Cytogenetic damage analysis in mice chronically exposed to low-dose internal tritium beta-particle radiation

Abstract

The aim of this study was to carry out a comprehensive examination of potential genotoxic effects of low doses of tritium delivered chronically to mice and to compare these effects to the ones resulting from equivalent doses of gamma-irradiation. Mice were chronically exposed for one or eight months to either tritiated water (HTO) or organically bound tritium (OBT) in drinking water at concentrations of 10 kBq/L, 1 MBq/L or 20 MBq/L. Dose rates of internal β-particle resulting from such tritium treatments were calculated and matching external gamma-exposures were carried out. We measured cytogenetic damage in bone marrow and in peripheral blood lymphocytes (PBLs) and the cumulative tritium doses (0.009 - 181 mGy) were used to evaluate the dose-response of OBT in PBLs, as well as its relative biological effectiveness (RBE). Neither tritium, nor gamma exposures produced genotoxic effects in bone marrow. However, significant increases in chromosome damage rates in PBLs were found as a result of chronic OBT exposures at 1 and 20 M Bq/L, but not at 10 kBq/L. When compared to an external acute gamma-exposure ex vivo, the RBE of OBT for chromosome aberrations induction was evaluated to be significantly higher than 1 at cumulative tritium doses below 10 mGy. Although found non-existent at 10 kBq/L (the WHO limit), the genotoxic potential of low doses of tritium (>10 kBq/L), mainly OBT, may be higher than currently assumed.

Keywords: chronicle; genotoxicity; low-dose; tritium exposure.

Figure 1

Frequencies of micronucleated PCE (MN-PCE) in bone marrow of mice treated in vivo with indicated doses of HTO or OBT in drinking water or γ-radiation for 1 (A) or 8 (B) months.

Figure 2

Chromosome damage rates measured using M-FISH in peripheral blood lymphocytes of mice treated in vivo with indicated doses of HTO or OBT in drinking water or γ-radiation for 1 (A) or 8 (B) months.

Figure 3. Dose-response for chromosome damages induced by in vivo chronic OBT exposure from both 1- and 8-month experiments

(A) chromosome damage rates resulting from chronic OBT exposure were plotted as a function of OBT-cumulative dose. The dotted line represents the curve fitted using a linear model (1). The full line represents the curve fitted using a logarithmic model (2). Error bars represent the upper and lower 95% confidence intervals assuming Poisson distribution. (B) Table summarizing the coefficients, p-value and AIC associated with each model (C), dose-response for OBT-induced chromosome damage that illustrates dose-dependence. Chromosome damage rates were plotted as a function of OBT-cumulative dose rescaled according to the logarithmic term in the equation (2). Error bars represent the upper and lower 95% confidence intervals assuming Poisson distribution.

Figure 4. RBE estimation of OBT with respect to chromosome damage rate induced in mouse PBLs

Color coded areas correspond to areas with different RBE estimates made using the shown dose-response fits. (A) Dose-response fits of chromosome damage rate in mouse PBLs upon chronic in vivo OBT or acute ex vivo γ-exposures. Upper and lower dotted lines show the 95% confidence intervals of the fitted dose-responses. Legend on the right describes RBE estimates of each of the color coded areas. (B) Upper and lower curves representing the 95% confidence intervals of the RBE derived from data in a.