Mitotic spindle destabilization and genomic instability in Shwachman-Diamond syndrome

Abstract

Deficiencies in the SBDS gene result in Shwachman-Diamond syndrome (SDS), an inherited bone marrow failure syndrome associated with leukemia predisposition. SBDS encodes a highly conserved protein previously implicated in ribosome biogenesis. Using human primary bone marrow stromal cells (BMSCs), lymphoblasts, and skin fibroblasts, we show that SBDS stabilized the mitotic spindle to prevent genomic instability. SBDS colocalized with the mitotic spindle in control primary BMSCs, lymphoblasts, and skin fibroblasts and bound to purified microtubules. Recombinant SBDS protein stabilized microtubules in vitro. We observed that primary BMSCs and lymphoblasts from SDS patients exhibited an increased incidence of abnormal mitoses. Similarly, depletion of SBDS by siRNA in human skin fibroblasts resulted in increased mitotic abnormalities and aneuploidy that accumulated over time. Treatment of primary BMSCs and lymphoblasts from SDS patients with nocodazole, a microtubule destabilizing agent, led to increased mitotic arrest and apoptosis, consistent with spindle destabilization. Conversely, SDS patient cells were resistant to taxol, a microtubule stabilizing agent. These findings suggest that spindle instability in SDS contributes to bone marrow failure and leukemogenesis.

The isochromosome i(7)(q10) carrying c.258+2t>c mutation of the SBDS gene does not promote development of myeloid malignancies in patients with Shwachman syndrome

“…Austin et al 5 demonstrated that the SBDS protein stabilizes the mitotic spindle preventing genomic instability; this observation may suggest that cells with the 258-iso7 are less prone to develop additional chromosome abnormalities, which in turn may lead to MDS/AML, in keeping with the observation reviewed above that the i(7)(q10) was rarely seen in SDS patients with MDS/AML.…”

“…This protein was shown to be relevant for ribosome biogenesis and, more recently, also for stabilization of the mitotic spindle. 5 A reduced but detectable quantity of the SBDS protein was shown to be present in fibroblasts of two SDS patients, this finding suggesting that the mutation found in these two cases, namely c.258 þ 2T4C, may lead to the presence of a small amount of alternatively spliced SBDS mRNA, encoding a functional protein. 6 Recently, Nicolis et al 7 demonstrated that wild-type (wt) mRNA can be found in patients carrying this mutation.…”

Ribosomopathies: human disorders of ribosome dysfunction

“…44 Although these findings provide evidence for a role of SBDS in ribosome biogenesis, SBDS is a multifunctional protein, and nonribosomal activities may play a dominant role in the clinical phenotype. In particular, SBDS has been shown to have a role in stabilizing the mitotic spindle, 46,47 which might indicate a role for SBDS in proliferation and/or chromosome segregation, thereby contributing to chromosomal instability and some component of bone marrow failure.…”

38 Uncoupling of GTP hydrolysis from eIF6 release on the ribosome causes Shwachman-Diamond syndrome

“…Although cosedimentation of human SBDS with free cytoplasmic 60S ribosomal subunits in sucrose gradients (Ganapathi et al 2007) would be consistent with a conserved role for SBDS in 60S subunit maturation, the current model in mammalian cells posits that eIF6 removal is triggered following phosphorylation of Ser 235 by protein kinase C (PKC) and RACK1 (receptor for activated protein C) (Ceci et al 2003). Furthermore, diverse alternate functions for SBDS in mammalian cells have been suggested, including mitotic spindle stabilization (Austin et al 2008), chemotaxis (Wessels et al 2006), Fas ligand-induced apoptosis (Rujkijyanont et al 2008), cellular stress responses (Ball et al 2009), and Rac2-mediated monocyte migration (Leung et al 2010). Thus, despite the prior genetic studies in yeast, the mechanism of eIF6 release in mammalian cells is controversial, biochemical evidence supporting direct catalysis of eIF6 release by SBDS and EFL1 in eukaryotic cells is currently lacking, and the specific function of the SBDS protein, its mode of action, and the molecular mechanism of the cooperative interaction with EFL1 remain obscure.…”