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#ZFN

3 public questions tagged with this topic.

Transgenic Plant -2

Major advantage of CRISPR over ZFN/TALEN is:

Major advantage prompting global adoption of CRISPR over zinc finger nucleases and TALEN is unparalleled ease of design, cloning and deployment reducing cost and technical expertise barrier. ZFN construction involves library of zinc finger modules each specific for 5'GNN 3' triplet; assembly of three to six modules suffers from context dependence where finger-finger junctions alter specificity requiring selection via bacterial two-hybrid or OPEN platforms taking months and low success rates for AT-rich targets. TALEN improves modularity with simple RVD code but still demands assembly of 15-20 repeat plasmids via Golden Gate requiring multiple ligations, sequence verification, 5-7 days per TALEN, limited by repeat instability in E. coli due to homologous recombination. CRISPR eliminates protein engineering entirely: chemically synthesized pair of oligonucleotides encoding 20 nt spacer annealed and ligated into guide RNA expression vector or synthesized as single guide RNA ribonucleoprotein complex ready for transfection electroporation within one day. No protein evolution. Commercial kits provide optimized SpCas9 protein, high-fidelity variants, base editors. Consequently labs worldwide adopted CRISPR for gene knockout, knockin, activation, leading to over 10000 publications yearly, crop edits and two Nobel prize.

Ref: Hsu et al. Cell 2014 157:1262 CRISPR ease vs ZFN TALEN; Zhang Feng Nat Protocols ease design.

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Transgenic Plant -2

Like ZFN, TALENs use which nuclease domain?

Like zinc finger nucleases, transcription activator-like effector nucleases utilize catalytic domain of FokI endonuclease for cleavage because DNA binding domain lacks nuclease activity. TALE repeats provide programmable anchoring but cannot cut phosphodiester backbone; fusion via flexible linker to C-terminal 196 aa FokI cleavage domain confers scission capability. FokI domain requires dimerization, so two TALEN monomers engineered to bind opposite strands in tail-to-tail orientation with spacer 12 to 21 bp allow FokI domains to associate forming active nuclease complex generating double-strand break with variable overhang within spacer. Truncation analysis identified optimal N-terminal 152 aa upstream of repeats and C-terminal 63 aa linker downstream essential for activity, influencing binding affinity and steric positioning of FokI. Introduction of heterodimeric FokI mutations ELD KKR improves specificity reducing homodimer off-target cutting. Use of shared FokI domain enables transfer of knowledge about dimerization requirement, spacer length optimization, and fidelity enhancements from ZFN to TALEN, streamlining both platforms and establishing architecture reused in dimeric CRISPR-FokI dCas9-FokI fusions for improved specificity via dual guide requirement.

Ref: Christian et al. Genetics 2010 186:757 TALEN FokI; Joung Sander Nat Rev Mol Cell Biol 2013 FokI.

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Gene Therapy

CRISPR, ZFN and TALEN are used in gene therapy for:

CRISPR-Cas9, ZFN, TALEN constitute programmable site-specific endonucleases enabling precise genome manipulation. CRISPR uses single-guide RNA 20 nt spacer complementary to genomic target adjacent NGG PAM recognized Streptococcus pyogenes Cas9 possessing HNH domain cleaving complementary strand RuvC noncomplementary generating blunt double-strand break 3 bp upstream PAM. Zinc-finger arrays modules each recognizing 3 bp triplets linked FokI nuclease dimerize cutting within spacer, while TALEN repeats repeat-variable di-residues HD recognizing cytosine NI adenine NG thymine NN guanine provide single-base flexibility. Cellular repair via error-prone nonhomologous end-joining introduces indels knocking out CCR5 for HIV resistance, while homology-directed repair donor template 400 bp homology arms flanking break precisely corrects sickle HBB glutamate 6 valine mutation or inserts therapeutic transgene into safe harbor AAVS1 chromosome 19 constitutive expression under endogenous promoter. Permanent genomic alteration distinguishes editing from transient augmentation offering curative potential single intervention approved exagamglogene autotemcel for sickle cell. This mechanistic insight guides vector optimization, dosing strategies, and clinical safety monitoring essential for translational development and regulatory evaluation.

Ref: NIH Genome Editing CRISPR ZFN TALEN Overview; Nature Biotech Comparison 2020; NCBI Gene Editing Therapy https://www.ncbi.nlm.nih.gov/books/NBK542207/.

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