Ph-positive leukemia: a transgenic mouse model

Animal models of leukemia: any closer to the real thing?

Animal models have been invaluable in the efforts to better understand and ultimately treat patients suffering from leukemia. While important insights have been gleaned from these models, limitations must be acknowledged. In this review, we will highlight the various animal models of leukemia and describe their contributions to the improved understanding and treatment of these cancers.

BCR-ABL oncogenic transformation of NIH 3T3 fibroblasts requires the IL-3 receptor

Oncogenic transformation of hematopoietic cells by the Bcr-Abl oncoprotein directly involves the activation Jak2 tyrosine kinase and the Stat5 transcription factor. Both proteins are normally linked to the interleukin (IL)-3/granulocyte-macrophage colony-stimulating factor receptors for growth and survival. Since fibroblastic cells are not targets of BCR-ABL-induced oncogenesis, we determined whether forced expression of the IL-3 receptor would allow oncogenic transformation of NIH 3T3 fibroblasts known to be resistant to transformation by BCR-ABL. NIH 3T3 cells transduced with the human IL-3 receptor alpha and beta chains were highly susceptible to oncogenic transformation by expression of BCR-ABL. Forced expression of both receptor chains but not either one alone allowed efficient foci formation of NIH 3T3 cells expressing BCR-ABL (triple positive cells), and these cells formed colonies in soft agar, whereas BCR-ABL+ NIH 3T3 cells lacking IL-3 receptor expression did not. Signaling studies indicate that the BCR-ABL/IL-3 receptor+ NIH 3T3 cells utilize the Gab2/PI-3 kinase pathway activated by Jak2, and the Stat5 pathway activated separately by Bcr-Abl, whereas BCR-ABL+ NIH 3T3 cells lacking the IL-3 receptor do not utilize the Jak2 pathway, but still maintain activation of Stat5. The Bcr-Abl kinase inhibitor imatinib mesylate (1 microM) and two Jak2 kinase inhibitors strongly inhibited agar colony formation and the activation of Gab2 caused by Jak2. All of these findings indicate that Bcr-Abl oncoprotein requires the IL-3 receptor/Jak2/Stat5 pathways for oncogenic transformation of NIH 3T3 fibroblasts.

Fusion oncogenes in tumor development

Currently, all identified fusion oncogenes are found in rare tumor forms, and most of them only in specific tumor types. Some fusion oncogenes are frequent in healthy individuals suggesting that they rarely induce tumor growth. Multiple double-strand breaks that cluster in time and space increases the risk for formation of fusion oncogenes genes. The normal cell type specific spatial distribution of chromatin and genes in interphase nuclei may affect the risk for fusion of specific genes. Transcriptional orientation, splicing of reading frames, size and sequences of breakpoint introns are other risk factors. The biological activity of fusion oncoproteins is the most important factor for penetrance. The effects in specific target cells may explain the tumor type specificity of most fusion oncogenes.

Spontaneous and genetically engineered animal models; use in preclinical cancer drug development

The preclinical development of anticancer drugs has been based primarily on the transplantation of murine or human cancers into mice. Alternatives to these transplantation models are animals that naturally develop cancers with features relevant to the human disease. The first group of these models arises in mice that are genetically engineered to develop cancer. The second group includes pet dogs and cats that naturally develop cancer. This review will discuss the use and integration of these spontaneous cancer models into a comprehensive and comparative approach to preclinical drug development. Examples of their successful use and an outline of their relative strengths and weaknesses will be provided.