24 .Radiation-Associated Loss of Heterozygosity at Ikaros Allele and Ikaros Mutation in Murine Thymic Lymphomas
Yoshiya Shimada, Mayumi Nishimura and Shizuko Kakinuma
Keywords: Ikaros, LOH, thymic lymphomas
Ionizing radiation is a potent carcinogenic agent. In order to find the tumor suppressor locus associated with radiation carcinogenesis, we determined the frequency and distribution of loss of heterozygosity (LOH) of X-ray-induced thymic lymphomas (TLs) of B6C3F1 mice using 58 microsatellite markers. We found a unique locus with frequent LOH in the centromeric region of chromosome 11 of X-ray-induced lymphomas. This locus has never been observed to be similarly altered in either N-ethylnitrosourea-induced or spontaneously developed lymphomas, suggesting radiation specific molecular alteration. The LOH patterns of individual TLs indicated that the common region of LOH was located within 1.6 cM between D11Mit62 and D11Mit204, a region syntenic to human chromosome 7p13. Linkage analysis revealed that the markers of the common LOH region were genetically linked to Ikaros. Ikaros encodes a Krüppel-type zinc-finger transcription factor that plays a critical role in both lineage commitment and differentiation of lymphoid cells. We next aimed to delineate the Ikaros inactivation with special reference to the LOH status, and to determine the relative contribution of each type of Ikaros inactivation in radiation-induced TLs. We demonstrated that Ikaros was frequently altered (in 50% of TLs), and that its inactivation was caused by a variety of mechanisms, which came under one of the following four categories: (i) null expression (14%); (ii) expression of unusual dominant-negative isoforms, (11%); (iii) amino acid substitutions in the N-terminal zinc-finger domain for DNA binding caused by point mutations (22%); (iv) lack of the Ik-1 isoform due to the creation of a stop codon by insertion of a dinucleotide in exon 3 (3%). The null expression, amino acid substitutions, and dinucleotide insertion inactivation types were well correlated with LOH at the Ikaros allele (86%), and were consistent with Knudson's two hit-theory. On the other hand, TLs expressing dominant-negative Ikaros isoforms retained both alleles. These results indicate that Ikaros inactivation takes place by a variety of mechanisms in radiation-induced murine TLs and is frequently associated with LOH, this association depending on the type of inactivation.
1) Shimada, Y., Nishimura, M., Kakinuma, S., Okumoto, M., Shiroishi, T., Clifton, K.H. and Wakana, S.: Radiat. Res. 154, 293-300, 2000.
2) Shimada, Y., Nishimura, M., Kakinuma, S., Takeuchi, T., Ogiu, T., Suzuki, G., Nakata, Y., Sasanuma, S., Mita, K. and Sado, T.: Int. J. Radiat. Biol. 77, 465-473, 2001.
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