Anti-FIV Gene Therapy: Lighting the Way to an AIDS Cure
Acquired Immune Deficiency Syndrome (AIDS) is a life-threatening, chronic condition triggered by the human immunodeficiency virus, or HIV. By damaging the immune system, the virus interferes with the body's capacity to fight disease-causing organisms. AIDS is responsible for almost 5,000 deaths every day, infecting a total of about 33 million people worldwide [1, 2]. The virus has been tracked in many different countries, but the epidemic is most strongly concentrated in sub-Saharan Africa, Central America, the Caribbean, and parts of Asia. While HIV is transmitted sexually, it can also be contracted through contact with infected blood, especially through needles used by drug users, or from a mother to her child . Acute symptoms usually surface in as little as two months after contracting the virus, although it may take up to several years to progress to AIDS.
HIV Life Cycle
HIV is a type of lentivirus, which is a slow retrovirus that takes a long time before causing effects on the body . Just like other viruses, HIV requires host cells in order to replicate and survive. The host cells of HIV are immune system cells known as the helper T cells, which are a type of CD4 cell . The role of CD4 cells is to recognize foreign antigens in the body and subsequently release cytokines to activate B and killer T cells to defend the body from those antigens. When HIV infects a helper T cell, the virus takes over the host cell machinery, turning it into an HIV-replicating cell . As a result, infected CD4 cells are unable to carry out their normal immune function, thereby making the body more susceptible to cancers and opportunistic infections .
Today's medicine offers no absolute cure for AIDS since the medications that have been developed typically merely slow the progression of the virus' activity. Furthermore, since HIV is cleverly able to undergo mutations, drugs constructed to combat a certain strain of HIV may not be able to recognize certain HIV mutants, thereby rendering those medications ineffective. HIV's high rate of mutation can be attributed to its high error rate of ribonucleic acid (RNA) replication; since HIV's genome is composed of RNA, the mutations in this genome can cause HIV enzymes and proteins to have a variety of substitutions. Since HIV-combating drugs target the virus' enzymes, these drugs are ineffective against new mutant enzyme strains [8, 9].
Administering three or more medications at the same time in an "antiretroviral cocktail" has proven to be a more effective anti-HIV treatment since mutant strains resistant to one drug may be inhibited by a different drug. Although antiretroviral cocktails have saved many lives in both developed and developing countries, their complexity lends them to side effects that doctors and patients, alike, must consider before prescription and administration . The most promising and potent treatment options on the horizon seek to kill the viruses while minimizing "collateral damage" to the patient's healthy tissue. One treatment option that meets these criteria is gene therapy, which enhances the patient's genetic makeup to fight HIV. One recent study takes the concept of genetic manipulation one step further, giving hope to scientists and drug developers.
Feline Immunodeficiency Virus (FIV): an Analog of HIV
Feline immunodeficiency Virus (FIV), a lentivirus affecting feline populations worldwide, has caused an epidemic analogous to that caused by HIV, presenting scientists with the opportunity to work on an esoteric problem to solve a more familiar, yet equally significant one [11, 12]. According to Eric Poeschla, M.D., a molecular virologist at the Mayo Clinic, "the world is suffering from two huge AIDS pandemics. Less well-known is the one in cats." Felines, chimpanzees, and humans are the only mammals that are in danger of contracting a "naturally acquired AIDS virus" .
Infecting almost 3% of all cats worldwide, including lions, tigers, and housecats, FIV is being targeted as the second-most prominent acquired immunodeficiency virus to affect mammals. The majority of cats fall victim to FIV because they cannot produce TRIM5α, a rhesus restriction factor that targets the viral capsid of FIV to prevent the virus from replicating its genome or undergoing replication itself [13, 14]. Poeschla and his research group aimed to render transgenic cats immune to FIV through a series of genetic modifications that would allow the cats to produce TRIMCyp, a macaque DNA restriction factor that blocks FIV by destroying the protective coatings of the virus when it tries to invade normal cells .
National Institutes of Health
Poeschla's work is a step towards genetically engineering immunity to FIV while shedding light on how to more effectively hinder the activity of HIV. His group genetically engineered kittens to express a gene that produces TRIMCyp proteins. A jellyfish gene that codes for green fluorescent protein (GFP) – that won the 2008 Nobel Prize in Chemistry – was also incorporated into the genome of the kittens to make them glow green under fluorescent light, confirming the success of the gene insertions [16, 17]. Thus, a green glow in the five-month-old transgenic cats indicated that they had successfully acquired the TRIM5? protective gene. Poeschla's group performed this genome transformation through a process known as gamete-targeted lentiviral transgenesis, which entailed the insertion of a viral vector containing both the TRIMCyp and GFP genes into unfertilized eggs from mature cat ovaries; this vector then incorporates its genes into the egg's genome . Then, after fertilizing the eggs, they transplanted them into female surrogate wombs to develop into transgenic kittens. The group found that the foreign genes were expressed in somatic and germ cell lines, enabling the kittens to pass them down to their own offspring. Since the antiviral protein and GFP genes were specially spliced into the viral vector in order to allow simultaneous expression, the green glow of the feline cells gave scientists visual evidence of antiviral protein production—that is, cells expressing the antiviral gene glowed green . Poeschla's group showed that the cats with the transformed genome produced kittens that also produced the TRIMcyp and GFP proteins, showing that genetic engineering process allowed cats to pass down the genes to future generations.
Poeschla and his colleagues found that FIV was unable to replicate well in blood cells from the transgenic cats . Past studies have also shown that TRIMcyp is able to confer cellular immunity against FIV [10, 18]. Poeschla's next goal is to determine if his group's transgenic cats have developed resistance to FIV infection or feline AIDS due to the action of TRIMcyp.
Application of FIV Research to the Fight Against HIV
While FIV and HIV are not mutually acquirable between felines and humans, the Poeschla project provides a clearer understanding of how the lentiviruses can be combated in both mammalian groups. By studying the response of the transgenic cats' cells to FIV, Poeschla and other research groups aim to closely document recognition of and battle with the virus. This data would ideally lead to safer and more effective treatment in both cats and humans .
Recent HIV research is exploring gene therapies as potential replacements of already-available drugs. During the past 15 years, scientists have used RNA- and protein-based gene therapy techniques to confer HIV resistance on hematopoietic cells, which are stem cells in the bone marrow that develop into blood cells, including T cells . Studies have shown that treatments based on gene therapy involve minimal toxicities when compared with other treatments, such as the antiviral cocktails given to patients with AIDS [10, 22]. According to geneticist Paula Cannon at the Keck School of Medicine, "In the future, we might engineer people's cells by adding a gene that encodes resistance, not to the whole body, but to the T-cells that are targeted by HIV" . Thus, researchers are hoping to develop gene therapy-based treatments against HIV that could inhibit the activity of this virus while also avoiding unnecessary side effects and toxicities . Much work in the fields of biomedical and genetic engineering is needed to initiate big changes in the paradigms scientists use to approach these problems. The direction of modern science, however, is taking the turn to interdisciplinary research teams, which will present fresh perspectives to the search for an HIV/AIDS cure.
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