cDNA‐library testing identifies transforming genes cooperating with c‐myc in mouse pre‐B cells

While c‐myc often contributes to the generation of B cell transformation, its transgenic overexpression alone does not lead to full transformation of B‐lineage cells. Synergistically acting second genes must cooperate. Here, we constructed doxycycline‐inducible cDNA‐libraries from pre‐B cell mRNA. These libraries were retrovirally transduced as single copies into single cells and overexpressed in fetal‐liver‐derived c‐myc‐overexpressing pre‐B cell lines. We scored transformation by survival and/or expansion of differentiating B‐lineage cells in vitro and in vivo. Only one double c‐myc/cDNA‐library‐expressing cell line was found in less than 5 × 10⁶ library‐transduced pre‐B cells surviving and expressing a cDNA‐library‐derived transcript in vitro. This transcript was identified as a shortened form of the Exosc1 gene, encoding the RNA exosome complex component CSL4. Transplantations of double c‐myc/Exosc1 short‐form‐ or c‐myc/Exosc1 full‐length‐transgenic cells into Rag1^?/? mice resulted in survival, differentiation to CD19⁺CD93^?sIgM⁺CD5^low/?CD11b⁺ mature B1 cells and, surprisingly, also vigorous expansion in vivo. Strikingly, after transplantations of c‐myc/cDNA‐library pre‐BI cells the frequencies of double‐transgenic pre‐B cells and their differentiated progeny, expanding in vivo to heterogeneous phenotypes, was at least tenfold higher than in vitro. In a first analysis Ptprcap, Cacybp, Ndufs7, Rpl18a, and Rpl35a were identified. This suggests a strong influence of the host on B‐cell transformation.