Diphtheria toxin resistance in human fibroblast cell strains from normal and cancer-prone individuals

Semirandom sampling to detect differentiation-related and age-related epigenome remodeling

With completion of the human genome project, patterns of higher order chromatin structure can be easily related to other features of genome organization. A well-studied aspect of chromatin, histone H4 acetylation, is examined here on the basis of its role in setting competence for gene activation. Three applications of a new hybrid genome sampling-chromatin immunoprecipitation strategy are described. The first explores aspects of epigenome architecture in human fibroblasts. A second focuses on chromatin from HL-60 promyelocytic leukemia cells before and after differentiation into macrophage-like cells. A third application explores age-related epigenome change. In the latter, acetylation patterns are compared in human skin fibroblast chromatin from donors of various ages. Two sites are reported at which observed histone H4 acetylation differences suggest decreasing acetylation over time. The sites, located in chromosome 4p16.1 and 4q35.2 regions, appear to remodel during late fetal-early child development and from preadolescence through adult life, respectively.

Mapping development-related and age-related chromatin remodeling by a high throughput ChIP-HPLC approach

Common to numerous differentiation pathways in vertebrate organisms is the regulation of key genes through epigenetic mechanisms. Less well studied is to what extent cells of a given differentiation state, but examined at different points within the life history of an organism, are distinct at the level of the epigenome. A few instances of such variation have been reported, and it would be of considerable value to have at hand a means to characterize additional examples more efficiently. We describe an integrated approach to this task, and further present evidence for regions of age-related histone H4 acetylation change extending over tens to hundreds of kilobases. Broad similarity between two distinct regions of such change suggests a previously unsuspected link between developmental programs and aging.

DNA damage processing defects and disease

Inherited defects in DNA repair or the processing of DNA damage can lead to disease. Both autosomal recessive and autosomal dominant modes of inheritance are represented. The diseases as a group are characterized by genomic instability, with eventual appearance of cancer. The inherited defects frequently have a specific DNA damage sensitivity, with cells from affected individuals showing normal resistance to other genotoxic agents. The known defects are subtle alterations in transcription, replication, or recombination, with alternate pathways of processing permitting cellular viability. Distinct diseases may arise from different mutations in one gene; thus, clinical phenotypes may reflect the loss of different partial functions of a gene. The findings indicate that partial defects in transcription or recombination lead to genomic instability, cancer, and characteristic disease phenotypes.

Large deletions at the HPRT locus associated with the mutator phenotype in a Bloom's syndrome lymphoblastoid cell line

Bloom's syndrome (BS) is an autosomal recessive disorder with a high cancer incidence. BS cells exhibit increased chromosomal instability and sister-chromatid exchange. The rate of spontaneous mutation at the locus encoding hypoxanthine phosphoribosyltransferase (HPRT) in a lymphoblastoid cell line derived from a BS patient, GM3403, was 1.39 x 10(-6) mutations/cell/generation, whereas that in TK6, a lymphoblastoid cell line derived from an individual who is not suffering from BS, was 1.75 x 10(-8) mutations/cell/generation. Molecular analysis of the HPRT gene in mutant clones by multiplex polymerase chain reaction revealed that 83.3% of the spontaneous mutants from GM3403 cells contained deletions at the HPRT locus, whereas 30.8% of mutants from TK6 cells had deletions. Approximately half of the BS mutants had lost the entire gene. Some mutant clones of GM3403 had also lost markers near the HPRT locus, although no mutant clones from TK6 cells had lost these markers. These results indicate that the mutator phenotype of BS cells is mainly due to an increase in large DNA alterations, reflecting the remarkable genomic instability that could be responsible for cancer proneness in this disease.

Bloom's syndrome. XIX. Cytogenetic and population evidence for genetic heterogeneity

Cells with abnormally high rates of sister-chromatid exchange (SCE) are uniquely characteristic of Bloom's syndrome (BS). However, in one in five persons a minor population of cells with a low-SCE phenotype circulates in the blood. The origin and significance of the low-SCE cells in BS have never been understood, although they are assumed to arise by somatic mutation. In the present investigation, the enigmatic high-SCE/low-SCE mosaicism was investigated by comparing the incidence in several subpopulations of persons in the Bloom's Syndrome Registry who exhibit the two types of cells, and a striking negative correlation emerged: in persons with BS whose parents share a common ancestor, the case in approximately half of registered persons, low-SCE cells are found only rarely; conversely, the mosaicism occurs almost exclusively in persons with BS whose parents are not known to share a common ancestor. Because those who share a common ancestor are predominantly homozygous-by-descent at the mutated BS locus, the negative correlation is interpreted to mean that the emergence of low-SCE cells in BS in some way depends on the pre-existence of compound heterozygosity. A corollary to this is that BS is genetically heterogeneous.

