Are Inducible Components Involved in the Repair of Irradiated Neuronal and Brain Tumour DNA?

Abstract

. IntroductionNondividing cerebellar neurons have been shown to restore their chromosomal DNA structure much more slowly than intracerebral 9L brain tumour cells after irradiation with doses of 50-0 Gy (Wang and Wheeler 1978) . There are several possible explanations for this phenomenon . First, 50-0 Gy is certainly not a biologically relevant dose and at lower, more relevant doses the differences in the DNA repair kinetics between neurons and tumour cells might disappear . Secondly, permanently nondividing cells like neurons might require the induction of components of the repair system that are already available in the predominantly dividing tumour cells as part of their DNA replication machinery . Thirdly, there may be differences in the quantity, quality or types of repair enzymes found in dividing and nondividing cells . Lastly, the chromatin structure in nondividing cells may restrict the accessibility of these repair enzymes to the lesions or interfere with their operation at these lesions . Materials and methodsThe entire heads of anaesthetized, 240-280 g (3-4 month old), male Fisher 344 rats bearing 100-200 mg intracerebral 9L brain tumours were irradiated with single doses of 12-5 or 25 . 0 Gy of Cs-137 y-rays at a dose rate of 9-21 Gy/min . The remainder of the body was shielded with lead . At various times after irradiation, the rats were killed by decapitation and the tumour and cerebellum were removed . The tumour and cerebellum were dissociated into a single cell suspension at 4°C by mechanical means and the cell suspensions layered onto a lytic zone over a 10-30% alkaline sucrose gradient in two Beckman Ti-15 zonal rotors . After sedimentation the DNA was collected in 20 ml fractions, precipitated with perchloric acid, reacted with diaminobenzoic acid dihydrochloride and the amount of DNA in each fraction determined by spectrofluorometrically measuring the fluorescence emitted at 500 nm after excitation at 409 rim . Complete descriptions of the tumour implantation procedure, tumour growth characteristics, dissociation procedure, gradients, sedimentation conditions, typical sedimentation profiles and the molecular properties of the sedimenting DNA have been published (Wang and Wheeler 1978, Leith et al . 1975, Wheeler and Wallen 1980 .The percentage of the DNA with sedimentation coefficients >250S was calculated from the DNA sedimentation profiles of both irradiated and unirradiated

DNA repair kinetics in irradiated undifferentiated and terminally differentiated cells

“…Previously we reported that intracerebral 9L/Ro cells apparently restore their DNA structure after irradiation more rapidly than cerebellar neurons [34,52]. The data reported here suggest that this phenomenon results from differences in both the DNA template structure and the quantity of the repair system (enzyme?)…”

“…3-5) would suggest that the neuronal genome contains approximately three times more of these less accessible regions than the tumor cell genome; a fact that is consistent with our expectations for a terminally differentiated cell that no longer needs to produce the many products required for cell growth and division. A three-fold increase in the less accessible portion of the genome certainly could be one major reason why neurons repair their radiation-induced DNA damage more slowly than intracerebral 9L tumor cells [34,52]. The displacement of the curves in Fig.…”

“…1). After every dose, restoration of the unirradiated DNA structure was faster in 9L tumor cells than in cerebellar neurons [34,52]. For this tumor and normal tissue, 1,250 rads is a biologically relevant dose [50,52].…”

Studies on DNA Damage and Repair in the Mammalian Brain

“…The high ADP-ribosyltransferase activities of the cell lines L1210 and NS 20Y may reflect a generally increased ability of immortal cells to repair their DNA. Certainly implanted intracerebral9L tumour cells have been found to repair y-ray-induced strand breakage more rapidly than nontransformed cells of rodent cerebellum (Wheeler et al, 1981). Somewhat in contrast, the "neuronal" fraction of cerebral nuclei, enriched in nuclei of nondividing neurones, had a higher transferase activity than the glial fraction.…”