42. Ras Gene Mutation in Spontaneous and Gamma-ray-induced Thymic Lymphomas of Scid Mice

Mayumi Nishimura, Shigeharu Wakana, Shizuko Kakinuma, Kazuhide Mita, Hiroko Ishii, Shigeru Kobayashi, Toshiaki Ogiu, Toshihiko Sado and Yoshiya Shimada

Keywords: thymic lymphoma, Scid mice, gamma-rays, ras gene mutation

Radiation carcinogenesis is a process in which cells acquire malignancy through cumulative genetic or epigenetic alterations. Characterization of genetic alterations in proto-oncogenes and tumor suppressor genes of spontaneous and radiation induced tumors of experimental animals will aid in further understanding the special gene change(s) underlying tumor development. From the viewpoint of radiation biology, severe combined immunodeficient (Scid) mice are unique in that they have a defect in rejoining of DNA double-strand breaks and thus exhibit not only complete absence of functional T and B cells, but also hypersensitivity to ionizing radiation. It has recently been found that scid mice are highly susceptible to radiation-induced lymphomagenesis. The purpose of the present study was to investigate the contribution of ras gene activation in radiation-induced thymic lymphomas in scid mice and to examine the mutation spectra of ras genes in these tumors.

The incidence of thymic lymphomas increased as a function of the dose after single exposures to gamma-rays at doses of 0.25 to 3 Gy. About 80% of the mice developed thymic lymphomas after the exposure to more than 1 Gy, whereas 40 out of 204 non-irradiated control mice (19.6%) developed thymic lymphomas within one year after exposure. Then both the frequency and the spectrum of Kras and Nras mutations in spontaneous and radiation-induced lymphomas were examined. As shown in Table 4, neither mutated Kras nor Nras genes were detected in spontaneous lymphomas, while Kras mutations increased in a dose-dependent manner in radiation-induced lymphomas. However, Kras mutations were rather infrequent and no mutations were detected in Nras genes, suggesting ras mutation was not significantly involved in the development of thymic lymphomas in scid mice. Analysis of the spectrum of Kras mutations demonstrated unique mutations in both codons 13 (GGC to GAC) and 61 (CAA to CTA) in addition to the commonly identified substitution of GAT for GGT in the codon 12 of Kras, whereas the activating mutation for the Kras gene was a G to A transition in codons 12 and 13 and an A to T transversion in codon 61.

Table 4. Ras gene point mutations.

Dose (Gy)	Lymphoma frequency	Number of tumors with mutation/ number of tumors examined	Kras		Nras
			Exon 1	Exon 2	Exon 1	Exon 2
0	40/204 (21%)	0/15 (0%)	nd	nd	nd	nd
0.25	32/104 (31%)	0/10 (0%)	nd	nd	nd	nd
1.0	72/106 (68%)	1/18 (6%)	nd	61CAA CTA	nd	nd
2.0	94/104 (86%)	2/16 (13%)	12GGT GAT 13GGC GAC	nd	nd	nd
3.0	78/204 (75%)	3/18 (17%)	12GGT GAT 12GGT GAT 12GGT GAT	nd	nd	nd

nd: Not detected

Publications:
Nishimura, M., Wakana, S., Kakinuma, S., Mita, K., Ishii, H., Kobayashi, S., Ogiu, T., Sado, T. and Shimada, Y.: Radiat. Res., 151, 142-149, 1999.