Shwachman-Bodian Diamond syndrome is a multi-functional protein implicated in cellular stress responses

Abstract

Shwachman-Diamond syndrome (SDS; OMIM 260400) results from loss-of-function mutations in the Shwachman-Bodian Diamond syndrome (SBDS) gene. It is a multi-system disorder with clinical features of exocrine pancreatic dysfunction, skeletal abnormalities, bone marrow failure and predisposition to leukemic transformation. Although the cellular functions of SBDS are still unclear, its yeast ortholog has been implicated in ribosome biogenesis. Using affinity capture and mass spectrometry, we have developed an SBDS-interactome and report SBDS binding partners with diverse molecular functions, notably components of the large ribosomal subunit and proteins involved in DNA metabolism. Reciprocal co-immunoprecipitation confirmed the interaction of SBDS with the large ribosomal subunit protein RPL4 and with DNA-PK and RPA70, two proteins with critical roles in DNA repair. Function for SBDS in response to cellular stresses was implicated by demonstrating that SBDS-depleted HEK293 cells are hypersensitive to multiple types of DNA damage as well as chemically induced endoplasmic reticulum stress. Furthermore, using multiple routes to impair translation and mimic the effect of SBDS-depletion, we show that SBDS-dependent hypersensitivity of HEK293 cells to UV irradiation can be distinguished from a role of SBDS in translation. These results indicate functions of SBDS beyond ribosome biogenesis and may provide insight into the poorly understood cancer predisposition of SDS patients.

Ubiquitin-proteasome-rich cytoplasmic structures in neutrophils of patients with Shwachman-Diamond syndrome

“…6 Thus, it seems likely that mutation-induced SBDS protein deprivation causes neutropenia in SDS patients, possibly through oxidative-stress-induced apoptosis. 2,5,6 Indeed, more apoptotic figures were observed in mutated SDS neutrophils and in control neutrophils treated with the ROS-generating xanthine/xanthine oxidase system than in untreated control neutrophils. In addition, the high concentration of polyubiquitinated proteins that we found in PaCS of SDS neutrophils, in apparent excess of their proteasome content, suggests a relative insufficiency of proteasome degradative function as a probable cause of disproportionate accumulation of polyubiquitinated proteins which, in turn, may activate autophagy and apoptosis.…”

“…2 Approximately 90% of the patients have a mutation in the SBDS gene, 3 coding for a multifunctional protein implicated in ribosome biogenesis, as well as in DNA metabolism and repair. 4,5 Of special interest is the observation that the SBDS protein plays a role in cellular stress responses. Indeed, SBDS depletion causes cellular hypersensitivity to a variety of stress conditions, including endoplasmic reticulum stress and DNA damage, which may help explain the patients' predisposition to MDS/AML.…”

“…Indeed, SBDS depletion causes cellular hypersensitivity to a variety of stress conditions, including endoplasmic reticulum stress and DNA damage, which may help explain the patients' predisposition to MDS/AML. 5 In fact, SBDS protein deficiency leads to markedly increased intracellular reactive oxygen species (ROS) in epithelial HeLa and myeloid TF1 cells, with accelerated apoptosis and reduced cell growth, both of which are rescued by antioxidants. 6 ROS are known to cause protein damage, with intracellular accumulation of misfolded and partially denatured proteins, which must be rapidly eliminated through hyperfunction of the ubiquitin-proteasome system (UPS) to preserve cell homeostasis and function.…”

Marrow failure: a window into ribosome biology

“…72 Observations of in vitro assays for abnormalities of marrow stroma from SDS patients have varied. 73,74 A variety of cellular phenotypes has been observed in SDS, including mitotic spindle destabilization, 75,76 Fas ligand-induced apoptosis, 77 heightened cellular stress responses 78 and Rac2-mediated monocyte migration, 79 decreased mitochondrial membrane potential and oxygen consumption, and increased the production of reactive oxygen species. 80,81 SBDS promotes the release of EIF6 from the pre-60S ribosome, which is required for the formation of a mature 80S functional ribosome [82][83][84] (Figure 2).…”

Defective ribosome assembly in Shwachman-Diamond syndrome

“…[4][5][6][7] At the cellular level in mammalian cells, diverse functions for SBDS have been proposed, including roles in chemotaxis, 8,9 mitosis, 10 and cellular stress responses. 11 Although multiple interaction partners for the SBDS protein have been suggested, 11,12 the functional relevance of these remains unclear and none of the proposed functions immediately suggests what the specific biochemical function of the SBDS protein might be.…”