The yield of radiation-induced chromosomal aberrations in first division human lymphocytes depends on the culture time

Abstract

Purpose: To investigate two long-held beliefs in radiation cytogenetics that were seemingly contradicted by reports that: (a) protracting γ-ray exposures over 0.5 h halves the induced aberration yield compared with acute exposure, and (b) that induced aberration yields in guaranteed first in vitro division metaphases (M1) vary with culture time.

Materials and methods: Replicate blood samples were exposed for 3 min to 3.0 Gy γ-rays and standard phytohaemagglutinin stimulated lymphocyte cultures were harvested at 10 times ranging from 45 – 72 h. Forty-eight hour cultures were also made from blood exposed to 3.0 Gy for 30 min. Slides were differentially stained, combining the harlequin method with fluorescent in-situ hybridization (FISH) painting of chromosomes 2, 3 and 5. M1 metaphases were scored for 1- and 2-way translocations involving the painted chromosomes and all unstable aberrations in the full genomes.

Results: Dicentric and translocation yields from the 30 min exposure were approximately 10% lower than in 48 h cultures from cells exposed for 3 min, although this reduction is not significant. Dicentric aberration yields from the 3 min exposed cells cultured over the range 45 – 72 h remained constant up to 51 h then rose to a different constant value beyond 60 h. The increase at 60 – 70 h compared with the yield at 48 h was about 50%. A marginal increase at later times was also observed for translocations.

Conclusion: The protracted exposure experiment produced results consistent with the G-function hypothesis that models the dose rate effect. Therefore the previous report of a marked departure from this model was not confirmed. The reports of aberration yields increasing with time of arrival at metaphase were confirmed. Possible explanations are discussed; the intercellular distributions of aberrations, or of doses to the cells or heterogeneous radiosensitivity of lymphocyte sub-populations. None alone seems sufficient quantitatively to explain the magnitude of the effect. The implications for biological dosimetry, which employs cultures times of approximately 48 h, are considered to be minor.

Keywords:

• Lymphocyte
• MI aberrations
• dose rate effect
• in vitro cycling effect