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

Abstract

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 Tgfβ, p15Ink4b 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 Tgfβ 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.

SBDS-Deficient Cells Have an Altered Homeostatic Equilibrium due to Translational Inefficiency Which Explains their Reduced Fitness and Provides a Logical Framework for Intervention

“…By sequential subcloning, we were unable to derive Sbds R126T/R126T MEFs due to early senescence (Fig 1A), whereas we normally derived wt cells. This result is in line with a recent work describing early senescence in the pancreas of SBDS mutants [8]. In contrast, immortalization of Sbds R126T/R126T MEFs by infection with a vector carrying the dominant negative p53 and Ras G12V was successful (Fig 1A; S1A and S1B Fig).…”

Shwachman-Diamond Syndrome Protein SBDS Maintains Human Telomeres by Regulating Telomerase Recruitment

“…A recent report further supports the idea that SDS belongs to ribosomopathies because defective ribosome synthesis and protein translation insufficiency was observed in an SBDS-deficient mouse model (Tourlakis et al, 2015). In this study, SBDS deficiency does not alter whole-cell telomerase activity, indicating that SBDS may not be involved in telomerase biogenesis.…”

Shwachman–Diamond syndrome presenting with early ichthyosis, associated dermal and epidermal intracellular lipid droplets, hypoglycemia, and later distinctive clinical SDS phenotype

“…In conditional knockout mice, replicating several features observed in human disease, both acinar hypoplasia and marked pancreatic fatty infiltration have been reproduced with loss of zymogen granules [Tourlakis et al, 2012]. This atrophic SDS pancreas phenotype was revealed to be p53-dependent, but has been rescued, including the fatty infiltration, by silencing the p53 [Tourlakis et al, 2015]. Interestingly, mutation-negative SDS patients have normal pancreatic MRI signal intensity despite ePI [Toiviainen-Salo et al, 2008] as for a recently described patient harboring a single SBDS missense mutation and a structural variation in the SBDS locus [Carvalho et al, 2014].…”

“…In an SDS-mouse model, embryos have decreased mass compared to littermate control and SDS brain was apoptotic with hypocellularity and neuronal cell death [Tourlakis et al, 2015]. Diffuse echogenicity of the pancreas indicating fatty infiltration was also present in the index patient at the age of 4 months and persisted at the age of 4 9/12 years of age.…”