Combined inactivation of H2ax and Artemis in p53-deficient thymocytes reduces the rate of mortality of mice from thymic lymphoma. (A) LAP and LAHP mice exhibit a block in thymocyte development. Shown is a representative anti-c-kit and anti-CD25 flow cytometric analysis of lineage-negative LAP and LAHP thymocytes. The percentage of cells within each quadrant is indicated. (B) p53 and H2ax/p53 are deleted in LAP and LAHP thymocytes. Southern blot analyses of p53 and H2ax status in genomic DNA isolated from thymocytes (Thy) or kidneys (Kid) of LAP or LAHP mice. Bands corresponding to the floxed (F) and deleted (Δ) alleles are indicated. (C) LAHP mice exhibit a lower rate of mortality from thymic lymphomas than LAP and LHP mice. Kaplan-Meier curves showing the percentage of tumor-free survival of LHP (n = 34), LAP (n = 38), and LAHP (n = 35) mice. A drop in the curve represents the death of an animal diagnosed at necropsy with thymic lymphomas (except for one LAP mouse, a sarcoma). Open circles represent the death of an animal resulting from prolapsed rectum, infections, or no apparent tumors at necropsy. Log-rank test: P(LAHP vs LHP) < .0001, P(LAHP vs LAP) = .0017, and P(LHP vs LAP) = .0062. (D) LAP and LAHP mice only develop TCR-β⁻ thymic lymphomas. Shown are anti-TCR-β and anti-CD4 and anti-CD8 flow cytometric analyses of LAP tumor 246 and LAHP tumor 215. The gates used to determine TCR-β, CD4, and CD8 status of tumors are shown. (E) LAP and LAHP tumors arise from the clonal expansion of single thymocytes. Southern blot analyses of Tcrβ rearrangements in HindIII-digested genomic DNA isolated from LAP and LAHP thymic lymphomas, as well as one LAHP tumor that spread into lymph nodes (no. 474 LN). Kidney (Kid) DNA from nontumor mice was used as controls. *Tumors of other genotypes (nos. 880 and 883, Lck-CreArtemis^+/−H2ax^+/floxp53^flox/flox; nos. 23 and 881, Lck-CreArtemis^+/−H2ax^+/+p53^flox/flox). The locations of bands corresponding to unrearranged germline (GL) Jβ1 and Jβ2 segments are indicated.