It is believed that naked DNA might be an effective option for gene therapy in future. The naked DNA is a free DNA without surrounded by any protein or lipid layer.

Also,

The naked DNA is called plasmid DNA and denoted as pDNA. It is a circular DNA comprises several important DNA sequences needed to integrate GOI into the host genome.

There are three major components of gene therapy: The gene of interest, a vector and a target site.

The vector is a vehicle used to deliver a gene of interest into the host genome or at a target location.

The gene therapy is categorised in the viral vector-mediated and non-viral vector-mediated gene therapy.

The viral vector-mediated gene therapy is restricted due to the use of the virus in gene therapy. Though it is more effective, it can infect the host cell too. Contrary to this, the non-viral vector-mediated gene therapy is less expressive but safer.

Naked DNA is one of the vectors categorized into the non-viral vector-mediated gene therapy.

If you want more information on gene therapy, read our previous article: What is Gene Therapy? and How Does it Work?

The present article is all about naked DNA, a gene therapy vehicle that is safer, effective and promising. The content of the topic is,

  • The belief
  • What is naked DNA?
  • Why naked DNA?
  • Why not naked DNA?
  • Conclusion

The belief…

Previously it is believed that the naked DNA cannot be used directly for the gene therapy, due to the similar negative charges on both DNA as well as the cell surface.

The DNA has a net negative charge on their backbone due to the presence of phosphate, also, the cell surface carries a negative charge too.

Due to the same negative affinity, the cell surface and the DNA repel each other, or it does not allow DNA to go inside the cell.

See the figure below,

The negatively charged cell surface does not allow naked DNA

However, in the year 1990 Wolff et al reported the first case of the intramuscular injection of naked DNA into the myofibers, accidentally.

The directly injected naked DNA into the myofibers of mice expressed very well and the new era of gene therapy arises.

The scientists are now trying to introduce the DNA directly into the cell.

None of the naked DNA gene therapy is still approved for clinical use. Despite, some veterinary products are now adopted for gene delivery in animals.

What is a naked DNA?

As we said earlier, “the naked DNA is a DNA without the outer membrane or proteinous-lipid layer.”

The plasmid DNA is one of the kinds of naked DNA.

Structurally, the pDNA or only plasmid DNA has only a DNA molecule nothing else. However, the DNA along cannot be inserted directly into the cell. That we will discuss in the earlier segment of this article.

The naked DNA mediated gene therapy is an in vivo method of gene application that can transfer the gene into the skin, cardiac muscle and thymus of up to 20kb.

Further, it is capable of penetrating in the muscle and liver cell directly.

Although, present gene therapy is difficult for gene transfer in the internal organs.

Generally, the DNA is settled inside the nucleus of the cell (some DNA also present in the cytoplasm as well as in some organelles). The naked DNA is released into the surrounding environment due to the cell bursting and cell breakage.

The circular plasmid DNA is present in the bacteria is also a naked DNA.

The naked DNA gene therapy is easy and simple to use and it is synthesised using the recombinant DNA technology.

Read more:

  1. Why is pcr used in the process of dna sequencing?
  2. “Primer Dimer”: Zones DNA amplification by pairing with foe oligo

Why naked DNA?

A high-risk factor is always associated with the viral-vector mediated gene therapy. The viral vectors induce a strong immune response of the host cell.

Furthermore, it can infect the host cell as well.

On the other side of it, naked DNA  delivery is safer for both recipients as well as the environment.

The efficiency of gene therapy is pretty much decent. It works finely for the muscle cells and for the production of therapeutic proteins such as insulin and clotting factors.

It can integrate a large amount of transgene (~15kb).

It is believed that the naked DNA is the future of DNA vaccines in which a genetically engineered synthetic DNA is used against a specific disease.

However, Injecting the naked DNA directly into the cell is not an easy process as we are discussing.

Two of the major problem occurs while injecting it,

First, it can not go inside without creating pores in the cell surface.

Second, if it is injected, it is engulfed by the cell and destroyed.

“Any foreign particle entered inside the cell is recognised by the cell defence mechanism and destroyed.” 

The figure below shows how foreign DNA destroyed inside the cell:

Due to the negative charges on both DNA as well as a cell surface, the cell membrane does not allow any foreign DNA into the cell.

We have to create pores in the cell wall to do so.

Although the potency of naked DNA and its expression is limited, several strategies can be opted to increase the potency of naked DNA.

  1. By adding some of the extra sequences such as promoters, polyadenylated DNA sequences, antibiotic-resistant gene and  ORF for specific protein increases the efficiency of the naked DNA intake.
  2. Due to the charge equality, the naked DNA cannot directly penetrate the cell membrane, using a synthetic capsule such as liposome naked DNA can efficiently transfer into the host cell. The capsule protects the naked DNA from nuclease attack.
  3. Using some of the artificial techniques just like electroporation, gene gun or particle bombardment, the rate of DNA potency can be increased.
  4. Other techniques like sonication, photoporation, megnetofection and hydroporation can also be used alternatively to electroporation.

The artificial techniques of electroporation and gene gun break the cell membrane very accurately.

Injecting naked DNA directly into the cells of internal organs is quite difficult by using the method of electroporation and gene gun, the naked DNA can be inserted efficiently into the cardiovascular tissues too.  

In the electroporation, pores are formed under the influence of higher electric field on the surface of a cell for DNA insertion.

The encapsulated DNA inserts in the cell via the pores created by the electroporation method.

After the insertion, the capsule transfers DNA into the nucleus via the nuclear membrane where it is integrated into the host genome.

The DNA is transcribed there and the mRNA moves back to the cytoplasm for translation of protein.

Naked DNA mediated gene therapy is now entered into the clinical trial phase for the “peripheral arterial occlusion disease”. 

The electroporation mediated naked DNA gene therapy can inject DNA efficiently into skin and muscle tissues.

Why not naked DNA?

The technique is not yet ready for clinical trials for all type of inherited diseases although it is safe.

The success rate of naked DNA mediated gene therapy is still very low having fewer gene expression rate.

In comparison with the viral-vector, the naked DNA shows lower transfection rate.

Also, the insertional mutagenesis rate is too low.

The pDNA can activate oncogenes which risks the environment as well as an individual. It may infect other individuals too.

The risk due to the naked DNA on the environment is negligible, although, it is believed that the naked DNA is responsible for the production of natural GMOs especially in the plant.

Some of the facts of naked DNA gene therapy are listed in the boxes below,

Injecting naked DNA via the tail vein in mice is used to test the expression vector designed for the naked DNA. 
Gene silencing is possible using the direct intravenous injection of siRNA.
A higher fold of gene transfer in liver obtained using the in vivo intravenous transfection.
Reducing CpG rich sequences and adding polyadenylated sequences to the plasmid increases the transgene expression by decreasing the immune response.

Related articles:

  1. Agarose gel electrophoresis
  2. DNA sequencing
  3. What is gene editing?

Conclusion:

Use of non-viral vector-mediated gene transfer is a safer option for the gene transfer. Naked DNA injection added a new dimension for this approach. The present technique has the potential to change the fate of gene therapy in future. The technique is simple yet effective.