In Vivo Senescence in the Sbds-Deficient Murine Pancreas: Cell-Type Specific Consequences of Translation Insufficiency.

Genetic models of ribosome dysfunction show selective organ failure, highlighting a gap in our understanding of cell-type specific responses to translation insufficiency. Translation defects underlie a growing list of inherited and acquired cancer-predisposition syndromes referred to as ribosomopathies. We sought to identify molecular mechanisms underlying organ failure in a recessive ribosomopathy, with particular emphasis on the pancreas, an organ with a high and reiterative requirement for protein synthesis. Biallelic loss of function mutations in SBDS are associated with the ribosomopathy Shwachman-Diamond syndrome, which is typified by pancreatic dysfunction, bone marrow failure, skeletal abnormalities and neurological phenotypes. Targeted disruption of Sbds in the murine pancreas resulted in p53 stabilization early in the postnatal period, specifically in acinar cells. Decreased Myc expression was observed and atrophy of the adult SDS pancreas could be explained by the senescence of acinar cells, characterized by induction of Tgfß, p15(Ink4b) and components of the senescence-associated secretory program. This is the first report of senescence, a tumour suppression mechanism, in association with SDS or in response to a ribosomopathy. Genetic ablation of p53 largely resolved digestive enzyme synthesis and acinar compartment hypoplasia, but resulted in decreased cell size, a hallmark of decreased translation capacity. Moreover, p53 ablation resulted in expression of acinar dedifferentiation markers and extensive apoptosis. Our findings indicate a protective role for p53 and senescence in response to Sbds ablation in the pancreas. In contrast to the pancreas, the Tgfß molecular signature was not detected in fetal bone marrow, liver or brain of mouse models with constitutive Sbds ablation. Nevertheless, as observed with the adult pancreas phenotype, disease phenotypes of embryonic tissues, including marked neuronal cell death due to apoptosis, were determined to be p53-dependent. Our findings therefore point to cell/tissue-specific responses to p53-activation that include distinction between apoptosis and senescence pathways, in the context of translation disruption.

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

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.