyperpentaploid tumor cells. The micronucleus formation was 2-5 times higher in tumors showing a good response to treatment than in carcinomas relapsing within two years.

Conclusions

The 5c-exceeding ratio measured by image cytometry and micronucleus formation proved to be good prognostic parameters for the clinical outcome of patients with locally advanced head and neck carcinomas.

Differential effects of methoxyamine on doxorubicin cytotoxicity and genotoxicity in MDA-MB-231 human breast cancer cells

Pharmacological inhibition of DNA repair is a promising approach to increase the effectiveness of anticancer drugs. The chemotherapeutic drug doxorubicin (Dox) may act, in part, by causing oxidative DNA damage. The base excision repair (BER) pathway effects the repair of many DNA lesions induced by reactive oxygen species (ROS). Methoxyamine (MX) is an indirect inhibitor of apurinic/apyrimidinic endonuclease 1 (APE1), a multifunctional BER protein. We have evaluated the effects of MX on the cytotoxicity and genotoxicity of Dox in MDA-MB-231 metastatic breast cancer cells. MX has little effects on the viability and proliferation of Dox-treated cells. However, as assessed by the cytokinesis-block micronucleus assay (CBMN), MX caused a significant 1.4-fold increase (P<0.05) in the frequency of micronucleated binucleated cells induced by Dox, and also altered the distribution of the numbers of micronuclei. The fluorescence probe dihydroethidium (DHE) indicated little production of ROS by Dox. Overall, our results suggest differential outcomes for the inhibition of APE1 activity in breast cancer cells exposed to Dox, with a sensitizing effect observed for genotoxicity but not for cytotoxicity.

Influence of DNA double‐strand break rejoining on clonogenic survival and micronucleus yield in human cell lines

PURPOSE

To examine the role of DNA double-strand break (DSB) rejoining in cell survival and micronucleus yield after 60Co gamma-irradiation.

MATERIALS AND METHOD

Thirteen human cell lines (six glioblastoma, five prostate, one melanoma, one squamous cell carcinoma) were irradiated with 60Co gamma-rays to doses of 0-10Gy for cell survival and micronucleus measurements and 0-100Gy for DSB rejoining. Measurements were performed using standard clonogenic, micronucleus and constant-field gel electrophoresis assays.

RESULTS

Radioresistance and micronucleus yield were positively correlated (r=0.74, p=0.004). A significant cell type-dependent correlation was demonstrated between total (0-20 h) DSB rejoining and cell survival (r=0.86, p=0.03 for glioblastomas; r=0.79, p=0.04 for other cell lines), with more resistant cell lines showing higher levels of DSB rejoining. No relationship was apparent between fast (0-2 h) or slow (2-20 h) DSB rejoining and clonogenic survival. While there was no relationship between total or slow DSB rejoining and micronucleus yield, a significant and cell type-specific correlation emerged between fast rejoining and micronucleus yield for the glioblastomas (r=0.89, p=0.04) and other cell lines (r=0.76, p=0.04). Cell lines with higher levels of DSB rejoining within 2 h of irradiation showed higher yields of micronuclei.

CONCLUSION

Fast DSB rejoining, possibly through interaction with slow DSB rejoining, appears to play an important role in the formation of micronuclei. However, total DSB rejoining reflects intrinsic radiosensitivity. Consideration of differences in DSB rejoining kinetics might contribute to a better understanding of the significance of cell survival and micronucleus data in the clinical and radiation protection setting.

Response to high LET radiation 12C (LET, 295 keV/microm) in M5 cells, a radio resistant cell strain derived from Chinese hamster V79 cells

PURPOSE

To study the effects of 12C-beam of 295 keV/microm (57.24 MeV) on M5 and Chinese hamster V79 cells by using cytogenetic assays like micronuclei (MN) induction, chromosomal aberrations (CA) and apoptosis. Additionally, the relative survival of these two cell lines was tested by the colony forming ability of the cells, with a view to understanding the mechanism of cellular damages that lead to difference in cell survival.

