HSCs: they can’t live without their SINs

In this issue of Blood, Cantor and David pinpoint Sin3B as an essential factor that regulates quiescence of long-term hematopoietic stem cells (LT-HSCs) and is required for their differentiation. The authors show that, under conditions of replicative stress, conditional deletion of Sin3B leads to an accumulation of LT-HSCs and a concomitant loss of differentiated blood cell types.¹ 

The capacity of LT-HSCs to undergo self-renewal upon cell division is a key trait that ensures a continuous output of various differentiated hematopoietic cell types in the blood system over the life span of each individual.²  Sudden changes in blood cell homeostasis (ie, as a consequence of inflammation, blood loss, or treatment with chemotherapeutics) require an immediate response from the LT-HSC compartment by switching from a quiescent to a proliferative state, to adapt to the new situation appropriately. The study presented here proposes a role for the transcription regulatory protein Sin3B in attenuating LT-HSC behavior under stress conditions where activation of LT-HSCs is required.

Sin3B belongs to a family of 2 proteins (Sin3A and Sin3B) that previously were shown to reside in the Sin3–histone deacetylase (HDAC) complex and are involved in transcriptional repression of target genes.³  Despite sharing 55% identity, murine knockout models suggested that both proteins have functionally diverged over the course of evolution. Whereas deletion of Sin3A caused early embryonic lethality, Sin3B deletion induced late embryonic lethality and impaired differentiation of, among others, erythroid and granulocytic progenitors.^4,5  Interestingly, conditional deletion of Sin3A in the bone marrow, using an Mx-Cre;Sin3A^Δ/Δ model, quickly induced bone marrow failure as a consequence of a loss of stem and progenitor cells.⁶  This phenotype was also observed upon simultaneous loss of HDAC1 and HDAC2, suggesting that Sin3A functions in HSCs within the context of the Sin3A/HDAC complex.

Here, Cantor and David assessed the role of Sin3B in the murine hematopoietic system using a Vav1-Cre;Sin3B^Δ/Δ conditional knockout (Sin3B^CKO) model. First, the authors showed that conditional deletion of Sin3B in steady-state animals resulted in a moderate accumulation of hematopoietic stem and progenitor cells in the bone marrow of these mice. In contrast, competitive transplant experiments showed a strong loss of the repopulating ability of Sin3B^CKO HSCs, whereas homing of the HSCs was not affected. Detailed analysis of the bone marrow cellularity of transplanted animals showed that, whereas more differentiated cell types were significantly decreased, there was an increase within the HSC compartment. This phenomenon became more apparent upon secondary and tertiary transplantation. Interestingly, the fraction of CD150^high HSCs was increased in Sin3B^CKO mice. High expression of CD150 is a hallmark of myeloid-biased HSCs, which have been shown to increase in aged mice and give rise to more myeloid output.⁷  This suggests that loss of Sin3B may induce an aged HSC phenotype. It will therefore be interesting to investigate the effects of Sin3B deletion on the HSC pool as an animal ages. Gene expression analysis in Sin3B^CKO LT-HSCs showed an increased expression of genes inducing proliferation. Based on both chromatin immune precipitation (ChIP) studies of Sin3B interaction partners in other cell types and protein-interaction studies in HPC-7 cells, the authors speculate that Sin3B represses these genes in the context of an E2F4/KDM5A/HDAC1 complex. This line of thinking is supported by earlier studies showing the existence of a Sin3B-specific complex containing, among others, KDM5A, but not Sin3A.^8,9  Interestingly, despite being expressed, Sin3A cannot compensate for the loss of Sin3B, which suggests that both proteins have nonredundant functions in HSCs and pointing toward a role for Sin3B in a protein complex independent of Sin3A. Consistent with pro-proliferative genes being deregulated, Sin3B^CKO HSCs displayed a loss of quiescence, which is reminiscent of previous observations showing Sin3B to be important for exiting the cell cycle into G₀.⁴  Finally, Sin3B^CKO mice displayed increases sensitivity to 5-fluorouracil–mediated myelosuppression as a consequence of increased proliferation and impaired differentiation of HSCs, which mimics the phenotype observed in the competitive transplant experiments.

Taken together, this study builds a case for Sin3B to regulate HSC quiescence and differentiation, likely in a Sin3A-independent manner. However, to convincingly show at the molecular level that Sin3A and Sin3B regulate distinct gene sets in specific protein complexes, additional experiments will be required. The authors demonstrated the existence of a KDM5A-containing Sin3B-specific complex in a hematopoietic precursor cell line. To address whether this Sin3B complex, but not Sin3A, indeed targets HSC pro-proliferative genes in LT-HSCs, ChIP experiments will be essential. In addition, it is of interest to investigate the epigenetic effects of loss of this Sin3B complex. Because the complex contains both histone deacetylase (HDAC1) and H3K4 demethylase (KDM5A) activity, it will be informative to study changes in both histone acetylation and H3K4me3 levels at specific target genes upon deletion of Sin3B. Alternatively, the observed reduction in Sin3B expression over the course of differentiation toward committed progenitors also provides a model to study loss of Sin3B complexes from their target genes. Finally, comparative analyses of Sin3A and Sin3B chromatin-binding profiles in both stem and progenitor populations will be vital to begin to understand how target gene specificity, and the chromatin-modifying actions of their respective complex partners, may explain the different phenotypic outcomes of the Sin3A and Sin3B conditional knockout models.

This study demonstrates that Sin3B deletion affects HSC differentiation and adds Sin3B to a growing list of genes that regulate HSC quiescence.¹⁰  It will be interesting to see to what extent there is cross-talk between Sin3B and other HSC quiescence-regulating genes. A comprehensive understanding of the function of these genes in HSC homeostasis will allow us to fully appreciate the complexity of HSC fate decisions and how this process is hijacked by leukemic stem cells.