Persistence of Chromatid Aberrations in the Cells of Solid Mouse Tissues Exposed to137Cs Gamma Radiation

Aneuploidy, polyploidy and ploidy reversal in the liver

Polyploidy has been described in the liver for over 100 years. The frequency of polyploid hepatocytes varies by age and species, but up to 90% of mouse hepatocytes and approximately 50% of human hepatocytes are polyploid. In addition to alterations in the entire complement of chromosomes, variations in chromosome copy number have been recently described. Aneuploidy in the liver is pervasive, affecting 60% of hepatocytes in mice and 30-90% of hepatocytes in humans. Polyploidy and aneuploidy in the liver are closely linked, and the ploidy conveyor model describes this relationship. Diploid hepatocytes undergo failed cytokinesis to generate polyploid cells. Proliferating polyploid hepatocytes, which form multipolar spindles during cell division, generate reduced ploidy progeny (e.g., diploid hepatocytes from tetraploids or octaploids) and/or aneuploid daughters. New evidence suggests that random hepatic aneuploidy can promote adaptation to liver injury. For instance, in response to chronic liver damage, subsets of aneuploid hepatocytes that are differentially resistant to the injury remain healthy, regenerate the liver and restore function. Future work is required to elucidate the mechanisms regulating dynamic chromosome changes in the liver and to understand how these processes impact normal and abnormal liver function.

Marked aneuploidy and loss of multiple chromosomes are common in autosomal mutants isolated from normal mouse kidney epithelium

Marked aneuploidy and loss of multiple chromosomes are hallmarks of cancer, but whether these events are only present in malignant cells is not known. In prior work, we showed that approximately half of spontaneous autosomal mutants isolated directly from normal kidney epithelium arose from loss of a marker chromosome 8 containing the wild type Aprt gene. Chromosome loss was detected by loss of heterozygosity (LOH) for all chromosome 8 polymorphic loci examined. To determine whether loss of chromosome 8 reflected a larger mitotic event, LOH was examined for polymorphic loci on 11 nonselected chromosomes in Aprt mutants that lost the selected chromosome 8 homologue. LOH events were detected for one or more nonselected chromosomes in 38% of these mutants. The additional LOH events also reflected apparent chromosome loss based on the molecular analysis. Metaphase spreads from mutants that lost chromosome 8 were markedly aneuploid, and chromosome painting revealed reduced levels for any chromosome shown to be lost with the LOH analysis. In contrast, LOH on nonselected chromosomes was infrequent in Aprt mutants exhibiting intragenic events or mitotic recombination for chromosome 8, and marked aneuploidy was absent. These observations suggest that the mechanism leading to chromosome loss in somatic mammalian cells is often not a simple nondisjunction event and instead could result from a single catastrophic event. They also suggest that cells with characteristics of malignancy are present in normal appearing tissue.

Age-related accumulation of autosomal mutations in solid tissues of the mouse is gender and cell type specific

Most cancers in solid tissues increase with age and invariably contain causal mutations eliminating expression of one or more autosomal tumor suppressor genes. However, very little is known about the effect of age on autosomal mutations, often large in size, in cells of solid tissues. In this study, the frequency and spectrum of autosomal mutations were examined as a function of age for kidney epithelial cells and ear mesenchymal cells in B6D2F1 mice heterozygous for the selectable Aprt locus. Aprt mutant frequencies were found to increase with age in the kidneys of both male and female mice, but at all ages the mutant frequencies were approximately twice as high in the females, which in this strain have shorter lifespans than the males. An age-related increase in Aprt mutant frequencies was also observed for ear cells from female mice, but no significant increases in mutant frequencies were observed for the ear cells of male mice. A molecular analysis showed that the kidney and ear mutational spectra were distinct and that the age-related increases in mutant frequencies did not involve significant shifts in the mutational spectra. In total, the results demonstrate both gender and cell-type-specific patterns of autosomal mutational accumulation as a function of age in two solid tissues of the mouse.

Cytogenetic effects of low-dose radiation with different LET in human peripheral blood lymphocytes

Chromosome damage and the spectrum of aberrations induced by low doses of gamma-irradiation, X-rays and accelerated carbon ions (195 MeV/u, LET 16.6 keV/microm) in peripheral blood lymphocytes of four donors were studied. G0-lymphocytes were exposed to 1-100 cGy, stimulated by PHA, and analyzed for chromosome aberrations at 48 h post-irradiation by the metaphase method. A complex nonlinear dose-effect dependence was observed over the range of 1 to 50 cGy. At 1-7 cGy, the cells showed the highest radiosensitivity per unit dose (hypersensitivity, HRS), which was mainly due to chromatid-type aberration. According to the classical theory of aberration formation, chromatid-type aberrations should not be induced by irradiation of unstimulated lymphocytes. With increasing dose, the frequency of aberrations decreased significantly, and in some cases it even reached the control level. At above 50 cGy the dose-effect curves became linear. In this dose range, the frequency of chromatid aberrations remained at a low constant level, while the chromosome-type aberrations increased linearly with dose. The high yield of chromatid-type aberrations observed in our experiments at low doses confirms the idea that the molecular mechanisms which underlie the HRS phenotype may differ from the classical mechanisms of radiation-induced aberration formation. The data presented, as well as recent literature data on bystander effects and genetic instability expressed as chromatid-type aberrations on a chromosomal level, are discussed with respect to possible common mechanisms underlying all low-dose phenomena.

Atm haploinsufficiency does not affect ionizing radiation mutagenesis in solid mouse tissues

Ataxia telangiectasia (AT) is a hereditary disease with autosomal recessive inheritance of ATM (ataxia telangiectasia mutation) alleles. AT is associated with severe sensitivity to ionizing radiation and a strong predisposition to develop cancer. A modest increase in cancer, particularly for the breast, has been shown for ATM carriers (i.e. heterozygotes), and a modest increase in radiation sensitivity has also been shown for those patients and their cells. However, the extent of these effects is unclear. Based on the well-established relationship between cancer and mutation, we used a mouse model for Atm haploinsufficiency to ask whether partial loss of Atm function could lead to an increased mutagenic response for solid tissues of mice exposed to radiation. The autosomal mouse Aprt gene was used as the mutational target and kidney and ear as the target tissues in B6D2F1 hybrids. Although induction of autosomal mutations was readily demonstrated in both tissues, a comparison of these data with those from an identical study performed with B6D2F1 mice that were wild-type for Atm (Cancer Res. 62, 1518-1523, 2002) revealed that Atm haploinsufficiency did not alter the radiation mutagenic response for the cells of either tissue. Moreover, no effect of Atm haploinsufficiency on reduced cellular viability due to radiation exposure was observed. The results demonstrate that Atm haploinsufficiency does not alter the radiation mutagenic response or decrease viability for normally quiescent cells in solid tissues of the mouse.