Clonal dominance of RadLV-induced lymphomas

Clonal growth of colorectal-carcinoma cell lines transplanted to nude mice

It is generally assumed that tumor progression is a microevolutionary process in which increasingly aggressive clones, generated through genetic instability, emerge in an initially monoclonal lesion. The present study was undertaken to determine how rapidly a dominant clone will emerge from an initial polyclonal situation, and whether dominance of these clones is a prerequisite for the onset of metastasis. To this end, colon-carcinoma cells were infected in culture with an amphotropic retroviral vector containing the neomycin-phosphotransferase gene, which makes cells resistant to neomycin. A heterogeneous population of neomycin-resistant cells carrying random retroviral integrations was xenografted to the subcutis and to the cecum of nude mice. The xenografts obtained, as well as the available metastases, were analyzed as to viral integrations by Southern blotting. The results show that, (i) clonal selection already takes place during growth of the primary tumor; (ii) dominant clones also generate metastases. The retroviral integration pattern of metastases turned out to be identical to that found in the primary xenografts. This pattern remained unchanged in tumors obtained after serial transplantations of cells cultured from metastases.

Clonal evolution of N-methylnitrosourea-induced C57BL/6J thymic lymphomas by analysis of multiple genetic alterations

C57BL/6J mice were treated with N-methylnitrosourea (NMU) and the evolution of leukemic T-cells clones into frank thymic lymphomas was followed in 42 animals using serology of T-cell markers, rearrangements of the T-cell receptor genes gamma, beta1 and beta2 and detection of carcinogen-induced Ki-ras mutations and trisomy of chromosome 15. During the latent period, multiple populations of T-cell clones were present in the thymus, many contained trisomy 15, but few had detectable Ki-ras mutations. Since most frank lymphomas consisted of a single T-cell clone with both a mutation of Ki-ras and trisomy 15, the results imply that these two events are critical for the evolution of T-cell clones from the preleukemic phase to a more malignant disease stage. Progression to frank lymphomas is coincident with changes in the expression pattern of the T-cell growth factor interleukin-2 receptor, which may play a role in the selection, expansion and thymus-independent growth of a T-cell clone.