Co-injection strategies to modify radiation sensitivity and tumor initiation in transgenic Zebrafish

Characterization of fluorescent eye markers for mammalian transgenic studies

Genotyping mice by DNA based methods is both laborious and costly. As an alternative, we systematically examined fluorescent proteins expressed in the lens as transgenic markers for mice. A set of eye markers has been selected such that double and triple transgenic animals can be visually identified and that fluorescence intensity in the eyes can be used to distinguish heterozygous from homozygous mice. Taken together, these eye markers dramatically reduce the time and cost of genotyping transgenics and empower analysis of genetic interaction.

Quantifying the frequency of tumor-propagating cells using limiting dilution cell transplantation in syngeneic zebrafish

Self-renewing cancer cells are the only cell types within a tumor that have an unlimited ability to promote tumor growth, and are thus known as tumor-propagating cells, or tumor-initiating cells. It is thought that targeting these self-renewing cells for destruction will block tumor progression and stop relapse, greatly improving patient prognosis¹. The most common way to determine the frequency of self-renewing cells within a tumor is a limiting dilution cell transplantation assay, in which tumor cells are transplanted into recipient animals at increasing doses; the proportion of animals that develop tumors is used the calculate the number of self-renewing cells within the original tumor sample^{2, 3}. Ideally, a large number of animals would be used in each limiting dilution experiment to accurately determine the frequency of tumor-propagating cells. However, large scale experiments involving mice are costly, and most limiting dilution assays use only 10-15 mice per experiment.

Zebrafish have gained prominence as a cancer model, in large part due to their ease of genetic manipulation and the economy by which large scale experiments can be performed. Additionally, the cancer types modeled in zebrafish have been found to closely mimic their counterpart human disease⁴. While it is possible to transplant tumor cells from one fish to another by sub-lethal irradiation of recipient animals, the regeneration of the immune system after 21 days often causes tumor regression⁵. The recent creation of syngeneic zebrafish has greatly facilitated tumor transplantation studies ^6-8. Because these animals are genetically identical, transplanted tumor cells engraft robustly into recipient fish, and tumor growth can be monitored over long periods of time. Syngeneic zebrafish are ideal for limiting dilution transplantation assays in that tumor cells do not have to adapt to growth in a foreign microenvironment, which may underestimate self-renewing cell frequency^{9, 10}. Additionally, one-cell transplants have been successfully completed using syngeneic zebrafish⁸ and several hundred animals can be easily and economically transplanted at one time, both of which serve to provide a more accurate estimate of self-renewing cell frequency.

Here, a method is presented for creating primary, fluorescently-labeled T-cell acute lymphoblastic leukemia (T-ALL) in syngeneic zebrafish, and transplanting these tumors at limiting dilution into adult fish to determine self-renewing cell frequency. While leukemia is provided as an example, this protocol is suitable to determine the frequency of tumor-propagating cells using any cancer model in the zebrafish.

High-throughput imaging of adult fluorescent zebrafish with an LED fluorescence macroscope

Zebrafish are a useful vertebrate model for the study of development, behavior, disease and cancer. A major advantage of zebrafish is that large numbers of animals can be economically used for experimentation; however, high-throughput methods for imaging live adult zebrafish had not been developed. Here, we describe protocols for building a light-emitting diode (LED) fluorescence macroscope and for using it to simultaneously image up to 30 adult animals that transgenically express a fluorescent protein, are transplanted with fluorescently labeled tumor cells or are tagged with fluorescent elastomers. These protocols show that the LED fluorescence macroscope is capable of distinguishing five fluorescent proteins and can image unanesthetized swimming adult zebrafish in multiple fluorescent channels simultaneously. The macroscope can be built and used for imaging within 1 day, whereas creating fluorescently labeled adult zebrafish requires 1 hour to several months, depending on the method chosen. The LED fluorescence macroscope provides a low-cost, high-throughput method to rapidly screen adult fluorescent zebrafish and it will be useful for imaging transgenic animals, screening for tumor engraftment, and tagging individual fish for long-term analysis.

Fluorescent imaging of cancer in zebrafish

Zebrafish are an ideal model organism to research cancer. Zebrafish embryos and larvae are optically translucent, which has made imaging multiple processes in development and disease possible. When coupled with fluorescent imaging techniques, zebrafish are fast becoming a model of choice for following tumor formation. This is highlighted by recent studies using fluorescent proteins to image xenograft transplantation, neovascularization, growth responses to drug treatments, and self-renewal. Fluorescent labeled tumors can be generated in zebrafish by multiple methods including chemical mutagenesis, oncogene expression by mosaic or stable transgenesis, or genetic mutations that are predisposing to cancer. In this chapter, we highlight the studies that have employed fluorescence to follow critical aspects of tumorigenesis, with particular focus on providing methods for labeling, isolating, transplanting, and imaging fluorescently labeled tumors in zebrafish.

