 | This article's factual accuracy is disputed .(May 2009) |
Gene therapy using lentiviral vectors was being explored in early stage trials as of 2009.[ needs update ]
| This article's factual accuracy is disputed .(May 2009) |
Gene therapy using lentiviral vectors was being explored in early stage trials as of 2009.[ needs update ]
In a Phase I clinical trial of three patients, two showed no improvement and one of them had some improvements. The study concluded further investigation is warranted for the use of the procedure to treat Leber's congenital amaurosis. [1] Other early trials have been used to explore the treatments potential, [2] including for therapeutic use of recombinant adeno-associated virus (rAAV) vectors. Many other possible viral vectors remain options for the treatment of various genetic disorders in the retina that lead to blindness. Retinal gene therapy using lentivirus vectors may be a way to treat a wider range of genetic disorders in the retina because of the various properties of the lentivirus that make it an attractive alternative to rAAV vectors.[ citation needed ]
Like rAAV vectors, lentiviral vectors offer many features that make it an excellent tool for molecular biology and possible medical treatments. Like many other vectors commonly used in the laboratory, lentiviral vectors allow for efficient transfer of foreign DNA (transgene) to target cells, long-lasting and stable expression of the foreign DNA, and a generally reduced ability to produce an immune response. [3] Like many other retroviral vectors, lentiviral vectors do not possess any of their original DNA content, allowing as little provocation of the immune response as possible. Unlike many retroviral vectors, though, lentiviral vectors offer the advantage of being able to successfully introduce a transgene to target cells whether or not the target cells proliferate (many retroviral vectors require replicating DNA to insert themselves into the host genome). [3]
An important consideration for the application of the lentiviral vector is the parent virus that gave rise to the vector. Not all lentiviral vectors are perfectly suited to every application, and sometimes it becomes necessary for the researcher to try work with a different lentiviral vector if one does not offer the desired transgene expression. Other times, it may be necessary to use another viral vector altogether. There are options to choose from between lentiviral vectors, though, and many popular lentiviral vectors have either a human immunodeficiency virus 1 (HIV-1) or equine infectious anemia virus (EIAV). [4]
Although both the lentiviral and rAAV vectors provide a high efficiency of gene transfer to cells in vivo, rAAV vectors do have some slight disadvantages that would preclude their use for certain diseases. rAAV vectors, for example, only allow genes less than 4 kb (4000 bases) for insertion into the vector; many genetic diseases, not only those the retina, have genes larger than 4 kb in length and thus does not allow the use of rAAV vectors. One such disease, Stargardt's disease (OMIM #601691), [5] can involve a mutation in the ATP-binding cassette transporter 4 ( ABCA4 ) [6] gene. This gene contains 50 exons with a coding region spanning 6.7 kb and thus requires a viral vector capable of handling such a relatively large insert. Lentiviral vectors, unlike rAAV vectors, are capable of efficiently incorporating and allowing expression of transgene fragments as large as 10 kb, and previous work suggests the lentiviral vector is a possible therapeutic option for patients with Stargardt's disease (see below). This is not to suggest lentiviral vectors do not efficiently transduce cells in vivo as well as rAAV vectors with transcripts less than 4 kb, though. Results of lentiviral gene transfer in mice for LCA-2 indicate gene therapy using lentiviral vectors is just as effective as using rAAV; [7] the decision to use a lentiviral vector versus an rAAV vector may simply be a matter of preference.
All transgene vectors have the risk of causing moderate to severe side effects with respect to the immune system, and lentiviral vectors are no exception. In the laboratory or clinical trials, one indication of an immune reaction to the vector is a drop in transgene expression. Often, this sudden loss of transgene expression is not due to a simple silencing of a transgene or loss of the vector from the cell, but loss of the cell itself. [3] The body has multiple methods of targeting and ridding itself of any cells infected with the lentivirus, all of them falling under either activity by the innate immune system or adaptive immune system. In the cases of some HIV-1-derived lentiviral vectors, both immune responses can occur. [3]
In an innate immune response, toll-like receptors (TLRs) found in many cells recognize particles and molecules normally produced by retroviruses like genomic DNA typically found in viruses, genomic RNA found in retroviruses, or double-stranded RNA found in still other retroviruses. [3] These TLRs can initiate downstream effects that can eventually result in the loss of the infected cell and complications in the treated patient. Other work by researchers suggests interferon may play an important role in preventing successful infection of the target cell. Although some researchers suggest treating interferon receptors in the patient with immunosuppressive drugs to allow for a greater response to lentiviral vector treatment, there is still very little data to suggest this approach would work or not. [3]
Expression of the transgene itself may cause an adaptive immune response in addition to any innate immune response initiated by the lentivirus. Because the transgene itself produces a protein either not produced by the cell normally or produces a protein in such a great quantity as compared to normal, the body may form antibodies specific to the transgene, producing further problems. [3] Although these immune system responses may present hurdles to future medical treatments, researchers may manage the issue with different methods.