Screening for genetic predisposition to mutagens in cancer patients

Both genetic and environmental factors are known to play an important role in the development of cancer. To determine whether, among individuals who develop cancers, some may have been more susceptible to the mutagenic effects of environmental agents, skin biopsies were taken from 79 cancer patients with different common types of cancers (e.g., lung, breast, bladder, colon, cervix, ovary, brain, vocal cord, uterus, skin, testis, stomach, basal cell carcinoma, leukemia, etc.). Fibroblast cultures have been established from skin explants from nearly all of the patients. The sensitivity of some of these cells as well as a number of other fibroblast strains established from "clinically normal" individuals to a battery of mutagenic agents (e.g., ethylmethane sulfonate, methylmethane sulfonate, ethidium bromide, actinomycin D, mitomycin C, bleomycin, camptothecin), which induce different kinds of DNA damage was examined. For the control group of fibroblasts, a normal range of toxicity for all of the above agents have been established. In contrast to other mutagens for which sensitivity of all of the control cell strains lay within a narrow range, large and interesting differences in sensitivity were observed for ethidium bromide. The fibroblast strains established from fetal tissue were found to be highly resistant to ethidium bromide, whereas fibroblasts from two clinically normal persons exhibited greatly enhanced sensitivity to this agent. The genetic or biochemical basis of increased sensitivity or resistance to ethidium bromide remains to be determined. The sensitivity of cells from 28 cancer patients to a number of the mutagenic agents was also examined. Most of these strains exhibited normal range of sensitivity to the mutagens; however, a few showed small but noticeable differences in sensitivity to specific agents. The fibroblast strains from cancer patients provide a useful resource to examine the genetic and metabolic factors that may be important determinants in cancer susceptibility.

Increased rate of spontaneous mitotic recombination in T lymphocytes from a Bloom's syndrome patient using a flow-cytometric assay at HLA-A locus

Bloom's syndrome (BS) is an autosomal recessive disorder conferring high propensity for cancer and displaying a high degree of genetic instability; the frequency of sister chromatid exchange is characteristically 10 times above background. The symmetrical four-armed chromatid interchanges are much more readily detected in peripheral blood lymphocytes of BS patients, suggesting that the frequency of somatic recombination is also increased. In the present study, the rate of spontaneous loss of HLA-A allele expression was estimated following fluctuation analysis in cultured T lymphocytes using a flow-cytometric assay. It was found to be 10 times or more higher than normal in lymphocytes from a BS patient. Molecular and chromosome analyses showed that all 13 independent variants from the patient were most likely derived from somatic recombinations. Further tests for loss of heterozygosity at a closely linked proximal locus, HLA-DQA1, showed that as many as half of the recombinants retained heterozygosity irrespective of the donor. The results suggest that the HLA region is hyperrecombinogenic in somatic cells and that the elevated recombination rate in BS cells results from the general increase at ordinary sites and not from random creation of unusual sites for recombination.

Two types of DNA ligase I activity in lymphoblastoid cells from patients with Bloom's syndrome

DNA ligases I and II were separated by hydroxylapatite (HA) column chromatography in cell-free extracts of lymphoblastoid cell lines (LCLs) derived from two unrelated patients with Bloom's syndrome (BS) and two healthy individuals. The specific activity of ligase I from the crude extract was consistently lower in GM3403, a BS LCL from an Ashkenazi Jewish patient, than in normal control LCLs. By contrast, the level of ligase I activity in BSL-2KA, another BS LCL derived from a Japanese patient, was equivalent to those in normal LCLs, although GM3403 and BSL-2KA shared the feature of exceedingly high frequency of spontaneous sister-chromatid exchange. The levels of total ligase activity in crude extracts without the separation into the two forms, however, were approximately two-fold higher for the two BS LCLs than for the normal LCLs. Partial purification by chromatography on a DEAE-cellulose 23 column and a phosphocellulose column did not affect the superiority of the two BS LCLs over the normal LCLs in the specific activity of the total ligases. Nonetheless, subsequent application to an HA column again resulted in much less elevation of the specific activity of ligase I for GM3403 than for BSL-2KA and control LCLs. The levels of ligase II activity, accounting for 4-13% of total ligase activity, were similar among the LCLs examined. Irrespective of the extent of purification, essentially no difference in the heat lability of DNA ligase I was detected among the four LCLs. These findings suggest that there may exist among BS LCLs at least two types of subtle abnormality of DNA ligase I itself and/or a putative substance modulating the enzyme function.

