The DNA damage response (DDR) can restrain the ability of oncogenes to cause genomic instability and drive malignant transformation. The gene encoding the histone H2AX DDR factor maps to 11q23, a region frequently altered in human cancers. Since H2ax functions as a haploinsufficient suppressor of B lineage lymphomas with c-Myc amplification and/or translocation, we determined the impact of H2ax expression on the ability of deregulated c-Myc expression to cause genomic instability and drive transformation of B cells. Neither H2ax deficiency nor haploinsufficiency affected the rate of mortality of Eμ-c-Myc mice from B lineage lymphomas with genomic deletions and amplifications. Yet H2ax functioned in a dosage-dependent manner to prevent unbalanced translocations in Eμ-c-Myc tumors, demonstrating that H2ax functions in a haploinsufficient manner to suppress allelic imbalances and limit molecular heterogeneity within and among Eμ-c-Myc lymphomas. Regardless of H2ax copy number, all Eμ-c-Myc tumors contained identical amplification of chromosome 19 sequences spanning 20 genes. Many of these genes encode proteins with tumor-promoting activities, including Cd274, which encodes the PD-L1 programmed death ligand that induces T cell apoptosis and enables cancer cells to escape immune surveillance. This amplicon was in non-malignant B and T cells and non-lymphoid cells, linked to the Eμ-c-Myc transgene, and associated with overexpression of PD-L1 on non-malignant B cells. Our data demonstrate that, in addition to deregulated c-Myc expression, non-malignant B lineage lymphocytes of Eμ-c-Myc transgenic mice may have constitutive amplification and increased expression of other tumor-promoting genes.
Keywords: DNA damage response; Eμ-c-Myc transgenic mice; genomic instability; histone H2AX; lymphoma.