CRISPR has been largely hailed as the breakthrough in genome editing, which helps us rewrite the DNA code to cure diseases, select for better crop variants, and a host of other applications.
However, one of the biggest limitations to the widespread use of genome-editing technology is the off-target effects. So far, CRISPR has been restricted to editing DNA in somatic cells because we cannot completely control any unintended changes. Extensive work on improving the editing efficiency has been at the heart of R&D in genetic engineering and gene therapy.
Apart from CRISPR, there have been many genome editing technologies that have been tried and tested. TALENs (Transcription activator-like effector nuclease) are widely used to scan, target, and edit DNA. They are similar to Cas9 in their recognition of signature DNA sequence but differ in their target-binding mode.
TALENs can be customized to recognize an array of 33-34 amino acid sequences, which enables DNA-binding to the target. A new report in Nature Communications demonstrated that TALEN could outperform CRISPR in certain regions of DNA.
Using single-cell microscopy and sequencing, a team of scientists from the University of Illinois, Urbana-Champaign, have assessed how TALENs and CRISPR perform gene-editing in live mammalian cells. Comparing the Cas9 and TALEN for their genome scanning abilities, the authors found that TALEN is five times more efficient in scanning heterochromatin (tightly-wound and condensed) regions of the DNA while CRISPR works better in navigating euchromatin (unfolded DNA, usually transcriptionally active).
TALENs use a combination of local search and 3D-diffusion to locate target sites, while CRISPR relies only on local search behavior. TALEN also demonstrated higher editing efficiency than CRISPR in heterochromatic regions.
According to the lead investigator Dr. Huimin Zhao, Professor of Chemical and Biomolecular Engineering, “the study adds to the evidence that a broader selection of genome-editing tools is needed to target all parts of the genome.”
This study underlines the importance of the availability of multiple strategies for genome editing, which can be combined to enhance the efficacy of gene therapy. It is particularly challenging to access and edit the dense heterochromatin DNA, which has been linked to diseases such as Fragile-X syndrome and sickle cell anemia. With the use of TALENs, there may be improved gene editing methods to address such disorders.
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