Genetic predisposition of transgenic mouse melanocytes to melanoma results in malignant melanoma after exposure to a low ultraviolet B intensity nontumorigenic for normal melanocytes.

Tyr-SV40E transgenic mice are susceptible to melanoma due to simian virus 40 oncogenic sequences specifically expressed in pigment cells. Skin melanomas form relatively late. Therefore, melanocyte cell lines have been established from very young transgenic animals, when they showed no skin lesions, so that the spontaneous and gradual progress of the cells toward tumorigenesis could be characterized under culture conditions in which wild-type cells of the same inbred strain remain untransformed. Melanocytes of an in vitro transgenic line were irradiated with very low intensities of ultraviolet B (UVB) (280- to 320-nm wavelength) light at culture passages when the cells had not achieved anchorage independence. After a single exposure to 0.7 mJ/cm2 of UVB radiation, the cells became anchorage independent and formed foci at confluence; however, cells propagated from the foci were not tumorigenic. After one exposure to 1.75 mJ/cm2, more numerous and larger foci resulted, and the cells grown from them yielded malignant melanomas in graft hosts. Wild-type melanocytes were not transformed at these UVB doses. At least two genetic changes contributing to malignant conversion--in addition to the initiating effect of the transgene--are likely to have occurred, one change leading to anchorage independence and another to further progress toward malignancy. Cells at these stages provide an opportunity to isolate the relevant genes and identify any molecular defects attributable to UVB. Tumorigenesis after a very low UVB dose in cells where an initiating stimulus is already present suggests that some other stimulus, such as a gene or a carcinogen, might lead to melanoma in conjunction with exposure to relatively little UVB. IMAGES:

Tyr-SV40E转基因小鼠（transgenic mice）因在色素细胞（pigment cells）中特异性表达猿猴病毒40致癌序列（simian virus 40 oncogenic sequences），易罹患黑色素瘤。其皮肤黑色素瘤的形成相对较晚。因此，研究人员从尚未出现皮肤病变的极年轻转基因动物体内分离建立了黑色素细胞系（melanocyte cell lines），以便在相同近交系（inbred strain）野生型细胞未发生转化的培养条件下，表征该细胞自发且逐步向肿瘤发生发展的动态过程。
在细胞尚未获得锚定非依赖性（anchorage independence）的培养传代阶段，对该体外转基因系的黑色素细胞施以极低强度的紫外线B（ultraviolet B, UVB，波长范围280~320 nm）辐射。单次暴露于0.7 mJ/cm²的UVB辐射后，细胞获得锚定非依赖性，并在汇合培养状态下形成细胞集落；但从该集落扩增得到的细胞并不具备致瘤性。单次暴露于1.75 mJ/cm²的UVB辐射后，可形成更多、体积更大的细胞集落，且从这些集落培养得到的细胞可在异体移植宿主中诱发恶性黑色素瘤。在上述UVB辐射剂量下，野生型黑色素细胞并未发生转化。
除转基因带来的起始致癌效应外，至少存在两种与恶性转化相关的遗传改变：其一可介导锚定非依赖性的获得，其二则推动细胞进一步向恶性表型进展。处于上述不同阶段的细胞为分离相关功能基因、鉴定归因于UVB暴露的分子缺陷提供了绝佳的研究模型。在已存在致癌起始刺激的细胞中，极低剂量UVB即可诱发肿瘤发生，这提示其他刺激因素（如特定基因或致癌物）或可与相对少量的UVB暴露协同作用，进而诱发黑色素瘤。
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