17. Enhancement of lntracisternal A-particle RNA in Regenerated Myeloid Cells after Sub-lethal Doses of X rays in C3H/He Mice

Hiroshi Ishihara, Izumi Tanaka, Masako Furuse and Kazuko Tsuneoka

Keywords: radiation damage, myeloid cells, retrotransposon, intracisternal A-particle, transposition

Intracisternal A-particle (NAP) DNA element in sizes of 5-7 kb is one of the mouse retrotransposons and it is closely related with endogenous retrovirus in structure. Normal mouse genome contains more than one thousand copies of the NAP element per haploid. When the IAP RNA is reversely transcribed and re-integrated into genome, genetic information of the cells is modified. Even though the retrotransposition is considered as a rare event, many cases of IAP mediated gene rearrangement are reported in tumor as an event III somatuc cells and in mutant mice as an event in the germ line.

Myeloud leukemia cells are derived from regenerated hematopoietic cells damaged by sub-lethal doses of x-ray irradiation in C3H/ He mice. We have already found that the most of the leukemia cell lines generated from different individuals undergo IAP mediated retrotransposition in the genomic DNA. This indicates that the IAP element is activated in the myeloid cells after x-irradiation.

To clarify the behaviour of IAP element in mouse tissue, quantitative analysis of the IAP-RNA was conducted. In C57BL6 and STS/ A inbred mice, basal IAP RNAs are detected only in the hematopoietie tissues such as thymus, spleen and bone marrow. In C3H/He inbred mouse, similar levels of IAP RNA are detected in non-hematoponetic inssues. In particular, the hematopoietic tissues of C3H mice possess 5to 10-fold higher levels of basal IAP RNA than other tissues. This means that the C3H/He inbred mouse is IAP expressive strain.

The decrease in the number of hematopoietic cells by whole body-irradiation of sublethal doses at 3Gy of x-rays recovers to the normal levels after 18 days. During and after the recovery step, the relative rate of IAP/-actin RNA is increased in hematopoietic cells but not in non-hematopoietic cells (Fig. 10) . The increase in the levels of NAP RNA is kept for at least 90 days after irradiation. This suggests that IAP tends to be activated in the recovered hematopoietic cells.

Among various types of IAP elements in the genome, all the retrotransposed ones at the rearranged sites in leukemia cells have a common structure in the nucleotide sequence at the regulator sequence of the promoter region. It is also found that the expression of this type of IAP is enhanced in all the leukemia cells. This shows that limited types in the IAP element population are activated in the leukemia cells. Since the IAP element behaves as an endogenous mutagen, the contribution of IAP in genomic instability should be considered.

Fig.10. IAP expression during hematopoietic regeneration Spleen, femur, liver, and blod were isolated on the days indicated after irradiation. For each period, 3 mice were analyzed separatery. The RNA ratio of IAP /-action to non-irradiated cells was calculated after quantitative Northern hyblidization. The calculated relative RNA ratio values in hematopoietic (spleen, bone marrow and peripheral blood) and non-hematopoietic cells (liver) are shown with mean and standard deviation values for the relative RNA ratio of IAP/-action among the 3 mice at each point. Asterisks(*) indicate data at p<0.05 versus control by t-test.

Publication:
Ishihara, H., Tanaka, I., Furuse, M. and Tsuneoka, K.: Radiat. Res. 153, 392-397, 2000.