Mutant ASXL1 Promotes Expansion of the Phenotypic Hematopoietic Stem Cell Compartment

Somatic mutations of the ASXL1 gene are recurrently detected in age-related clonal hematopoiesis (CH). However, how ASXL1 mutations causes CH are not understood. Here, using knockin (KI) mice expressing a C-terminally truncated form of ASXL1-mutant (ASXL1-MT), we investigated the effect of ASXL1-MT on physiological aging in hematopoietic stem cells (HSCs).

　To examine the influence of ASXL1-MT on hematopoiesis, we bred the ASXL1-MT-KI mice with Vav-Cre transgenic mice. Young ASXL1-MT-KI mice (6-12 weeks) did not show significant changes in hematological parameters and differentiation status of peripheral blood. We observed the decreased frequency of hematopoietic stem and progenitor cells (HSPCs), including long-term HSCs (LT-HSCs). Competitive transplantation assays showed the reduced repopulation ability in ASXL1-MT-KI HSPCs. Thus, ASXL1-MT decreased the number and impaired the function of HSPCs in young mice.

　Next, we examined age-related changes in hematopoiesis caused by ASXL1-MT. Aged ASXL1-MT-KI mice displayed a myeloid-biased differentiation and hypocellular bone marrow, indicating the dysfunction of hematopoiesis. Interestingly, ASXL1-MT markedly increased the frequency of phenotypic LT-HSCs (pLT-HSCs) in aged mice (20-24 months). Competitive transplantation assays showed the impaired repopulation potential of pLT-HSCs from aged ASXL1-MT-KI mice. These data demonstrate that the increased pLT-HSCs in aged ASXL1-MT-KI mice are not functional HSCs with long-term repopulation potential.

　To elucidate how ASXL1-MT drives HSPC dysfunction, we conducted RNA-Seq analysis using HSPCs from young mice. This analysis revealed upregulation of mitochondrial genes in ASXL1-MT-KI HSPCs. In addition, MitoTracker staining, extracellular flux analyses and metabolome analyses demonstrated the enhanced mitochondrial metabolism in ASXL1-MT-KI HSPCs. We also found that the aberrantly elevated mitochondrial activity induced ROS overproduction and increased DNA damage, resulting in HSPC dysfunction.

　As a mechanism underlying the enhanced mitochondrial activity of ASXK1-MT-KI HSPCs, we revealed that ASXL1-MT activated the Akt/mTOR pathway in HSPCs. Treatment with an Akt inhibitor perifosine or an mTOR inhibitor rapamycin normalized the mitochondrial membrane potential and ROS levels in ASXL1-MT-KI HSPCs. Moreover, rapamycin treatment improved engraftment of ASXL1-MT-KI bone marrow cells after transplantation. These data indicate that the activated Akt/mTOR signaling leads to the enhanced mitochondrial activity, elevated ROS levels, and HSPC dysfunction in ASXL1-MT-KI mice.

　To assess the impact of the enhanced Akt/mTOR signaling on age-related changes in ASXL1-MT-KI mice, we administered rapamycin to aged ASXL1-MT-KI mice. Intriguingly, rapamycin treatment decreased the frequency of pLT-HSCs, and normalized the bone marrow cellularity in aged ASXL1-MT-KI mice. Cell cycle analysis revealed that pLT-HSCs in G0 phase were decreased in aged ASXL1-MT-KI mice, which was normalized by rapamycin treatment. These data demonstrate that the activated Akt/mTOR pathway provokes the aberrant expansion of pLT-HSCs in aged ASXL1-MT-KI mice.

　We next attempted to clarify the underlying mechanism of Akt activation in ASXL1-MT-KI mice. Immunoprecipitation experiments revealed that ASXL1-MT/BAP1 complex deubiquitinated AKT in 293T cells. To determine the role of endogenous Bap1 on Akt signaling, we assessed the effect of Bap1 deletion in murine bone marrow cells transformed by combined expression of SETBP1-D868N and ASXL1-MT (cSAM cells). A time course experiments showed Akt phosphorylation induced by IL-3 stimulation was attenuated and shortened in Bap1-depleted cSAM cells. These data suggest that ASXL1-MT/BAP1 complex deubiquitinate and stabilize phosphorylated Akt.

　In summary, we demonstrated that ASXL1-MT cooperated with BAP1 to promote AKT deubiquitination and activation. The activated Akt/mTOR pathway led to enhanced mitochondrial metabolism, elevated ROS levels and increased DNA damage. These molecular bases underlie the age-associated expansion of the pLT-HSC compartment. Our results underscore the possibility that CH can originate from a pLT-HSC with a limited repopulation potential. A pharmacological inhibition of the Akt/mTOR pathway could be a promising therapeutic intervention to individuals with CH harboring ASXL1 mutations.