Radiosensitivity of fibroblasts from patients with retinoblastoma and chromosome-13 anomalies

Fibroblasts from Retinoblastoma Patients Show Radiosensitivity Linked to Abnormal Localization of the ATM Protein

PURPOSE/AIM OF THE STUDY

Retinoblastoma (Rb) is a rare form of pediatric cancer that develops from retina cells. Bilateral and some unilateral forms of Rb are associated with heterozygous germline mutations of the (retinoblastoma 1) RB1 gene. RB1 mutations are also associated with a significant risk of secondary malignancy like head and neck tumors. Hence, to date, even if Rb patients are less subjected to radiotherapy to treat their primary ocular tumors, their healthy tissues may be exposed to significant doses of ionizing radiation during the treatment against their secondary malignancies with a significant risk of adverse tissue reactions (radiosensitivity) and/or radiation-induced cancer (radiosusceptibility). However, the biological role of the Rb protein in response to radiation remains misunderstood. Since the ataxia telangiectasia mutated (ATM) protein is a key protein of radiation response and since untransformed skin fibroblasts are a current model to quantify cellular radiosensitivity, we investigated here for the first time the functionality of the ATM-dependent signaling and repair pathway of the radiation-induced DNA double-strand breaks (DSB) in irradiated skin fibroblasts derived from Rb patients.

MATERIALS AND METHODS

The major biomarkers of the DSB repair and signaling, namely clonogenic cell survival, micronuclei, nuclear foci of the phosphorylated forms of the X variant of the H2A histone (γH2AX), the phosphorylated forms of the ATM protein (pATM) and the meiotic recombination 11 nuclease (MRE11) were assessed in untransformed skin fibroblasts derived from three Rb patients.

RESULTS

Skin fibroblasts from Rb patients showed significant cellular radiosensitivity, incomplete DSB recognition, delay in the ATM nucleo-shuttling and exacerbated MRE11 nuclease activity. Treatment with statin and bisphosphonates led to significant complementation of these impairments.

CONCLUSIONS

Our findings strongly suggest the involvement of the ATM kinase in the radiosensitivity/radiosusceptibility phenotype observed in Rb cases.

Intrinsic Radiosensitivity and PLD Repair in Osteosarcoma Cell Lines

The response to radiation of seven osteosarcoma cell lines was analysed by in vitro colony-forming assay and compared with that of eight human fibroblast strains. The values of D0, the surviving fraction after 2 Gy (S2Gy), and the mean inactivation dose (D) of osteosarcoma cells in log-phase culture were significantly higher than those of fibroblast strains (p less than 0.01). PLD (potentially lethal damage) repair of osteosarcoma cells evaluated in the plateau phase of growth showed great variation for enhancement of survival, although all of the values were maximized within 12 h after irradiation. In the osteosarcoma, intrinsic radiosensitivity in vitro reflected the clinical response to radiation. However, the capacity for PLD repair might not be a good indicator for predicting the results of radiation therapy.

Proficient repair of potentially lethal damage sensitive to hypertonic treatment in osteosarcoma cells

Fast-repairing potentially lethal damage (PLD) in seven osteosarcoma cell lines was analyzed after treatment with a hypertonic 0.5 M NaCl solution for 20 min and compared to that in seven human fibroblast strains. Fixation of PLD after exposure to ionizing radiation was observed without exception in both the osteosarcoma cells and the fibroblast strains. The percentages by which the D(o)'s of the osteosarcoma cells decreased were significantly higher than the percentage decreases in the C(o)'s of the fibroblast strains (P < 0.01). Hypertonic treatment resulted in radiosensitization due to fixation of PLD in all of the osteosarcoma cell lines, demonstrating that osteosarcoma cells can repair PLD better than normal fibroblast cells. The radiobiological response of the osteosarcoma cells, with enhanced killing after hypertonic treatment, was similar to that of normal untreated fibroblast cells.

A review of human cell radiosensitivity in vitro

The survival curves of 694 human cell lines irradiated in exponentially growing phase in vitro were collected from the literature. Among them, 271 were derived from tumors, 423 were nontransformed fibroblasts and other normal cell strains from healthy people or people with some genetic disorders. Seventy-six different cell types are identified, and a specific radiosensitivity could be associated with each, using D and surviving fraction at 2 Gy. Technical factors such as culture medium, feeder cells, and scoring method were found to affect intrinsic radiosensitivity. In particular, the cell type is not a discriminating factor when cells are studied in agar. Results obtained with cells irradiated in agar must be used cautiously, depending on how the cells were prepared for the experiments. The use of feeder cells narrows the range of radiosensitivity of human cells. For cells irradiated as monolayer, it was possible to build a scale of radiosensitivity according to cell type, ranging, in terms of D from 0.6 Gy for the most sensitive cell lines to more than 4 Gy for the most resistant. Considering that, in most cases, we could estimate the variation of radiosensitivity within each cell type, our classification among cell types can be used by researchers to place their results in the context of the literature.