Molecular evidence that homologous recombination occurs in proliferating human somatic cells

A strategy has been developed to detect and characterize certain heritable genomic alterations that occur as cells proliferate in vitro. Multiple subclones of cells were isolated from two clonal lymphoblastoid cell lines--one from a boy with Bloom's syndrome (BS), a cancer-predisposing condition known to feature excessive somatic mutation, the other from a normal man. The DNAs from the cell lines were hybridized to a panel of probes that can detect restriction fragment length polymorphisms, and the patterns of polymorphism in the primary clones were compared with that in each of the secondary clones. In one of the BS secondary clones three loci, positioned distally on the long arm of chromosome 3 and that are heterozygous in the donor and all other cell lines derived from the primary clone, had lost heterozygosity and apparently had become homozygous; in contrast, heterozygous loci more proximal on 3q had retained their heterozygosity, as had those on 3p. Taking into account the pattern of chromosome instability uniquely characteristic of BS, the most plausible explanation for the alterations in the altered clone is that somatic recombination had occurred in vitro, via homologous chromatid interchange. Such spontaneous recombinational events in nonneoplastic, nonmutagenized cells may contribute to the high cancer incidence in BS and, by analogy, to cancer that arises in the general population.

Immunological alteration of the Bloom's syndrome uracil DNA glycosylase in Epstein-Barr virus-transformed human lymphoblastoid cells

The immunological reactivity of the uracil DNA glycosylase was investigated in three Epstein-Barr virus-transformed human lymphoblastoid cell lines. Two were derived from normal human lymphocytes while the third was derived from a Bloom's syndrome patient. A panel of 3 anti-human placental uracil DNA glycosylase monoclonal antibodies (37.04.12, 40.10.09 and 42.08.07) was used. Immunological reactivity was determined in a double-blind enzyme-linked immunosorbent assay (ELISA); by inhibition of enzyme activity; and by immunoblot analysis. In the ELISA, the glycosylase from each lymphoblastoid cell line was recognized by glycosylase antibodies 37.04.12 and 42.08.07. In contrast, antibody 40.10.09 failed to recognize the glycosylase from the Bloom's syndrome cell line. Further analysis demonstrated that the 40.10.09 antibody was unable to inhibit catalysis by the Bloom's syndrome lymphoblast glycosylase. In contrast, the 40.10.09 antibody inhibited the activity of the two normal human lymphoblast enzymes. Denaturation of the Bloom's syndrome lymphoblast glycosylase rendered that protein immunoreactive with the 40.10.09 antibody. These results demonstrated that: (1) the immunological alteration in the Bloom's syndrome uracil DNA glycosylase was detected in hematopoietic cells; and (2) viral transformation did not affect the immunoreactivity of the enzyme from either normal human or Bloom's syndrome cells.

Frequency of variant erythrocytes at the glycophorin-A locus in two Bloom's syndrome patients

Blood type MN is determined by a glycoprotein termed glycophorin A (GPA) which exists on the surface of erythrocytes, and the difference between the M and N types is derived from the presence of 2 different amino acids in the amino-terminal portion (Furthmayer, 1978). Using a pair of fluorescence-labeled monoclonal antibodies specific to each GPA, somatic mutations in erythrocytes of MN heterozygotes at the GPA-M and -N alleles can be quantitatively determined using a flow sorter (Langlois et al., 1986). Our results for 2 Bloom's syndrome (BS) patients showed that variants either lost expression of one allele (simple gene inactivation or loss) or expressed only one allele at twice the normal level (most probably somatic recombination) occurring at a frequency of about 1-3 per 10(3) erythrocytes. The flow cytometric patterns of erythrocytes from the BS patients showed a typical smear of variants bearing intermediate levels of expression of one GPA allele, indicating that the real variant frequency is even greater than that measured. On the other hand, the parents heterozygous for the BS gene showed variant frequencies (1-8 x 10(-5)) within the normal range. These data strongly support the hypothesis that the cancer proneness of BS patients is due to their increased frequency of spontaneous mutations and somatic recombination.

Enhanced transfection efficiency and improved cell survival after electroporation of G2/M-synchronized cells and treatment with sodium butyrate

To achieve high transfection efficiency in human fibroblasts with good preservation of proliferative capacity we developed an electroporation procedure that combines two distinct modalities: use of recipient cells synchronized in the late G2/mitotic phase of the cell cycle and treatment of cells post-electroporation with 5 mM butyrate. This combination enabled reduction of plasmid DNA concentration and electroporation voltage, both associated with cytotoxicity, while greatly enhancing transfection efficiencies. Although the method was primarily developed for transient expression it was also found to improve stable expression. This procedure should have wide applicability, particularly in studies seeking to identify DNA sequences that lead to inhibition of DNA synthesis and proliferation in human fibroblasts and other cells refractory to transfection.

