Ribosomal protein control of hematopoietic stem cell transformation through direct, non-canonical regulation of metabolism.

We report here that expression of the ribosomal protein, RPL22, is frequently reduced in human myelodysplastic syndrome (MDS) and acute myelogenous leukemia (AML); reduced RPL22 expression is associated with worse outcomes. Mice null for Rpl22 display characteristics of an MDS-like syndrome and develop leukemia at an accelerated rate. Rpl22-deficient mice also display enhanced hematopoietic stem cell (HSC) self-renewal and obstructed differentiation potential, which arises not from reduced protein synthesis but from increased expression of the Rpl22 target, ALOX12, an upstream regulator of fatty acid oxidation (FAO). The increased FAO mediated by Rpl22-deficiency also persists in leukemia cells and promotes their survival. Altogether, these findings reveal that Rpl22 insufficiency enhances the leukemia potential of HSC via non-canonical de-repression of its target, ALOX12, which enhances FAO, a process that may serve as a therapeutic vulnerability of Rpl22 low MDS and AML leukemia cells. Highlights: RPL22 insufficiency is observed in MDS/AML and is associated with reduced survivalRpl22-deficiency produces an MDS-like syndrome and facilitates leukemogenesisRpl22-deficiency does not impair global protein synthesis by HSCRpl22 controls leukemia cell survival by non-canonical regulation of lipid oxidation eTOC: Rpl22 controls the function and transformation potential of hematopoietic stem cells through effects on ALOX12 expression, a regulator of fatty acid oxidation.

Loss of Ribosomal Protein Paralog Rpl22-like1 Blocks Lymphoid Development without Affecting Protein Synthesis.

Ribosomal proteins are thought to primarily facilitate biogenesis of the ribosome and its ability to synthesize protein. However, in this study, we show that Rpl22-like1 (Rpl22l1) regulates hematopoiesis without affecting ribosome biogenesis or bulk protein synthesis. Conditional loss of murine Rpl22l1 using stage or lineage-restricted Cre drivers impairs development of several hematopoietic lineages. Specifically, Tie2-Cre-mediated ablation of Rpl22l1 in hemogenic endothelium impairs the emergence of embryonic hematopoietic stem cells. Ablation of Rpl22l1 in late fetal liver progenitors impairs the development of B lineage progenitors at the pre-B stage and development of T cells at the CD44-CD25+ double-negative stage. In vivo labeling with O-propargyl-puromycin revealed that protein synthesis at the stages of arrest was not altered, indicating that the ribosome biogenesis and function were not generally compromised. The developmental arrest was associated with p53 activation, suggesting that the arrest may be p53-dependent. Indeed, development of both B and T lymphocytes was rescued by p53 deficiency. p53 induction was not accompanied by DNA damage as indicated by phospho-γH2AX induction or endoplasmic reticulum stress, as measured by phosphorylation of EIF2α, thereby excluding the known likely p53 inducers as causal. Finally, the developmental arrest of T cells was not rescued by elimination of the Rpl22l1 paralog, Rpl22, as we had previously found for the emergence of hematopoietic stem cells. This indicates that Rpl22 and Rpl22l1 play distinct and essential roles in supporting B and T cell development.

Disruption of Thrombocyte and T Lymphocyte Development by a Mutation in ARPC1B.

Regulation of the actin cytoskeleton is crucial for normal development and function of the immune system, as evidenced by the severe immune abnormalities exhibited by patients bearing inactivating mutations in the Wiskott-Aldrich syndrome protein (WASP), a key regulator of actin dynamics. WASP exerts its effects on actin dynamics through a multisubunit complex termed Arp2/3. Despite the critical role played by Arp2/3 as an effector of WASP-mediated control over actin polymerization, mutations in protein components of the Arp2/3 complex had not previously been identified as a cause of immunodeficiency. Here, we describe two brothers with hematopoietic and immunologic symptoms reminiscent of Wiskott-Aldrich syndrome (WAS). However, these patients lacked mutations in any of the genes previously associated with WAS. Whole-exome sequencing revealed a homozygous 2 bp deletion, n.c.G623DEL-TC (p.V208VfsX20), in Arp2/3 complex component ARPC1B that causes a frame shift resulting in premature termination. Modeling of the disease in zebrafish revealed that ARPC1B plays a critical role in supporting T cell and thrombocyte development. Moreover, the defects in development caused by ARPC1B loss could be rescued by the intact human ARPC1B ortholog, but not by the p.V208VfsX20 variant identified in the patients. Moreover, we found that the expression of ARPC1B is restricted to hematopoietic cells, potentially explaining why a mutation in ARPC1B has now been observed as a cause of WAS, whereas mutations in other, more widely expressed, components of the Arp2/3 complex have not been observed.

Regulatory Roles of Rpl22 in Hematopoiesis: An Old Dog with New Tricks.

Ribosomal proteins have long been known to serve critical roles in facilitating the biogenesis of the ribosome and its ability to synthesize proteins. However, evidence is emerging that suggests ribosomal proteins are also capable of performing tissue-restricted, regulatory functions that impact normal development and pathological conditions, including cancer. The challenge in studying such regulatory functions is that elimination of many ribosomal proteins also disrupts ribosome biogenesis and/or function. Thus, it is difficult to determine whether developmental abnormalities resulting from ablation of a ribosomal protein result from loss of core ribosome functions or from loss of the regulatory function of the ribosomal protein. Rpl22, a ribosomal protein component of the large 60S subunit, provides insight into this conundrum; Rpl22 is dispensable for both ribosome biogenesis and protein synthesis yet its ablation causes tissue-restricted disruptions in development. Here we review evidence supporting the regulatory functions of Rpl22 and other ribosomal proteins.