What is gene therapy?
    The aim of gene therapy is to modify the genetic material of living cells for therapeutic purposes (Amado and Chen, 1999).  Gene therapy involves the insertion of a functional gene or another molecule that contains and information sequence into a cell to achieve a therapeutic effect.  Thus, the gene serves as a drug (Lasic, 1997).  There are two types of gene therapy: somatic cell and germ line.  Somatic cell gene therapy is the only technique now in use.  The purpose of the procedure is to eliminate the clinical consequences of a disease and the inserted gene is not passed on to the patient's offspring.  In germ line gene therapy a healthy gene is inserted into the fertilized egg of an animal that has a genetic effect.  Every cell that develops from this egg, including the reproductive cells, will have the new gene.  However, there are very serious social and ethical considerations with this type of gene therapy (Nichols, 1998).

    Before 1996 scientists relied mainly on modified retroviruses such as Moloney murine leukemia virus when gene transfer into the chromosomes of target cells was needed, and adenovirus vectors when such integration was not needed.  However, there has been little success in gene transfer with such virus vectors because even though the vectors can enter into their target cells, the cells need to be dividing, so that their nuclear membrane are broken down, for the gene to enter and integrate into the chromosome (Sikorski and Peters, 1998; CFAR at UC San Diego).  However, scientists soon realized that members of the subfamily lentivirus, such as the retrovirus human immunodeficiency virus (HIV), would have the same ability to transfer genetic material into the genomes of cells, but could do this with non-dividing, dormant cells in vivo and growth-arrested cells in vitro (Amado and Chen, 1999; CFAR at UC San Diego).  Exploring this new method of gene therapy has been the work of many labs in the past few years.

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What are lentiviral vectors?
    Lentiviral vectors are a type of retrovirus that can infect both dividing and nondividing cells because their preintegration complex (virus “shell”) can get through the intact membrane of the nucleus of the target cell.  Lentiviruses can be used to provide highly effective gene therapy as lentiviruses can change the expression of their target cell's gene for up to six months.  They can be used for nondividing or terminally differentiated cells such as neurons, macrophages, hematopoietic stem cells, retinal photoreceptors, and muscle and liver cells, cell types for which previous gene therapy methods could not be used.  HIV is a very effective lentiviral vector because it has evolved to infect and express its genes in human helper T cells and other macrophages.  The only cells lentiviruses cannot gain access to are quiescent cells (in the G0 state) because this blocks the reverse transcription step (Amado and Chen, 1999).  To understand how HIV is a good vector for gene therapy, we must understand the structure of HIV and how it functions and infects its host.

Structure of HIV

Structure of HIV

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Why HIV is a good vector for gene therapy?
    The preintegration complex of the human immunodeficient virus (HIV), which allows the vector assess inside human cells, dividing or non-diving, is composed of the enzyme integrase, the product of the vpr gene (an accessory gene), and a protein encoded by the gag gene (an essential structural gene) called matrix.  This matrix protein contains a localization sequence which is recognized by the import machinery of the nucleus of a cell.  The virus is surrounded by a lipid bilayer with protruding membrane proteins.  One of these proteins, gp120, is recognized by the host helper T cell CD4 receptor protein.   Then HIV binds to a secondary receptor (CCR5 or CXCR4) and triggers a membrane fusion-mechanism with the gp41 transmembrane protein.  This allows the virus asses to the cell interior and the virus content is released into the cytoplasm of the cell (Adler, Gifford, and Sumner; Schmidt, The HIV Page).  Once inside of the cell in the cytoplasm, the matrix protein of the HIV contains a localization sequence that is recognized by the nuclear import machinery, which docks the complex at a nuclear membrane pore.  This enables the preintegration complex of the HIV lentiviral vector to pass into the nucleus (Amado and Chen, 1999).     It is useful to understand the components of HIV and how it affects its host cell.  The major protein components of the HIV virus can be seen in Table 1.
Table 1: The major protein components, which are expressed by all retroviruses and are necessary for virus replication. They are encoded by three major transcripts: gag, pol, env. These proteins are synthesized as fusion proteins, which are post-translationally cleaved by the virus-encoded protease. HIV has some additional genes (from Schmidt, The HIV Page).
Name:        Protein:                      Fuction:
MA            Matrix                           Matrix protein (gag gene); lines envelope
CA             Capsid                          Capsid protein (gag gene); protects the core; most
                                                       abundant protein in virus particle
NC             Nucleocapsid                Capsid protein (gag gene); protects the genome;
                                                        forms the core
PR              Protease                        Essential for gag protein cleavage during maturation
RT              Reverse transcriptase     Reverse transcribes the RNA genome; also has
                                                        RNAseH activity
IN               Integrase                       Encoded by the pol gene; needed for integration of
                                                        the provirus
SU             Surface glycoprotein       The outer envelope glycoprotein; major virus
TM            Transmembrane protein   The inner component of the mature envelope
                                                         glycoprotein     Lentiviruses are the only type of virus that are diploid; they have two strands of RNA.  Thus, HIV contains a diploid single stranded positive sense RNA-genome that is approximately 10 kb long.  The ends are flanked with long terminal repeats (LTRs).  A Psi-sequence is found near the 5’ end of the RNA-genome which is necessary for packaging viral RNA into virus capsids to continue the infection of HIV in its host (Schmidt, The HIV Page).  However, the HIV’s genetic information is integrated into the DNA of the host cell, so its RNA must be converted into DNA inside of the host for viral replication to be successful.  This is done by reverse transcription of the RNA into DNA, and some of the proteins described in Table 1 are essential for this process.  Reverse transcriptase synthesizes the first strand of DNA from the RNA template, and the host DNA polymerase synthesizes the second strand to produce dsDNA.  Thus, quiescent cells do not have the ability to perform this second step in the reverse transcription process, so the RNA is not turned into DNA in cells in the G0 state.  This is the reason for the limitation on gene therapy with HIV vectors.  The DNA copy just made, which contains the genes gag, env, and pol, is inserted by integrase into the host genome (Adler, Gifford, and Sumner).  LTRs are also necessary for integration of the dsDNA into the host chromosome.  LTRs also serve as part of the promoter for transcription of the viral genes (Schmidt, The HIV Page).  Thus, the virus is protected from attack by the immune system.  It is this ability of the HIV to integrate its genetic material into a host cell which scientists would like to harness to put towards gene therapy.  It has been shown that the HIV vector has an even higher rate of expression in its hosts cells than other retroviruses.  HIV gene therapy vectors also do not trigger immune reactions, making them very attractive delivery systems (Adler, Gifford, and Sumner).

