Honors & Awards
Office of Graduate Education Travel Grant, Stanford Office of Graduate Education (2016)
President's Undergraduate Research Fellowship, University of California, Davis (3/6/2011-9/6/2012)
NSF fellowship, National Science Foundation (2013)
UC Davis College of Biological Sciences Summer Research Grant, University of California, Davis (6/1/2011-9/1/2011)
Chancellor's Award for Excellence in Undergraduate Research, University of California, Davis (05/2012-6/2012)
Ronald and Lydia Baskin College Medal for Original and Outstanding Research, University of California, Davis (05/2012-6/2012)
Doctor of Philosophy, Stanford University, GENE-PHD (2017)
PhD, Stanford University, Genetics (2017)
Master of Science, Stanford University, BIOM-MS (2017)
B.S., University of California, Davis, Genetics (2012)
On the Origin of Reverse Transcriptase-Using CRISPR-Cas Systems and Their Hyperdiverse, Enigmatic Spacer Repertoires.
2017; 8 (4)
Cas1 integrase is the key enzyme of the clustered regularly interspaced short palindromic repeat (CRISPR)-Cas adaptation module that mediates acquisition of spacers derived from foreign DNA by CRISPR arrays. In diverse bacteria, the cas1 gene is fused (or adjacent) to a gene encoding a reverse transcriptase (RT) related to group II intron RTs. An RT-Cas1 fusion protein has been recently shown to enable acquisition of CRISPR spacers from RNA. Phylogenetic analysis of the CRISPR-associated RTs demonstrates monophyly of the RT-Cas1 fusion, and coevolution of the RT and Cas1 domains. Nearly all such RTs are present within type III CRISPR-Cas loci, but their phylogeny does not parallel the CRISPR-Cas type classification, indicating that RT-Cas1 is an autonomous functional module that is disseminated by horizontal gene transfer and can function with diverse type III systems. To compare the sequence pools sampled by RT-Cas1-associated and RT-lacking CRISPR-Cas systems, we obtained samples of a commercially grown cyanobacterium-Arthrospira platensis Sequencing of the CRISPR arrays uncovered a highly diverse population of spacers. Spacer diversity was particularly striking for the RT-Cas1-containing type III-B system, where no saturation was evident even with millions of sequences analyzed. In contrast, analysis of the RT-lacking type III-D system yielded a highly diverse pool but reached a point where fewer novel spacers were recovered as sequencing depth was increased. Matches could be identified for a small fraction of the non-RT-Cas1-associated spacers, and for only a single RT-Cas1-associated spacer. Thus, the principal source(s) of the spacers, particularly the hypervariable spacer repertoire of the RT-associated arrays, remains unknown.IMPORTANCE While the majority of CRISPR-Cas immune systems adapt to foreign genetic elements by capturing segments of invasive DNA, some systems carry reverse transcriptases (RTs) that enable adaptation to RNA molecules. From analysis of available bacterial sequence data, we find evidence that RT-based RNA adaptation machinery has been able to join with CRISPR-Cas immune systems in many, diverse bacterial species. To investigate whether the abilities to adapt to DNA and RNA molecules are utilized for defense against distinct classes of invaders in nature, we sequenced CRISPR arrays from samples of commercial-scale open-air cultures of Arthrospira platensis, a cyanobacterium that contains both RT-lacking and RT-containing CRISPR-Cas systems. We uncovered a diverse pool of naturally occurring immune memories, with the RT-lacking locus acquiring a number of segments matching known viral or bacterial genes, while the RT-containing locus has acquired spacers from a distinct sequence pool for which the source remains enigmatic.
View details for DOI 10.1128/mBio.00897-17
View details for PubMedID 28698278
View details for PubMedCentralID PMC5513706
High-Throughput Characterization of Cascade type I-E CRISPR Guide Efficacy Reveals Unexpected PAM Diversity and Target Sequence Preferences.