Bloom's syndrome. XII. Report from the Registry for 1987

Bloom's syndrome has been known as a clinical entity for 34 years. Careful records of cases diagnosed throughout the world have been maintained since its recognition as an entity, and most instances of cytologically verified Bloom's syndrome have been accessioned to what has been referred to as the Bloom's Syndrome Registry since the mid-1960s. Presented here is the fourth in a series of progress reports from the Registry of information accumulated during this long-term surveillance of affected families, along with mention of selected recent advances that have been made in the understanding of the syndrome. 130 persons had been accessioned to the Registry by the end of 1987; 96 of these were alive, their mean age being 18.9 years. Although a number of clinical complications occur in Bloom's syndrome, the most important is malignant neoplasia. In the 130 persons in the Registry, 57 malignant neoplasms had been detected, the mean age at diagnosis being 24.8 years. Neoplasia in Bloom's syndrome is noteworthy not only because of its frequency and exceptionally early age of emergence but for its variety of histological types and sites of origin.

Evidence for increased in vivo mutation and somatic recombination in Bloom's syndrome

The glycophorin A assay was used to estimate the frequency of mutations that accumulate in vivo in somatic cells of persons with Bloom's syndrome (BS). This assay measures the frequency in persons of blood type MN of variant erythrocytes that lack the expression of one allelic form of glycophorin A, presumably due to mutational or recombinational events in erythroid precursor cells. Samples of blood from persons with BS showed dramatic 50- to 100-fold increases in the frequency of variants of three types, those with a hemizygous phenotype, those with a homozygous phenotype, and those with what appears to be partial loss of the expression of one locus. The high frequency of homozygous variants, genetic evidence for altered allelic segregation of a specific biochemical locus, provides evidence for increased somatic crossing-over in vivo in BS. An increased generation of functional hemizygosity and homozygosity in their somatic cells may play an important role in the extreme cancer risk of persons with BS.

Immunological lesions in human uracil DNA glycosylase: association with Bloom syndrome

Three monoclonal antibodies that react with uracil DNA glycosylase of normal human placenta were tested to determine whether one of the antibodies could be used as a negative marker for Bloom syndrome. As defined by enzyme-linked immunosorbent assay, monoclonal antibody 40.10.09, which reacts with normal human glycosylase, neither recognized nor inhibited native uracil DNA glycosylase from any of five separate Bloom syndrome cell strains. Immunoblot analyses demonstrated that the denatured glycosylase protein from all five Bloom syndrome cell strains was immunoreactive with the 40.10.09 antibody. Further, each native enzyme was immunoreactive with two other anti-human placental uracil DNA glycosylase monoclonal antibodies. In contrast, ELISA reactivity was observed with all three monoclonal antibodies in reactions of glycosylases from 5 normal human cell types and 13 abnormal human cell strains. These results experimentally verify the specificity of the aberrant reactivity of the Bloom syndrome uracil DNA glycosylase. The possibility arises that determination of the lack of immunoreactivity with antibody 40.10.09 may have value in the early diagnosis of Bloom syndrome.

Carcinogenic potency in rodents versus genotoxic potency in E. coli: a correlation analysis for bifunctional alkylating agents

The mutagenic (M), recombinagenic (R) and SOS inducing (I) potencies of 6 bifunctional directly acting alkylating agents (mitomycin C, thiotepa, chlorambucil, nitrogen mustard, bis(2-chloroethyl)ether and bis(2-chloroethyl)nitrosourea) were measured in an E. coli test system (E. coli multitest) as the integral under the yield-dose curve obtained for each event. This potency corresponds to the cumulative yield of the affected cell population over the entire effective dose range of the chemical treatment. A weak mutagenic activity was detected only for mitomycin C and thiotepa. Except for bis(2-chloroethyl)ether, all agents were recombinagenic and SOS inducing. When the 3 genotoxic potencies (M, R and I) of these bifunctional alkylating agents were correlated, separately or in combination, with the respective carcinogenic potencies in rodents, a highly significant correlation was obtained with both the recombinagenic and SOS inducing potencies.

Effects of Bloom's syndrome fibroblasts on genetic recombination and mutagenesis of herpes simplex virus type 1

The effects of Bloom's syndrome (BS) fibroblasts on genetic recombination and the mutation frequency of herpes simplex virus type 1 (HSV-1) was determined by employing two factor crosses of selected temperature-sensitive (ts) mutants. A significant increase in the recombination frequency (RF) was observed in seven of nine crosses when multiple BS fibroblast lines were compared to normal human fibroblasts. The RF of HSV-1 ts mutants increased following 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment of normal, but not BS fibroblasts, suggesting that BS fibroblasts express higher constitutive levels of genetic recombination activity. HSV-1 ts mutants demonstrated significantly higher reversion frequencies to the nontemperature sensitive (ts+) phenotype following growth in BS rather than normal fibroblasts, indicating that exogenous viral DNA encoding many of the enzymes necessary for its own replication is affected by the mutator phenotype of BS.

