Ott1(Rbm15)-Deficient Hematopoietic Stem Cells Are Unable to Maintain Quiescence During Replicative Stress and Display Features of Premature Aging,
Authors: Xiao Nan , Jani Kaushal , Jesneck Jonathan L , Raffel Glen D 
Published: October 2019

Abstract
Abstract 3433
With age, hematopoietic stem cells (HSCs) have numerical expansion, skewing towards myeloid development, loss of lymphoid potential, an underlying pro-inflammatory state and loss of self-renewal potential thus severely limiting responses to hematopoietic stress, ultimately leading to bone marrow failure. The mechanisms and pathways responsible for these changes in aged HSCs are incompletely understood. Using a conditional allele of Ott1, a gene originally isolated as the 5' fusion partner in t(1;22) acute megakaryocytic leukemia, we previously found a global regulatory role for the gene in hematopoiesis. Deletion of Ott1 in adult mice utilizing Mx1-cre recapitulated certain aspects of aging hematopoiesis including increased Lin−Sca1+c-Kit+ (LSK) population, myeloid expansion and decreased lymphopoiesis. The LSK compartment was further characterized using SLAM and CD34/Flk2 markers and demonstrated normal levels of LT-HSCs and increased ST-HSCs. Despite sufficient LT-HSC numbers, Ott1-deleted bone marrow was unable to competitively or non-competitively repopulate irradiated recipients. To exclude a homing or engraftment effect, Ott1flox/null Mx1-cre bone marrow was transplanted with competitor then excised post-engraftment. The rapid loss of the Ott1-deficient graft demonstrated Ott1 is required for maintenance under competitive stress. In contrast, primary mice undergoing Ott1 excision lived a normal lifespan and were able to maintain sufficient hematopoiesis although with a partial reduction in bone marrow clonagenicity showing loss of Ott1 is not limiting under steady state conditions. To test the HSC requirement for Ott1 under replicative stress, Ott1 knockout mice were challenged with 5-fluorouracil (5-FU). Ott1-deleted mice treated with 5-FU displayed delayed peripheral blood neutrophil recovery and showed accelerated bone marrow failure. Cell cycle analysis of steady state Ott1 knockout HSCs showed a similar profile to wild type controls, however, after 5-FU treatment, the G0 fraction was dramatically reduced. The G0 fraction is associated with the quiescent, self-renewing HSC population, therefore, Ott1 is required for maintaining HSC quiescence during replicative stress but not steady state hematopoiesis.
To more specifically assess whether the functional hematopoietic changes seen after loss of Ott1 were accompanied by alterations in known aging-associated pathways, Gene Set Enrichment Analysis comparing Ott1-deleted HSCs in steady state to aged HSCs was performed and showed a highly enriched gene expression signature (NES 2.02 p<0.0001). Physiologic sequelae of HSC aging were observed after Ott1 excision including activation of NFκβ, elevation of reactive oxygen species (ROS), increase in DNA damage (γH2A.X levels) and activation of p38Mapk. Although ROS was elevated under steady state conditions, neither apoptosis, senescence or proliferation was significantly different from wild type control HSCs. Furthermore, anti-oxidant treatment with N-acetyl-cysteine was unable to rescue the HSC maintenance defect of the Ott1 knockout, signifying additional requirements in HSCs for Ott1 beyond regulation of ROS. An observed increase of mitochondrial mass in Ott1-deleted HSCs suggests an upstream function for Ott1 in metabolic control, potentially contributing to ROS generation or degradation. In summary, we have demonstrated an essential role for Ott1 in maintaining HSC quiescence during replicative stress and shown loss of Ott1 leads to the acquisition of key gene expression patterns and pathophysiologic changes associated with aging. These data suggest Ott1 functions in part to oppose specific consequences of aging in the hematopoietic compartment. Ott1 and Ott1-dependent pathways therefore represent a potential therapeutic target to prevent the morbidity and mortality arising from age-related defects in hematopoiesis.

Disclosures:
No relevant conflicts of interest to declare.