1016 – REGULATION OF RIBOSOME BIOGENESIS IN HEMATOPOIETIC STEM CELLS

Abstract

Dysregulation of ribosome biogenesis is the underlying etiology in a group of bone marrow failure syndromes called ribosomopathies. As prominently illustrated by one type of ribosomopathies, Shwachman-Diamond Syndrome (SDS), germline biallelic mutations in three assembly factors essential for the maturation of the 60S ribosome subunit result in progressive hematopoietic stem and progenitor cell (HSPC) failure, suggesting that HSPCs are sensitive to perturbations in ribosome assembly. However, how ribosome assembly is regulated in HSPCs remains poorly understood, as are approaches to restore in hematopoietic stem cell (HSC) function in the context of defective ribosome biogenesis. Here we uncover a novel role for the E3 ubiquitin ligase, HectD1, in regulating HSC function via ribosome biogenesis. HSCs from hematopoietic-specific <i>Hectd1</i> knockout mice exhibit a striking defect in transplantation ability and self-renewal. <i>Hectd1</i>-deficient HSCs fail to be maintained ex vivo, concomitant with a reduction in global protein synthesis and growth rate. The mechanism underlying HSC dysfunction upon <i>Hectd1</i> deficiency is directly linked to aberrant ribosome assembly. We demonstrate that HectD1 ubiquitinates and degrades an assembly factor for the polypeptide exit tunnel of the 60S ribosome in the cytoplasm. Depletion of <i>HectD1</i> led to an accumulation of the anti-association factor eIF6 on the 60S and decreased 80S to 60S ratio, consistent with a subunit joining defect associated with SDS-like diseases. Importantly, we found that restoration of ribosome assembly in <i>HectD1</i>-deficient cells promoted protein synthesis and enhanced HSC reconstitution capacity. These findings highlight the importance of ribosome assembly in HSC regeneration and represent a rare <i>in vivo</i> example of genetic suppression of HSC defects associated with dysfunctional ribosome biogenesis.