Abstract LB555: Augmented Txnip-p53-p21 axis preserves Dnmt3a mutant hematopoietic stem cells during inflammatory stress
Authors: Zhang Christine R. , Ostrander Elizabeth , Koh Won Kyun , kukhar Ostap , Celik Hamza , Magee Jeffrey , Challen Grant 
Published: June 2022

Abstract
Clonal hematopoiesis (CH), the age-related expansion of specific clones in the blood system has been considered as a pre-malignant condition of blood cancers. Approximately 50% of CH manifests from DNMT3A mutations acquired in the hematopoietic stem cells (HSCs), suggesting these mutations may convey a fitness advantage to HSCs during hematopoietic stress. However, a unifying molecular mechanism to illuminate how DNMT3A-mutant HSCs outcompete their counterparts is still lacking. Here, we used interferon-gamma (IFNg) as a model to study the mechanisms by which Dnmt3a mutations enhances HSC fitness under recurrent inflammatory stress.
To represent the spectrum of DNMT3A variants found in humans, mouse genetic models were generated by specifically deleting Dnmt3a heterozygously (Vav-Cre;Dnmt3afl/+ = Dnmt3aHET) or homozygously (Vav-Cre;Dnmt3afl/fl = Dnmt3aKO) from the hematopoietic system. A hematopoietic system-specific knockin model analogous to the hotspot point mutation most prevalent in AML (Vav-Cre;Dnmt3aR878H/+ = Dnmt3aR878) was also included in the analysis.
Competitive transplantation assays coupled with inflammatory and proliferative challenges suggested that Dnmt3aKO and Dnmt3aR878 HSCs were specifically tolerant of the deleterious effects of IFNg on HSC self-renewal and clonal expansion. When the competition between Dnmt3a-mutant and control HSCs under IFNg exposure was directly quantified in a novel mouse model, we found Dnmt3a-mutant HSCs resisted IFNg-mediated depletion due to an enhanced fitness advantage. Mechanistically, DNA hypomethylation-associated over-expression of Txnip in Dnmt3a-mutant HSCs was identified by coupling single-cell RNA-sequencing and Whole-Genome Bisulfite sequencing. The sustained Txnip levels in Dnmt3aKO HSCs lead to p53 stabilization and upregulation of p21 under IFNg challenge, further correlated with a retained quiescence due to a prolonged G0 exit in response to IFNg exposure. Implementing biochemical studies, we observed Txnip mediated an enrichment of p53 at p21 promoter under IFNg exposure in Dnmt3aKO but not control 32D cells. Knocking down Txnip by shRNA normalized p53 occupancy at p21 promoter and rescued IFNg-associated p21 upregulation in Dnmt3aKO 32D cells. Functionally, knocking down Txnip and p21 sensitized Dnmt3aKO HSCs to IFNg-induced cell cycle activation ex vivo. In vivo, down-regulation of p21 had no effect on control HSCs in response to IFNg exposure, but it completely primed Dnmt3aKO and Dnmt3aR878 HSCs to IFNg-associated exhaustion in a transplantation experiment.
In summary, our data suggest an augmented Txnip-p53-p21 axis preserves the functional potential of Dnmt3a-mutant HSCs under inflammatory stress, which highlights a novel mechanism to explain the increased fitness of Dnmt3a-mutant HSCs and supports rationale for developing interventions to mitigate expansion of pre-malignant clones as a method of blood cancer prevention.
Citation Format: Christine R. Zhang, Elizabeth Ostrander, Won Kyun Koh, Ostap kukhar, Hamza Celik, Jeffrey Magee, Grant Challen. Augmented Txnip-p53-p21 axis preserves Dnmt3a mutant hematopoietic stem cells during inflammatory stress [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB555.