Importance of orthotopic transplantation procedures in assessing the effects of transfected genes on human tumor growth and metastasis.

Assessment of the function of putative dominantly-acting oncogenes or recessive tumor-suppressor genes in human tumor development and progression must ultimately involve xenografting experiments using immune deficient animals such as nude mice. Most human tumor xenograft experiments have employed conventional subcutaneous injection procedures. However, despite the simplicity of this procedure, it poses some serious potential drawbacks as most types of human tumor will not readily grow or metastasize from a subcutaneous ('ectopic') site of injection. In contrast, 'orthotopic' injection procedures will often enhance the tumorigenic and/or metastatic ability of tumor cell populations. An example of this is summarized in the context of human malignant melanoma where the effects of subcutaneous versus subdermal injection are compared. Despite the seeming subtle and minor change in injection site, superior growth of human melanomas can be obtained by the latter, orthotopic-like, route of injection. It therefore follows that induction of tumorigenic or metastatic properties in a given human cell population by gene transfection may not be detected if the transfected cells are assayed in vivo only by subcutaneous injection procedures. An example of this is provided by experiments involving transfection of normal or mutated ras genes into a low-grade, well-differentiated human bladder carcinoma cell line, called RT-4. Thus overexpression of normal or mutated (valine 12) c-H-ras resulted in acquisition of a clinical-like invasive phenotype. However, this was clearly seen only if the cells were injected into the bladders (i.e. 'intravesically') of nude mice. In contrast, conventional subcutaneous injection of the high ras expressing transfected RT-4 cell lines did not reveal acquisition of invasive properties: all cell lines grew locally as well-encapsulated tumor masses. It is argued that similar orthotopic injection procedures should be employed when assessing the suppressive effects of various wild-type tumor-suppressor genes on human tumor growth in vivo. Utilization of subcutaneous injection procedures may grossly exaggerate the growth suppressive effects of such genes. This could explain the paradox of why, on the one hand, alterations involving many different genes (including different suppressor genes) appear to be involved in human carcinoma tumorigenesis while on the other hand, complete suppression of tumorigenicity can be caused by transfer of a single wild-type suppressor gene. Such complete suppressions might be observed only after ectopic (usually subcutaneous) injection procedures.