MATERIALS AND METHODS

Confluent cells were irradiated with 12C-beam at various doses using 15UD Pelletron accelerator. Cell survival was studied by the colony forming ability of cells. MN assay was done by fluorescent staining. Different types of chromosomal aberrations in metaphase cells were scored at 12 h after irradiation. Apoptosis was measured at different post irradiation times as detected by nuclear fragmentation and DNA ladder was prepared after 48 h of incubation.

RESULTS

Dose-dependent decrease in surviving fractions was found in both the cell lines. However, the surviving fractions were higher in M5 cells in comparison to V79 cells when exposed to the same radiation doses. On the other hand, induced MN frequencies, CA frequencies and apoptosis percentages were less in M5 cells than V79 cells. Very good correlations between surviving fractions and induced MN frequencies or induced total CA or induced apoptosis percentages were obtained in this study.

CONCLUSIONS

The cell strain M5 showed relatively more radio-resistance to 12C-beam compared to Chinese hamster V79 cells in this study. As the MN formation, CA and apoptosis induction were less in M5 cells as compared to parental V79 cells, the higher cell survival in the former could possibly be attributed to their better repairing ability leading to higher cell survival.

Genotoxic effects in M5 cells and Chinese hamster V79 cells after exposure to 7Li-beam (LET=60 keV/microm) and correlation of their survival dynamics to nuclear damages and cell death

Chinese hamster V79 cell and a cell strain M5, derived from V79 cells and reported to be relatively resistant to gamma-ray, hydrogen peroxide, and N-methyl-N-nitro-N-nitrosoguanidine (MNNG; a potent human carcinogen), were exposed to high LET (7)Li-beam (LET=60 keV/microm) at approximately 90% confluent state in the dose range of 0-1 Gy. Effects of (7)Li-beam exposure on cell survival, micronuclei induction (MN), chromosomal aberrations (CA) and apoptosis were compared in both the cell lines. A dose-dependent decline in survival for both the cell lines was noted, relatively less in M5 cells (mostly p<0.01) indicating greater radio-resistance in this strain. The MN, CA and apoptosis increased in a dose-dependent manner in both V79 and M5 cells. Significant differences in various other parameters between these two cell lines were also noted. The relative intensity of DNA ladder, which is a useful marker for the determination of the extent of apoptosis induction, was much higher in V79 cells. A good correlation between the reduction of the surviving fractions and the increase in frequencies of MN or CA or apoptosis was noted for both the cell lines.

The bystander effect-induced formation of micronucleated cells is inhibited by antioxidants, but the parallel induction of apoptosis and loss of viability are not affected

X-rays induce various DNA damages including strand breaks that lead to formation of micronuclei and chromosomal aberrations as well as increased number of apoptotic cells. Similar effects appear when non-irradiated cells are treated with medium collected from cultures of irradiated cells (irradiation conditioned medium - ICM). This phenomenon was termed "bystander effect". A number of studies suggest that bystander effect appears to be associated with up-regulation of oxidative metabolism. We thus compared the effects of antioxidant Vitamins C and E on the frequency of micronuclei and apoptotic cells in both directly irradiated cell cultures and in cultures exposed to ICM. Addition of Vitamins C or E (1-40 microg/ml) to culture medium after exposure to radiation or ICM reduced the frequency of micronuclei in a concentration-dependent manner. These vitamins had no effect on cell viability, clonogenic survival or the frequency of apoptotic cells under both conditions tested. These results show that the bystander effect causes micronucleation in addition to other known effects and suggest that the factors causing micronucleation by X-irradiation, oxidative DNA damage and incomplete repair, are regulated by apoptosis-independent pathways.

Radiation therapy induced micronuclei in cervical cancer—does it have a predictive value for local disease control?

OBJECTIVE

To evaluate the predictive value of serial changes in micronuclei induction during external radiotherapy (EXRT) in cervical cancer with respect to local response at the end of EXRT and local disease free survival (LDFS).