O6-methylguanine methyltransferase increases before S phase in normal human cells but does not increase in hypermutable Bloom's syndrome cells

The regulation of the O6-methylguanine methyltransferase was examined during cell proliferation in hypermutable Bloom's syndrome fibroblasts and normal human skin fibroblasts. During synchronous growth following serum stimulation normal human cells enhanced methyltransferase activity 2.4-fold in the absence of exogenous damage as a normal regulatory event during the cell cycle. Methyltransferase activity was increased prior to the induction of DNA replication and of DNA polymerase and was diminished when each replicative activity was maximal. In contrast, although methyltransferase levels in quiescent cells are equivalent, hypermutable Bloom's syndrome cells did not increase methyltransferase at any interval in the cell cycle.

Spontaneous 6-thioguanine-resistant lymphocytes in Fanconi anemia patients and their heterozygous parents

The incidence of spontaneous 6-thioguanine-resistant (TGr) lymphocytes was studied in the peripheral blood collected from seven Fanconi anemia (FA) patients and five of their heterozygous parents using an autoradiographic or a lymphocyte cloning method. Five of the seven patients showed a significantly elevated incidence of TGr lymphocytes as compared to age- and sex-matched healthy controls. There was, however, no difference between FA heterozygotes and controls. These results suggest some variability among the patients similar to those reported in clinical and cytogenetic investigations. The basis for the increase in TGr cells in the patients is not known, but the inherent genomic instability reflected as increased frequencies of chromosomal aberrations is one possible explanation.

Autoradiographic detection of diphtheria toxin resistant mutants in human diploid fibroblasts

An autoradiographic procedure for the detection of diphtheria toxin (DT) resistant (DipR) mutants in human diploid fibroblast (HDF) cells has been developed. The assay is based on the observation that when HDFs from confluent cultures are seeded in medium containing 0.01 flocculating units/ml or higher concentration of DT, protein synthesis in sensitive cells is severely inhibited by 4-6 hr. If at this or later time, a radiolabeled protein precursor (eg, 3H-leucine) is added to the culture, it is almost exclusively incorporated into the resistant cells, which are then readily identified by autoradiography. The DipR cells can also be identified by labeling in the presence of 3H-thymidine, although a higher background is observed in these experiments. Reconstruction experiments using DipS and DipR HDFs show that the frequency of heavily labeled cells that are detected by autoradiography show an excellent correlation with the number of DipR cells added and to the number of DipR cells as detected by conventional colony forming assay. These studies provide strong evidence that the labeled cells identified by autoradiography are bona fide DipR mutants. The detection of DipR cells by autoradiography is apparently not affected by the presence of the sensitive cells in the mixtures. The spontaneous frequency of DipR cells in HDFs has been found to be in the range of 1-5 X 10(-6), and this increases in a dose dependent manner upon treatment with the mutagen ethyl methanesulfonate. These results indicate that the autoradiographic assay could be used for quantitative mutagenesis. Since the autoradiographic assay does not depend on cell division, it may prove useful in estimating the incidence of pre-existing mutations in cell populations that either do not divide or have very limited growth potential (eg, lymphocytes, muscle cells, neurons, senescent fibroblasts, etc).

Quantitative genetic analysis of tumor progression

Metastasis and resistance to chemotherapy are common features of progressed cancers. With respect to the latter phenotype, it is thought that during tumor growth drug-resistant cells arise spontaneously at rates characteristic of the genetic alterations involved. On application of chemotherapy, such variant tumor cells are more likely to survive, and they may eventually dominate, resulting in a non-responsive malignancy. Aspects of this model have been confirmed in a number of experimental systems and in patients. In contrast to our understanding of drug resistance, steps involved in the progression to metastatic spread of tumor cells are much less well-understood. In this review we describe methodologies of quantitative genetic analysis with reference to development of drug resistance. We then describe attempts by ourselves and others to use a similar approach to investigate metastatic properties. Based on these studies, we have proposed the quantitative 'dynamic heterogeneity' model of tumor metastasis, which is presented here. Using an 'experimental' metastasis assay and Luria-Delbruck fluctuation analysis, we determined that in murine KHT fibrosarcoma and B16 melanoma lines, 'metastatic' variants with a distinct phenotype are generated at high rates. These variants are relatively unstable resulting in a dynamic equilibrium between generation and loss of metastatic variants. The metastatic ability of such a tumor population is thus dependent on the frequency of a subpopulation of metastatic variants which are turning over rapidly. This dynamic heterogeneity model is able to quantitatively provide a unifying explanation for a wide range of observations concerning tumor heterogeneity and clonal instability. Genetic mechanisms involving rapid rates have been characterized in drug-resistant variants. We speculate that similar processes may be involved in different aspects of tumor progression such as those resulting in metastasis.

