Is TALEN or CRISPR/Cas9 best for your project?

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The gold standard for genetically engineering mouse models is ES-cell based homologous recombination. However, this approach is very time-consuming and costly. Recently, TALEN and CRISPR/Cas9 systems have been harnessed to edit genomes of cultured cells, mice and rats1,2. Both systems can be used to create knockouts, and to introduce point mutations or small insertions, but each has distinct advantages (see Table 1). TALENs are chimeric proteins composed of site-specific DNA-binding domains fused to the non-specific endonuclease FokI. CRISPR/Cas9 uses a site-specific single guide RNA (sgRNA) to direct the Cas9 nuclease to its target locus.

TALENCRISPR/Cas9
OriginPlant pathogenic bacteria (Xanthomonas)Diverse bacteria
ComponentsPairs of TALE-FokI fusion proteinsGuide RNA and Cas9
EfficiencyHighHigh but variable
Off-target effectsMinorModerate to high
Target site availabilityNo restrictionRequires PAM (NGG) motif
Time required for vector engineeringOne week1-3 days
MultiplexingModerateEfficient

Off-target effects

Because TALEN pairs bind opposite sides of the target site, TALEN-mediated cleavage at other sites in the genome is unlikely4. In contrast, off-target effects have been reported using CRISPR/Cas9 in cell lines3, but analyses of CRISPR/Cas9 knockout mice suggest lower off-target frequency in vivo5.

Efficiency and multiplexing

Genome modifications can be introduced by directly injecting RNAs encoding TALENs or Cas9 protein and gRNA, into one-cell stage fertilized eggs5,6. Mutations can also be introduced at multiple loci by coinjecting multiple gRNAs with Cas94.

Target sites

TALENs can be generated to specifically target nearly any sequence in the genome. In contrast, target site selection for CRISPR/Cas9 is limited by the requirement for a PAM (NGG) sequence7. Since either DNA strand can be targeted, this is no barrier for gene knockout, but may present difficulties in site-specific mutations or insertions.

Target design and construction

Because targeting of CRISPR/Cas9 relies on simple RNA/DNA hybridization, gRNAs are easier to design and construct than TALENs, taking only 1-3 days. However, currently available TALEN recognition modules have greatly reduced work required to clone TALEN vectors.

Both TALEN and CRISPR/Cas9 systems show great promise- Which approach should you choose? To generate a single- or double-knockout quickly, try CRISPR/Cas9. Otherwise, TALEN offers fewer off-target effects and target sequence requirements. Unfortunately, for the generation of conditional and inducible alleles, ES cell-based gene targeting remains the only option.

References

  1. Gaj T, Gersbach CA, Barbas CF 3rd (2013) ZFN, TALEN, and CRISPR/Cas-based methods for genome engineering. Trends Biotechnol. 31: 397-405
  2. Shalem O, Sanjana NE, Hartenian E, Shi X, Scott DA, Mikkelsen TS, Heckl D, Ebert BL, Root DE, Doench JG, Zhang F (2014) Genome-scale CRISPR-Cas9 knockout screening in human cells. Science 343: 84-87
  3. Fu Y, Foden JA, Khayter C, Maeder ML, Reyon D, Joung JK, Sander JD (2013) High-frequency off-target mutagenesis induced by CRISPR-Cas nucleases in human cells. Nat Biotechnol. 31: 822-826.
  4. Park CY, Kim J, Kweon J, Son JS, Lee JS, Yoo JE, Cho SR, Kim JH, Kim JS, Kim DW (2014) Targeted inversion and reversion of the blood coagulation factor 8 gene in human iPS cells using TALENs. Proc Natl Acad Sci USA 111: 9253-9258
  5. Wang H, Yang H, Shivalila CS, Dawlaty MM, Cheng AW, Zhang F, Jaenisch R (2013) One-step generation of mice carrying mutations in multiple genes by CRISPR/Cas-mediated genome engineering. Cell 153:910-918
  6. Tesson L, Usal C, M\A8\A6noret S, Leung E, Niles BJ, Remy S, Santiago Y, Vincent AI, Meng X, Zhang L, Gregory PD, Anegon I, Cost GJ (2011) Knockout rats generated by embryo microinjection of TALENs. Nat Biotechnol. 29: 695-696
  7. Hwang WY, Fu Y, Reyon D, Maeder ML, Tsai SQ, Sander JD, Peterson RT, Yeh JR, Joung JK (2013) Efficient genome editing in zebrafish using a CRISPR-Cas system. Nat Biotechnol. 31: 227-229
Original Source: Cyagen Biosciences

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