2017; 206 (4): 1727–38
Interactions between Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) RNAs and CRISPR-associated (Cas) proteins form an RNA-guided adaptive immune system in prokaryotes. The adaptive immune system utilizes segments of the genetic material of invasive foreign elements in the CRISPR locus. The loci are transcribed and processed to produce small CRISPR RNAs (crRNAs), with degradation of invading genetic material directed by a combination of complementarity between RNA and DNA and in some cases recognition of adjacent motifs called PAMs (Protospacer Adjacent Motifs). Here we describe a general, high-throughput procedure to test the efficacy of thousands of targets, applying this to the Escherichia coli type I-E Cascade (CRISPR-associated complex for antiviral defense) system. These studies were followed with reciprocal experiments in which the consequence of CRISPR activity was survival in the presence of a lytic phage. From the combined analysis of the Cascade system, we found that (i) type I-E Cascade PAM recognition is more expansive than previously reported, with at least 22 distinct PAMs, with many of the noncanonical PAMs having CRISPR-interference abilities similar to the canonical PAMs; (ii) PAM positioning appears precise, with no evidence for tolerance to PAM slippage in interference; and (iii) while increased guanine-cytosine (GC) content in the spacer is associated with higher CRISPR-interference efficiency, high GC content (>62.5%) decreases CRISPR-interference efficiency. Our findings provide a comprehensive functional profile of Cascade type I-E interference requirements and a method to assay spacer efficacy that can be applied to other CRISPR-Cas systems.
View details for DOI 10.1534/genetics.117.202580
View details for PubMedID 28634160
View details for PubMedCentralID PMC5560783
- Distinct patterns of Cas9 mismatch tolerance in vitro and in vivo NUCLEIC ACIDS RESEARCH 2016; 44 (11): 5365-5377
Cas9 Variants Expand the Target Repertoire in Caenorhabditis elegans.
2016; 202 (2): 381-388
The proliferation of CRISPR/Cas9-based methods in Caenorhabditis elegans has enabled efficient genome editing and precise genomic tethering of Cas9 fusion proteins. Experimental designs using CRISPR/Cas9 are currently limited by the need for a protospacer adjacent motif (PAM) in the target with the sequence NGG. Here we report the characterization of two modified Cas9 proteins in C. elegans that recognize NGA and NGCG PAMs. We found that each variant could stimulate homologous recombination with a donor template at multiple loci and that PAM specificity was comparable to that of wild-type Cas9. To directly compare effectiveness, we used CRISPR/Cas9 genome editing to generate a set of assay strains with a common single-guide RNA (sgRNA) target sequence, but that differ in the juxtaposed PAM (NGG, NGA, or NGCG). In this controlled setting, we determined that the NGA PAM Cas9 variant can be as effective as wild-type Cas9. We similarly edited a genomic target to study the influence of the base following the NGA PAM. Using four strains with four NGAN PAMs differing only at the fourth position and adjacent to the same sgRNA target, we observed that efficient homologous replacement was attainable with any base in the fourth position, with an NGAG PAM being the most effective. In addition to demonstrating the utility of two Cas9 mutants in C. elegans and providing reagents that permit CRISPR/Cas9 experiments with fewer restrictions on potential targets, we established a means to benchmark the efficiency of different Cas9::PAM combinations that avoids variations owing to differences in the sgRNA sequence.
View details for DOI 10.1534/genetics.115.185041
View details for PubMedID 26680661
View details for PubMedCentralID PMC4788222
DNA polymerases d and ? cooperate in repairing double-strand breaks by microhomology-mediated end-joining in Saccharomyces cerevisiae.
Proceedings of the National Academy of Sciences of the United States of America
2015; 112 (50): E6907-16
Maintenance of genome stability is carried out by a suite of DNA repair pathways that ensure the repair of damaged DNA and faithful replication of the genome. Of particular importance are the repair pathways, which respond to DNA double-strand breaks (DSBs), and how the efficiency of repair is influenced by sequence homology. In this study, we developed a genetic assay in diploid Saccharomyces cerevisiae cells to analyze DSBs requiring microhomologies for repair, known as microhomology-mediated end-joining (MMEJ). MMEJ repair efficiency increased concomitant with microhomology length and decreased upon introduction of mismatches. The central proteins in homologous recombination (HR), Rad52 and Rad51, suppressed MMEJ in this system, suggesting a competition between HR and MMEJ for the repair of a DSB. Importantly, we found that DNA polymerase delta (Pol δ) is critical for MMEJ, independent of microhomology length and base-pairing continuity. MMEJ recombinants showed evidence that Pol δ proofreading function is active during MMEJ-mediated DSB repair. Furthermore, mutations in Pol δ and DNA polymerase 4 (Pol λ), the DNA polymerase previously implicated in MMEJ, cause a synergistic decrease in MMEJ repair. Pol λ showed faster kinetics associating with MMEJ substrates following DSB induction than Pol δ. The association of Pol δ depended on RAD1, which encodes the flap endonuclease needed to cleave MMEJ intermediates before DNA synthesis. Moreover, Pol δ recruitment was diminished in cells lacking Pol λ. These data suggest cooperative involvement of both polymerases in MMEJ.
