Zebrafish Xenotransplant Cancer Model for Drug Screening - Zebrafish: Methods for Assessing Drug Safety and Toxicity - page 212

xenograft models. After cell injection, presence of a tumor, defined as a subcutaneous
lump, is assessed. Tumor growth is then quantitated by measuring tumor size after
surgical removal (Yang et al., 1997). Because tumor development takes weeks and
animal survival time varies, the experimental period can span several months. In
order to avoid cell rejection, human tumor cells are typically transplanted into nude
or SCID immunosuppressed mice (Yang et al., 1997; Greiner et al., 1998; Katsanis
et al., 1998), which are expensive to maintain, less hardy than normal mice, and more
susceptible to infection and drug toxicity. An additional disadvantage of mouse
cancer models is that a large number of cells ( 10 6 ) are required to generate a

tumor. Although this is not a concern using cell lines, it is often difficult to procure a
sufficient number of viable cells from primary tumors. Moreover, it is extremely
difficult to generate mouse xenograft metastasis models (van Weerden and
Romijn, 2000).

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17.2.2 Zebrafish: A Predictive Cancer Model

In the past decade, zebrafish has emerged as an important model organism for
biomedical research. Although the primary focus of zebrafish studies has been on
developmental biology, most human diseases, including cancers, have now been
modeled in this organism. Since 2000, more than 100 manuscripts describing
zebrafish cancer models have been published. Research strategies have included
mammalian cancer cell xenotransplant, forward genetic screens for assessing pro-
liferation or genomic instability, reverse genetic target-selected mutagenesis to
inactivate tumor suppressor genes, and generation of transgenic animals expressing
human oncogenes. Zebrafish have been shown to exhibit comparable signaling
pathways, assessed by microarrays, and similar morphology, assessed by histology,
as mammalian cancer models. However, compared to rodents, zebrafish-specific
tumor incidence is low and tumors develop at later stages. Despite these disadvan-
tages, zebrafish has created its own niche in cancer research, complementing
conventional animal models with unique experimental advantages.

17.2.3 Advantages of Zebrafish Xenotransplant
Models

The technique for cell transplantation in zebrafish is well established. Genetic
mosaics are usually generated by transplanting cells from a donor embryo labeled
with a lineage-specific marker into unlabeled blastula stage host embryos (Ho and
Kane, 1990). These studies have been crucial for addressing fundamental questions in
developmental biology including (a) when do cells commit to a certain lineage? and
(b) which cell interactions are involved in establishing this commitment? Lineage
markers or tracer dyes such as fluorescent dextrans or lipophilic carbocyanine tracers,
including CM-DiI, can be used to distinguish donor from host-derived cells and to
visualize fate of transplanted cells in transparent zebrafish. In these studies, xeno-
transplant cells are usually homografts (cells from the same species).