DOE Low Dose Radiation Research Program Workshop I Abstracts November 10-12, 1999, Washington, D.C.

11. Current Cytogenetic Issues Pertaining to Low Dose and Low Dose Rate

Michael Cornforth and Bradford Loucas
University of Texas Medical Branch, Department Radiation Oncology, 344 Gail Borden Building, Route F-56, Galveston, TX 77555-0656
mcornfor@utmb.edu

Summary: For both theoretical and pragmatic reasons that impact low dose issues, we propose to study radiation-induced chromosome exchanges in human cells that require three or more breakpoints for their formation, utilizing combinatorial whole chromosome labeling techniques that allow each chromosome to be "painted" a unique color.

Abstract: By providing a quantitative, sensitive and relevant measure of genotoxic damage, the study of chromosomal exchange-type aberrations has served to guide theoretical models of radiation action for half a century. Until recently, it was universally assumed that the vast majority of radiation-induced exchanges between different chromosomes were simple reciprocal events, involving pairs of chromosomes. Newer "chromosome painting" studies suggest that a large fraction of exchange aberrations are actually "complex", involving three or breakpoints among two or more chromosomes. Putting this discovery in the context of contemporary models of radiation action requires a number of assumptions, because conventional chromosome painting techniques reveal only a partial picture of complex rearrangements. Differing assumptions lead to grossly divergent predictions about the true extent of aberration complexity. This controversy impacts fundamental tenets of radiation action, including prominent generalized theories of radiation action that form the cornerstone concepts currently applied to issues of radiation protection.

A key aspect of the project is the use of multicolor combinatorial painting (m-FISH) to eliminate discrepancies surrounding the interpretation of complex aberration data. This will allow the testing of two contemporary cytogenetic models. The models chosen are particularly important to the issue of low dose effects, because their predictions have been used to buttress diametrically opposed views of how chromosome aberrations are produced by ionizing radiation. Also described are experiments to examine the feasibility of using complex aberrations as a biomarker of past exposure to densely ionizing radiations.

Specific Aims (abbreviated)

1. Over doses of 137-Cs gamma-rays ranging from 1 to 4 Gy, evaluate the frequency of simple versus complex exchanges in first post-irradiation metaphases of primary human cells. Determine the frequency of total exchanges that are complex (the relative complex fraction; RCF).

2. Measure the frequencies of simple reciprocal and complex aberrations following exposure to 5 Gy of gamma-rays, delivered at the limiting low dose rate of 0.1 cGy/min. Determine the RCF, in order to test the hypothesis that single-tracks from low LET radiations cannot produce complex aberrations.

3. As a function of dose ranging from 0.2 to 1.5 Gy, determine the RCF for 238-Pu alpha particles and for 0.43 MeV neutrons. Test the hypothesis that the RCF is independent of dose for both alpha particles and neutrons, and explore the use of this parameter as a potentially robust cytogenetic biomarker for high LET radiation exposure.

By quantifying the true extent of exchange complexity, m-FISH allows for the testing of two models used to buttress diametrically opposed mechanisms of aberration formation. There are two key issues at stake for the Low Dose Program. The first is whether complex exchanges display a near-quadratic dependency on dose, as this implies a "virtual threshold" for their formation at low doses. Secondly, the model chosen will dramatically affect estimates for low-dose yields of simple exchanges. The development of a cytogenetic "signature" of prior high LET exposure would have far- reaching implications relating to radiation quality factors used in risk management.