Publications by authors named "Joshua Z Johnson"

3 Publications

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Minimized human telomerase maintains telomeres and resolves endogenous roles of H/ACA proteins, TCAB1, and Cajal bodies.

Elife 2016 08 15;5. Epub 2016 Aug 15.

Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States.

We dissected the importance of human telomerase biogenesis and trafficking pathways for telomere maintenance. Biological stability of human telomerase RNA (hTR) relies on H/ACA proteins, but other eukaryotes use other RNP assembly pathways. To investigate additional rationale for human telomerase assembly as H/ACA RNP, we developed a minimized cellular hTR. Remarkably, with only binding sites for telomerase reverse transcriptase (TERT), minimized hTR assembled biologically active enzyme. TERT overexpression was required for cellular interaction with minimized hTR, indicating that H/ACA RNP assembly enhances endogenous hTR-TERT interaction. Telomere maintenance by minimized telomerase was unaffected by the elimination of the telomerase holoenzyme Cajal body chaperone TCAB1 or the Cajal body scaffold protein Coilin. Surprisingly, wild-type hTR also maintained and elongated telomeres in TCAB1 or Coilin knockout cells, with distinct changes in telomerase action. Overall, we elucidate trafficking requirements for telomerase biogenesis and function and expand mechanisms by which altered telomere maintenance engenders human disease.
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http://dx.doi.org/10.7554/eLife.18221DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5005035PMC
August 2016

Human stem cell-based disease modeling: prospects and challenges.

Curr Opin Cell Biol 2015 Dec 11;37:84-90. Epub 2015 Nov 11.

Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720, USA. Electronic address:

Human stem cell-based disease models have great promise to advance our understanding of human disease. These models can be derived from patients with genetic disorders and manipulated with genome editing and myriad differentiation protocols to model pathologies in vitro. However, several challenges have impeded the full potential of stem cell-based in vitro disease modeling. Many genetically predisposed diseases take time to manifest and occur in specific tissue microenvironments, and these parameters are often not adequately modeled using conventional shorter-term monolayer cultures. These challenges must be overcome especially for cases where animal models also incompletely recapitulate the complex pathologies found in humans. As prominent ways to tackle these challenges we discuss here how advanced genome editing tools in human stem cells and human organoid cultures, specifically the example of intestinal organoids, contribute genetically defined models that recapitulate phenotypes of disease.
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http://dx.doi.org/10.1016/j.ceb.2015.10.007DOI Listing
December 2015

Cancer-associated TERT promoter mutations abrogate telomerase silencing.

Elife 2015 Jul 21;4. Epub 2015 Jul 21.

Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States.

Mutations in the human telomerase reverse transcriptase (TERT) promoter are the most frequent non-coding mutations in cancer, but their molecular mechanism in tumorigenesis has not been established. We used genome editing of human pluripotent stem cells with physiological telomerase expression to elucidate the mechanism by which these mutations contribute to human disease. Surprisingly, telomerase-expressing embryonic stem cells engineered to carry any of the three most frequent TERT promoter mutations showed only a modest increase in TERT transcription with no impact on telomerase activity. However, upon differentiation into somatic cells, which normally silence telomerase, cells with TERT promoter mutations failed to silence TERT expression, resulting in increased telomerase activity and aberrantly long telomeres. Thus, TERT promoter mutations are sufficient to overcome the proliferative barrier imposed by telomere shortening without additional tumor-selected mutations. These data establish that TERT promoter mutations can promote immortalization and tumorigenesis of incipient cancer cells.
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http://dx.doi.org/10.7554/eLife.07918DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4507476PMC
July 2015
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