Premature aging and immunodeficiency: Mulvihill-Smith syndrome?

We report on a 30-year-old woman with premature aging, immunodeficiency, and other abnormalities. She had many manifestations of the Mulvihill-Smith syndrome, a disorder that has been described in 4 sporadic individuals, ranging in age from 4 to 17 years. The common manifestations include short stature, microcephaly, a senile face with an underdeveloped lower half, diminished facial subcutaneous fat, multiple pigmented nevi, sensorineural hearing loss, and a low IgG level. Our patient also had severe mental retardation, brachydactyly, severe T cell dysfunction, and suffered from severe verruca vulgaris and a chronic, active Epstein-Barr virus infection. The fact that her parents were first cousins suggests autosomal recessive inheritance of her disorder. Two alternative possibilities were considered: the disorder in the patient represents the Mulvihill-Smith syndrome with immune deficiency as a sign of its advanced stage, or a hitherto undescribed syndrome.

The Japan Society of Human Genetics award lecture. Cytogenetic aspects of cancer-predisposing genes

Some human monogenic syndromes are characterized by a high cancer propensity, and provide a pathway to the understanding of genetic origin of human cancers. In this review, cytogenetic insights into the Fanconi's anemia (FA) and retinoblastoma (RB) genes have been presented as the model for recessively and dominantly transmitted cancer-predisposing genes, respectively. Some recessively transmitted genes are unequivocally associated with abnormalities in DNA metabolisms and homozygosity increases the genome instability, and some dominantly inherited genes are categorized into loss-of-function mutation of tumor suppressing genes. While the identified syndromes themselves are uncommon and constitute a minor fraction in population, the heterozygote expression of recessive genes is our major concern of genetic predisposition to cancer, and mutation of tumor suppressing genes, identified as somatic mutation, plays a crucial role in the development of a variety of common cancers. Moreover, their possible non-Mendelian inheritance of mutational susceptibility leads us to an even more rewarding area of cancer genetics.

X-ray sensitivity of fibroblasts from patients with hereditary retinoblastoma and their families

The in vitro response to X-irradiation of cultured human fibroblasts was studied using a colony forming assay. A comprehensive reference range was established, giving a median D0 value of 98.5 cGy with an interquartile range of 86.5-110.5 cGy. Cells from 3 retinoblastoma family pedigrees were studied and the cell survival after exposure to X-rays was compared between affected (11 samples) and unaffected (26 samples) family members. No significant differences in response to ionising radiation were found between the controls, the affected and the unaffected members of the 3 families. The affected members had a median D0 of 97.5 cGy (interquartile range 87.5-107.5 cGy) and the unaffected members had a median D0 of 102 cGy (interquartile range 93-111 cGy). Thus radiosensitivity is not a useful marker for the detection of the retinoblastoma gene.

X-radiation-induced chromosome breakage in retinoblastoma lymphocytes

We have examined the spontaneous and X-radiation-induced chromosomal damage in normal humans and in patients with retinoblastoma using the BudR-Giemsa technique in lymphocytes cultured for 48 h. 9 sporadic unilateral non-hereditary cases, 11 hereditary cases (8 bilateral sporadic and 3 unilateral hereditary cases) and 20 healthy individuals were studied simultaneously. No difference in the spontaneous frequency of chromatid and chromosome aberrations was observed between patients and controls. After treatment with 150 rad the frequency of chromosome exchange aberrations was higher in unilateral hereditary cases than the controls (42.0% +/- 5.3 and 22.3% +/- 2.6 respectively; p = 0.05). In bilateral sporadic retinoblastoma 2 different groups were observed. A hypersensitive group showed a significant increment in radiation-induced chromosomal exchange aberrations over the control group (46.2% +/- 5.4 and 24.2% +/- 2.1 respectively; p = 0.01). The other group had a chromosomal exchange frequency similar to normal individuals (26.5% +/- 2.0 and 24.2% +/- 0.4 respectively; p = 0.10). Sporadic unilateral non-hereditary retinoblastoma had an exchange chromosomal aberration frequency similar to control individuals (26.1% +/- 2.8 and 24.6% +/- 2.7 respectively; p greater than 0.10). These results suggest that: There is no relationship between spontaneous chromosome fragility and retinoblastoma. Sporadic unilateral non-hereditary retinoblastoma has normal chromosome sensitivity to X-irradiation. Some hereditary cases of retinoblastoma are sensitive to X-rays while others behave like normals. A mutation or a submicroscopic deletion at a DNA repair locus which is independent of the retinoblastoma gene may cause this radiosensitivity.