METHODS

Twenty-five patients of squamous cell cancer of the cervix were treated by 50 Gy of EXRT delivered over 5 weeks followed by intracavitary brachytherapy. Serial cytological smears were taken from cervical growth at weekly intervals during the course of EXRT and stained by Giemsa and May-Grunwald's stain. Micronuclei induction were scored as (a) number of cells expressing micronuclei (MN), and (b) total number of micronuclei (TMN) in 1000 tumor cells from each of the serial smears.

RESULTS

A significant rise in micronuclei count was seen for both MN and TMN from pretreatment (week 0) to successive weeks of EXRT. For those having a near total tumor regression by end of EXRT, a significant rise in micronuclei was evident even at the end of first week of EXRT (MN: P = 0.05, TMN: P = 0.04). A superior LDFS was observed in patients showing greater than 50% increment in MN value in the first week (median survival for <50% vs. > or =50% rise: 5 months vs. not reached, P = 0.21), while it reached significance for a similar rise of TMN (median survival <50% vs. > or =50% rise: 5 months vs. not reached, P = 0.04).

CONCLUSIONS

The significant rise of micronuclei at the end of first week of EXRT in cervical cancers as observed from serial cytological smears could predict for a better local response and LDFS.

Relationship between clonogenic radiosensitivity, radiation-induced apoptosis and DNA damage/repair in human colon cancer cells

The intrinsic radiation sensitivity of normal and tumour tissue is a major determinant of the outcome of radiotherapy. There is currently no established test that can be used routinely to measure the radiosensitivity of the cells in an individual patient's cancer in a manner that can inform treatment planning. The purpose of this study was to evaluate, in four human colorectal adenocarcinoma cell lines, two possible end points as surrogate markers of radiation response – apoptosis and induction of DNA single-strand breaks – and to compare the results with those of a conventional clonogenic assay. Cell lines (SW707 SW480, SW48 and HT29) known to differ in radiosensitivity were exposed to single doses of X-rays ranging from 0.5 to 5 Gy and cell survival was measured using the clonogenic assay. Apoptosis was determined on the basis of morphology under fluorescent microscopy and DNA damage/repair was measured, as tail moment, using an adaptation of the alkaline comet assay. The relationship between surviving fraction at 2 Gy (SF₂) and the percentage of apoptotic cells 24 h after the same dose was complex, but apoptosis accurately predicted the order of radiosensitivities as measured by SF₂. Initial damage measured after 2 Gy using the alkaline comet assay gave a close correlation with SF₂ (r²=0.95), whereas there was no correlation between initial DNA damage repair rate and SF₂.

Micronucleus response of human glioblastoma and neuroblastoma cells toward low-LET photon and high-LET p(66)/Be neutron irradiation

The identification of photon resistant tumors that are sensitive to neutrons is still an unresolved problem, and no radiobiological criteria have been developed that could help the selection of patients for neutron therapy. The micronucleus (MN) assay has been evaluated for this purpose in a panel of human glioblastoma and neuroblastoma cell lines spanning a wide range of photon sensitivities defined by mean inactivation doses ([Latin capital letter D with macron above][gamma]) of 1.25-3.21 Gy. We show that the relative biologic effectiveness (RBE) of the p(66)/Be neutrons is significantly correlated with inherent photon sensitivity (r = 0.89, p < 0.01), indicating that the panel of cell lines used is suitable to study the differential biologic response to neutrons and photons. We find that p(66)/Be neutrons are 1.43 to 5.29 times more effective per unit dose in inducing micronuclei than 60Co [gamma]-rays. Surprisingly, cells that are inherently photon resistant tend to show a higher yield of micronuclei following exposure to either photons or neutrons, but no significant correlation could be demonstrated. However, RBE values based on micronucleus yield were found to strongly correlate with RBE values derived from cell survival data (r = 0.91, p < 0.01). It is concluded that although micronucleus yield does not reflect intrinsic sensitivity to either photons or neutrons, the strong correlation between RBE calculated from micronucleus formation and RBE derived from cell survival demonstrates that the micronucleus endpoint has a potential for detecting photon resistant cells that show increased sensitivity to neutrons.

