Clonal origin of tumors induced by ultraviolet radiation

Skin tumors were induced in (C3H/HeN X C3H/HeN-PGK-1a)F1 female mice, heterozygous at the X-linked phosphoglycerate kinase-1 (PGK-1) locus, by exposure to the carcinogenic influence of ultraviolet radiation (UVR), 3-methylcholanthrene [(MCA) CAS: 56-49-5], or benz[a]pyrene [(BP) CAS: 50-32-8]. An assessment of the clonal origin of these tumors was accomplished through an analysis of the PGK-1 enzyme phenotype expressed by the transformed cells. In vitro culture was employed as a means of depleting nontransformed cells of host origin from the induced tumors. Cultured lines derived from tumors induced by each of the above agents were found to express only one of the two enzyme forms encoded by the host genotype, consistent with the probability that all the UVR-, MCA-, and BP-induced tumors examined in this study were monoclonal in origin. For further substantiation of the monoclonality of UVR-induced tumors, 2 UVR-induced tumors were enzymatically dissociated immediately following excision from the primary hosts, and the resulting cell suspensions were cloned in soft agar. Upon analysis, each set of clones selected in soft agar expressed only a single PGK-1 enzyme form. To rule out the possibility that the apparent monoclonality of culture-adapted tumor lines was due to in vitro selection, tumors that arose in UVR-treated PGK-1a/b female heterozygote mice were transplanted into PGK-1a and PGK-1b homozygous recipients. These transplanted tumors expressed a single PGK-1 allozyme following growth in recipients that were genetically homozygous for the major PGK-1 enzyme form expressed by the tumor prior to transplantation. These data strongly support the concept that most, if not all, UVR-induced tumors are derived from the progeny of a single transformed cell. This observation is important to the understanding of the nature of tumor-specific transplantation antigens expressed by individual UVR-induced tumors and indicates that such antigens also are clone specific. In addition, the results of this study indicate that polyclonality does not play a significant role in the generation of cellular and phenotypic heterogeneity known to be present within individual UVR-induced tumors.