UV-induced melanoma: A karyotype with a single translocation is stable after allografting and metastasis

Metastatic melanoma cell lines were derived from a lymph node of a laboratory opossum, Monodelphis domestica, which had been exposed to mid-wavelength ultravillet radiation (UVB) initially as a suckling young, and subsequently as a shaved juvenile and adult. The melanoma cell lines were dendritic and pigmented in vitro and contained a t(6;8)(p13;q13) as the only cytogenetic abnormality. The translocation was detected in 15% of primary cultures (passage 2) from the affected lymph node and in 100% of two ring-clone-derived lines, L1 and L2. The breakpoint or resulting partial trisomy of chromosomes 8 may have played a functional role in the tumorigenesis or metastasis of the tumor. The t(6;8) served as a convenient cytogenetic marker for allogeneic grafting studies in Monodelphis. The L2 cells were allografted subcutaneously (s.c.) into genetically diverse suckling young at 3 weeks of age and resulted in the growth of invasive, pigmented, primary and metastatic lesions affecting lymph nodes, lung, and other tissues. Metastatic variant cell lines, M1 and M3, were derived from the affected lungs of two animals and both lines demonstrated the same t(6;8), without additional numerical or structural chromosomal abnormalities. The maintenance of karyotypic stability with a single translocation during in vivo tumor growth and dissemination in this new allografting model is quite remarkable, as most human metastatic melanomas exhibit multiple structural and numerical cytogenetic abnormalities.