Science - 27.03.2020

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SCIENCE

form has been developed that allows the chi-

rality of the phosphorothioate modification to

be controlled during ASO synthesis. It is re-

ported that “stereopure” ASOs have improved

activity, stability, and specificity compared

with stereoisomer mixtures ( 7 ). Two clinical

trials were initiated in 2017 to assess two dif-

ferent stereopure ASOs targeting specific HD-

associated SNPs. These studies require precise

genotyping to ensure that the targeted SNP is

accurately phased to the HTT allele with the

CAG expansion. Not all individuals with HD

can be treated with this SNP-based approach,

and it is estimated that the two ASOs being

tested are applicable to ~65% of HD patients

from North America or Europe. Each ASO

is given as a monthly intrathecal bolus over

three consecutive months, and it was recently

reported that at the highest dose tested, one

of the ASOs caused a modest 12.4% reduction

in CSF concentration of mHTT. An additional

higher-dose cohort will now be added to this

trial (NCT03225833).

It is not known whether HTT lowering or

mHTT-selective lowering will be most effec-

tive. Both approaches have distinct strengths

and limitations, and this question will ulti-

mately be answered empirically in clinical

efficacy trials. Preclinical studies in a hu-

manized transgenic HD mouse model found

that the benefits of ASO-mediated lowering

of total HTT concentrations by 75% were

similar to those of ASOs that selectively re-

duced mHTT. These and other studies have

suggested that the degree of mHTT lowering

is the most critical parameter for preclinical

efficacy ( 8 ). A substantial advantage of HTT-

lowering approaches compared to SNP-based

SCIENCE

selective mHTT targeting is the potential to

develop a single therapeutic agent for the en-

tire HD population.

There is optimism that HTT-targeting

ASOs may lead to a viable disease-modify-

ing therapy for HD, as well as the develop-

ment of ASOs for other neurodegenerative

diseases associated with aberrant protein

production. Following a substantial preclini-

cal development program, an ASO targeting

superoxide dismutase 1 (SOD1) was found to

be safe and well tolerated after lumbar in-

trathecal infusion in a phase 1 trial for the

treatment of SOD1 mutated ALS ( 9 ). A more

potent MOE-modified ASO (ISIS-SOD1Rx) is

being evaluated in a phase 1/2a clinical trial

(NCT02623699). Promising preclinical data

have also been generated using ASO-based

approaches for Parkinson’s disease, target-

ing leucine-rich repeat kinase 2 (LRRK2,

NCT03976349) and a-synuclein ( 10 ); for

Alzheimer’s disease by targeting Tau protein

( 11 ), which is currently in a phase 1 clinical

trial (NCT03186989); and for prion diseases

by targeting the prion protein PRP ( 12 ).

ASO-based therapies are also of interest

in diseases whose etiology is similar to that

of HD, such as the polyglutamine protein–

related forms of spinocerebellar ataxia ( 13 )

that are caused by polyglutamine inclusions,

and in frontotemporal dementia (FTD) with

Tau or TAR DNA-binding protein 43 (TDP-

43) pathologic inclusions. Indeed, ASOs that

lower ataxin 2 expression have shown ben-

efit in mouse models of both spinocerebel-

lar ataxia 2 and TDP-43–related FTD ( 14 ).

The most common genetic cause of ALS and

FTD is a GGGGCC repeat expansion in the

C9ORF72 gene that induces RNA-mediated

neurotoxicity. ASOs that selectively target

these repeat-containing RNAs may be a use-

ful therapeutic approach to this class of dis-

eases ( 15 ); clinical trials are in development.

ASOs have already changed the landscape

of therapeutic development for neurodegen-

erative diseases. Their advancement in the

clinic will require continued development

and research, including optimization of tar-

get sequence selection, improving biological

activity, testing new delivery technologies,

and maintaining an appropriate safety pro-

file. Improving the delivery of ASOs to target

cells is an important area of future devel-

opment, including intrathecal pumps and

the use of lipid-based and polymer-based

nanocarriers. Such delivery systems will po-

tentially improve the controlled release of

ASOs and cell and tissue specificity, and may

provide additional protection from nuclease

degradation. Beyond potency and specificity,

another crucial feature of a good candidate

molecule is the ability to reach its intracel-

lular target at sufficient concentration. Given

the substantially increased potency of MOE-

modified ASOs, there are safety concerns re-

garding excessive on-target lowering of pro-

teins (such as wild-type HTT) and potential

off-target effects. It is critical that extensive

preclinical assessment of both potency and

off-target effects is performed in the develop-

ment of new ASO therapies.

The ASO RG6042 is the result of more than

a decade of extensive preclinical assessment

in multiple model systems and is a testament

to effective academic and industry collabora-

tion in drug development. With a growing

number of ASO therapeutics being tested in

clinical trials, this exciting technology holds

the potential to change the therapeutic land-

scape for many neurological and non-neu-

rological conditions (including cancer, and

cardiovascular, infectious, and pulmonary

diseases) in the near future. j

REFERENCES AND NOTES

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3. G. P. Bates et al., Nat. Rev. Dis. Primers 1 , 15005 (2015).


4. H. B. Kordasiewicz et al., Neuron 74 , 1031 (2012).


5. S. J. Tabrizi et al. N Eng. J. Med. 380 , 2307 (2019).


6. N. H. Skotte et al., PLOS ONE 9 , e107434 (2014).


7. N. I w a m o t o et al., Nat. Biotechnol. 35 , 845 (2017).


8. A. L. Southwell et al., Sci. Transl. Med. 10 , eaar3959 (2018).


9. T. M. Miller et al., Lancet Neurol. 12 , 435 (2013).


10. H. T. Zhao et al., Mol. Ther. Nucleic Acids 8 , 508 (2017).


11. S. L. DeVos et al., J. Neurosci. 33 , 12887 (2013).


12. G. J. Raymond et al., JCI Insight 5 , 131175 (2019).


13. P. Gonzalez-Alegre, Hum. Mol. Genet. 28 (R1), R80 (2019).


14. L. A. Becker et al., Nature 544 , 367 (2017).


15. J. Jiang et al., Neuron 90 , 535 (2016).
    ACKNOWLEDGMENTS
    Both authors have undertaken paid consultancy regarding
    ASOs for Ionis Pharmaceuticals, Takeda Pharmaceuticals,
    and F. Hoffman-La Roche. B.R.L. is cofounder of Incisive
    Genetics Inc. and has associated patents pending.

10.1126/science.aba4624

Target mutations

ASOs can target RNA

transcripts that produce

disease-causing proteins.

Reduced protein amounts

Targeted degradation of

RNA or modulation of

splicing or translation

reduces the expression of

disease-causing proteins.

Target splice sites

Unique sequences at

splice sites in pre-mRNAs

can allow ASOs to

modulate RNA splicing.

Target translation

start sites

ASOs can selectively

target translation start

sites in mRNAs, which

prevents protein translation.

RNase H1 degrades

RNAs in DNA-RNA hybrids

mRNA

ASO

Ribosome

Intron

Translation

start site

pre-mRNA

Exon

Pathological

protein production

27 MARCH 2020 • VOL 367 ISSUE 6485 1429

Reducing pathological protein expression

Antisense oligonucleotides (ASOs) are small, single-stranded DNAs that can bind specific RNA sequences on

precursor messenger RNAs (pre-mRNAs) and mRNAs. The resulting RNA-DNA hybrid can induce ribonuclease

H1 (RNase H1) degradation of the targeted RNA, modulation of splicing, or blockade of translation.