Professional Education

  • Bachelor of Science, Ohio State University (2009)
  • Doctor of Philosophy, University of California San Francisco (2016)

Stanford Advisors


All Publications

  • Stress-mediated exit to quiescence restricted by increasing persistence in CDK4/6 activation. eLife Yang, H. W., Cappell, S. D., Jaimovich, A., Liu, C., Chung, M., Daigh, L. H., Pack, L. R., Fan, Y., Regot, S., Covert, M., Meyer, T. 2020; 9


    Mammalian cells typically start the cell-cycle entry program by activating cyclin-dependent protein kinase 4/6 (CDK4/6). CDK4/6 activity is clinically relevant as mutations, deletions, and amplifications that increase CDK4/6 activity contribute to the progression of many cancers. However, when CDK4/6 is activated relative to CDK2 remained incompletely understood. Here we developed a reporter system to simultaneously monitor CDK4/6 and CDK2 activities in single cells and found that CDK4/6 activity increases rapidly before CDK2 activity gradually increases, and that CDK4/6 activity can be active after mitosis or inactive for variable time periods. Markedly, stress signals in G1 can rapidly inactivate CDK4/6 to return cells to quiescence but with reduced probability as cells approach S phase. Together, our study reveals a regulation of G1 length by temporary inactivation of CDK4/6 activity after mitosis, and a progressively increasing persistence in CDK4/6 activity that restricts cells from returning to quiescence as cells approach S phase.

    View details for DOI 10.7554/eLife.44571

    View details for PubMedID 32255427

  • Putting the brakes on the cell cycle: mechanisms of cellular growth arrest. Current opinion in cell biology Pack, L. R., Daigh, L. H., Meyer, T. 2019; 60: 106?13


    Precise regulation of cellular proliferation is critical to tissue homeostasis and development, but misregulation leads to diseases of excess proliferation or cell loss. To achieve precise control, cells utilize distinct mechanisms of growth arrest such as quiescence and senescence. The decision to enter these growth-arrested states or proliferate is mediated by the core cell-cycle machinery that responds to diverse external and internal signals. Recent advances have revealed the molecular underpinnings of these cell-cycle decisions, highlighting the unique nature of cell-cycle entry from quiescence, identifying endogenous DNA damage as a quiescence-inducing signal, and establishing how persistent arrest is achieved in senescence.

    View details for DOI 10.1016/

    View details for PubMedID 31252282

  • EMI1 switches from being a substrate to an inhibitor of APC/CCDH1 to start the cell cycle. Nature Cappell, S. D., Mark, K. G., Garbett, D., Pack, L. R., Rape, M., Meyer, T. 2018


    Mammalian cells integrate mitogen and stress signalling before the end of G1 phase to determine whether or not they enter the cell cycle1-4. Before cells can replicate their DNA in S phase, they have to activate cyclin-dependent kinases (CDKs), induce an E2F transcription program and inactivate the anaphase-promoting complex (APC/CCDH1, also known as the cyclosome), which is an E3 ubiquitin ligase that contains the co-activator CDH1 (also known as FZR, encoded by FZR1). It was recently shown that stress can return cells to quiescence after CDK2 activation and E2F induction but not after inactivation of APC/CCDH1, which suggests that APC/CCDH1 inactivation is the point of no return for cell-cycle entry 3 . Rapid inactivation of APC/CCDH1 requires early mitotic inhibitor 1 (EMI1)3,5, but the molecular mechanism that controls this cell-cycle commitment step is unknown. Here we show using human cell models that cell-cycle commitment is mediated by an EMI1-APC/CCDH1 dual-negative feedback switch, in which EMI1 is both a substrate and an inhibitor of APC/CCDH1. The inactivation switch triggers a transition between a state with low EMI1 levels and high APC/CCDH1 activity during G1 and a state with high EMI1 levels and low APC/CCDH1 activity during S and G2. Cell-based analysis, in vitro reconstitution and modelling data show that the underlying dual-negative feedback is bistable and represents a robust irreversible switch. Our study suggests that mammalian cells commit to the cell cycle by increasing CDK2 activity and EMI1 mRNA expression to trigger a one-way APC/CCDH1 inactivation switch that is mediated by EMI1 transitioning from acting as a substrate of APC/CCDH1 to being an inhibitor of APC/CCDH1.

    View details for PubMedID 29875408

  • Opposing Chromatin Signals Direct and Regulate the Activity of Lysine Demethylase 4C (KDM4C). journal of biological chemistry Pack, L. R., Yamamoto, K. R., Fujimori, D. G. 2016; 291 (12): 6060-6070


    Histone H3 lysine 4 trimethylation (H3K4me3) and histone H3 lysine 9 trimethylation (H3K9me3) are epigenetic marks with opposing roles in transcription regulation. Whereas colocalization of these modifications is generally excluded in the genome, how this preclusion is established remains poorly understood. Lysine demethylase 4C (KDM4C), an H3K9me3 demethylase, localizes predominantly to H3K4me3-containing promoters through its hybrid tandem tudor domain (TTD) (1, 2), providing a model for how these modifications might be excluded. We quantitatively investigated the contribution of the TTD to the catalysis of H3K9me3 demethylation by KDM4C and demonstrated that TTD-mediated recognition of H3K4me3 stimulates demethylation of H3K9me3 in cis on peptide and mononucleosome substrates. Our findings support a multivalent interaction mechanism, by which an activating mark, H3K4me3, recruits and stimulates KDM4C to remove the repressive H3K9me3 mark, thus facilitating exclusion. In addition, our work suggests that differential TTD binding properties across the KDM4 demethylase family may differentiate their targets in the genome.

