Abstract

The contribution of microglia in neurological disorders is emerging as a leading disease driver rather than a consequence of pathology. RNAseT2‐deficient leukoencephalopathy is a severe childhood white matter disorder affecting patients in their first year of life and mimicking a cytomegalovirus brain infection. The early onset and resemblance of the symptoms to a viral infection suggest an inflammatory and embryonic origin of the pathology. There are no treatments available for this disease as our understanding of the cellular drivers of the pathology are still unknown. In this study, using a zebrafish mutant for the orthologous rnaset2 gene, we have identified an inflammatory signature in early development and an antiviral immune response in mature adult brains. Using the optical transparency and the ex utero development of the zebrafish larvae we studied immune cell behavior during brain development and identified abnormal microglia as an early marker of pathology. Live imaging and electron microscopy identified that mutant microglia displayed an engorged morphology and were filled with undigested apoptotic cells and undigested substrate. Using microglia‐specific depletion and rescue experiments, we identified microglia as drivers of this embryonic phenotype and potential key cellular player in the pathology of RNAseT2‐deficient leukoencephalopathy. Our zebrafish model also presented with reduced survival and locomotor defects, therefore recapitulating many aspects of the human disease. Our study therefore placed our rnaset2 mutant at the forefront of leukodystrophy preclinical models and highlighted tissue‐specific approaches as future therapeutic avenues.

Highlights

  • In the central nervous system (CNS), developmental apoptosis is critical for the formation of a healthy brain, eliminating excess neurons produced during development (Dekkers, Nikoletopoulou, & Barde, 2013; Meier, Finch, & Evan, 2000; Nijhawan, Honarpour, & Wang, 2000)

  • This study identified dysfunctional microglia as a new early marker of the pathology in Ribonuclease T2 (RNAseT2)-deficient leukoencephalopathy

  • Our results demonstrate that microgliaspecific interventions can rescue microglial defects in rnaset2deficient zebrafish, highlighting a cell specific therapeutic approach to be investigated in patients

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Summary

| INTRODUCTION

In the central nervous system (CNS), developmental apoptosis is critical for the formation of a healthy brain, eliminating excess neurons produced during development (Dekkers, Nikoletopoulou, & Barde, 2013; Meier, Finch, & Evan, 2000; Nijhawan, Honarpour, & Wang, 2000). Mouse models of Krabbe's disease, another fatal leukodystrophy, showed similar microglial activation before myelin loss, with elevated innate immune markers even before microglial activation (Potter et al, 2013; Snook et al, 2014) The phenotype of another common leukodystrophy, Aicardi Goutières Syndrome (AGS), resembled a cytomegalovirus brain infection with early upregulation of type I interferon being central to the pathology (Crow & Manel, 2015). We hypothesized that the early onset of RNAseT2-deficient leukoencephalopathy is caused by abnormal microglial activity during neurodevelopment In this current study, we utilize the ex utero development of zebrafish larvae and its optical transparency to study the impact of loss of rnaset early during brain development. Approaches, opening the possibility of gene therapy and bone marrow transplant to treat this rare leukodystrophy

| METHODS
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Findings
| DISCUSSION
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