A novel two-step genome editing strategy with CRISPR-Cas9 provides new insights into telomerase action and TERT gene expression.

Linghe Xi,Jens C Schmidt,Dante R Ascarrunz,Arthur J Zaug,Thomas R Cech

doi:10.1186/s13059-015-0791-1

Abstract

BackgroundTo facilitate indefinite proliferation, stem cells and most cancer cells require the activity of telomerase, which counteracts the successive shortening of telomeres caused by incomplete DNA replication at the very end of each chromosome. Human telomerase activity is often determined by the expression level of telomerase reverse transcriptase (TERT), the catalytic subunit of the ribonucleoprotein complex. The low expression level of TERT and the lack of adequate antibodies have made it difficult to study telomerase-related processes in human cells.ResultsTo overcome the low CRISPR-Cas9 editing efficiency at the TERT locus, we develop a two-step “pop-in/pop-out” strategy to enrich cells that underwent homologous recombination (HR). Using this technique, we fuse an N-terminal FLAG-SNAP-tag to TERT, which allows us to reliably detect TERT in western blots, immunopurify it for biochemical analysis, and determine its subcellular localization by fluorescence microscopy. TERT co-localizes detectably with only 5–7 % of the telomeres at a time in S-phase HeLa cells; no nucleolar localization is detected. Furthermore, we extend this approach to perform single base-pair modifications in the TERT promoter; reverting a recurrent cancer-associated TERT promoter mutation in a urothelial cancer cell line results in decreased telomerase activity, indicating the mutation is causal for telomerase reactivation.ConclusionsWe develop a two-step CRISPR-Cas9 genome editing strategy to introduce precise modifications at the endogenous TERT locus in human cell lines. This method provides a useful tool for studying telomerase biology, and suggests a general approach to edit loci with low targeting efficiency and to purify and visualize low abundance proteins.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-015-0791-1) contains supplementary material, which is available to authorized users.

Highlights

To facilitate indefinite proliferation, stem cells and most cancer cells require the activity of telomerase, which counteracts the successive shortening of telomeres caused by incomplete DNA replication at the very end of each chromosome
Modification of the endogenous telomerase reverse transcriptase (TERT) protein with an N-terminal FLAG-SNAP-tag We found that the efficiency of genome editing in the TERT 5′ region was very low
We designed a two-step protocol to introduce the sequence coding for a FLAG-SNAP-tag into the TERT locus (Fig. 1a)

Summary

Introduction

Stem cells and most cancer cells require the activity of telomerase, which counteracts the successive shortening of telomeres caused by incomplete DNA replication at the very end of each chromosome. Human telomerase activity is often determined by the expression level of telomerase reverse transcriptase (TERT), the catalytic subunit of the ribonucleoprotein complex. The low expression level of TERT and the lack of adequate antibodies have made it difficult to study telomerase-related processes in human cells. Like stem cells and cancer cells, require a mechanism to compensate for telomere attrition during continuous division [1]. Most often this requirement is fulfilled by the telomerase enzyme. Somatic cells lack telomerase activity, due to the transcriptional inactivation of the gene encoding TERT, the catalytic subunit of the telomerase holoenzyme [2,3,4]. CRISPR-Cas9-mediated genome editing provides an alternative approach, allowing tagging of the endogenous TERT protein with a welldefined epitope tag, for which well-characterized antibodies are available

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Discussion

Conclusion