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Dirk Hockemeyer

Dirk Hockemeyer

Assistant Professor of Cell and Developmental Biology

Lab Homepage: http://mcb.berkeley.edu/labs/hockemeyer/

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Research Interests

Our goal is to shed light on the key functions of telomeres and telomerase in tissue homeostasis, tumorigenesis and aging. Telomeres are the repetitive DNA sequences at the end of linear eukaryotic chromosomes that allow a cell to distinguish the natural chromosome end from aberrant DNA breaks. Telomeric DNA repeats can be generated de novo by the enzyme telomerase thereby providing a compensatory mechanism that counteracts terminal sequence loss caused by the end replication problem. Figure 1As a result, telomeres and telomerase are essential to genomic integrity and their disruption is associated with cancer and aging. The use of genetic mouse models has been a powerful way to gain insight into the fundamental mechanisms of how the telomere evades recognition by the DNA-damage machinery, the consequences of telomerase loss, and how the single stranded telomeric overhang is established. However, telomere shortening naturally occurs only in human somatic cells, but not in mouse cells. This telomere shortening, which functions as a tumor suppressor mechanism by limiting the replicative potential of human cells, is the result of selective silencing of telomerase expression in human cells upon their differentiation. Notably, this process is reversed and telomerase reactivated in about 90% of all human tumors after which telomerase expression becomes essential for their proliferation.

Figure 2Human pluripotent stem cells (hPSC) are an ideal model system to study telomerase regulation, as they are telomerase-positive, but can be rapidly differentiated into telomerase-negative cells. Moreover, the reverse process of telomerase reactivation can be studied during the process of reprogramming somatic cells into induced pluripotent stem cells (iPSCs). Owing to their constitutive telomerase expression, PSCs are immortal and can be expanded indefinitely in culture. However, in contrast to other model systems such as tumor cells, they have functional DNA surveillance and cell cycle checkpoints and therefore are ideal for studying the effects of telomere shortening and dysfunction on these pathways. Over the last few years we have developed robust protocols to derive and genetically modify human PSCs. In collaboration with Sangamo Bioscience we developed the use of site-specific nucleases, specifically Zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs), to genetically engineer human PSCs. Prior to this, primary human cells were resilient to conventional homologous recombination making gene targeting extremely inefficient and time consuming.

Figure 3
Due to these technical advancements we can now overexpress or silence genes, correct disease-causing mutations, and engineer reporter genes in human PSCs. In my laboratory, we will use these gene targeting strategies to study the biology of human telomeres to address:

  1. What is the mechanism of telomerase regulation in human stem cells, upon their differentiation and during tumor formation?
  2. What are the molecular mechanisms underlying the recruitment of telomerase to the telomere in human cells?
  3. What are the consequences of telomere shortening in stem cells and how does this impact tumor formation?

Selected Publications

Hockemeyer D*, Wang H*, Kiani S, Lai CS, Gao Q, Cassady JP, Cost GJ, Zhang L, Santiago Y, Miller JC, Zeitler B, Cherone JM, Meng X, Hinkley SJ, Rebar EJ, Gregory PD, Urnov FD, Jaenisch R. (2011) Genetic engineering of human pluripotent cells using TALE nucleases. Nature biotechnology 29: 731-4 *equal contribution

Soldner F, Laganiere J, Cheng AW, Hockemeyer D, Gao Q, Alagappan R, Khurana V, Golbe LI, Myers RH, Lindquist S, Zhang L, Guschin D, Fong LK, Vu BJ, Meng X, Urnov FD, Rebar EJ, Gregory PD, Zhang HS, Jaenisch R. (2011) Generation of isogenic pluripotent stem cells differing exclusively at two early onset Parkinson point mutations. Cell 146: 318-31

Hockemeyer D, Jaenisch R. (2010) Gene targeting in human pluripotent cells. Cold Spring Harbor symposia on quantitative biology 75: 201-9

DeKelver RC, Choi VM, Moehle EA, Paschon DE, Hockemeyer D, Meijsing SH, Sancak Y, Cui X, Steine EJ, Miller JC, Tam P, Bartsevich VV, Meng X, Rupniewski I, Gopalan SM, Sun HC, Pitz KJ, Rock JM, Zhang L, Davis GD, Rebar EJ, Cheeseman IM, Yamamoto KR, Sabatini DM, Jaenisch R, Gregory PD, Urnov FD. (2010) Functional genomics, proteomics, and regulatory DNA analysis in isogenic settings using zinc finger nuclease-driven transgenesis into a safe harbor locus in the human genome. Genome Research 20: 1133-42

Soldner F*, Hockemeyer D*, Beard C, Gao Q, Bell GW, Cook EG, Hargus G, Blak A, Cooper O, Mitalipova M, Isacson O, Jaenisch R. (2009) Parkinson's disease patient-derived induced pluripotent stem cells free of viral reprogramming factors. Cell 136: 964-77 *equal contribution

Hockemeyer D*, Soldner F*, Beard C, Gao Q, Mitalipova M, DeKelver RC, Katibah GE, Amora R, Boydston EA, Zeitler B, Meng X, Miller JC, Zhang L, Rebar EJ, Gregory PD, Urnov FD, Jaenisch R. (2009) Efficient targeting of expressed and silent genes in human ESCs and iPSCs using zinc-finger nucleases. Nature Biotechnology 27: 851-7 *equal contribution

Hockemeyer D, Soldner, F., Jaenisch, R. (2009) Direct reprogramming of somatic cells to a pluripotent state. In In Essentials of Stem Cell Biology, Second edition, pp. pp. 29-32. (San Diego, USA, Academic Press, Elsevier): Robert Lanza, John Gearhart, Brigid Hogan, Douglas Melton, Roger Pederson, E. Donnall Thomas, James Thomson, and Sir Ian Wilmut

Hockemeyer D*, Soldner F*, Cook EG, Gao Q, Mitalipova M, Jaenisch R. (2008) A drug-inducible system for direct reprogramming of human somatic cells to pluripotency. Cell Stem Cell 3: 346-53 *equal contribution

Hockemeyer D, Palm W, Wang RC, Couto SS, de Lange T. (2008) Engineered telomere degradation models dyskeratosis congenita. Genes & Development 22: 1773-85

Hockemeyer D, Palm W, Else T, Daniels JP, Takai KK, Ye JZ, Keegan CE, de Lange T, Hammer GD. (2007) Telomere protection by mammalian Pot1 requires interaction with Tpp1. Nature Structural & Molecular Biology 14: 754-61

Hockemeyer D, Daniels JP, Takai H, de Lange T. (2006) Recent expansion of the telomeric complex in rodents: Two distinct POT1 proteins protect mouse telomeres. Cell 126: 63-77

Hockemeyer D, Sfeir AJ, Shay JW, Wright WE, de Lange T. (2005) POT1 protects telomeres from a transient DNA damage response and determines how human chromosomes end. The EMBO journal 24: 2667-78

Ye JZ, Hockemeyer D, Krutchinsky AN, Loayza D, Hooper SM, Chait BT, de Lange T. (2004) POT1-interacting protein PIP1: a telomere length regulator that recruits POT1 to the TIN2/TRF1 complex. Genes & Development 18: 1649-54

Last Updated 2012-10-02