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 Table of Contents  
Year : 2018  |  Volume : 4  |  Issue : 2  |  Page : 73-76

Oligonucleotides: A new approach against undruggable cardiovascular disorders

Department of Cardiology, AIIMS, New Delhi, India

Date of Web Publication10-Sep-2018

Correspondence Address:
Dr. Arvind Dambalkar
Department of Cardiology, AIIMS, New Delhi
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jpcs.jpcs_40_18

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Oligonucleotides are short DNA or RNA molecules used for a variety of indications in medicine. They can be used for artificial gene synthesis, polymerase chain reactions, and DNA sequencing and can be used as molecular probes. They are usually found as small RNA molecules that function in the regulation of gene expression (e.g., microRNA). These antisense oligonucleotides, based on DNA or RNA chemistries, regulate gene expression by binding to an RNA target.

Keywords: Antisense oligonucleotide, oligonucleotides, siRNA

How to cite this article:
Dambalkar A. Oligonucleotides: A new approach against undruggable cardiovascular disorders. J Pract Cardiovasc Sci 2018;4:73-6

How to cite this URL:
Dambalkar A. Oligonucleotides: A new approach against undruggable cardiovascular disorders. J Pract Cardiovasc Sci [serial online] 2018 [cited 2021 Jun 25];4:73-6. Available from: https://www.j-pcs.org/text.asp?2018/4/2/73/240967

  Introduction Top

Oligonucleotides have recently emerged as an effective therapy against a variety of diseases. These molecules are rapidly gaining importance in therapy for various genetic disorders, viral infections, cancers, and recently cardiovascular disorders. The molecules have overcome a lot of hurdles during their development over decades and are now being used in various undruggable diseases.

Oligonucleotides are small messenger RNA (mRNA) which can target any gene by posttranslational modification of the mRNA [Figure 1].
Figure 1: Oligonucleotides.

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They are mainly of two types – antisense oligonucleotide (ASO) and small-interfering RNA (siRNA). Both these molecules are different in structure but share fundamental principle; an oligonucleotide binds a target RNA through Watson–Crick base pairing, resulting duplex directs degradation/modulation of the target mRNA.

In 1978, Zamecnik first reported a synthetic oligonucleotide complementary to Rous sarcoma virus 35 S RNA inhibiting its protein expression. Another oligonucleotide called short-interfering RNA (siRNA) was discovered by Craig Mello and Andrew Fire, for which they were awarded the Nobel Prize in 2006 for its discovery.

Further developments were slow initially primarily due to instability and nonselective distribution of the drug. The last decade witnessed milestones in developing the molecule as a targeted therapy for a wide range of diseases.

Oligonucleotides were conjugated to specific molecules via triantennary N-acetylgalactosamine (GalNAc) which facilitates targeted delivery to hepatocytes. The oligonucleotides have also been linked to synthetic antigen binding (Fab) fragment of antibody against CD markers which target them to specific organs. CD71 is a specific marker for heart and muscle cells.

  Mechanism of Action Top

Antisense oligonucleotide

The ASO consists of complementary base pairs of the targeted mRNA. The ASO is modified further by GalNAc conjugated with a therapeutic ASO, which are internalized and then released inside the cell, where they can hybridize to their cognate pre-mRNA and induce cleavage of the RNA–DNA heteroduplex mainly. However, multiple mechanisms have been observed which lead to inhibition of gene expression.

ASO, the single-stranded DNA oligonucleotide, passes through the cell membrane and enters into the nucleus from the cytoplasm. In the cell nucleus, the ASO binds or hybridizes to the target mRNA, resulting in the formation of a sense–antisense duplex. If ASO is completely complementary to the mRNA, the formation of the duplex initiates the recruitment of the RNase H and enzyme, an endogenous nuclease. RNase H degrades the target mRNA, which results in inhibition of target mRNA expression. The oligonucleotide moves on and then hybridizes to another target mRNA. If ASO is not completely complementary to target mRNA, it causes steric hindrance to attachment of ribosomes or leads to abnormal splicing of target mRNA.

