Author reply

Field: Biochemistry, Genetics and Molecular Biology (Life Sciences)

Pratesi and Tortoreto have made some interesting suggestions, including the use of cytogenetic techniques, tumor growth properties, and enzymatic assay for the identification of interspecies cell line contamination. They published an article in 1989 describing their experience of murine host cell transformation with human small cell lung carcinoma. We somehow missed citing this work in our 1998 report in Cancer. Their experience indicates that there are alterations in growth properties, with doubling time drastically decreasing in the transformed host cell tumor. They also suggest the use of a lactate dehydrogenase (LDH) isoenzyme assay for distinguishing between the human and the transformed host cells. In the many samples that we have analyzed since early 1970, we have observed that this phenomenon is limited to not only human cancer cells (breast, colon, osteosarcoma, prostate, brain, melanoma, and renal cell) and transformed human embryonic lung cells, but also tumors of other mammalian species, including melanomas of swine origin, marsupial skin tumor cells induced by ultraviolet light (our unpublished data), murine leukemia cells, and transformed rat cells injected into baby Syrian hamsters—all have the capability of host cell transformation.1-5 Furthermore, it is not always the case that only 100% transformed host cells are present. There could be a mixture of transformed host cells as well as the original injected human tumor cells in various proportions. It is, therefore, not easy to identify and differentiate the two different cell populations based on growth properties and isoenzyme patterns, especially when a situation such as 95% human cells and 5% noncycling mouse cells is present. We recently demonstrated that a human osteosarcoma xenograft in nude mice, when removed and cultured, contained all metaphases of human origin, but a total human DNA fluorescence in situ hybridization (FISH) analysis showed that a small percentage of noncycling murine interphase cells was also present.6 In this study we clearly demonstrated that conventional cytogenetics with banding was not sufficient alone for identifying interspecies cell line contamination. In this same tumor at a later passage, we found both mouse and human metaphases. Only at this stage would a visible change in growth properties be noticed or an isoenzyme assay be effective. We observed a similar phenomenon in 1974, in distinguishing between rat and human contaminated tumor cells that were received from Miami (our unpublished data). These dynamic changes occurring in the cell populations can be rapidly identified by conventional as well as molecular cytogenetics. However, conventional cytogenetics alone may fail to identify the nondividing murine host cells due to the lack of such metaphase spreads in the preparations.6 Our main intentions were to demonstrate that 1) murine host cells can be transformed in some cases by the tumor cells originally inoculated, and 2) this might be happening in some human cancer patients as well. It is, therefore, very important to analyze those metastatic human tumors that are either partially or completely nonregressing with conventional therapy and/or radiation therapy. We therefore recommend that human or nonmurine cancer cells that are to be xenographed into nude mice or rats be studied not only with conventional cytogenetics or isoenzyme (LDH) assay, as suggested by Pratesi and Tortoreto, but also by the molecular cytogenetic FISH technique using species specific total DNA probes. In this way, even a small percentage of nondividing cells of another species, if present, can be definitely identified. Sen Pathak Ph.D.*, Margit A. Nemeth M.S.*, Asha S. Multani Ph.D.*, * Department of Cancer Biology, The University of Texas M. D. Anderson, Cancer Center, Houston, Texas