Photo: R. J. Baker

Chornobyl and Genotoxicity:
Indications from mtDNA Heteroplasmy

Jeffrey K. Wickliffe1, Brenda E. Rodgers1, Ronald K. Chesser2,3, and Robert J. Baker1

1Texas Tech University, Lubbock, TX, U.S.A.

2Savannah River Ecology Laboratory, Aiken, SC,

U.S.A.

3University of Georgia, Athens, GA, U.S.A.

B. E. Rodgers

What is mitochondrial DNA heteroplasmy?

Presence of both wild-type and mutant mtDNA’s within a cell or organism (Wallace 1995)

Abstract

We examined heteroplasmy in the cytochrome b gene from a chronically-irradiated, wild bank vole (Clethrionomys glareolus) and her embryos collected from a highly radioactive area within the Chornobyl exclusion zone. Heteroplasmy is defined as the presence of wild type and mutant mtDNA’s within a cell or organism. Compared to levels of heteroplasmy in a relatively non-radioactive female and her six embryos, the irradiated animals had an increased number of DNA base substitutions, a higher incidence of heteroplasmy, and more DNA base deletions. Though not statistically significant, these results suggest those animals which are chronically exposed to the Chornobyl environment might be experiencing an increased mutation pressure relative to unexposed animals. Moreover, our data indicate that mtDNA heteroplasmy is a potentially diagnostic endpoint in detecting genotoxicity resulting from chronic exposure to environmental irradiation.

SERIES

Figure 2. Total number of mutations per series.

Homoplasmic Condition 

Normal mitochondria

 

Heteroplasmic Condition

Normal and Mutant Mitochondria

SERIES

Figure 3. Proportion of mtDNA cytochrome b heteroplasmy. Represents the frequency of observed mutated molecules in each series.

Cartoon diagram depicting heteroplasmy

*Black circle represents the nucleus

*Colored circles represent mitochondria

Conclusions

Previous research on humans, birds, and mice has been equivocal in determining molecular genetic damage associated with Chornobyl radiation (Dubrova et al. 1996, 1998; Ellegren et al. 1997; Baker et al. 1996a, 1996b). These studies have focused on repetitive nuclear DNA elements and mitochondrial DNA. Baker et al. (1999) pioneered the use of mitochondrial DNA heteroplasmy in genotoxicity research (Fig. 1). Their study design was very similar to ours, however they investigated this phenomenon in another vole species (Microtus arvalis) experiencing a substantially lower radiation exposure. They found more DNA mutations (50%) in the exposed animals relative to the reference animals. This study indicates more DNA mutations (18.8%; Fig. 2) and a higher incidence of heteroplasmy (8.7%) in the experimental (exposed) animals (Fig. 3). In addition, the experimental animals exhibited more DNA deletions (400%) than the reference animals (Fig. 4). Though not statistically significant, the increase in molecular genetic alterations suggests the consequences of inhabiting the Chornobyl environment may involve sublethal impacts to the genetic material of small mammals.

SERIES

Figure 4. Number of deletions observed in each series. Deletions in the experimental series include 10 single nucleotide deletions, 1 double nucleotide deletion, and 2 triple nucleotide (codon) deletions.

Experimental Design and Methods

Two wild caught female bank voles and embryos

Experimental

Series

1) 73,083Bq/g IM Cs

2) 53.1mGy/day

3) 6 embryos

Reference

Series

1) 427Bq/g IM Cs

2) 0.3mGy/day

3) 6 embryos

 

Acknowledgments

Natural Science Research Laboratory, Texas Tech University, U.S.A.

U. S. Department of Energy, U.S.A.

International Radioecology Laboratory, UA

Ministry of Emergency Situations, UA

Texas GAP Analysis Program, Wildlife and Fisheries Coop Unit, U.S.A.

 

Figure 1. Cartoon representation of the process used  to estimate heteroplasmy. Colored dots represent different mitochondrial DNA sequences.

    

  

 

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