Micronucleus assay in vivo provides significant prognostic information in human cervical carcinoma; the updated analysis

PURPOSE

Reanalysis after a 5-year follow-up previously presented relationships between spontaneous and radiation-induced micronucleus frequencies in tumour cells and the clinical outcome of patients with advanced stages (II B-IV B) of cervix carcinomas treated with radiotherapy.

MATERIALS AND METHODS

Spontaneous and induced in vitro and in vivo micronucleus frequencies were determined and related to clinical parameters. Data were analysed by the univariate Kaplan-Meier method and multivariate Cox proportional hazards model.

RESULTS

In univariate analysis stage, spontaneous micronucleus frequency before radiotherapy (MNSP) and per cent increment of micronucleus level in vivo after 20 Gy in relation to spontaneous pretreatment level were statistically significant predictors of 5-year recurrence-free, disease-free and overall survival. Neither micronucleus frequency (MN/BNC at 2 Gy) nor proliferating fraction (%BNC at 0 Gy) estimated in vitro (in primary culture) were related to radiotherapy outcome. The age of patients was not associated with clinical results. Multivariate analysis demonstrated that the clinical stage of disease, the high frequency of spontaneous micronuclei and low-induced micronucleus frequency were independent and significantly unfavourable predictive factors for disease-free and overall survival. But for local control, only high MNSP and low-induced MN frequency were significant negative predictive variables.

CONCLUSIONS

A high frequency of micronuclei before radiotherapy and a slight increase of micronucleus frequency during radiotherapy measured after 10 fractions of 2 Gy were independent on stage, statistically significant adverse predictors of clinical outcome in cervical carcinoma patients treated with radiotherapy.

Radiobiologic significance of apoptosis and micronucleation in quiescent cells within solid tumors following gamma-ray irradiation

PURPOSE

To determine the frequency of apoptosis in quiescent (Q) cells within solid tumors following gamma-ray irradiation, using four different tumor cell lines. In addition, to assess the significance of detecting apoptosis in these cell lines.

METHODS AND MATERIALS

C3H/He mice bearing SCC VII or FM3A tumors, Balb/c mice bearing EMT6/KU tumors, and C57BL mice bearing EL4 tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. The mice then received gamma-ray irradiation at a dose of 4--25 Gy while alive or after tumor clamping. Immediately after irradiation, the tumors were excised, minced, and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labeling (= Q cells) was determined using immunofluorescence staining for BrdU. Meanwhile, 6 hours after irradiation, tumor cell suspensions obtained in the same manner were fixed. The apoptosis frequency in Q cells was also determined with immunofluorescence staining for BrdU. The MN and apoptosis frequency in total (P + Q) tumor cells were determined from the tumors that were not pretreated with BrdU.

RESULTS

In total cells, SCC VII, FM3A, and EMT6/KU cells showed reasonable relationships between MN frequency and surviving fraction (SF). However, fewer micronuclei were induced in EL4 cells than the other cell lines. In contrast, a comparatively close relationship between apoptosis frequency and SF was found in total cells of EL4 cell line. Less apoptosis was observed in the other cell lines. Quiescent tumor cells exhibited significantly lower values of MN and apoptosis frequency probably due to their large hypoxic fraction, similar to total tumor cells on clamped irradiation.

CONCLUSION

gamma-ray irradiation induced MN formation in SCC VII, FM3A, and EMT6/KU tumor cells, and the apoptosis was marked in EL4 cells compared with the other cell lines. Our method for detecting the Q cell response to gamma-ray irradiation using P cell labeling with BrdU and the MN frequency assay was also applicable to apoptosis detection assay.