 | This section needs to be updated.(June 2023) |
Lentiviral vectors may offer substantial promise for the treatment of many genetic disorders manifesting themselves in the retina, such as LCA-2 and Stargardt disease.[ citation needed ]
LCA-2, for example, involves a loss of function in both copies of a gene known as RPE65 . In a normal, healthy retina, this protein acts as an isomerase, converting all-trans-retinol to 11-cis-retinol in the visual cycle. Loss of this protein results in an early-onset retinal degeneration in which affected patients become blind. [8] Swiss researchers used a lentiviral vector containing a copy of the human RPE65 gene under control of an 800 bp fragment of the human promoter to maintain cone and visual function to mice. Although there appears to be a relatively narrow treatment window (after birth but before the retinal degeneration becomes too severe), mice showed expression and cone function four months after treatment. [7] While addition of a functional RPE65 protein to cones helps slow the rate of visual loss but cannot halt or reverse the damage, treatment in humans may help prolong functional vision to patients with this disease.[ citation needed ]
Stargardt disease patients may also one day benefit from lentiviral gene therapy. Unlike rAAV vectors which can only carry relatively small genes, lentiviral vectors can carry larger genes, making them the vector of choice for possible therapy with a functional copy of the ABCA4 gene which is not functional in Stargardt disease patients. Two copies of a nonfunctional ABCA4 gene result in a buildup of a retinoid compound known as A2E, which is believed to act like a detergent inside cells, causing massive cellular damage. [4] As A2E buildup from the photoreceptor cells collects in the retinal pigment epithelium, severe visual loss occurs. When researchers treated Stargardt disease-affected mice with a lentiviral vector containing a functional ABCA4 gene, A2E buildup in the retinal pigment epithelium decreased. More importantly, mice regained some loss of vision. [4] Future lentiviral vector treatments in humans may help preserve vision in these patients.[ citation needed ]
Gene therapy is a medical technology that aims to produce a therapeutic effect through the manipulation of gene expression or through altering the biological properties of living cells.
The retina is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then processes that image within the retina and sends nerve impulses along the optic nerve to the visual cortex to create visual perception. The retina serves a function which is in many ways analogous to that of the film or image sensor in a camera.
Retinitis pigmentosa (RP) is a genetic disorder of the eyes that causes loss of vision. Symptoms include trouble seeing at night and decreasing peripheral vision. As peripheral vision worsens, people may experience "tunnel vision". Complete blindness is uncommon. Onset of symptoms is generally gradual and often begins in childhood.
Virotherapy is a treatment using biotechnology to convert viruses into therapeutic agents by reprogramming viruses to treat diseases. There are three main branches of virotherapy: anti-cancer oncolytic viruses, viral vectors for gene therapy and viral immunotherapy. These branches use three different types of treatment methods: gene overexpression, gene knockout, and suicide gene delivery. Gene overexpression adds genetic sequences that compensate for low to zero levels of needed gene expression. Gene knockout uses RNA methods to silence or reduce expression of disease-causing genes. Suicide gene delivery introduces genetic sequences that induce an apoptotic response in cells, usually to kill cancerous growths. In a slightly different context, virotherapy can also refer more broadly to the use of viruses to treat certain medical conditions by killing pathogens.
Adeno-associated viruses (AAV) are small viruses that infect humans and some other primate species. They belong to the genus Dependoparvovirus, which in turn belongs to the family Parvoviridae. They are small replication-defective, nonenveloped viruses and have linear single-stranded DNA (ssDNA) genome of approximately 4.8 kilobases (kb).
Lentivirus is a genus of retroviruses that cause chronic and deadly diseases characterized by long incubation periods, in humans and other mammalian species. The genus includes the human immunodeficiency virus (HIV), which causes AIDS. Lentiviruses are distributed worldwide, and are known to be hosted in apes, cows, goats, horses, cats, and sheep as well as several other mammals.
