Induction of chromosome damage by Neurospora endonuclease in repair-inhibited quiescent normal human fibroblasts

Radiation Research Society. 1952-2002. Historical and current highlights in radiation biology: has anything important been learned by irradiating cells?

Around 30 years ago, a very prominent molecular biologist confidently proclaimed that nothing of fundamental importance has ever been learned by irradiating cells! The poor man obviously did not know about discoveries such as DNA repair, mutagenesis, connections between mutagenesis and carcinogenesis, genomic instability, transposable genetic elements, cell cycle checkpoints, or lines of evidence historically linking the genetic material with nucleic acids, or origins of the subject of oxidative stress in organisms, to name a few things of fundamental importance learned by irradiating cells that were well known even at that time. Early radiation studies were, quite naturally, phenomenological. They led to the realization that radiations could cause pronounced biological effects. This was followed by an accelerating expansion of investigations of the nature of these radiobiological phenomena, the beginnings of studies aimed toward better understanding the underlying mechanisms, and a better appreciation of the far-reaching implications for biology, and for society in general. Areas of principal importance included acute tissue and tumor responses for applications in medicine, whole-body radiation effects in plants and animals, radiation genetics and cytogenetics, mutagenesis, carcinogenesis, cellular radiation responses including cell reproductive death, cell cycle effects and checkpoint responses, underlying molecular targets leading to biological effects, DNA repair, and the genetic control of radiosensitivity. This review summarizes some of the highlights in these areas, and points to numerous examples where indeed, many things of considerable fundamental importance have been learned by irradiating cells.

Induction of chromosomal aberrations by restriction endonucleases encapsulated in liposomes

We used liposomes to deliver the restriction endonucleases BamHI and SmaI into human heteroploid HEp-2 cells. With this method very low concentrations of enzymes (2 units/ml) were active in the production of chromosomal aberrations. SmaI, which produces blunt-ended double-strand breaks in the DNA molecule, induces chromosomal aberrations more effectively than BamHI, which produces cohesive ends. Our results indicate that liposomes are suitable vectors for introducing restriction endonucleases into cultured human cells.

Inducible expression and cytogenetic effects of the EcoRI restriction endonuclease in Chinese hamster ovary cells

The cytogenetic endpoints sister chromatid exchange (SCE) and chromosome aberrations are widely used as indicators of DNA damage induced by mutagenic carcinogens. Chromosome aberrations appear to result directly from DNA double-strand breaks, but the lesion(s) giving rise to SCE formation remains unknown. Most compounds that induce SCEs induce a spectrum of lesions in DNA. To investigate the role of double-strand breakage in SCE formation, we constructed a plasmid that gives rise to one specific lesion, a staggered-end ("cohesive") DNA double-strand break. This plasmid, designated pMENs, contains a selectable marker, neo, which is a bacterial gene for neomycin resistance, and the coding sequence for the bacterial restriction endonuclease EcoRI attached to the mouse metallothionein gene promoter. EcoRI recognizes G decreases AATTC sequences in DNA and makes DNA double-strand breaks with four nucleotides overhanging as staggered ends. Cells transfected with pMENS were resistant to the antibiotic G418 and contained an integrated copy of the EcoRI gene, detectable by DNA filter hybridization. The addition of the heavy metal CdSO4 resulted in the intracellular production of EcoRI, as measured by an anti-EcoRI antibody. Cytogenetic analysis after the addition of CdSO4 indicated a dramatic increase in the frequency of chromosome aberrations but very little effect on SCE frequency. Although there was some intercellular heterogeneity, these results confirm that DNA double-strand breaks do result in chromosome aberrations but that these breaks are not sufficient to give rise to SCE formation.

Inducible expression and cytogenetic effects of the EcoRI restriction endonuclease in Chinese hamster ovary cells

The cytogenetic endpoints sister chromatid exchange (SCE) and chromosome aberrations are widely used as indicators of DNA damage induced by mutagenic carcinogens. Chromosome aberrations appear to result directly from DNA double-strand breaks, but the lesion(s) giving rise to SCE formation remains unknown. Most compounds that induce SCEs induce a spectrum of lesions in DNA. To investigate the role of double-strand breakage in SCE formation, we constructed a plasmid that gives rise to one specific lesion, a staggered-end ("cohesive") DNA double-strand break. This plasmid, designated pMENs, contains a selectable marker, neo, which is a bacterial gene for neomycin resistance, and the coding sequence for the bacterial restriction endonuclease EcoRI attached to the mouse metallothionein gene promoter. EcoRI recognizes G decreases AATTC sequences in DNA and makes DNA double-strand breaks with four nucleotides overhanging as staggered ends. Cells transfected with pMENS were resistant to the antibiotic G418 and contained an integrated copy of the EcoRI gene, detectable by DNA filter hybridization. The addition of the heavy metal CdSO4 resulted in the intracellular production of EcoRI, as measured by an anti-EcoRI antibody. Cytogenetic analysis after the addition of CdSO4 indicated a dramatic increase in the frequency of chromosome aberrations but very little effect on SCE frequency. Although there was some intercellular heterogeneity, these results confirm that DNA double-strand breaks do result in chromosome aberrations but that these breaks are not sufficient to give rise to SCE formation.

Non random distribution of lesions induced by deoxyribonuclease I in human chromosomes

We studied the action of deoxyribonuclease I on human lymphocytes in order to determine the localization of the DNAase induced aberrations. Our results indicate a non-random distribution of the lesions on chromosome regions which may reflect a differential pattern of sensitivity to the enzyme. Furthermore we observed a correspondence between the preferential DNAase induced breaks and fragile sites that are expressed in lymphocytes maintained in medium without folic acid. A possible interpretation of our findings is that the accessibility to DNAase and/or the efficiency of the repair systems depend on the chromatin structure that influences also the expression of some common fragile sites.

Chromosomal aberrations induced in human cultured cells by liposome-encapsulated deoxyribonuclease I

Experiments of incorporation of a nucleolytic enzyme into human cells cultured in vitro have been carried out with the aim of inducing structural chromosome variations. Human heteroploid cells, either as asynchronous populations or enriched in mitoses, and PHA-stimulated lymphocytes were used as recipients. We found that all these cells when exposed to pancreatic DNAase I encapsulated in liposomes, either of multilamellar (MLV) or of small unilamellar (SUV) type, show an incidence of chromosome damage higher than that induced by the enzyme free in the incubation buffer. Our results indicate that liposomes are suitable vehicles for the transfer of an exogenous nuclease into human cultured cells. The enzyme remains functionally active and interacts with nuclear DNA, giving rise to chromosome lesions.