Abstract
Thymic lymphoma is very common in mouse malignancies and provides a model to delineate aberrant cells at the early precancerous stage. The existence of prelymphoma cells or lymphoma-initiating cells in γ-ray induced mouse atrophic thymus has been noted by the finding that transplantation of thymocytes in atrophic thymus can produce thymic lymphomas. Importance of leukemia/lymphoma-initiating cells is underlined in the analysis of relapsed acute lymphoblastic leukemia (CML) in humans, which showed that the cells responsible for relapse are ancestral to the primary leukemia cells. Here we study phenotypic and genetic changes of the probable prelymphoma cells in atrophic thymus. Also, we investigate changes in DNA damage checkpoint because a hallmark of precancerous cells in major human cancer types is aberrant stimulation of cell proliferation and the subsequent activation of DNA damage checkpoint. For atrophic thymuses at 40 and 80 days after fractionated whole-body γ-irradiation to mice, we examined clonality by assaying D-J rearrangement patterns at the TCRβ locus. Clonally expanded thymocytes (designated as C type) were found in approximately 40% (43/111 in 40 day thymus and 21/45 in 80 day thymus), while others exhibited similar D-J rearrangement patterns to normal thymus (T type). The C type thymocytes mostly consisted of CD4+CD8+ double positive (DP) cells despitess clonal expansion, suggesting that the thymocytes are aberrant DP cells having passed b-selection. C type thymocytes exhibited pausing at an early G1 phase of cell cycle progression but did not show the activation of DNA damage checkpoints such as γHA2X, Chk1/2, or p53. This is unexpected because the checkpoint activation is assumed to function as an inducible barrier against g-ray induced genomic instability. Of interest, is that 17 of the 52 Ttype thymuses at 40days after showed allelic loss at Bcl11b tumor suppressor locus. This suggest that allelic loss contributes to clonal expansion of the thymocytes that still possess the capacity to differentiate. These results suggest that formation of prelymphoma cells probably requires changes in two distinct steps, cell proliferation and differentioation arrest, as described in human CML and lymphoma malignancies.
