The Scientist - USA (2021-12)

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Other companies, such as San Diego–

based Locanabio, are also coupling Cas

enzymes with ADAR to improve targeting.

But there could be disadvantages to using

Cas and other bacterially derived proteins,

according to Erez Levanon, a computa-

tional biologist with Bar-Ilan University

in Israel and former consultant for the San

Diego company ADARx Pharmaceuticals,

which is developing ADAR-based RNA-

targeting therapies. Compared to ADARs,

which aren’t foreign to human cells, Cas

enzymes are more likely to set off immune

reactions, he says.

Some groups are tweaking ADARs

directly to get the same specificity benefit

without using Cas. University of Califor-

nia, San Francisco, biologist Leanna Mon-

teleone and her colleagues, for instance,

have developed a unique ADAR that only

edits As when there’s no base attached to

the ribose backbone across from them in

double-stranded RNA—a situation that is

extremely rare in naturally occurring RNA,

but that can be engineered into a guide

RNA. In vitro testing showed that this so-

called “bump-hole” strategy led to fewer

off-target edits, says Monteleone, who has

applied for a patent on the technology.

Other teams are exploring the possibil-

ity of using a person’s own ADAR enzymes,

rather than delivering them alongside a

guide RNA. “You can deliver only a guide

RNA molecule, and recruit the endogenous

wild-type ADAR that’s already present

inside the cell, and do that at very high effi-

ciency and specificity,” says Huss—some-

thing first demonstrated back in 1995, and

something that a few companies, includ-

ing Shape, are exploring. “That kind of

approach is beautiful,” Rosenthal says. “In

therapeutics, the simpler the system, the

better, in general.” Nakae agrees, adding

that the elegance of a system that uses a

guide RNA alone to recruit ADARs already

present in cells is “one of the reasons why

ADAR, I think, is attracting attention.”

Strides are being made on generat-

ing guide RNAs, too. Shape Therapeu-

tics, for example, has invested heavily in

designing guides, making use of machine-

learning techniques and high-throughput

screening, says Huss. Such approaches

could open up the possibility of editing

“any adenosine in the transcriptome”—

and with it, the ability to manipulate pro-

teins in novel ways, he adds. Altering A

residues involved in splicing reactions,

for instance, could promote exon skip-

ping or alternative splicing. RNA editing

could also tamp down protein production

by converting AUG start codons to GUGs,

blocking translation initiation. It may even

become trivial to tweak proteins at will—to

add or remove a phosphorylation site, for

instance, or alter protein cleavage points.

Experts in the field are quick to note

that the utility of RNA editing extends

far beyond the genetic diseases that have

been the focus of biotech interest until

now. “The much more nuanced and I

think powerful approaches to this down

the line are going to be really manipulat-

ing systems,” Rosenthal says. His group is

looking into the possibility of editing neu-

ronal RNAs to temporarily dull pain, for

instance, as an alternative to opiates.

Huss says that one of the big chal-

lenges in the near term will be not put-

ting the cart before the horse. “I think

Now that biotech com panies are involved... I’m hopeful that

things are going to accelerate with RNA editing quickly.

—Gail Mandel, Oregon Health & Science University

RNA EDITING APPROACHES: Endogenous adenosine enzymes acting on RNA (ADARs) edit genetic

material in the cell by attaching to naturally occurring double-stranded RNAs, including mRNAs,

and switching out A bases with I bases (left). Therapeutic RNA editing platforms based on this mech-

anism fall into one of two categories: either they use engineered enzymes, which generally consist of

the editing part of the ADAR enzyme attached to another protein such as Cas13 that boosts specific-

ity, alongside a guide RNA that targets the enzyme to the desired location (middle); or they consist

of a guide RNA alone, which recruits an endogenous ADAR to edit the target sequence (right).

A that will be

switched to an I

mRNA

Cas13 Endogenous ADAR

Guide RNA

RNA-binding

domains of ADAR

Guide RNA

AA A

Base-editing

domain of ADAR