Antisense oligonucleotide therapy plays a vital role in cancer and genetic disorders. This therapy involves the use of oligonucleotides that are complementary to specific target DNA or RNA in the nucleus. Oligonucleotides bind the complementary sequence and inhibit the synthesis of specific proteins by different mechanisms. Thus, downregulating the expression of target genes.
What are Oligonucleotides?Oligo means “Few”. These are short double stranded RNA molecules which consist of sequences of AGCU nucleotides storing the genetic information.
Types of Oligonucleotides
- siRNA, Primers (Natural) and
- Antisense Oligonucleotides
What are Oligonucleotides used for?Oligonucleotides are used in treatments for gene defects and Metabolic disorders. Therapeutic Oligonucleotides target the mRNA and change gene expression (proteins).
Antisense OligonucleotidesThese are Oligonucleotides which cuts the expression of proteins with in a cell.
Antisense Oligonucleotide Therapy (AOT)
The use of oligonucleotides that are complementary to our target mRNA in order to reduce the expression of some specific gene in a cell is called antisense therapy. These oligonucleotides bind in the 5 to 3 directions with mRNA and do not allow it to translate into proteins. So, a low quantity of protein is formed, and loss of function occurs in the cell.
Many antisense therapeutic drugs are available in the market and are being used for the treatment of multiple disorders like cancer, Alzheimer’s disease, Parkinson’s disease, and arthritis. All these drugs are approved by FDA.
The oligonucleotides that are available naturally are usually unstable or with low stability and efficacy as they have a lot of side effects. So, oligonucleotides used for antisense therapy should be modified in such a way that they have high stability and low side effects. This can be done by chemically modifying the oligonucleotides like CpG, anti-gene, small non-coding RNAs, etc.).
The chemical modifications allow the oligonucleotides to enter the nucleus and bind their complementary RNA which is also an RNA of interest. They will not allow RNA to form proteins, or they may destroy the RNA molecule thus downregulating the target gene.
Antisense therapy is more effective than other available therapeutic
modalities because oligonucleotides can easily be modified, they have a high affinity to bind targeted mRNA, they are highly specific, and have no side effects.
Gendicine was the first antisense therapeutic drug that was approved for head and neck cancer in China. This and other drugs available for the treatment of different cancer types usually target the cancer-promoting genes in the cell (Sharad, 2019).
Role of Antisense Oligonucleotide Therapy in CancerThe use of oligonucleotides for the treatment of cancer patients is found to be a very effective therapy. many drugs approved by Food and Drug Regulatory Authority are available in the market and are being used commonly.
Antisense therapy is effective in the sense that it can distinguish between the normal and mutated oncogenes in cells and targets only mutated oncogenes without downregulating the expression of normal genes. It can also recognize a single mRNA class in the cell and the specific location of targeted DNA as well.
When oligonucleotide-based drugs are given to the cancer patient, the drugs target the specific mutated genes in cancerous cells. They bind the complementary RNA that will be either degraded in the cell or will not be able to translate into proteins thus affecting the expression of oncogenes in the cell. When these tumor-promoting genes are downregulated in the cell, they will not be able to promote cancer in that cell. Thus, oligonucleotides stop the metastasis and invasion of tumors (Jansen & Zangemeister-Wittke, 2002).
The oligonucleotide sequence binds the mRNA in the cell and prevents the synthesis of specific proteins from it. This prevention may result when oligonucleotides cause inhibition of the translation process, they may prevent the spicing of exons, or they inhibit the transport of mRNA from the nucleus into the cytoplasm which is important for translation to occur.
The oligonucleotides are of specific sequences complementary to the target RNA or DNA sequence. The process is based on finding that a DNA sequence of 17 or more than 17 base pairs is found once in the genome. Similarly, an RNA sequence of 13 base pairs or more is also found only once in the whole genome. So, there is no chance of multiple complementary sites for oligonucleotides in the genome and hence no harm to normal cells. These oligonucleotides must be delivered into the nucleus to the target nucleic acid either DNA or RNA. For this, they have to enter the cell, cross the cytoplasm and move into the nucleus.
The oligonucleotides may cross the cell membrane by the formation of endosomes. This process requires energy. Sometimes the drug can cross the cell membrane without the expenditure of energy if the plasma membrane is permeable for that drug.
In tissue culture techniques, lipophilic molecules are used called liposomes for the effective delivery of drugs into the cell. As most of the drugs are anionic in nature and they cannot cross the plasma membrane.
Current research on antisense oligonucleotides (ASOs) in cancer
Many new therapeutic modalities are being introduced to target cancer in the patient’s body. One of the most effective therapies whose use is growing exponentially day by day is the use of antisense oligonucleotides against cancer. It is a promising therapy against cancer and genetic disorders that specifically targets RNA. It inhibits the RNA and thus prevents the synthesis of many important proteins in the cell.
Currently, a drug named AZD8701 is in clinical trials that acts as an antisense inhibitor against the FOXP3 gene, preventing the synthesis of protein and thus activating the immune system against cancer cells (Shang et al., 2022).
Similarly, an antisense inhibitor against heat shock protein 27 (Hsp27) is also in clinical trials for the treatment of prostate cancer patients. Hsp27 is found to be involved in the progression of prostate cancer. So, targeting the RNA of Hsp27 and inhibiting its synthesis will obviously slow down the progression of prostate cancer in patients and this can act as a promising therapy (Le et al., 2022).
The use of oligonucleotides for downregulating the expression of specific genes is an important therapeutic modality against cancer. Many inhibitory drugs are approved by the FDA and have multiple types of cancer. They inhibit protein synthesis by various mechanisms. Some drugs are in clinical trials against many cancers. The use of ASOs can be a promising therapy against cancer.
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