Inhibited intercellular communication as a mechanistic link between teratogenesis and carcinogenesis

Teratogenesis and carcinogenesis share many characteristics, leading to the speculation that they may also share pathogenic mechanisms. Direct intercellular communication mediated by membrane junctions is known to occur between a variety of cells and may play an important role in the control of cell growth and differentiation. Inhibition of junctional communication may be a mechanism common to both teratogenesis and carcinogenesis whereby cells and tissues are diverted from their normal differentiation paths. The multistage model of carcinogenesis predicts that the irreversibly initiated cell is at least partially regulated by the surrounding cells of a tissue, and that the initiated cell remains inactive until stimulated to proliferate by a tumor promotor. Tumor promoters may release the initiated cell from control of the surrounding tissue by interrupting intercellular communication, since many tumor promoters have now been shown to interfere with junctional communication in cultured mammalian cells. Furthermore, many tumorigenic cells have compromised junctional communication abilities. Similarly, it has been reasoned that the cells of an embryo must be able to communicate with each other to define tissue specificity and pattern formation, and to coordinate morphogenetic events. Many studies have chronicled alterations in junctional communication that occur coincident with major developmental events and some studies suggest that junctional communication may be modified at boundaries of morphogenetic fields. A recent in vivo study has provided evidence that inhibition of junctional communication may interfere with embryonic development, and several teratogens are known to interrupt junctional communication in mammalian cells in culture. These observations suggest that inhibition of junctional intercellular communication may be a shared mechanism of carcinogenesis and teratogenesis.

Diphtheria toxin resistance in human lymphocytes and lymphoblasts in the in vivo somatic cell mutation test

It has been shown that circulating peripheral blood lymphocytes can be used for the enumeration of 6-thioguanine-resistant cells that presumably arise by mutation in vivo. This somatic cell mutation test has been studied in lymphocytes from human populations exposed to known mutagens and/or carcinogens. The sensitivity of the test could be further enhanced by including other gene markers, since there is evidence for locus-specific differences in response to mutagens. Resistance to diphtheria toxin (Dipr) seemed like a potential marker to incorporate into the test because the mutation acts codominantly, can readily be selected in human diploid fibroblasts and Chinese hamster cells with no evidence for cell density or cross-feeding effects, and can be assayed for in nondividing cells by measuring protein synthesis inhibition. Blood samples were collected from seven individuals, and fresh, cryopreserved, or Epstein-Barr virus (EBV)-transformed lymphocytes were tested for continued DNA synthesis (3H-thymidine, autoradiography) or protein synthesis (35S-methionine, scintillation counting). Both fresh and cryopreserved lymphocytes, stimulated to divide with phytohemagglutinin (PHA), continued to synthesize DNA in the presence of high doses of diphtheria toxin (DT). Similarly, both dividing (PHA-stimulated) and nondividing fresh lymphocytes carried on significant levels of protein synthesis even 68 hr after exposure to 100 flocculating units (LF)/ml DT. The findings were confirmed in cord blood lymphocytes, ruling out the possibility that diphtheria immunization could have led to a selection of Dipr lymphocytes. One lymphoblast line (EBV-transformed lymphocytes) showed a reduction in protein synthesis to 0.2% of controls only at 192 hr after exposure to 100 LF/ml. The results suggest that human T and B lymphocytes may not be as sensitive to DT protein synthesis inhibition as human fibroblast and Chinese hamster cells. For this reason, Dipr may not be a suitable marker for the somatic cell mutation test.

Autoradiographic assay of mutants resistant to diphtheria toxin in mammalian cells in vitro

Diphtheria toxin kills mammalian cells by ribosylating elongation factor 2, a protein factor necessary for protein synthesis. The frequency of cells able to form colonies in the presence of the toxin can be used as an assay for mutation to diphtheria toxin resistance. We report here that resistance to diphtheria toxin can also be detected autoradiographically in cells exposed to [3H]leucine after treatment with the toxin. In cultures of Chinese hamster ovary cells, the frequency of such resistant cells is increased by exposure of the cells to gamma-rays, ultraviolet light, ethylnitrosourea, mitomycin c, ethidium bromide, and 5-bromo-2'-deoxyuridine in a dose- and time-dependent manner. The resistant cells form discrete microcolonies if they are allowed to divide several times before intoxication, which indicates that they are genuine mutants. The assay is potentially adaptable to any cell population that can be intoxicated with diphtheria toxin and labeled with [3H]leucine, whether or not the cells can form colonies. It may be useful, therefore, for measuring mutation rates in slowly growing or nondividing cell populations such as breast, brain, and liver, as well as in cells that do divide but cannot be readily cloned, such as the colonic epithelium.