Small-interfering RNA

The mechanism of action of siRNA is analogous to ASO; however, there are subtle differences. The siRNA molecule consists of two strands of complementary oligonucleotides. The guide strand is complementary to mRNA, and the passenger strand acts as prodrug and stabilizes the molecule. The loaded RNA-induced silencing complex (RISC) has a long half-life, and as few as 100–200 loaded RISC complexes per cell are sufficient to eliminate expression of the targeted gene. Once siRNA enters the cell a helicase separates the sense and antisense strands of the oligonucleotide. RISC, an endogenous conglomerate of functional components, associates with the ASO. The antisense strand of the oligonucleotide hybridizes to the target mRNA, resulting in the formation of a sense–antisense duplex. The nuclease component of RISC is an endogenous nuclease that degrades the target mRNA. This results in the inhibition of target mRNA expression

Oligonucleotides in cardiology

Oligonucleotide therapies are being increasingly investigated in various diseases of cardiology, and the success achieved by these drugs has been phenomenal. Recently, the phase II and III trials were completed siRNA (inclisiran)[1] for dyslipidemia in high cardiovascular risk patients which showed favorable response rates and minimal side effects profile. Proprotein convertase subtilisin/kexin type 9 (PCSK9), an enzyme (serine protease) encoded by the PCSK9 gene, is predominantly produced in the liver. PCSK9 binds to the low-density lipoprotein receptor (LDL-R) on the surface of hepatocytes, leading to the degradation of the LDL-R and higher plasma LDL-cholesterol (LDL-C) levels. Antibodies to PCSK9 interfere with its binding of the LDL-R, leading to higher hepatic LDL-R expression and lower plasma LDL-C levels [Figure 2].[2] There are several strategies to lower free plasma PCSK9, and inclisiran is one such attempt.
Figure 2: Inclisiran effective in reducing low-density lipoprotein cholesterol.

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These trials conclusively demonstrated successful improvement of lipid profile with dosing as less as twice per year and have thus been recently approved by the United States Food and Drug Administration.

Familial homozygous hypercholesterolemia, a genetic disorder leading to dyslipidemia and accelerated atherosclerosis, was deemed undruggable till now as statin was ineffective in these patients. Mipomersen, an ASO, has been studied in these patients in a randomized trial and found significant improvement in dyslipidemia.[3] Mipomersen consists of a 20-mer 2′-O-methoxyethyl-modified nucleotide complementary and specific to human apolipoprotein B (apoB)-100 mRNA. It distributes mainly to the liver, where it forms a duplex with the target mRNA, causing the mRNA to be cleaved by RNase H and therefore unable to be translated to apoB-100. Hepatic apoB mRNA silencing causes reduction in hepatic apoB and plasma total cholesterol, LDL-C, and apoB concentrations [Figure 3].
Figure 3: Mipomersen for low-density lipoprotein reduction.

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The molecule being synthetic could prove to be an economical way of lipid reduction in future patients with dyslipidemia, in addition to improved compliance to the drug.

Volanesorsen[4] targeting APO3 is in phase III trial for hypertriglyceridemia, familial chylomicronemia syndrome, and familial partial lipodystrophy.

Eteplirsen[5] has been approved for Duchene's muscular dystrophy. It induces skipping of exon 51 of the dystrophin gene by binding to a sequence within the dystrophin pre-mRNA and restored the translational reading frame to dystrophin transcripts in patients with Duchene's muscular dystrophy.

These molecules are also being investigated in hypertension, targeting the RAS pathway. Promising results have been shown in preclinical studies.

Idiopathic pulmonary hypertension is also being investigated as a target for oligonucleotide therapy and has met with success in diagnosing and treating idiopathic pulmonary arterial hypertension in animal studies.

