Evaluation of tumor clonality with chemical carcinogenesis in a mouse model of visualized X chromosome inactivation

Tumor clonality is determined by somatic mutations in genes that regulate cell proliferation, and this can be either monoclonal or multiclonal. Assays based on X-chromosome inactivation that exploit the random inactivation of one of the two X chromosomes in female embryos have been used to evaluate tumor clonality. However, these methods require technically complex procedures and are not easily applicable to various types of tumors. Here, we visualized the clonality of tumors induced by chemical substances <i>in vivo</i> using X-linked <i>LacZ</i> heterozygous transgenic female mice that displayed a blue or white mosaic pattern of tissue on X-gal staining. In a model of colorectal tumors induced by 1, 2-dimethylhydrazine dihydrochloride and dextran sulfate sodium salt, 18 blue, 20 unstained (white), and seven mixed-colored tumors in intestinal tissues from 20 mice were observed after X-gal staining. Similarly, in a model of diethylnitrosamine-induced liver tumors, multiple blue or white nodules were observed. These findings demonstrated that this is a simple and effective method for visualizing tumor clonality <i>in vivo</i>. This approach may be readily applicable to models of chemically induced carcinogenesis and useful for evaluating the clonality of multifocal lesions.