View details for DOI 10.1073/pnas.1507833112
View details for PubMedID 26607450
View details for PubMedCentralID PMC4687552
Landscape of target:guide homology effects on Cas9-mediated cleavage.
Nucleic acids research
2014; 42 (22): 13778-13787
To study target sequence specificity, selectivity, and reaction kinetics of Streptococcus pyogenes Cas9 activity, we challenged libraries of random variant targets with purified Cas9::guide RNA complexes in vitro. Cleavage kinetics were nonlinear, with a burst of initial activity followed by slower sustained cleavage. Consistent with other recent analyses of Cas9 sequence specificity, we observe considerable (albeit incomplete) impairment of cleavage for targets mutated in the PAM sequence or in 'seed' sequences matching the proximal 8 bp of the guide. A second target region requiring close homology was located at the other end of the guide::target duplex (positions 13-18 relative to the PAM). Sequences flanking the guide+PAM region had measurable (albeit modest) effects on cleavage. In addition, the first-base Guanine constraint commonly imposed by gRNA expression systems has little effect on overall cleavage efficiency. Taken together, these studies provide an in vitro understanding of the complexities of Cas9-gRNA interaction and cleavage beyond the general paradigm of site determination based on the 'seed' sequence and PAM.
View details for DOI 10.1093/nar/gku1102
View details for PubMedID 25399416
View details for PubMedCentralID PMC4267615
- Efficient Marker-Free Recovery of Custom Genetic Modifications with CRISPR/Cas9 in Caenorhabditis elegans GENETICS 2014; 198 (3): 837-U842
Alleles of the homologous recombination gene, RAD59, identify multiple responses to disrupted DNA replication in Saccharomyces cerevisiae.
2013; 13: 229-?
In Saccharomyces cerevisiae, Rad59 is required for multiple homologous recombination mechanisms and viability in DNA replication-defective rad27 mutant cells. Recently, four rad59 missense alleles were found to have distinct effects on homologous recombination that are consistent with separation-of-function mutations. The rad59-K166A allele alters an amino acid in a conserved α-helical domain, and, like the rad59 null allele diminishes association of Rad52 with double-strand breaks. The rad59-K174A and rad59-F180A alleles alter amino acids in the same domain and have genetically similar effects on homologous recombination. The rad59-Y92A allele alters a conserved amino acid in a separate domain, has genetically distinct effects on homologous recombination, and does not diminish association of Rad52 with double-strand breaks.In this study, rad59 mutant strains were crossed with a rad27 null mutant to examine the effects of the rad59 alleles on the link between viability, growth and the stimulation of homologous recombination in replication-defective cells. Like the rad59 null allele, rad59-K166A was synthetically lethal in combination with rad27. The rad59-K174A and rad59-F180A alleles were not synthetically lethal in combination with rad27, had effects on growth that coincided with decreased ectopic gene conversion, but did not affect mutation, unequal sister-chromatid recombination, or loss of heterozygosity. The rad59-Y92A allele was not synthetically lethal when combined with rad27, stimulated ectopic gene conversion and heteroallelic recombination independently from rad27, and was mutually epistatic with srs2. Unlike rad27, the stimulatory effect of rad59-Y92A on homologous recombination was not accompanied by effects on growth rate, cell cycle distribution, mutation, unequal sister-chromatid recombination, or loss of heterozygosity.The synthetic lethality conferred by rad59 null and rad59-K166A alleles correlates with their inhibitory effect on association of Rad52 with double-strand breaks, suggesting that this may be essential for rescuing replication lesions in rad27 mutant cells. The rad59-K174A and rad59-F180A alleles may fractionally reduce this same function, which proportionally reduced repair of replication lesions by homologous recombination and growth rate. In contrast, rad59-Y92A stimulates homologous recombination, perhaps by affecting association of replication lesions with the Rad51 recombinase. This suggests that Rad59 influences the rescue of replication lesions by multiple recombination factors.
View details for DOI 10.1186/1471-2180-13-229
View details for PubMedID 24125552
View details for PubMedCentralID PMC3852934
- MicrobiologyOpen Rad59 regulates association of Rad52 with DNA double-strand breaks 2012