Fibroblasts from patients with inherited predisposition to retinoblastoma exhibit normal sensitivity to the mutagenic effects of ionizing radiation

Retinoblastoma (RB) is a cancer of the retina which characteristically occurs in early childhood. Bilateral RB is an inherited form of this disease. Such patients are at greatly increased risk of subsequently developing second tumors in mesenchymal tissue, especially in areas exposed to ionizing radiation therapy. Fibroblasts from bilateral RB patients have been reported to be more sensitive than normal fibroblasts to the cytotoxic effects of ionizing radiation. Because xeroderma pigmentosum patients have a hereditary predisposition to UV-induced cancer and the cells of such patients are abnormally sensitive to the cytotoxic and mutagenic effects of UV radiation, we compared fibroblasts from 6 bilateral RB patients and 3 normal individuals for their sensitivity to the mutagenic effects of cobalt 60, using resistance to 6-thioguanine (TG) as the genetic marker. The results showed no statistically significant difference between the two types of cell lines. The slope of the weighted least squares line representing the frequency of TG-resistant cells induced in the RB populations as a function of dose was 17 +/- 6 (S.E.)/10(6) cells/Gy with an intercept of 0.09 Gy; that for the normal cells was 17 +/- 7/10(6) cells/Gy with an intercept of 0.14 Gy. We also compared 8 bilateral RB cell lines and 9 age-matched normal cell lines for their sensitivity to the cytotoxic effect of 60Co, using survival of colony-forming ability. The cloning efficiency of the unirradiated RB cell lines ranged from 22% to 76% with an average of 52%; that of the normal cell lines from 21% to 89% with an average of 64%. The results showed the RB cells were somewhat more sensitive than the normal cells. The mean D0 for the RB cell lines ranged from 0.99 +/- 0.01 (S.E.) to 1.69 +/- 0.04 Gy with a weighted average of 1.44 +/- 0.08 Gy; that of the normal cell lines ranged from 1.42 +/- 0.17 to 2.24 +/- 0.10 Gy, with a weighted average of 1.79 +/- 0.11 Gy. The difference in means was estimated to be 0.34 +/- 0.14. The mean for the RB cell lines is statistically significantly lower than the mean for the normal cell lines, at a significance level ca. 1%.

Re-evaluation of in vitro radiosensitivity of human fibroblasts of different genetic origins

A statistical analysis of the radiosensitivity of 204 different survival curves of nontransformed human fibroblast cell strains of different genetic origins was made using three criteria: the multi-target one-hit model (characterized by parameters n and D0), the surviving fraction for a 2 Gy dose (S2) and the mean inactivation dose (D). D is found to be the best parameter for characterization of anomalous radiosensitivity linked to a genetic disorder and for discrimination between groups of cell strains of differing radiosensitivity. Its use allows the description of a range of 'normal' radiosensitivity for control fibroblasts and the classification of the various genetic disorders as a function of their mean radiosensitivity expressed in terms of D. Nine groups of cell strains appear to exhibit radiosensitivity which differs significantly from that of the controls: seven groups are hypersensitive (ataxia-telangiectasia homozygotes and heterozygotes, Cockayne's syndrome, Gardner's syndrome, 5-oxoprolinuria homozygotes and heterozygotes, Fanconi's anaemia) and two groups are more radioresistant (fibroblasts from retinoblastoma patients and from individuals with chromosome 13 anomalies). Since the coupled parameter n and D0 failed to discriminate between the radiosensitivity of the different genetic groups, we recommend the use of D to make an intercomparison of intrinsic radiosensitivity of nontransformed human fibroblasts.