Frequency of radiation-induced micronuclei in neuronal cells does not correlate with clonogenic survival

It is generally assumed that radiation-induced micronuclei (MN) in cytokinesis-blocked cells are an expression of cellular radiosensitivity. Therefore, radiosensitive cells should have a high frequency of MN and radioresistant cells should show lower levels. We have irradiated cells of a panel of 13 neuronal cell lines of widely differing radiosensitivity [human neuroblastomas: N2alpha, SHSY5Y, SK-N-SH, KELLY and SK-N-BE(2c); murine neuroblastomas: OP-6 and OP-27; human glioblastomas: G120, G60, G28, G112, G44 and G62] and compared their radiation response using the micronucleus and standard clonogenic assays. It was found that micronucleus frequency was much higher in some of the radioresistant cell lines (N2alpha, G28, G120 and G44; SF2 >/= 0.60). These cell lines showed a high frequency of more than 0.32 MN per gray of (60)Co gamma radiation per binucleated cell. On the other hand, the more radiosensitive cell lines (OP-27 and SK-N-SH, SF2 </= 0.27) produced 0.08 +/- 0.01 and 0.04 +/- 0.01 MN per gray, respectively. OP-6, SK-N-BE(2c), G112, G62, G60 and KELLY cells constituted an intermediate group and displayed a micronucleus formation index between 0.10 and 0.24 MN per gray per binucleated cell. SHSY5Y cells showed no detectable formation of MN. In two groups [OP-6, SK-N-BE(2c), G112, G62, N2alpha and G28 or G120, G60, OP-27 and SK-N-SH], the more resistant cell lines produced more MN per unit dose. Another group [OP-6, SK-N-BE(2c), G112, G62, G44 and G120] showed no correlation between micronucleus formation and radiosensitivity. We conclude that the relationship between cell survival and micronucleus formation is not straightforward and that it would be simplistic to translate micronucleus frequency into radiosensitivity.

The increment of micronucleus frequency in cervical carcinoma during irradiation in vivo and its prognostic value for tumour radiocurability

A potential usefulness of micronucleus assay for prediction of tumour radiosensitivity has been tested in 64 patients with advanced stage (II B-IV B) cervical carcinoma treated by radiotherapy. The study of cellular radiosensitivity in vitro was conducted in parallel with the study of cellular damage after tumour irradiation in vivo. Radiosensitivity of in vitro cultured primary cells isolated from tumour biopsies taken before radiotherapy was evaluated using cytokinesis-block micronucleus assay. Frequency of micronuclei per binucleated cell (MN/BNC) at 2 Gy was used as a measure of radiosensitivity. Radiation sensitivity in vivo was expressed as per cent increment of micronucleus frequency in cells isolated from biopsy taken after 20 Gy (external irradiation, 10 x 2 Gy) over the pre-treatment spontaneous micronucleus level and was called MN20. Very low correlation (r = 0.324) was observed between micronucleus frequency in vitro and in vivo. Although micronucleus frequency at 2 Gy differed widely between tumours evaluated (mean MN/BNC was 0.224; range 0.08-0.416), no significant correlation was observed between this parameter and clinical outcome. The average increment of micronucleus frequency after 20 Gy amounted to 193% of spontaneous level (range 60-610%) and was independent of spontaneous micronucleation before radiotherapy. In contrast to in vitro results, these from in vivo assay seem to have a predictive value for radiotherapy of cervix cancer. The micronucleus increment in vivo that reached at least 117.5% of pretreatment value (first quartile for MN20 data set) correlated significantly with better tumour local control (P < 0.008) and overall survival (P < 0.045). Our results suggest that evaluation of increment of micronucleus frequency during radiotherapy (after fixed tested dose of 20 Gy) offers a potentially valuable approach to predicting individual radioresponsiveness and may be helpful for individualization of treatment strategy in advanced stage cervical cancer.

