A role for Tau protein in maintaining ribosomal DNA stability and cytidine deaminase-deficient cell survival

Elias Bou Samra,Xavier Veaute,Christelle Machon,Michael R Green,Jérôme Guitton,Claude Gazin,Olivier Alibert,Mounira Amor-Guéret,Rosine Onclercq-Delic,Géraldine Buhagiar-Labarchède

doi:10.1038/s41467-017-00633-1

Abstract

Cells from Bloom’s syndrome patients display genome instability due to a defective BLM and the downregulation of cytidine deaminase. Here, we use a genome-wide RNAi-synthetic lethal screen and transcriptomic profiling to identify genes enabling BLM-deficient and/or cytidine deaminase-deficient cells to tolerate constitutive DNA damage and replication stress. We found a synthetic lethal interaction between cytidine deaminase and microtubule-associated protein Tau deficiencies. Tau is overexpressed in cytidine deaminase-deficient cells, and its depletion worsens genome instability, compromising cell survival. Tau is recruited, along with upstream-binding factor, to ribosomal DNA loci. Tau downregulation decreases upstream binding factor recruitment, ribosomal RNA synthesis, ribonucleotide levels, and affects ribosomal DNA stability, leading to the formation of a new subclass of human ribosomal ultrafine anaphase bridges. We describe here Tau functions in maintaining survival of cytidine deaminase-deficient cells, and ribosomal DNA transcription and stability. Moreover, our findings for cancer tissues presenting concomitant cytidine deaminase underexpression and Tau upregulation open up new possibilities for anti-cancer treatment.

Highlights

Cells from Bloom’s syndrome patients display genome instability due to a defective BLM and the downregulation of cytidine deaminase
We searched for genes potentially required for the viability and proliferation of Bloom’s syndrome (BS) cells, by conducting a genome-wide RNAi screen with a human shRNA library comprising ∼60,000 shRNAs directed against ∼27,000 human genes[12]
As Tau expression was correlated with pre-ribosomal RNA (rRNA) levels, we investigated the mechanism by which the decrease in Tau levels decreased 45 S pre-rRNA synthesis, by performing chromatin immunoprecipitation (ChIP)-qPCR assays to analyze the effect of Tau downregulation on UBTF recruitment to ribosomal DNA (rDNA) repeats

Summary

Introduction

Cells from Bloom’s syndrome patients display genome instability due to a defective BLM and the downregulation of cytidine deaminase. We use a genome-wide RNAi-synthetic lethal screen and transcriptomic profiling to identify genes enabling BLM-deficient and/or cytidine deaminase-deficient cells to tolerate constitutive DNA damage and replication stress. The imbalance in the nucleotide pool resulting from the CDA defect, either in BLM-deficient BS cells or BLM-proficient HeLa cells, reproduced several aspects of the genetic instability associated with BS condition[7, 9] These data suggest that BS cells lacking both BLM and CDA, and CDA-deficient HeLa cells have developed mechanisms for tolerating endogenous DNA damage and replication stress. Tau depletion is sufficient to cause genomic instability, and its coupling with CDA deficiency aggravates this instability These results reveal a function for Tau in rDNA metabolism, and indicate that Tau is critical for the survival of CDA-deficient cells, through its contribution to the safeguarding of genome integrity

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