The great puzzles of medicine often first appear to be easy to solve. Take, for example, lysosomal storage diseases (LSDs). LSDs, including lysosomal acid lipase deficiency (LAL-D) and Pompe disease, are characterized by the missing activity of the lysosome enzyme, which causes various pathologies in the body. Patients with these diseases have certain genetic mutations; hence, the replacement of these defective genes should tentatively amount to a curative solution. In theory, this should be an easy problem to solve: replace what is defective and things go back to normal, right?

In reality, gene therapy is a nascent field that is still the subject of copious amounts of research. At present, we simply do not have the technology to “replace” a defective gene in the same way one might imagine replacing a bottle of milk in the fridge. 

The key difficulty is that the delivery of missing genes requires a “carrier” to safely deliver the missing gene to its intended target. This is where lentiviral vectors come in. 


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Introducing Foreign DNA

First, let’s review the goals of gene therapy. Gene therapy involves introducing foreign DNA into the cell genome. The aim is for the DNA to replicate when the host cell begins to divide. However, when using nonintegrating vectors, the DNA introduced into the nucleus remains foreign in an episomal form.

There are 2 ways in which foreign genetic material can be introduced into a patient’s cell: in vivo or ex vivo. In vivo gene therapy is rather straightforward: the gene is transferred directly into the patient and transduction occurs inside the patient. 

With regards to ex vivo gene therapy, the first step is to perform an extraction of the patient’s cells. This means that subsequent vector transduction that contains the gene of interest occurs in a laboratory setting. After that, scientists need to select and amplify the transducer cells in order to reach the threshold number needed to obtain a therapeutic outcome. Only then are the modified cells reinfused into the patient. 

Read more about LAL-D etiology 

Before we discuss the role of lentiviral vectors in the gene therapy of LSDs, it is worth mentioning hematopoietic stem cell transplantation (HSCT)—another potential therapy that has gained widespread attention lately. In this approach, cells are extracted from a healthy donor and repopulated in the recipient patient. This results in the secretion of enzymes within the periphery of the organism. 

Rintz and colleagues wrote, “HSCT could protect or ameliorate [central nervous system (CNS)] damage in some LSDs if it is conducted at an early stage; however, it still does not correct CNS impairment in other diseases.” In addition, the risk of graft-vs-host disease remains. 

The Merits and Downsides of Lentiviral Vectors 

Let us now return to discussing lentiviral vectors, a proposed adjunct of gene therapy that can have massive therapeutic implications in LSDs and other diseases in the years to come. First, a definition: “Lentiviruses are RNA-based and replicate thanks to a retrotranscription process, leading the genetic integration into a DNA host genome,” Martínez-Molina and colleagues wrote. 

In gene therapy research today, scientists recognize lentiviruses and adeno-associated viruses as the most promising vectors for the replacement of missing genes. They both have merits and downsides.

“One of the significant discoveries in [lentiviral vectors] is that these viruses infect both dividing and nondividing cells and integrate into the host chromosome,” Rintz and colleagues wrote. “Therefore, [lentiviral vectors] display stable and long-term expression of the transcript.”

Read more about Pompe disease patient education

Studies on lentiviral vectors have spanned the last few decades and there are now 3 generations of these vectors. The successful administration of lentiviral vectors depends on choosing the right promoters. Promoters are responsible for the overall degree of gene expression, as well as direct tissue or cell target specificity. In the last few years, scientists have experimented with different promoters in order to identify the ones best suited to treating LSDs. 

“The promoter is a major cis-acting element found at the beginning of the gene to be transcribed,” Rintz et al wrote. “Attaching RNA polymerase to the binding site of the promoter activates the transcription of the desired gene.”

Widespread Clinical Research Ongoing

Lentiviral vectors remain a subject of intense medical research. Martínez-Molina reported that in 2000, more than 100 clinical trials were being carried out involving lentiviral vectors. That number is expected to be higher today. 

The reason for the intense scientific interest in lentiviral vectors is that they provide hope for a cure in a number of diseases aside from LSDs, including sickle cell disease, hemophilia, COVID-19, and various malignancies. Hence, it offers tremendous promise and serves as a window into the future of gene therapy. 

An area of lentiviral vector research that continues to be a subject of intense investigation is the pairing of the right promoter with the right lentiviral vectors. 

On future directions in lentiviral vector research, Rintz et al wrote, “Choosing an appropriate promoter is crucial for obtaining the desirable effects of lentiviral gene therapy. The proper choice depends on the disease, target tissue, and organs. In some cases, a combination of promoters or the construction of fused and engineered promoters might give the best results.”

References

Rintz E, Higuchi T, Kobayashi H, Galileo DS, Wegrzyn G, Tomatsu S. Promoter considerations in the design of lentiviral vectors for use in treating lysosomal storage diseases. Mol Ther Methods Clin Dev. 2021;24:71-87. doi:10.1016/j.omtm.2021.11.007

Martínez-Molina E, Chocarro-Wrona C, Martínez-Moreno D, Marchal JA, Boulaiz H. Large-scale production of lentiviral vectors: current perspectives and challenges. Pharmaceutics. 2020;12(11):1051. doi:10.3390/pharmaceutics12111051