In a randomized controlled trial, factor XI (FXI)-ASO,[6] an ASO that specifically reduces FXI levels, was found to be superior to enoxaparin to prevent deep vein thrombosis in patients undergoing total knee arthroplasty. There was decreased risk of bleeding with these agents 2%–3% compared to 8% seen in enoxaparin. FXI-ASO could be used in triple therapy (dual antiplatelet therapy (DAPT) + anticoagulation) in view of better safety profile.

Diseases with disorders of protein accumulation such as amyloidosis can also be potentially treated with oligonucleotides. Animal studies have shown positive response in transthyretin and senile systemic amyloidosis with this therapy, which need to be further confirmed in human trials.

Adverse effects

Toxicity study of phosphorothioate oligodeoxynucleotide[2] and its analogs done on animals has shown to cause thrombocytopenia, dose-dependent elevation of liver transaminases, reduction of the levels of alkaline phosphatase, albumin, and total protein. Splenomegaly, lymphoid hyperplasia, diffused multiorgan mixed mononuclear cell infiltrates, lymph nodes necrosis, cytological alterations and necrosis in hepatocytes, and renal tubule regeneration were also observed.

However, the recent studies[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24] have failed to replicate the concerns generated by preclinical trials. Injection site reactions and flu-like syndrome were the most commonly reported adverse effects in these trials. Creatine kinase elevation (5%) and myalgia (8%) were observed in patients receiving inclisiran. Liver enzymes were not significantly different compared to placebo. The study provides only 210-day follow-up of these patients and long-term safety profile data will be available in due course of time.

