Erg is required for self-renewal of hematopoietic stem cells during stress hematopoiesis in mice

Abstract

Hematopoietic stem cells (HSCs) are rare residents of the bone marrow responsible for the lifelong production of blood cells. Regulation of the balance between HSC self-renewal and differentiation is central to hematopoiesis, allowing precisely regulated generation of mature blood cells at steady state and expanded production at times of rapid need, as well as maintaining ongoing stem cell capacity. Erg, a member of the Ets family of transcription factors, is deregulated in cancers; and although Erg is known to be required for regulation of adult HSCs, its precise role has not been defined. We show here that, although heterozygosity for functional Erg is sufficient for adequate steady-state HSC maintenance, Erg ؉/Mld2 mutant mice exhibit impaired HSC self-renewal after bone marrow transplantation or during recovery from myelotoxic stress. Moreover, although mice functionally compromised for either Erg or Mpl, the receptor for thrombopoietin, a key regulator of HSC quiescence, maintained sufficient HSC activity to sustain hematopoiesis, Mpl ؊/؊ Erg ؉/Mld2 compound mutant mice displayed exacerbated stem cell deficiencies and bone marrow failure. Thus, Erg is a critical regulator of adult HSCs, essential for maintaining self-renewal at times of high HSC cycling. (Blood. 2011;118(9): 2454-2461) IntroductionAdult hematopoietic stem cells (HSCs) are a rare population of bone marrow cells responsible for the production of multiple functionally specialized lineages of blood cells. In addition to maintaining exquisitely regulated numbers of cells during the normal daily replacement of billions of aged or expended blood cells, HSCs are also critical for regeneration and maintenance of hematopoiesis in response to major hematologic stresses, such as after myelotoxic therapy or stem cell transplantation. Life-long maintenance of the blood cell system requires HSC self-renewal, a specialized cell division in which one or both daughter cells retain full long-term HSC pluripotency. Regulation of the appropriate balance between HSC self-renewal and differentiation is the essence of hematopoiesis, allowing production of sufficient effector blood cells while protecting sufficient pluripotential reserve capacity for future hematopoietic supply. Analysis of the effects of genetic mutation or gene dysregulation have defined diverse molecular regulators of physiologic and stress-induced HSC selfrenewal, including transcription factors, epigenetic regulators, cell cycle modulators, and cell extrinsic regulators and their intracellular signal transducers. 1,2 The E-twenty-six (ETS)-related gene (ERG, OMIM ID 165080) is a member of the ETS family of transcription factors. 3 A key role for ERG in cancer has emerged with the observation that up to 75% of prostate cancers are characterized by ERG deregulation. This commonly occurs via a chromosomal rearrangement that fuses the 5Ј untranslated region of the TMPRSS2 (androgen-regulated transmembrane protease, serine 2, OMIM ID 602060) gene with the 3Ј region of ERG, resulting in expression o...

Structural and dynamic studies of the transcription factor ERG reveal DNA binding is allosterically autoinhibited

“…Although ERG autoinhibition has been biochemically characterized in a recent study (29), our current report presents structures of ERG and characterization of the molecular mechanism of autoinhibition. The changes in ERG activity we identified here, ∼fivefold weaker binding in ERGi and threefold weaker binding in full-length ERG, are highly relevant in a biological context, because a change in ERG expression of as little as 50% is sufficient to alter angiogenesis (13), myeloproliferation (8), and maintenance of the hematopoietic stem cell population (7,12).…”

“…ERG in particular is critical for maintenance of the hematopoietic stem cell (HSC) population. In a mouse model, the HSC population cannot recover after being challenged by sublethal radiation in heterozygous ERG animals, whereas the loss of both functional copies of ERG is embryonic lethal (7). Overexpression of the ERG gene product, located on chromosome 21, has also been implicated in the myeloproliferative disorders frequently seen in Down Syndrome patients (8), has been shown to have a dedifferentiating effect (9), and confers greater invasiveness to cell lines (10,11).…”

Transposon mutagenesis reveals cooperation of ETS family transcription factors with signaling pathways in erythro-megakaryocytic leukemia

“…ETS proteins are a family of over 20 helix-loop-helix domain transcription factors with diverse biological roles, including important functions in hematopoiesis and leukemia. In normal hematopoiesis, Erg is required for hematopoietic stem cell self-renewal during times of high hematopoietic stem cell cycling (11)(12)(13). Whereas the ERG locus has been studied extensively in prostate cancer, in which translocation of ERG to the TMPRSS2 locus and expression of a TMPRSS2-ERG fusion oncoprotein occurs in the majority of cases (8), ERG is also known to be translocated to the fused in sarcoma/translocated in liposarcoma (FUS/TLS) locus in rare cases of acute myeloid leukemia (AML).…”

ERG promotes the maintenance of hematopoietic stem cells by restricting their differentiation

“…Erg Mld2/+ mice (i.e., heterozygous carriers of the mutant Erg allele) had slightly reduced numbers of immunophenotypical HSCs that appeared functionally compromised in transplantation experiments. In addition, follow-up work demonstrated that although Erg haploinsufficiency was compatible with lifelong HSC self-renewal, it strongly impaired stress hematopoiesis following exposure to myelotoxic treatment (Ng et al 2011). Collectively, these data suggest that ERG could be particularly important during the expansion of HSCs.…”

“…The E-twenty-six (ETS)-related gene (ERG) is a member of the ETS family of transcription factors, of which several, including PU.1, have been shown to play a role in HSC maintenance (Loughran et al 2008;Ng et al 2011). Erg was identified in a sensitized genetic screen as a gene involved in HSC function, and the molecular defect was assigned to a point mutation in the DNA-binding domain of ERG (Loughran et al 2008).…”