A new gene, designated KiSS-1, has been isolated from cells of malignant melanoma, in which metastatic potential was suppressed by the introduction of normal human chromosome 6. According to the research report in the December 4th Journal of the National Cancer Institute, KiSS-1 substantially suppresses the metastasis of melanoma in laboratory animals and may prove useful in the clinical setting for distinguishing metastatic from nonmetastatic melanomas.
In the report, Jeong-Hyung Lee, PhD, The Pennsylvania State University College of Medicine, and colleagues explain that tumor metastasis is now understood to be a multistep process involving complex interactions between tumor cells and host cells. To metastasize, say the authors, tumor cells must dissociate from the tumor mass, be transported to another site in the body, and establish themselves at that site, invading the surrounding tissue and responding to growth signals at the secondary site. Unless each step in this pathway is successfully accomplished, they state, metastases will not develop. Both positive and negative regulators exist for each step in this cascade of events, add Lee and coworkers, implicating the involvement of dozens of different genes.
In previous studies, Lee and colleagues found that introducing a normal human chromosome 6 into the highly metastatic human melanoma cell line C8161 almost entirely suppressed its metastatic properties, although the cells still were tumor-producing. This finding suggested that metastatic capability was the result of genetic changes occurring after the cells became tumorigenic, that at least some of these later mutations occurred on chromosome 6, and that one or more metastasis-suppressor genes might be located on chromosome 6 or regulated by genes on that chromosome.
Tumor-Suppressor Gene Isolated and Tested
In the current study, the researchers used a process known as subtractive hybridization to attempt to isolate the gene(s) in nonmalignant clones of C8161 cells responsible for suppressing metastasis. A previously unidentified gene, designated KiSS-1, was isolated and then introduced into unaltered (parental) malignant C8161 cells. Resulting clones expressing different levels of KiSS-1 were selected and each type was injected into separate groups of immune-deficient mice by two methods (either into the skin or directly into the circulatory system) designed to elicit spontaneous and induced (experimental) metastases, respectively. As controls, additional groups of mice were similarly injected with C8161 cells into which only the vector used to carry KiSS-1 was introduced.
In the test of spontaneous metastasis (measuring the ability of intradermally injected cells to metastasize to distant sites), the KiSS-1 clones were consistently less able to colonize lung or regional lymph nodes than the control C8161 cells. Parental C8161 cells resulted in an average of 50 lung metastases per mouse, and all mice had lymph node metastases. By contrast, the KiSS-1-expressing clones resulted in as few as one and no more than six lung metastases per animal and few instances of regional lymph node involvement.
In addition, the researchers tested various body tissues for the expression of messenger RNA (mRNA) by the KiSS-1 gene. KiSS-1 mRNA was abundant in placental tissue, and weak expression was found in the kidney. The authors note that KiSS-1 expression was also detected in normal human melanocytes, suggesting that it functions in normal melanoma precursor cells and that loss of the gene's expression may indicate greater potential for melanoma metastasis. Although the predicted protein product of KiSS-1 resembles protein products involved in certain cell signaling processes and/or cytoskeletal organization, the researchers caution that further study of the role of KiSS-1 in melanoma progression is needed. Additional experiments are also needed, they conclude, to determine whether KiSS-1 has a role in other cancers and whether it may be a useful marker for assessing melanoma progression in the clinical setting.
Translation of Findings to the Clinic Called Challenging
In an editorial accompanying this report, Isaiah J. Fidler, DVM, PhD, and Robert Radinsky, PhD, The University of Texas M. D. Anderson Cancer Center, Houston, observe that the identification of metastasis-suppressing genes can be expected to have far-reaching implications for diagnosing and treating disseminated cancer. But, say Fidler and Radinsky, the translation of these findings to clinical reality still faces serious challenges.
By the time of diagnosis, they say, many human tumors contain different types of cells with varying metastatic potential, which may limit the ability to predict a given tumor's metastatic potential using the expression level of metastasis-suppressing genes. Since metastasis can be produced from a small subpopulation of cells within the primary tumor, they add, the high expression level of metastasis-suppressing genes by the majority of the nonmetastatic cells in a primary tumor could mask a clinically relevant population of metastatic cells.
Moreover, say Fidler and Radinsky, detailed understanding of the function and regulation of metastasis-suppressing genes is needed to direct the development of therapeutic approaches designed to increase their expression. Despite these challenges, they maintain, a growing understanding of the molecular biology of cancer metastasis and the advent of new technologies offer unprecedented opportunities for inhibiting and treating cancer metastasis.