Chromosome breakage and cell lethality in human hepatoma cells irradiated with X rays and carbon-ion beams

Prediction of radiosensitivity would be valuable for heavy-ion radiotherapy. Premature chromosome condensation (PCC) technique has been a potential predictive assay in photon radiotherapy, but has not been investigated for hepatomas receiving heavy ions. Two human hepatoma cell lines, i.e., HLE and HLF, were irradiated with either 290 MeV/u carbon ions or 200 kVp X rays. Cell lethality was assayed by colony formation and compared with the unrejoined fraction of chromatin breaks as measured by PCC technique. Carbon ions at linear energy transfer (LET) of 76 keV/micron produced cell death more effectively than those of 13 keV/micron and X rays. For the cell killing, the relative biological effectiveness (RBE) of 13 and 76 keV/micron carbon ions compared with X rays was 1.10-1.24 and 2.57-2.59, respectively. Mean number of chromosomes in HLE and HLF cells was similar to each other, i.e., 60.48 and 60.28. RBEs for chromatin breaks of 13 and 76 keV/micron carbon ions were 1.30-1.31 and 2.64-2.79, respectively. A strong correlation between unrejoined chromatin breaks and cell killing for human hepatoma cells was observed irrespective of radiation quality. We conclude that PCC provides a potential predictor for the radiosensitivity of individual hepatoma that are treated with photon as well as heavy ion irradiation.

Prediction of radiosensitivity of oral cancers by serial cytological assay of nuclear changes

BACKGROUND AND PURPOSE

To identify the relationship between the radiosensitivity of oral cancers and the induction of micronucleation, nuclear budding and multinucleation (polynucleation) evaluated by serial cytology during fractionated radiotherapy.

MATERIALS AND METHODS

Forty-four patients with epidermoid cancer of the oral cavity receiving radiotherapy (60 Gy in 25 fractions over 5 weeks) were studied. Serial scrape smears were taken from the tumour before and during radiotherapy and stained by Giemsa and the frequency of micronucleated cells (MNC), nuclear budded cells (NBC) and multinucleated cells (PNC) was evaluated by light microscopy. After a minimum follow-up period of 30 months the patients were classified as having resistant or sensitive tumours, depending on whether the primary tumour had recurred or not within that time. Within-group and between-group analysis on the induction of the above individual parameters and two combined parameters, the micro- or multinucleated cell (MPC) count and the abnormally nucleated cell (ANC) count, was done. The counts were expressed per 1000 uni-nucleated cells.

RESULTS

In both groups each parameter showed a statistically significant increase with dose, the increase being higher in the sensitive group. The ANC count showed the greatest increase, the mean counts before treatment and after 28.8 Gy being 24.3 and 157.8 (P < 0.0005), respectively, in the sensitive group and 21.0 and 65.2 (P < 0.0005), respectively, in the resistant group. After 28.8 Gy the sensitive tumours had significantly higher ANC (P = 0.01), MPC (P < 0.05) and PNC (P < 0.05) counts.

CONCLUSION

The study shows that serial cytological assay of nuclear changes (SCANCing) during radiotherapy is a potentially useful test to predict radiosensitivity. The fact that multinucleation showed the greatest relation with radiosensitivity suggests that injury to the cytokinetic apparatus is important in determining tumour radiosensitivity.

Radiation-induced micronuclei formation in human breast cancer cells: dependence on serum and cell cycle distribution

Micronuclei (MN) formation was defined as a form of radiation-induced damage in MCF-7 cells. MN appeared post-mitosis and were scored in bi-nucleated cells of cytochalasin B treated cultures. MN were surrounded by an envelope composed of inner and outer membranes, and contained fragmented chromosomes. However, typical features of apoptosis, such as chromatin margination or condensation were not observed. Reducing serum concentration resulted in a decreased MN formation, suggesting that serum factors directly affected MN formation and/or that serum depletion decreased the availability of radiation sensitive MN-forming cells for mitosis. Irradiation of G1 and S phase enriched populations revealed that S phase cells were more prone to MN formation than G1 cells. Radiation-induced chromosomal aberration can therefore be modulated by altering serum level and cell cycle distribution.

