We continued our studies on reciprocal chromosomal translocations that deregulate the proto-oncogene c-myc in BALB/c plasmacytomas and their precursor cells. Applying a newly developed, ultrasensitive method of nested PCR with hybridization-enriched templates, Dr. Jurgen R. Muller was able to show that interchromosomal c-myc-deregulating recombinations occur in B cells of both plasmacytoma-susceptible mice, such as BALB/cAnPt, and plasmacytoma-resistant mice, such as DBA/2N (Cancer Res. 56, 419-423, 1996). This finding led us to conclude that the molecular machinery for generating B-cell specific, c-myc-deregulating recombinations can probably be found in many strains of mice and that the genetic susceptibility of BALB/c to the development of plasmacytoma must be determined at a later stage of B-cell transformation, following constitutive activation of c-myc. Dr. Alexander L. Kovalchuk analyzed the genetic recombination between Ig mu and c-myc in an uncommon primary BALB/c plasmacytoma, TEPC 1194, and used the result of the study to propose the hypothesis that recombinations between Ig mu and c-myc can serve as the molecular precursors for plasmacytoma-typical recombinations between Igalpha and c-myc (Immunogenetics 44, 151-156, 1996). To complement our PCR-based studies by molecular cytogenetics, we established a collaboration with Dr. Thomas Ried, allowing us to employ a recently developed chromosome painting method that provides a multicolor spectral karyotype of the entire mouse chromosome complement. Mr. Allen E. Coleman from our laboratory used spectral karyotyping to study mouse plasmacytomas and observed consistent chromosomal aberrations involving the distal region of chromosome 4 in four tumors. This result may provide an important lead, since the distal part of chromosome 4 is known to contain at least two plasmacytoma susceptibility loci, Ptcr1 and Ptcr2. Another collaborative effort has been initiated with Dr. Gerald E. Marti and is aimed at using flow cytometry to elucidate the long standing question of when during plasmacytoma development tetraploidization takes place. Experiments have been started to search for abnormal, aneuploid B cells in the spleen and peritoneal cavity of pristane-primed BALB/c mice and to determine the ploidy status of atypical plasma cells in plasmacytic foci. Another major project in our lab is directed at clarifying whether pristane-induced BALB/c plasmacytomagenesis takes place in a mutagenic environment. To address this problem, Dr. Klaus Felix chose to employ a new in vivo mutagenesis assay that is based on a phage lambda-derived lacI/alpha-lacZ transgene. Since we want to study mutation frequencies in B cells on a genetic background that is conducive for plasmacytoma research, he backcrossed the lambda-lacI/alpha-lacZ transgene onto BALB/cAnPt and DBA/2N. The congenic mice are currently being used to determine the mutagenicity of pristane, the DNA damage in B cells caused by neighboring inflammatory cells, and, in collaboration with Dr. Georg W. Bornkamm, the increase in the rate of mutations in B cells that are compromised in their antioxidative defense due to depletion of intracellular glutathione.
