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2015 ; 16
(ä): 231
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A novel two-step genome editing strategy with CRISPR-Cas9 provides new insights
into telomerase action and TERT gene expression
#MMPMID26553065
Xi L
; Schmidt JC
; Zaug AJ
; Ascarrunz DR
; Cech TR
Genome Biol
2015[Nov]; 16
(ä): 231
PMID26553065
show ga
BACKGROUND: 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. 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. RESULTS: To 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. CONCLUSIONS: We 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.