2022: landmark year for CRISPR as CRISPR 2.0 enters human trials

CRISPR is a powerful genome-editing tool that allows researchers to alter DNA sequences to modify gene function easily. CRISPR can correct genetic defects and treat and prevent disease spread.

CRISPR-Cas is a natural defense system that microbes use. Our scientists have converted it into a molecular biology tool. CRISPR genome editing involves targeting a specific DNA sequence to delete or insert genetic material such as new genes at that precise location.

CRISPR stands for 'Clustered Regularly Interspaced Short Palindromic Repeats.' It represents a sequence of letters in DNA.

Cas means 'CRISPR-associated protein.' Numerous Cas proteins are available with various functions. One of them, Cas9, is an enzyme that cuts specific DNA sites.

CRISPR sequences and Cas proteins work in combination - they are referred to as a CRISPR-Cas system, often abbreviated to just CRISPR.

The original CRISPR vs CRISPR 2.0

In CRISPR–Cas9 genome editing, the Cas9 enzyme breaks both DNA strands at the site to be edited. When the cell's DNA-repair processes stitch the strands back together, errors may happen, leading to unwanted DNA sequences. Hence, unwanted 'off-target' effects are a big challenge in the standard CRISPR–Cas9 system.

Base editing, by contrast, is more precise. It cuts only one DNA strand rather than both. The Cas9 directs the base-editing enzyme to the correct location in the genome, where only one edit takes place.

Base editing, in other words, chemically converts one DNA letter directly into another, e.g., converting a T to an A or a G to a C, without breaking both DNA strands. This is something that standard CRISPR–Cas9 can't do.

Base editing was introduced in 2016. In the last six years, researchers have designed multiple base editors that alter DNA with greater efficiency and better precision, i.e., with lower chances of introducing unwanted genetic changes. Some scientists call it CRISPR 2.0.

Two trials are going on wrt base-editing.
1. Base-editing in a cholesterol-regulating gene (by Verve Therapeutics).
2. Base editing in sickle-cell disease (by Beam Therapeutics).

Both trials involve single letter DNA edits without breaking both DNA strands, as CRISPR–Cas9 would do. Results are expected in 2023.

Sekar Kathiresan, CEO and co-founder of Verve Therapeutics, announced recently that Verve has dosed the first patient with base editing.

In short, we will soon see the results of precise, single-letter edits in DNA sequence.

Base-editing vs Prime-editing:

Base-editing has its own challenges - it can't help genetic disorders caused by multi-letter mutations. Tay–Sachs disease is typically caused by the insertion of four DNA letters into the HEXA gene. Here you need greater accuracy and precision.

A newer, more precise technique called prime editing can help you here by letting you do only the edits you want. You can alter individual DNA letters, delete the letters you don't want, or insert a series of letters - all these with minimal damage to DNA strands.

That could make prime-editing-based gene therapies safer for use in people.

RNA editing:

RNA editing is another technique where we alter an RNA sequence without changing the sequence or integrity of genomic DNA. Although discovered more than 30 years ago, “RNA editing” - the site-specific substitution of RNA, has garnered increasing attention in recent years.

References:

1. Ledford H. CRISPR: gene editing is just the beginning. Nature. 2016 Mar 10;531(7593):156-9. doi: 10.1038/531156a. PMID: 26961639.
2. Ledford H. Beyond CRISPR: A guide to the many other ways to edit a genome. Nature. 2016 Aug 11;536(7615):136-7. doi: 10.1038/536136b. PMID: 27510203.
3. Ledford H. Super-precise new CRISPR tool could tackle a plethora of genetic diseases. Nature. 2019 Oct;574(7779):464-465. doi: 10.1038/d41586-019-03164-5. PMID: 31641267.
4. Ledford H. CRISPR 'cousin' put to the test in landmark heart-disease trial. Nature. 2022 Jul 15. doi: 10.1038/d41586-022-01951-1. Epub ahead of print. PMID: 35840676.
5. Kung CP, Maggi LB Jr, Weber JD. The Role of RNA Editing in Cancer Development and Metabolic Disorders. Front Endocrinol (Lausanne). 2018 Dec 18;9:762. doi: 10.3389/fendo.2018.00762. PMID: 30619092; PMCID: PMC6305585.

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