The effect of 50 Hz electromagnetic fields on the formation of micronuclei in rodent cell lines exposed to gamma radiation

Low frequency electromagnetic fields (EMF) do not produce enough energy to damage DNA, in contrast to ionizing radiations. Any relationship between increased incidence of cancer and EMF must therefore be explained by a promoting effect on cellular transformation by ionizing radiation. The aim of this study was to investigate using the cytokinesis-blocked micronucleus assay a possible amplification of the genotoxic effects of ionizing radiations in cells exposed to combined static and power-frequency electromagnetic fields. Rat tracheal epithelial cell lines were first exposed in vitro to 60Co gamma rays (0, 2 and 6 Gy) and cells were then cultured for 24 h in a homogeneous sinusoidal 50 Hz magnetic field (flux density: 100 microTrms) combined with an artificial geomagnetic-like field created by the use of horizontal and vertical pairs of Helmholtz coils. Control cells were cultured in an adjacent incubator where the background EMF was about 0.1 microTrms. Under our in vitro experimental conditions, EMF appeared to have no significant direct effect on micronucleus induction in rat tracheal cell lines. However, an increased frequency of binucleated cells with micronuclei was observed in cells exposed to 6 Gy of gamma rays and EMF, compared with gamma irradiation alone. This could enhance radiation-induced genomic alterations and increase the probability of neoplastic transformation.

Determination of radiosensitivity in established and primary squamous cell carcinoma cultures using the micronucleus assay

In this study, the cytokinesis-block micronucleus assay (CBMN) was used to measure radiosensitivity in three established cell lines (SCC-61, V175 and V134) and 10 primary cell cultures of squamous cell carcinoma (SCC) of the head and neck. Assessment involved optimisation of the assay to determine cytochalasin-B (CB) concentration and sampling time postirradiation. A much closer correlation between dose-response data measured in the clonogenic and micronucleus assays was found when the micronucleus assay was performed under standardised conditions for each cell line (2 micrograms/ml CB: 48 h postirradiation) instead of predetermined optimised assay conditions. This indicates that, for these SCC cell lines, the CBMN assay may be able to predict in vitro radiosensitivity. To be of clinical use in predicting radiosensitivity, the CBMN assay also needs to be evaluated with primary cell cultures. In this study, no relationship between micronucleus frequency at 2 or 6 Gy and patient clinical outcome 12 months following surgery and radiotherapy was seen. Similarly, no association between patient outcome and tumour stage, nodal stage and histology was observed. These CBMN assay data from the primary cell cultures are presently inconclusive as a measure of patient tumour radiosensitivity.

Serial cytological assay of micronucleus induction: a new tool to predict human cancer radiosensitivity

BACKGROUND AND PURPOSE

The micronucleus test, generally done in cultured tumour cells irradiated in vitro, has not gained wide acceptance in predicting human cancer radiosensitivity. The purpose of this study was to see if micronucleus assay by serial scrape smear cytology can predict oral cancer radiosensitivity.

MATERIALS AND METHODS

Forty nine oral cancer patients given radiotherapy (60 Gy/25 fractions/5 weeks) form the study population. Serial scrape smears were taken from their tumours before treatment and after delivery of 2, 5, 8 and 12 fractions, stained by Giemsa and the number of micronucleated cells (MNC) noted. The patients were grouped to those who developed tumour recurrence ('Resistant') and those who did not ('Sensitive'), and the pattern of micronucleus induction compared.

RESULTS

Both groups of tumours had MNC even before treatment, with statistically significant dose-related increase with radiotherapy. The sensitive group had a higher mean increase in MNC count than the resistant group (6.1 times and 3.6 times the pre-treatment value, respectively) and better correlation with dose (r = 0.54 vs. 0.43). The increase in MNC count occurred earlier in the resistant group than in the sensitive, the TMNC (time for the pre-treatment value to double) being 3.3 days and 7.6 days, respectively. Also, the resistant group showed a plateauing of the MNC count which the sensitive group lacked.