Ataxia telangiectasia with generalized skin pigmentation and early death

A female infant with clinical and laboratory features of ataxia telangiectasia (AT) showed two clinical features exceptional for the disease, i.e. generalized skin pigmentation and an unusually early death at the age of 15 months. Her clinical features supportive of the diagnosis of AT included growth and developmental retardation and muscle weakness. Findings indicating immunodeficiency included recurrent pulmonary infections, failure of PHA stimulation of PB lymphocytes, decreased levels of serum IgM and IgA and on autopsy, an atrophic thymus without Hassall's corpuscles. Her cultured skin fibroblasts showed increased spontaneous chromosome breakages and hypersensitivity to X-ray irradiation, as would be expected for AT fibroblasts. She showed elevated blood HbF levels, macrocytic anaemia, granulocytopenia and thrombocytopenia, findings suggestive of a preleukaemic or leukaemic process. Yet aspirates of her bone marrow revealed no malignant cells. Autopsy revealed bilateral Pneumocystis carinii pneumonia, telangiectatic lesions in all the internal organs studied, sparse and degenerative Purkinje cells in the cerebellar cortex and atrophic ovaries. In view of these findings, it was concluded that the patient had a hitherto undescribed variant of ataxia telangiectasia.

Cellular studies on retinoblastoma

Retinoblastoma may be hereditary or non-hereditary. The hereditary form involves either a predisposing gene transmissible as an autosomal dominant or a deletion at chromosome 13q14. An abnormal cellular response to ionizing radiation was suggested by the occurrence of secondary neoplasms within the field of therapeutic radiation in hereditary retinoblastoma patients. Hereditary retinoblastoma patients also show a predisposition to second neoplasms not related to therapy. In vitro studies on the radiation response of cells from retinoblastoma patients have generated conflicting results. Some laboratories, including our own, find that survival following ionizing irradiation of fibroblasts is within the normal range, other laboratories find an abnormal decrease in cell survival. X-ray-induced chromosome damage in G0-irradiated lymphocytes was slightly elevated compared to control subjects. Recent studies using chromosome 13 genetic markers suggest that retinoblastoma tumour cells are homo- or hemi-zygous for the mutant retinoblastoma gene. It seems unlikely that the mutant gene causes sensitivity to ionizing radiation but any tendency to chromosomal rearrangement in a gene carrier would increase the probability of tumour development.

Radiation sensitivity of fibroblasts of bilateral retinoblastoma patients as determined by micronucleus induction in vitro

The radiation sensitivity of fibroblasts isolated from bilateral retinoblastoma (RB) patients was investigated using an in vitro micronucleus assay. Bilateral RB is an autosomal dominant disease associated with a single locus, RB-1; therefore, all cells in an affected individual carry the germ line mutation. The ability to identify gene carriers made it possible to study the effect of the RB-1 mutation in the heterozygous state on the sensitivity of the cells to chromosome breakage by gamma-rays. The micronucleus assay was chosen for this study since it is a quick and easy measure of chromosomal aberrations. The fibroblasts from bilateral RB patients did not differ systematically from the normal fibroblasts in either the spontaneous or the induced rates of micronucleus production. Thus, bilateral RB fibroblasts are not more sensitive to the clastogenic effects of gamma-radiation than the controls.

Retinoblastoma: a model of oncogenesis

Early events in oncogenesis can be understood in retinoblastoma (RB) because of several unusual clinical features: intraocular location of the tumor results in early diagnosis and frequent cure by surgery; survivors demonstrate a dominantly transmitted hereditary predisposition to RB; heritable and nonheritable subgroups can usually be distinguished clinically. Mathematical analysis of the clinical data lead Knudson to formulate the hypothesis that two mutations are required for RB tumor production. The first mutation (M1) occurs in the germ line of hereditary RB patients and in a somatic retinal cell in nonhereditary RB patients; for all RB patients, the second mutation (M2) occurs in the somatic retinal cell that becomes malignant. The locus of M1 (the RB locus), suspected on the basis of deletion patients to be at 13q14, was confirmed by linkage to the esterase D (ESD) gene locus in hereditary families. Studies utilizing multiple polymorphic markers, (ESD isoenzymes, restriction fragment length polymorphisms and karyotypic heteromorphisms) have shown that a somatic change from heterozygosity in constitutional cells to homozygosity in RB tumors occurs frequently for chromosome 13q but not for other chromosomes. Thus, M2 produces malignancy by somatic loss of the normal allele on the homologous chromosome 13. The normal allele at the RB locus probably regulates differentiation. In its absence, uncontrolled proliferation occurs. The genetic mechanisms defined in RB for expression of a dominantly inherited mutation may well apply to other malignancies and other dominantly inherited diseases.

