Chromosomal instability in human lymphocytes after low dose rate gamma -irradiation and delayed mitogen stimulation

Abstract

Purpose: To study the possibility that radiation induced chromosomal instability in human lymphocytes is promoted by a conflict between mitogen-induced growth stimulation and radiationinduced genotoxic stress. Materials and methods: Peripheral blood lymphocytes were exposed to low LET-irradiation at: (1) low-dose rate (LDR, 1-3 Gy, 0.024 Gy h1) in order to minimize genotoxic stress; (2) high dose rate (HDR, 1-3 Gy, 45 Gy h1)followed by immediate mitogen stimulation; and (3) HDR followed by a recovery period of 5 days before mitogen stimulation. Subsequent analyses included cell viability and clonogenic cell survival, chromosome aberrations at the first post-irradiation mitosis, and karyotype analysis of long term cultured cells, 11-57 days after mitogen stimulation. Results: Dose (1-3 Gy) and dose rate (LDR and HDR) effects on the frequency of dicentric chromosomes at the first postirradiation mitosis were in agreement with published data, with a pronounced dose rate effect of 2 and 3 Gy exposures. G-banded karyotypes after 11 days of growth in vitro showed increased frequencies of chromosome breaks and rearrangements in all irradiated cell cultures. Clones with complex karyotype abnormalities and increased frequencies of de novo aberrations developed in the irradiated cultures during extended growth for 22-57 days. These results show that: (1) LDR-irradiation induces chromosomal instability in primary human lymphocytes; (2) mitogen stimulation rescues HDR-irradiated cells from death at the expense of an increased level of chromosome aberrations; and (3) HDR-irradiated cells that are allowed 5 days of recovery before mitogen stimulation develop chromosomal instability during subsequent long-term proliferation. Conclusions: Neither the acute genotoxic stress of HDR-irradiation compared with LDR-irradiation, nor the hypothesized conflict between mitogen-induced growth stimulation and irradiationinduced growth arrest, seem to be critical conditions for the development of chromosomal instability in primary human T lymphocytes. Post-irradiation incubation allowing apoptotic processes to remove damaged cells does not prevent the subsequent development of chromosomal instability during long-term cell proliferation.