CONCLUSIONS

The higher MNC induction in the sensitive tumours suggests the usefulness of the assay as a test of radiosensitivity. The differing patterns of MNC increase suggest that differences in proliferation rate is an important cause of tumour failure. Serial cytological assay of micronucleus induction can identify both radiosensitivity and proliferation characteristics of tumours, and thus may turn out to be a useful test of radiocurability.

Micronucleus induction in 10 human tumour cells after high- and low-dose radiation

A number of data measuring survival of animal or human cells to low LET ionizing radiation have demonstrated that these cells may be hypersensitive to doses below 1 Gy, possibly due to the absence of an inducible repair mechanism, which is observed at higher doses. The production of micronuclei (MN) in cells exposed to ionizing radiation reflects genotoxic damage. Moreover, the micronucleus assay is sensitive to low radiation doses. We have exposed 10 human tumour cell lines to doses ranging between 0.12 and 4 Gy. Using cytochalasin B to block the cells in a binucleate phase, we have scored the fraction of binucleate cells (BNC) expressing MN, as well as the number of MN per BNC, as a function of gamma-ray dose. Experimental points were fitted with a binomial equation. Doses from 1 to 4 Gy were fitted separately from those below 1 Gy, and the initial slopes after both fits were compared. Taken together, the initial slopes of MN induction after low-dose (LD) irradiation were not different from those after high-dose (HD) irradiation. Only in one cell line was a significant increase in MN production detected after LD irradiation. This cell line had the shallowest linear term after HD irradiation. It appeared that the likeliness of expressing hypersensitivity at LD was correlated with the quadratic term of MN induction at HD, which does not contradict an inducible repair hypothesis. However, the failure of observation of a significant hypersensitivity at LD for nine cell lines, and the high variability of response at LD suggests that this occasional effect may be influenced by other factors as well.

Selective assessment of in vitro radiosensitivity of tumour cells and fibroblasts from single tumour biopsies using immunocytochemical identification of colonies in the soft agar clonogenic assay

The assumed selective growth of tumour cells has formed the basis for the use of the soft agar clonogenic assay to test in vitro radio- and chemosensitivity of tumours. However, recent studies have demonstrated that fibroblasts proliferate in soft agar in addition to tumour cells. The present study was initiated to quantify the contaminating growth of non-malignant cells in the modified form of the Courtenay-Mills soft agar assay, in order to establish a reliable assay for estimating tumour cell radiosensitivity in squamous cell carcinomas of the head and neck. DNA flow cytometry analysis confirmed that 'tumour fibroblasts' (fibroblasts obtained from tumour biopsies) grow in soft agar. In contrast, white blood cells did not form colonies. Different media were tested with soft agar, but a selective medium for tumour cells was not found. Therefore, a colony filter-technique combined with an immunocytochemical analysis was developed to quantify the number of tumour cell and fibroblast colonies. In 12 tumour biopsies, 2-33% of the colonies were Cytokeratin AE1-3 positive, whereas 83-100% of the colonies were 5B5 fibroblast antibody positive. The parameter normally reported, the overall SF2 (surviving cell fraction at 2 Gy) based on colonies in agar, was found to be statistically significantly correlated to the fibroblast SF2, but not to the tumour cell SF2. The overall SF2 was significantly different from the tumour cell SF2 in half of the tumours. Furthermore, the tumour cell SF2 was not correlated to fibroblast SF2. In consequence of our findings, correcting for fibroblast contamination is a necessity, when studying in vitro sensitivity of tumour cells. Combining the soft agar clonogenic assay with the new colony filter-technique and the immunocytochemical analysis appear to be useful for making this routine correction and for measuring the in vitro radiosensitivity of both tumour cells and fibroblasts from single tumour biopsies, which is of interest in future clinical studies on predictive assays.