Leber congenital amaurosis (LCA) is a rare inherited eye disease that appears at birth or in the first few months of life.
Choroideremia is a rare, X-linked recessive form of hereditary retinal degeneration that affects roughly 1 in 50,000 males. The disease causes a gradual loss of vision, starting with childhood night blindness, followed by peripheral vision loss and progressing to loss of central vision later in life. Progression continues throughout the individual's life, but both the rate of change and the degree of visual loss are variable among those affected, even within the same family.
Viral vectors are modified viruses designed to deliver genetic material into cells. This process can be performed inside an organism or in cell culture. Viral vectors have widespread applications in basic research, agriculture, and medicine.
Stargardt disease is the most common inherited single-gene retinal disease. In terms of the first description of the disease, it follows an autosomal recessive inheritance pattern, which has been later linked to bi-allelic ABCA4 gene variants (STGD1). However, there are Stargardt-like diseases with mimicking phenotypes that are referred to as STGD3 and STGD4, and have a autosomal dominant inheritance due to defects with ELOVL4 or PROM1 genes, respectively. It is characterized by macular degeneration that begins in childhood, adolescence or adulthood, resulting in progressive loss of vision.
ATP-binding cassette, sub-family A (ABC1), member 4, also known as ABCA4 or ABCR, is a protein which in humans is encoded by the ABCA4 gene.
Retinal pigment epithelium-specific 65 kDa protein is a retinoid isomerohydrolase enzyme of the vertebrate visual cycle. RPE65 is expressed in the retinal pigment epithelium and is responsible for the conversion of all-trans-retinyl esters to 11-cis-retinol during phototransduction. 11-cis-retinol is then used in visual pigment regeneration in photoreceptor cells. RPE65 belongs to the carotenoid oxygenase family of enzymes.
The Llura Liggett Gund Award honors researchers for career achievements that have significantly advanced the research and development of preventions, treatments and cures for eye disease.
Gene therapy for color blindness is an experimental gene therapy of the human retina aiming to grant typical trichromatic color vision to individuals with congenital color blindness by introducing typical alleles for opsin genes. Animal testing for gene therapy began in 2007 with a 2009 breakthrough in squirrel monkeys suggesting an imminent gene therapy in humans. While the research into gene therapy for red-green colorblindness has lagged since then, successful human trials are ongoing for achromatopsia. Congenital color vision deficiency affects upwards of 200 million people in the world, which represents a large demand for this gene therapy.
Retinal gene therapy holds a promise in treating different forms of non-inherited and inherited blindness.
Lentiviral vectors in gene therapy is a method by which genes can be inserted, modified, or deleted in organisms using lentiviruses.
Voretigene neparvovec, sold under the brand name Luxturna, is a gene therapy medication for the treatment of Leber congenital amaurosis.
Occult macular dystrophy (OMD) is a rare inherited degradation of the retina, characterized by progressive loss of function in the most sensitive part of the central retina (macula), the location of the highest concentration of light-sensitive cells (photoreceptors) but presenting no visible abnormality. "Occult" refers to the degradation in the fundus being difficult to discern. The disorder is called "dystrophy" instead of "degradation" to distinguish its genetic origin from other causes, such as age. OMD was first reported by Y. Miyake et al. in 1989.
Congenital blindness refers to blindness present at birth. Congenital blindness is sometimes used interchangeably with "Childhood Blindness." However, current literature has various definitions of both terms. Childhood blindness encompasses multiple diseases and conditions present in ages up to 16 years old, which can result in permanent blindness or severe visual impairment over time. Congenital blindness is a hereditary disease and can be treated by gene therapy. Visual loss in children or infants can occur either at the prenatal stage or postnatal stage. There are multiple possible causes of congenital blindness. In general, 60% of congenital blindness cases are contributed from prenatal stage and 40% are contributed from inherited disease. However, most of the congenital blindness cases show that it can be avoidable or preventable with early treatment.
Jean Bennett is the F. M. Kirby Professor of Ophthalmology in the Perelman School of Medicine at the University of Pennsylvania. Her research focuses on gene therapy for retinal diseases. Her laboratory developed the first FDA approved gene therapy for use in humans, which treats a rare form of blindness. She was elected a member of the National Academy of Sciences in 2022.