Clonal analysis of stable chromosome rearrangements in Bloom's syndrome fibroblasts

In situ cytogenetic analysis was performed on colonies derived from single cells of cultured skin fibroblast-like strains from two patients with Bloom's syndrome (GM 1492 and GM 2520). Metaphases in all of the colonies displayed structural chromosome rearrangements. Among 212 metaphases from 24 colonies of GM 1492, only 16% were pseudodiploid, and there was a high incidence of de novo rearrangements within individual colonies. There were two "families" of 16 and five colonies, respectively, each containing identical or related aneusomies, and these could be arranged into pedigrees showing clonal evolution. The heterochromatic region of chromosome #1 and the telomeric regions of chromosome arms 2q, 3q, 4p, and 11q were most frequently involved in the rearrangements. In contrast, strain GM 2520 showed less intraclonal variation, was primarily pseudodiploid, and displayed only three clonal types, one of which had extensive subclonal variation (19 of 24 clones). A remarkable finding in GM 2520 was that, in some clones, extra copies of specific chromosome segments were present as translocations. These results caution against the use of strain GM 1492 as a prototype Bloom's syndrome strain for cell biological studies.

Bloom's syndrome XI. Progress report for 1983

During the 30 years since its description as a clinical entity, Bloom's syndrome has been diagnosed in more than 100 persons. It is believed that most of these have been accessioned to the Bloom's Syndrome Registry, which now includes 103 persons. Of those 103, 80 are alive, with a mean age of 18.2 years. Twenty-eight malignant neoplasms have been detected, at a mean age of 20.7 years. Periodically, progress reports are being made in this journal of the long-term surveillance of affected families.

Altered temporal expression of DNA repair in hypermutable Bloom's syndrome cells

The temporal regulation of DNA repair during synchronous cell proliferation was examined in normal human skin fibroblasts and in Bloom's syndrome skin fibroblasts. Normal human cells regulated DNA repair in a defined temporal sequence prior to the induction of DNA replication. Nucleotide-excision repair was stimulated prior to the induction of base-excision repair, which itself was increased prior to the induction of DNA replication. This temporal sequence was observed (i) by quantitation of the induction of the base-excision repair enzyme uracil DNA glycosylase during cell proliferation in the absence of cellular insult and (ii) by quantitation of nucleotide-excision repair after UV irradiation or base-excision repair after exposure to methylmethane sulfonate. In contrast, Bloom's syndrome cells were characterized by specific alterations in this temporal sequence of gene regulation, such that DNA repair was not enhanced prior to the induction of DNA replication. Nucleotide-excision repair, base-excision repair, and the uracil DNA glycosylase were induced in a temporal sequence identical to that observed for DNA polymerase and for DNA replication. The inability of Bloom's syndrome cells to enhance DNA repair prior to DNA replication suggests that miscoding lesions remain in DNA and are replicated during cell proliferation.

Bloom's syndrome: evidence for an increased mutation frequency in vivo

The incidence of lymphocytes resistant to the purine analog 6-thioguanine was studied in seven patients with Bloom's syndrome. The mean frequency was 17.3 X 10(-4). The mean incidence in age- and sex-matched controls was 2.1 X 10(-4), so approximately eight times the normal number of 6-thioguanine-resistant lymphocytes were detected in Bloom's syndrome blood. The basis for this increase is unknown, but the inherent genomic instability demonstrated in the form of chromosomal aberrations is one possible explanation.

Association of mutator activity with UV sensitivity in an aphidicolin-resistant mutant of Chinese hamster V79 cells

The spontaneous mutation rates of an ultraviolet light (UV)-sensitive aphidicolin-resistant mutant (aphr-4-2) and its revertants have been determined by 2 techniques. By using the fluctuation analysis, the mutant and its thymidine (TdR)-prototrophic 'revertant' were found to exhibit elevated spontaneous mutation rates at the 6-thioguanine- and diphtheria-toxin-resistant loci. In contrast, the TdR-auxotrophic 'revertant' did not show this property. Similar results were obtained by the multiple replating technique. From these comparative studies and other previous characterizations, it appears that a single gene mutation is responsible for the following pleiotropic phenotype: slow growth. UV sensitivity, high UV-induced mutability, high frequency of site-specific bromodeoxyuridine (BrdU)-dependent chromosome breaks and enhanced spontaneous mutation rate. Recent studies indicate that the mutation may be on the gene for DNA polymerase alpha. The results further indicate that thymidine auxotrophy or imbalance in nucleotide pools is not necessarily associated with the mutator activity in mammalian cells.

