Chronic Radiation-induced Alteration in Hematopoietic Repair during Preclinical Phases of Aplastic Anemia and Myeloproliferative Disease: Assessing Unscheduled DNA Synthesis Responses I

Protracted, low-daily-dose T-ray exposure (3.8--7.5 cGy/day) segregates canines into separate survivaland pathology-based subgroups by the early elicitation of distinct, repair-mediated hemopathological response pathways. In this study, we verified the blood and marrow responses of two major subgroups prone to either aplastic anemia or myeloproliferative disease, along with two variants, and extended our analyses of hematopoietic repair to include studies of DNA repair in bone marrow blasts using an autoradiographicaily based unscheduled DNA synthesis (UDS) assay. The myeloproliferative disease-prone subgroup exhibited extended survival (>200 days), related to partial, gradual restoration of blood leukocyte, platelet, and marrow progenitor levels following an initial phase of acute suppression. Marrow blasts taken during the restoration phase showed expanded and qualitatively modified UDS relative to marrow blasts of age-matched control animals. The amount of UDS per blast (signal strength) increased significantly, as did the number of UDS-positive cells and their sensitivities to high-dose UV induction and 1-/3-1)arabinofuranosylcytosine chemical inhibition. A nonevolving myeloproliferative disease-prone variant having prolonged survival (>200 days) and restored blood cells and marrow progenitor levels also had marrow blasts with expanded UDS responses, but these were uniquely evoked by low (but not high) doses of UV inducer. The aplastic anemia-prone subgroup was characterized by short survival (<200 days), progressive decline (without restoration) in all measured blood and marrow compartments, and largely nonsignificant changes in UDS responses of marrow blasts. A variant of this aplastic anemia-prone subgroup (with comparable short survival due to markedly ineffective hematopoiesis, but expressing select preleukemic features) exhibited reduced numbers (relative to age-matched controls) of highly responsive, UDS-positive marrow blasts (in terms of UDS signal strength and increased sensitivity to 1-13-D-arabinofuranosylcytosine-induced UDS inhibition). From these observations we conclude that: (a) the UDS response of marrow blasts, a correlate of hematopoietic progenitorial repair, is altered differentially within selected subgroups of animals under chronic radiation exposure; and (b) the nature of altered UDS repair response patterns appears to be largely related to the preclinical status/ predisposition of the individual animal and thus may provide prognostically useful information in the clinical monitoring of chronically irradiated individuals with minimal but evolving hematological disease. I N T R O D U C T I O N An increased awareness is being developed concerning the relationship(s) between DNA repair sufficiency and the potency of a given toxicant to cause pathological conditions (1). In a number of welldocumented genetic diseases of man (e.g., xeroderma pigmentosum, ataxia-telangiectasia, Bloom's syndrome, Fanconi's anemia, Cockayne's syndrome, etc.), insufficient DNA repair capacity has been related clinically to a hypersensitivity to a variety of physical and chemical toxicants, as well as causally associated with a marked predisposition to several significant pathological conditions, including cancer (2). Multiple gene locus-based defects in various steps along Received 2/1/93; accepted 7/26/93. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. i Work supported by the United States Department of Energy, Office of Health and Environmental Research, under Contract W-31-109-ENG-38. 2 To whom requests for reprints should be addressed, at 9700 South Cass Avenue, Argonne, IL 60439-4833. the major repair pathways (e.g., excision, recombination, postreplication repair pathways) appear responsible for promoting selected types/ frequencies of DNA lesions and chromosomal rearrangements and, in turn, manifesting increased cell death, mutation, and transformation (3). The potential for multiple sites of repair dysfunction lends itself to a range of repair capacities in patients suffering from such genetic diseases. Cells of patients with xeroderma pigmentosum, for example, exhibit a wide range of, but generally suppressed, UDS 3 activity and, in turn, an overall reduced repair capacity stemming from altered DNA polymerase function (4). Xeroderma pigmentosum complementation groups B and G exhibit UDS responses less than 10% of the normal control levels, whereas groups C, E, and F have UDS levels between 10 and 50% of control levels, and a variant subpopulation, group D, has nearly normal UDS responses. By contrast, assays of xeroderma pigmentosum repair function based on measures of "end step" ligase activity often show "normal" levels of activity (1). Such differential, repair pathway-specific changes have been associated with other disease states as well (e.g., in therapy-induced leukemias, the capacity to repair toxicant-elicited DNA lesions is compromised due to differential alterations along the repair pathway; UDS-related DNA polymerase activity is amplified, whereas strand ligase activity is suppressed) (5). In the above pathological conditions, preexisting genetic/epigenetic lesions clearly underlie and perhaps promote the toxicant-directed changes in repair capacity. An important question that arises from the latter is, "What promotes the toxicant-mediated repair responses within "'normal'" individuals that outwardly lack signs of genetic/ epigenetic-based diseases?" It would appear that the normal individual's repair capacity, in terms of both magnitude and fidelity, can be differentially altered under varying parameters of toxicant exposure (dose, dose rate, exposure time, etc.) (6-8). The work by Tuschl et al. (9) indicates that selected repair functions (UDS/polymerase responses) can be significantly amplified after prolonged courses of very low dose toxicant exposures (i.e., two qualities of ionizing radiation). The late pathological consequences of this amplified toxicant-elicited repair is, however, unclear and needs to be examined. We have previously identified distinct subgroups of experimental, outbred dogs within a closed colony with selected predispositions to various types of hematopathological conditions (e.g., A A , myeloid leukemia, and related MPD) under chronic ionizing radiation exposure (10). Further work demonstrated that such pathological predispositions were largely based on differences in the magnitude and plasticity of hematopoietic repair under chronic toxic stress (11-13). This relationship serves as a basis of a working hypothesis suggesting that early aberration of repair of chronic hematopoietic injury mediates and promotes early stages of evolving myeloproliferative disease. The intent of this study, therefore, was to use UDS as a repair correlate in an attempt to monitor and qualitatively assess repair capacity in essential bone marrow elements of chronically irradiated 3 The abbreviations used are: UDS, unscheduled DNA synthesis; AA, aplastic anemia; MPD, myeloproliferative disease; ara-C, 1-~-o-arabinofuranosyicytosine; AA/PLS, aplastic anemia/preleukemia syndrome. 4518 Research. on April 18, 2017. © 1993 American Association for Cancer cancerres.aacrjournals.org Downloaded from UDS REPAIR UNDER CHRONIC IRRADIATION individual animals prone to the major pathological conditions of interest, namely, AA, myelo id leukemia, and related MPD. M A T E R I A L S A N D M E T H O D S