Wilms' tumour: association with cellular sensitivity to mitomycin C in patients and first-degree relatives

To investigate whether predisposition to Wilms' tumour is associated with a particular defect in the handling of DNA damage, cell-lines from families in which the tumour had occurred were tested for sensitivity to a variety of DNA-damaging agents. Lymphoblastoid lines from both Wilms' tumour patients and their first-degree relatives showed increased sensitivity to the cross-linking agent, mitomycin C, but normal sensitivity to ultraviolet (UV) and gamma irradiation. Thus sensitivity to mitomycin C damage can be associated with the Wilms'-tumour-susceptible genotype and could be a genetic factor responsible for the modification of expression of this genotype.

A decreasing tendency for cytogenetic abnormality in peripheral lymphocytes of retinoblastoma patients with 13q14 deletion mosaicism

A significant decrease in the proportion of 13q14-deleted cells over a 9-month period was observed in a boy with retinoblastoma and 13q14 deletion mosaicism. To evaluate whether this phenomenon is generally the case, the bloods of three retinoblastoma patients with 13q14 deletion mosaicism reported in 1981 and 1982 were reexamined. A significant decrease in the proportion of abnormal cells was observed in three of four patients including the present case, suggesting that a 13q14 deletion mosaicism might disappear with age in some individuals.

Comparison of kinetics of X-ray-induced cell killing in normal, ataxia telangiectasia and hereditary retinoblastoma fibroblasts

Survival, cumulative labeling indices and chromosomal aberrations were studied in normal, ataxia telangiectasia (AT) and hereditary retinoblastoma fibroblasts after X-irradiation during density-inhibition of growth and immediate release by subculture to low density. The D0 of the survival curves were: normal strains, 150-160 rad; Retinoblastoma strains AG 1880, 95 rad; AG 1978, 40-50 rad (sensitive fraction); AT5BI, 45 rad. Mainly chromosome-type Aberrations were induced in normal and retinoblastoma cells. The frequency of X-ray-induced chromosomal aberrations was much higher in AT5BI cells, and 33-45% were of the chromatid type. Normal and retinoblastoma cells showed a measureable X-ray induced G1 delay before entering S. In addition, a fraction of the cells showed an apparently irreversible G1 block; these cells did not initiate DNA synthesis up to 120 h post-irradiation and subculture. The G1 block was much more marked in retinoblastoma cells; after 400 rad about 70% of retinoblastoma cells did not enter S as compared with only 20% of normal cells. Neither a G1 delay nor a G1 block was observed in AT cells irradiated with up to 400 rad despite their hypersensitivity to cell killing by X-rays and evidence of severe chromosome damage. These results suggest different mechanisms for the X-ray hypersensitivity of AT and retinoblastoma cells.

Significance of retinoma and phthisis bulbi for retinoblastoma

Genetic counseling of retinoblastoma (RB) patients and their families is difficult and inaccurate. The data in this paper show that two ocular lesions, retinoma and phthisis bulbi, are associated with RB and can be useful in diagnosis and counseling. Phthisis bulbi is the end result of many conditions and can only be linked to RB if other evidence is present. Retinomas, on the other hand, have three distinctive clinical characteristics (irregular translucent retinal mass, calcification, and pigment epithelial disturbance) and quite accurately predict the presence of the RB gene. Recognition of the significance of retinoma and phthisis bulbi with respect to the RB gene is critical in genetic counseling. Of 34 individuals studied, 32 with retinoma and five with phthisis bulbi, two thirds had a family history of RB or RB in the other eye, and 23/37 of their offspring developed RB. The clinical course of RB in the six children known to be at risk on the basis of detection of the gene in the parent was significantly better than in 17 children that were symptomatic at the time of diagnosis.

Normal repair of gamma radiation-induced single-strand and double-strand DNA breaks in retinoblastoma fibroblasts

The formation and repair of gamma radiation-induced DNA damage were examined in fibroblasts from individuals with retinoblastoma. Utilizing the alkaline elution technique to study single-strand DNA breaks and the nondenaturing elution technique to study double-strand DNA breaks, cultured cell strains from two patients with bilateral retinoblastoma and one patient with sporadic retinoblastoma were shown to develop the same number of DNA breaks as cells from normal individuals after exposure to various gamma radiation doses. Furthermore, the rate and extent of rejoining of both single- and double-strand DNA breaks induced by gamma radiation in retinoblastoma fibroblasts were also normal Defective DNA break repair does not appear to explain the marked predisposition to second malignancies and the possible in vitro radiation sensitivity which are associated with hereditary retinoblastoma.