Transgenic shuttle vector assays for determining genetic differences in oxidative B cell mutagenesis in vivo

Selenium Deficiency Abrogates Inflammation-Dependent Plasma Cell Tumors in Mice

The role of the micronutrient, selenium, in human cancers associated with chronic inflammations and persistent infections is poorly understood. Peritoneal plasmacytomas (PCTs) in strain BALB/c (C), the premier experimental model of inflammation-dependent plasma cell transformation in mice, may afford an opportunity to gain additional insights into the significance of selenium in neoplastic development. Here, we report that selenium-depleted C mice (n = 32) maintained on a torula-based low-selenium diet (5-8 micro g of selenium/kg) were totally refractory to pristane induction of PCT. In contrast, 11 of 26 (42.3%) control mice maintained on a selenium adequate torula diet (300 micro g of selenium/kg) and 15 of 40 (37.5%) control mice fed standard Purina chow (440 micro g of selenium/kg) developed PCT by 275 days postpristane. Abrogation of PCT was caused in part by the striking inhibition of the formation of the inflammatory tissue in which PCT develop (pristane granuloma). This was associated with the reduced responsiveness of selenium-deficient inflammatory cells (monocytes and neutrophils) to chemoattractants, such as thioredoxin and chemokines. Selenium-deficient C mice exhibited little evidence of disturbed redox homeostasis and increased mutant frequency of a transgenic lacZ reporter gene in vivo. These findings implicate selenium, via the selenoproteins, in the promotion of inflammation-induced PCT and suggest that small drug inhibitors of selenoproteins might be useful for preventing human cancers linked with chronic inflammations and persistent infections.

Moderate Hypermutability of a TransgeniclacZReporter Gene inMyc-Dependent Inflammation-Induced Plasma Cell Tumors in Mice

Mutator phenotypes, a common and largely unexplained attribute of human cancer, might be better understood in mouse tumors containing reporter genes for accurate mutation enumeration and analysis. Previous work on peritoneal plasmacytomas (PCTs) in mice suggested that PCTs have a mutator phenotype caused by Myc-deregulating chromosomal translocations and/or phagocyte-induced mutagenesis due to chronic inflammation. To investigate this hypothesis, we generated PCTs that harbored the transgenic shuttle vector, pUR288, with a lacZ reporter gene for the assessment of mutations in vivo. PCTs exhibited a 5.5 times higher mutant frequency in lacZ (40.3 +/- 5.1 x 10(-5)) than in normal B cells (7.36 +/- 0.77 x 10(-5)), demonstrating that the tumors exhibit the phenotype of increased mutability. Studies on lacZ mutant frequency in serially transplanted PCTs and phagocyte-induced lacZ mutations in B cells in vitro indicated that mutant levels in tumors are not determined by exogenous damage inflicted by inflammatory cells. In vitro studies with a newly developed transgenic model of inducible Myc expression (Tet-off/MYC) showed that deregulated Myc sensitizes B cells to chemically induced mutations, but does not cause, on its own, mutations in lacZ. These findings suggested that the hypermutability of PCT is governed mainly by intrinsic features of tumor cells, not by deregulated Myc or chronic inflammation.

Bcl-2 reduces mutant rates in a transgenic lacZ reporter gene in mouse pre-B lymphocytes

To assess mutagenesis during early B-lymphocyte development in vitro, progenitor B cells (pre-B cells) were obtained from fetal livers of BALB/c mice and DBA/2N mice that harbored the transgenic shuttle vector, pUR288, with a lacZ reporter gene for the determination of mutant frequencies (MFs). Differentiation-arrested pre-B cells demonstrated a marked dose-dependent increase in lacZ mutant levels after exposure to gamma-irradiation with a peak MF of 250 x 10(-5) at 2.5 Gy. Without genotoxic treatment, pre-B cells undergoing spontaneous differentiation into surface IgM expressing immature B cells exhibited lacZ mutant levels of up to 95 x 10(-5). The mutational pattern was dominated in both experiments by illegitimate recombination mutations of lacZ, not point mutations. Likewise, in both experiments, the enforced expression of Bcl-2 resulted in a striking reduction of lacZ mutations. These findings indicated that mouse pre-B cells are prone to accumulate induced and self-inflicted mutations, particularly recombinations. Additionally, our studies revealed a heretofore unknown role of Bcl-2 in inhibiting mutagenesis during early B-cell development in mice.