  Conclusions Top

Diseases ranging from viral infections such as HIV, to degenerative disorders such as amyotrophic lateral sclerosis to various cancers and inflammatory diseases such as Crohn's disease and rheumatoid arthritis are being investigated, and approximately 40 drugs have been tested till now of which 20 have been tested in phase II and III trials. The success of oligonucleotide therapy might be the dawn of new era in pharmacotherapy and could be a hope for patients suffering from undruggable diseases.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Ray KK, Landmesser U, Leiter LA, Kallend D, Dufour R, Karakas M, et al. Inclisiran in patients at high cardiovascular risk with elevated LDL cholesterol. N Engl J Med 2017;376:1430-40.  Back to cited text no. 1
Iannitti T, Morales-Medina JC, Palmieri B. Phosphorothioate oligonucleotides: Effectiveness and toxicity. Curr Drug Targets 2014;15:663-73.  Back to cited text no. 2
Raal FJ, Santos RD, Blom DJ, Marais AD, Charng MJ, Cromwell WC, et al. Mipomersen, an apolipoprotein B synthesis inhibitor, for lowering of LDL cholesterol concentrations in patients with homozygous familial hypercholesterolaemia: A randomised, double-blind, placebo-controlled trial. Lancet 2010;375:998-1006.  Back to cited text no. 3
Gouni-Berthold I, Alexander V, Digenio A, DuFour R, SteinhagenThiessen E, Martin S, et al. Apolipoprotein C-III inhibition with volanesorsen in patients with hypertriglyceridemia (COMPASS): A randomized, double-blind, placebo-controlled trial. J Clin Lipidol 2017;11:794-5. Available from: https://doi.org/10.1016/j.jacl.2017.04.038.  Back to cited text no. 4
Mendell JR, Rodino-Klapac LR, Sahenk Z, Roush K, Bird L, Lowes LP, et al. Eteplirsen for the treatment of duchenne muscular dystrophy. Ann Neurol 2013;74:637-47.  Back to cited text no. 5
Büller HR, Bethune C, Bhanot S, Gailani D, Monia BP, Raskob GE, et al. Factor XI antisense oligonucleotide for prevention of venous thrombosis. N Engl J Med 2015;372:232-40.  Back to cited text no. 6
Liyanage KE, Burnett JR, Hooper AJ, van Bockxmeer FM. Familial hypercholesterolemia: Epidemiology, neolithic origins and modern geographic distribution. Crit Rev Clin Lab Sci 2011;48:1-8.  Back to cited text no. 7
Müller C. Xanthomata, hypercholesterolemia, angina pectoris. Acta Med Scand 1938;89:75.  Back to cited text no. 8
Khachadurian AK. The inheritance of essential familial hypercholesterolemia. Am J Med 1964;37:402-7.  Back to cited text no. 9
Brown MS, Kovanen PT, Goldstein JL. Regulation of plasma cholesterol by lipoprotein receptors. Science 1981;212:628-35.  Back to cited text no. 10
Goldstein JL, Brown MS. The LDL receptor. Arterioscler Thromb Vasc Biol 2009;29:431-8.  Back to cited text no. 11
Brown MS, Goldstein JL. A receptor-mediated pathway for cholesterol homeostasis. Science 1986;232:34-47.  Back to cited text no. 12
Bilheimer DW, Stone NJ, Grundy SM. Metabolic studies in familial hypercholesterolemia. Evidence for a gene-dosage effect in vivo. J Clin Invest 1979;64:524-33.  Back to cited text no. 13
Cummings MH, Watts GF, Umpleby M, Hennessy TR, Quiney JR, Sönksen PH, et al. Increased hepatic secretion of very-low-density-lipoprotein apolipoprotein B-100 in heterozygous familial hypercholesterolaemia: A stable isotope study. Atherosclerosis 1995;113:79-89.  Back to cited text no. 14
Millar JS, Maugeais C, Ikewaki K, Kolansky DM, Barrett PH, Budreck EC, et al. Complete deficiency of the low-density lipoprotein receptor is associated with increased apolipoprotein B-100 production. Arterioscler Thromb Vasc Biol 2005;25:560-5.  Back to cited text no. 15
Tremblay AJ, Lamarche B, Ruel IL, Hogue JC, Bergeron J, Gagné C, et al. Increased production of VLDL apoB-100 in subjects with familial hypercholesterolemia carrying the same null LDL receptor gene mutation. J Lipid Res 2004;45:866-72.  Back to cited text no. 16
Faiz F, Hooper AJ, van Bockxmeer FM. Molecular pathology of familial hypercholesterolemia, related dyslipidemias and therapies beyond the statins. Crit Rev Clin Lab Sci 2012;49:1-7.  Back to cited text no. 17
Soutar AK, Naoumova RP. Mechanisms of disease: Genetic causes of familial hypercholesterolemia. Nat Clin Pract Cardiovasc Med 2007;4:214-25.  Back to cited text no. 18
Leigh SE, Foster AH, Whittall RA, Hubbart CS, Humphries SE. Update and analysis of the university college London low density lipoprotein receptor familial hypercholesterolemia database. Ann Hum Genet 2008;72:485-98.  Back to cited text no. 19
Soria LF, Ludwig EH, Clarke HR, Vega GL, Grundy SM, McCarthy BJ, et al. Association between a specific apolipoprotein B mutation and familial defective apolipoprotein B-100. Proc Natl Acad Sci U S A 1989;86:587-91.  Back to cited text no. 20
Seftel HC, Baker SG, Sandler MP, Forman MB, Joffe BI, Mendelsohn D, et al. A host of hypercholesterolaemic homozygotes in South Africa. Br Med J 1980;281:633-6.  Back to cited text no. 21
Fahed AC, Safa RM, Haddad FF, Bitar FF, Andary RR, Arabi MT, et al. Homozygous familial hypercholesterolemia in lebanon: A genotype/phenotype correlation. Mol Genet Metab 2011;102:181-8.  Back to cited text no. 22
Moorjani S, Roy M, Gagné C, Davignon J, Brun D, Toussaint M, et al. Homozygous familial hypercholesterolemia among French Canadians in Québec province. Arteriosclerosis 1989;9:211-6.  Back to cited text no. 23
Slack J. Risks of ischaemic heart-disease in familial hyperlipoproteinaemic states. Lancet 1969;2:1380-2.  Back to cited text no. 24


  [Figure 1], [Figure 2], [Figure 3]


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