Zebrafish models of rhabdomyosarcoma

Rhabdomyosarcoma (RMS), an aggressive malignant neoplasm that shows features of skeletal muscle, is the most common soft tissue tumor of childhood. In children, the major subtypes are embryonal and alveolar. Although localized disease responds to a multimodal treatment, the prognosis for patients with high-risk features and metastasis remains dismal. Several in vivo models of RMS have been developed in mouse, human xenografts, zebrafish, and Drosophila to better understand the underlying mechanisms governing malignancy. The findings so far have indicated the potential role of skeletal muscle precursor cells in malignant transformation. To better understand histogenesis and different aspects of tumorigenesis in RMS, we have previously developed a robust zebrafish model of kRAS-induced RMS, which shares morphologic and immunophenotypic features with the human counterpart. Cross-species mircroarray comparisons confirm that conserved genetic pathways drive RMS growth. The ease for ex vivo manipulation allows the development of different transgenic and co-injection strategies to induce tumor formation in zebrafish. In contrast to other vertebrate model systems, the tumor onset in zebrafish is short, allowing for efficient study of different tumor processes including tumor growth, self-renewal, and maintenance.

Zebrafish as a model for cancer self-renewal

Self-renewal is the process by which normal stem cells and cancer cells make more of themselves. In cancer, this process is ultimately responsible for the infinite replicative potential of malignant cells and is likely found in residual cell populations that evade conventional therapy. Two intrinsically opposing hypotheses have emerged to explain how self-renewal occurs in cancer. The cancer stem cell hypothesis states that self-renewal is confined to a discrete subpopulation of malignant cells, whereas the stochastic model suggests that all tumor cells have the potential to self-renew. Presently, the gold standard for measuring cancer self-renewal is limiting dilution cell transplantation into immune-matched or immune-deficient animals. From these experiments, tumor-initiating frequency can be calculated based on the number of animals that engraft disease following transplantation of various doses of tumor cells. Here, we describe how self-renewal assays are performed, summarize the current experimental models that support the cancer stem cell and stochastic models of cancer self-renewal, and enumerate how the zebrafish can be used to uncover important pathways in cancer self-renewal.

Zebrafish tumor assays: the state of transplantation

Tumor transplant studies are important tools for studying cancer biology in a model organism. Transplantation is especially important for assaying tumor cell malignancy and migration capabilities, and is critical for identifying putative cancer stem cell populations. In this review, we discuss the current state of tumor transplantation studies performed in the zebrafish. We address several zebrafish-specific considerations for development of the transplant assay, including choosing recipient animals, transplant methods, and post-transplant observation. We also examine how the zebrafish is an advantageous model for transplantation, particularly with development of the translucent fish. Transplantation has already been critical for characterizing zebrafish models of leukemia, rhabdomyosarcoma, and melanoma. With further development of imaging techniques and other tools, zebrafish tumor transplantation will continue to contribute to our understanding of tumor cell biology.

High-throughput cell transplantation establishes that tumor-initiating cells are abundant in zebrafish T-cell acute lymphoblastic leukemia

Self-renewal is a feature of cancer and can be assessed by cell transplantation into immune-compromised or immune-matched animals. However, studies in zebrafish have been severely limited by lack of these reagents. Here, Myc-induced T-cell acute lymphoblastic leukemias (T-ALLs) have been made in syngeneic, clonal zebrafish and can be transplanted into sibling animals without the need for immune suppression. These studies show that self-renewing cells are abundant in T-ALL and comprise 0.1% to 15.9% of the T-ALL mass. Large-scale single-cell transplantation experiments established that T-ALLs can be initiated from a single cell and that leukemias exhibit wide differences in tumor-initiating potential. T-ALLs also can be introduced into clonal-outcrossed animals, and T-ALLs arising in mixed genetic backgrounds can be transplanted into clonal recipients without the need for major histocompatibility complex matching. Finally, high-throughput imaging methods are described that allow large numbers of fluorescent transgenic animals to be imaged simultaneously, facilitating the rapid screening of engrafted animals. Our experiments highlight the large numbers of zebrafish that can be experimentally assessed by cell transplantation and establish new high-throughput methods to functionally interrogate gene pathways involved in cancer self-renewal.

Heritable T-cell malignancy models established in a zebrafish phenotypic screen

T cell neoplasias are common in pediatric oncology, and include acute lymphoblastic leukemia (T-ALL) and lymphoblastic lymphoma (T-LBL). These cancers have worse prognoses than their B cell counterparts, and their treatments carry significant morbidity. While many pediatric malignancies have characteristic translocations, most T lymphocyte-derived diseases lack cytogenetic hallmarks. Lacking these informative lesions, insight into their molecular pathogenesis is less complete. Although dysregulation of the NOTCH1 pathway occurs in a substantial fraction of cases, many other genetic lesions of T cell malignancy have not yet been determined. To address this deficiency, we pioneered a phenotype-driven forward-genetic screen in zebrafish (Danio rerio). Using transgenic fish with T lymphocyte-specific expression of enhanced green fluorescent protein (EGFP), we performed chemical mutagenesis, screened animals for GFP⁺ tumors, and identified multiple lines with a heritable predisposition to T cell malignancy. In each line, patterns of infiltration and morphologic appearance resembled human T-ALL and T-LBL. T cell receptor analyses confirmed their clonality. Malignancies were transplantable and contained leukemia-initiating cells (LIC), like their human correlates. In summary, we have identified multiple zebrafish mutants that recapitulate human T cell neoplasia and show heritable transmission. These vertebrate models provide new genetic platforms for the study of these important human cancers.