    View details for DOI 10.1074/jbc.M115.696864

    View details for PubMedID 26747609

  • Product Binding Enforces the Genomic Specificity of a Yeast Polycomb Repressive Complex CELL Dumesic, P. A., Homer, C. M., Moresco, J. J., Pack, L. R., Shanle, E. K., Coyle, S. M., Strahl, B. D., Fujimori, D. G., Yates, J. R., Madhani, H. D. 2015; 160 (1-2): 204-218


    We characterize the Polycomb system that assembles repressive subtelomeric domains of H3K27 methylation (H3K27me) in the yeast Cryptococcus neoformans. Purification of this PRC2-like protein complex reveals orthologs of animal PRC2 components as well as a chromodomain-containing subunit, Ccc1, which recognizes H3K27me. Whereas removal of either the EZH or EED ortholog eliminates H3K27me, disruption of mark recognition by Ccc1 causes H3K27me to redistribute. Strikingly, the resulting pattern of H3K27me coincides with domains of heterochromatin marked by H3K9me. Indeed, additional removal of the C. neoformans H3K9 methyltransferase Clr4 results in loss of both H3K9me and the redistributed H3K27me marks. These findings indicate that the anchoring of a chromatin-modifying complex to its product suppresses its attraction to a different chromatin type, explaining how enzymes that act on histones, which often harbor product recognition modules, may deposit distinct chromatin domains despite sharing a highly abundant and largely identical substrate-the nucleosome.

    View details for DOI 10.1016/j.cell.2014.11.039

    View details for Web of Science ID 000347923200019

    View details for PubMedID 25533783

  • Presteady State Kinetic Investigation of the Incorporation of Anti-Hepatitis B Nucleotide Analogues Catalyzed by Noncanonical Human DNA Polymerases CHEMICAL RESEARCH IN TOXICOLOGY Brown, J. A., Pack, L. R., Fowler, J. D., Suo, Z. 2012; 25 (1): 225-233


    Antiviral nucleoside analogues have been developed to inhibit the enzymatic activities of the hepatitis B virus (HBV) polymerase, thereby preventing the replication and production of HBV. However, the usage of these analogues can be limited by drug toxicity because the 5'-triphosphates of these nucleoside analogues (nucleotide analogues) are potential substrates for human DNA polymerases to incorporate into host DNA. Although they are poor substrates for human replicative DNA polymerases, it remains to be established whether these nucleotide analogues are substrates for the recently discovered human X- and Y-family DNA polymerases. Using presteady state kinetic techniques, we have measured the substrate specificity values for human DNA polymerases ?, ?, ?, ?, ?, and Rev1 incorporating the active forms of the following anti-HBV nucleoside analogues approved for clinical use: adefovir, tenofovir, lamivudine, telbivudine, and entecavir. Compared to the incorporation of a natural nucleotide, most of the nucleotide analogues were incorporated less efficiently (2 to >122,000) by the six human DNA polymerases. In addition, the potential for entecavir and telbivudine, two drugs which possess a 3'-hydroxyl, to become embedded into human DNA was examined by primer extension and DNA ligation assays. These results suggested that telbivudine functions as a chain terminator, while entecavir was efficiently extended by the six enzymes and was a substrate for human DNA ligase I. Our findings suggested that incorporation of anti-HBV nucleotide analogues catalyzed by human X- and Y-family polymerases may contribute to clinical toxicity.

    View details for DOI 10.1021/tx200458s

    View details for Web of Science ID 000299028700025

    View details for PubMedID 22132702

  • Efficiency and fidelity of human DNA polymerases lambda and beta during gap-filling DNA synthesis DNA REPAIR Brown, J. A., Pack, L. R., Sanman, L. E., Suo, Z. 2011; 10 (1): 24-33


