International Mammalian Genome Society

F35 Positional Cloning of Two Tumor Suppressor Genes Involved in Gamma-Ray Induced Mouse Thymic Lymphomas

Kominami R.¹, Okano H.¹, Matsuki A.¹, Wakabayashi Y.¹, Shinbo T.¹, Kosugi S.¹, Takahashi Y.¹, and Niwa O². ¹Department of Biochemistry, Niigata University School of Medicine, Asahimachi 1-757, Niigata 951-8122; ²Radiation Biology Center, Kyoto University, Yoshida-Konoecho, Sakyou-Ku 606-831, Japan

Mouse genetic systems have a number of advantages for allelic loss (or LOH) mapping of tumors that leads to identification of new chromosomal loci harboring tumor suppressor genes. Our previous genome-wide analysis of allelic loss for thymic lymphomas that were induced by gamma-ray irradiation in F₁ hybrid mice between BALB/c and MSM strains suggested three regions of frequent allelic losses: (1) the centromeric region (D11Mit71) on chromosome 11, (2) near the D12Mit279 locus (Tlsr4) on chromosome 12, and (3) around the D16Mit162 on chromosome 16. By constructing a physical map of each locus and fine LOH mapping, we have succeeded in cloning of candidate genes from two of the three loci. One is the Ikaros gene on chromosome 11 through positional candidate approach, and the other is a novel gene on chromosome 12 that was isolated by extensive sequencing of the LOH-peak region. Ikaros is an transcriptional regulator that was postulated to participate in oncogenic process from the study of Ikaros knockout mice. We show that the critical region of allelic loss in lymphomas was centered at the Ikaros locus, that nine lymphomas failed to give PCR-amplification for either of two exon primer pairs, indicative of homozygous deletion, and that 11 mutations, six of which were frameshift or nonsense mutations, were detected in the zinc finger and the activation domain. On the other hand, construction of a physical map of Tlsr4 on chromosome 12 with 15 BAC clones and isolation of BAC-derived polymorphic probes have led to very fine allelic loss mapping; the commonly lost region is a 20 kb interval with a NotI site. Sequence analysis of this region, exon search and cDNA isolation revealed a novel gene encoding a zinc finger protein comprising approximately 800 amino acids. Involvement of these two genes in tumorigenesis is discussed.