a, MPP8 loss increased γH2A.X foci (green) relative to control (sgNT) in AML cells, whereas 3TC treatment (10 µM for 3 days) abrogated the DNA damage response. b, Quantification of γH2A.X foci in control and MPP8-deficient AML cells with vehicle or 10 µM of 3TC. Results are mean ± SD (N = 20 cells over 3 independent experiments) and analyzed by a two-way ANOVA with Dunnett’s test. c, L1 reactivation increased γH2A.X foci in AML cells, which were abrogated by 3TC treatment (10 µM for 3 days). d, Quantification of γH2A.X in control and L1-reactivated AML cells. Results are mean ± SD (N = 20 cells over 3 experiments) and analyzed by a two-way ANOVA with Dunnett’s test. e, MPP8 loss increased p21 expression in MOLM-13 and OCI-AML3 cells 3 days after Dox-induced MPP8 KO. f, L1 reactivation increased p21 expression in AML cells 3 days after transduction of sgRNAs. g, Quantification of γH2A.X by intracellular staining of MLL-AF9-transformed AML cells. The mean % of γH2A.X-positive cells from 3 independent mice is shown. P values by a two-sided t test. h, Quantification of p21 by intracellular staining. The mean % of p21-positive cells from 3 independent mice is shown. P values by a two-sided t test. i, Kaplan-Meier survival curves of recipient mice after transplantation of AE9a-transformed BM lineage-negative cells from WT (N = 7), MPP8−/− (N = 9), or MPP8−/−p21−/− (N = 7) mice. P values by a log-rank Mantel-Cox test. Leukemia burden in PB, BM and spleen at day 60 post-transplantation is shown. Results are mean ± SEM (N = 5 mice) and analyzed by a two-way ANOVA with Tukey’s test. j, Kaplan-Meier survival curves of recipients engrafted with AE9a-transformed AML cells. N = 7, 10, and 7 WT, Mx1-Cre+;MPP8f/f, and Mx1-Cre+;MPP8f/f;p21−/− mice, respectively. P values by a log-rank Mantel-Cox test. Leukemia burden at day 90 is shown. Results are mean ± SEM (N = 5 mice) and analyzed by a two-way ANOVA with Tukey’s test. k, Schematic of the working model.