    The base excision repair (BER) pathway coordinates the replacement of 1-10 nucleotides at sites of single-base lesions. This process generates DNA substrates with various gap sizes which can alter the catalytic efficiency and fidelity of a DNA polymerase during gap-filling DNA synthesis. Here, we quantitatively determined the substrate specificity and base substitution fidelity of human DNA polymerase ? (Pol ?), an enzyme proposed to support the known BER DNA polymerase ? (Pol ?), as it filled 1-10-nucleotide gaps at 1-nucleotide intervals. Pol ? incorporated a correct nucleotide with relatively high efficiency until the gap size exceeded 9 nucleotides. Unlike Pol ?, Pol ? did not have an absolute threshold on gap size as the catalytic efficiency for a correct dNTP gradually decreased as the gap size increased from 2 to 10 nucleotides and then recovered for non-gapped DNA. Surprisingly, an increase in gap size resulted in lower polymerase fidelity for Pol ?, and this downregulation of fidelity was controlled by its non-enzymatic N-terminal domains. Overall, Pol ? was up to 160-fold more error-prone than Pol ?, thereby suggesting Pol ? would be more mutagenic during long gap-filling DNA synthesis. In addition, dCTP was the preferred misincorporation for Pol ? and its N-terminal domain truncation mutants. This nucleotide preference was shown to be dependent upon the identity of the adjacent 5'-template base. Our results suggested that both Pol ? and Pol ? would catalyze nucleotide incorporation with the highest combination of efficiency and accuracy when the DNA substrate contains a single-nucleotide gap. Thus, Pol ?, like Pol ?, is better suited to catalyze gap-filling DNA synthesis during short-patch BER in vivo, although, Pol ? may play a role in long-patch BER.

    View details for DOI 10.1016/j.dnarep.2010.09.005

    View details for Web of Science ID 000286547400005

    View details for PubMedID 20961817

  • Pre-Steady-State Kinetic Analysis of the Incorporation of Anti-HIV Nucleotide Analogs Catalyzed by Human X- and Y-Family DNA Polymerases ANTIMICROBIAL AGENTS AND CHEMOTHERAPY Brown, J. A., Pack, L. R., Fowler, J. D., Suo, Z. 2011; 55 (1): 276-283


    Nucleoside reverse transcriptase inhibitors (NRTIs) are an important class of antiviral drugs used to manage infections by human immunodeficiency virus, which causes AIDS. Unfortunately, these drugs cause unwanted side effects, and the molecular basis of NRTI toxicity is not fully understood. Putative routes of NRTI toxicity include the inhibition of human nuclear and mitochondrial DNA polymerases. A strong correlation between mitochondrial toxicity and NRTI incorporation catalyzed by human mitochondrial DNA polymerase has been established both in vitro and in vivo. However, it remains to be determined whether NRTIs are substrates for the recently discovered human X- and Y-family DNA polymerases, which participate in DNA repair and DNA lesion bypass in vivo. Using pre-steady-state kinetic techniques, we measured the substrate specificity constants for human DNA polymerases ?, ?, ?, ?, ?, and Rev1 incorporating the active, 5'-phosphorylated forms of tenofovir, lamivudine, emtricitabine, and zidovudine. For the six enzymes, all of the drug analogs were incorporated less efficiently (40- to >110,000-fold) than the corresponding natural nucleotides, usually due to a weaker binding affinity and a slower rate of incorporation for the incoming nucleotide analog. In general, the 5'-triphosphate forms of lamivudine and zidovudine were better substrates than emtricitabine and tenofovir for the six human enzymes, although the substrate specificity profile depended on the DNA polymerase. Our kinetic results suggest NRTI insertion catalyzed by human X- and Y-family DNA polymerases is a potential mechanism of NRTI drug toxicity, and we have established a structure-function relationship for designing improved NRTIs.

    View details for DOI 10.1128/AAC.01229-10

    View details for Web of Science ID 000285577400033

    View details for PubMedID 21078938

  • Identification of Critical Residues for the Tight Binding of Both Correct and Incorrect Nucleotides to Human DNA Polymerase lambda JOURNAL OF MOLECULAR BIOLOGY Brown, J. A., Pack, L. R., Sherrer, S. M., Kshetry, A. K., Newmister, S. A., Fowler, J. D., Taylor, J., Suo, Z. 2010; 403 (4): 505-515


    DNA polymerase ? (Pol ?) is a novel X-family DNA polymerase that shares 34% sequence identity with DNA polymerase ?. Pre-steady-state kinetic studies have shown that the Pol ?-DNA complex binds both correct and incorrect nucleotides 130-fold tighter, on average, than the DNA polymerase ?-DNA complex, although the base substitution fidelity of both polymerases is 10(-)(4) to 10(-5). To better understand Pol ?'s tight nucleotide binding affinity, we created single-substitution and double-substitution mutants of Pol ? to disrupt the interactions between active-site residues and an incoming nucleotide or a template base. Single-turnover kinetic assays showed that Pol ? binds to an incoming nucleotide via cooperative interactions with active-site residues (R386, R420, K422, Y505, F506, A510, and R514). Disrupting protein interactions with an incoming correct or incorrect nucleotide impacted binding to each of the common structural moieties in the following order: triphosphate?base>ribose. In addition, the loss of Watson-Crick hydrogen bonding between the nucleotide and the template base led to a moderate increase in K(d). The fidelity of Pol ? was maintained predominantly by a single residue, R517, which has minor groove interactions with the DNA template.

    View details for DOI 10.1016/j.jmb.2010.09.014

    View details for Web of Science ID 000284440500002

    View details for PubMedID 20851705

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