Species specific differences in the toxicity of mithramycin, chromomycin A3, and olivomycin towards cultured mammalian cells

Three structurally related anticancer drugs, mithramycin, chromomycin A3, and olivomycin, showed large unexpected differences (up to more than 1000 fold) in their toxicity towards cultured cells from various species (human, Chinese hamster, Syrian hamster, and mouse). Among the cell types examined, human cells (both a diploid fibroblast cell strain and HeLa cells) were maximally sensitive to all these drugs, followed by the Syrian hamster kidney cells (BHK 21). The mouse (LMTK- cells) and Chinese hamster (CHO) cells, which were more resistant, showed interesting differences in their sensitivity towards these drugs. For example, whereas the mouse cells were more resistant to mithramycin than CHO cells, the sensitivity pattern was reversed for both chromomycin A3 and olivomycin. In cell extracts derived from human, mouse, and Chinese hamster cells RNA synthesis, which is the cellular target of these drugs, showed identical sensitivity to both mithramycin and chromomycin A3, indicating that the species specific differences in the toxicity to these drugs are at the level of cellular entry of these compounds. Based on the structures of these glycosidic antibiotics and their patterns of toxicity, it is suggested that the intracellular transport of these drugs involves specific interactions between the sugar residues on these compounds and some type of cell surface receptor(s), which differ among different cell types. Some implications of these results for toxicity studies are discussed.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

Human fibroblast strains showing increased sensitivity or resistance to ethidium bromide

The sensitivity of a number of human fibroblast cell strains towards the DNA intercalating and mutagenic agent ethidium bromide has been examined. Among the cell strains investigated, 3 were of fetal origin, 6 from clinically normal adult persons, 2 from Lesch-Nyhan individuals and 1 each from persons with 3 genetic disorders, xeroderma pigmentosum, Fanconi anemia and Bloom syndrome, which are known to predispose to cancer. Results of our studies show that cells derived from 2 otherwise normal individuals exhibits a marked sensitivity towards ethidium bromide as compared to the rest of the group. At the same time all 3 cell strains of fetal origin were found to be highly resistant to killing by this agent. These results are discussed in relation to the toxic/mutagenic effects of ethidium bromide.

Podophyllotoxin resistance: a codominant selection system for quantitative mutagenesis studies in mammalian cells

Mutants resistant to the microtubule inhibitor podophyllotoxin (PodR), a codominant marker, can be readily selected in various mammalian cell lines such as, CHO, HeLa, mouse L cells, Syrian hamster cells (BHK21) and a mouse teratocarcinoma cell line OC15. In CHO cells, the recovery of PodR mutants is not affected by cell density (up to 1 X 10(6) cells per 100-mm diameter dish), and after treatment with the mutagen ethyl methanesulfonate maximum mutagenic effect is achieved after a relatively short expression time (40-48 h). The frequency of PodR mutants in various cell lines increased in a dose-dependent manner in response to treatment with the mutagens ethyl methanesulfonate and N-methyl-N'-nitro-N-nitrosoguanidine. The PodR selection system thus provides a new genetic marker which should prove useful in studies of quantitative mutagenesis in mammalian cells.

Elevated spontaneous mutation rate in Bloom syndrome fibroblasts

The rates of spontaneous mutation to 6-thioguanine resistance were determined in fibroblasts derived from normal and two Bloom syndrome individuals (GM 2548 and GM 1492). Two methods were utilized to determine the rates. Method I obtained the spontaneous mutation rate from the increase in the mutation frequency of a cell population in logarithmic-phase growth over 10 days. The two Bloom syndrome strains had spontaneous mutation rates of 16 X 10(-6) and 17 X 10(-6) mutations per cell per generation, whereas two normal strains had rates of 1.5 X 10(-6) and 1.1 X 10(-6). Method II utilized fluctuation analysis to measure the rate of spontaneous mutation. This method resulted in rates of 19 X 10(-6) and 23 X 10(-6) mutations per cell per generation in Bloom syndrome cells, compared to rates of 4.6 X 10(-6) and 4.9 X 10(-6) in the control strains. These data suggest that Bloom syndrome may be a mutator mutation, a previously unrecognized phenomenon in humans, and that an elevated spontaneous mutation rate in vivo may be responsible for the clinical phenotype of primordial dwarfism and increased cancer incidence.