HIV Provirus Used to Construct HIV Based Gene Therapy Vector

    With the new genes from the HIV vector, DNA copy duplication, excision, and integration of the virus can take place.  After infection and integration of the virus into its host regulatory proteins let the retroviral DNA exist in three stages—the latent period with inactivity, the stage where the virus gradually infects helper T cells, and then rapid production of infective viral particles that are released into the blood by the host cell lysis to infect other cells (Adler, Gifford, and Sumner).  Researchers must curtail these second and third phases of HIV infection or HIV cannot be used as a gene therapy vector as patients would be infected with not only the therapeutic gene product but also the AIDS disease.

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How are HIV lentiviral vectors made?
    To obtain a lentiviral gene therapy vector, a reporter gene or therapeutic gene is cloned into a vector sequence that is flanked by LTRs and the Psi-sequence of HIV.  The LTRs are necessary to integrate the therapeutic gene into the genome of the target cell, just as the LTRs in HIV integrate the dsDNA copy of the virus into its host chromosome.  The Psi-sequence acts as a signal sequence and is necessary for packaging RNA with the reporter or therapeutic gene in virions. Viral proteins which make virus shells  are provided in the packaging cell line, but are not in context of the LTRs and Psi-sequences and so are not packaged into virions.  Thus, virus particles are produced that are replication deficient, so are designed to be unable to continue to infect their host after they deliver their therapeutic content (Schmidt, HIV as a Vector for Gene Therapy).

HIV based gene therapy vector

    Lentiviral vectors are usually created in a transient transfection system in which a cell line is transfected with three separate plasmid expression systems.  These include the transfer vector plasmid ( portions of the HIV provirus), the packaging plasmid or construct, and a plasmid with the heterologous envelop gene (ENV) of a different virus (Amado and Chen, 1999).  The three plasmid components of the vector are put into a packaging cell which is then inserted into the HIV shell (Kalapana, 1999).  The virus portions of the vector contain insert sequences so that the virus cannot replicate inside the cell system (Adler, Gifford, and Sumner).

Transfer Vector Plasmid
    The transfer vector plasmid contains cis-acting genetic sequences necessary for the vector to infect the target cell and for transfer of the therapeutic (or reporter) gene and contains restriction sites for insertion of desired genes.  The 3’ and 5’ LTRs, the original envelop proteins, and gag sequence promoter have been removed (Adler, Gifford, and Sumner; Naldini et al., 1996).

Transfer Vector

Packaging Plasmid
    The packaging plasmid is the backbone of the virus system and is also known as pCMVAR9.  In this plasmid are found the elements required for vector packaging such as structural proteins, HIV genes (except the gene env which codes for infection of T cells, or the vector would only be able to infect these cells), and the enzymes that generate vector particles (Amado and Chen, 1999).  Also contained is the human cytomegalovirus (hCMV) which is responsible for the expression of the virus proteins during translation.  The packaging signals and their adjacent signals are removed so the parts responsible for packaging the viral DNA have been separated from the parts that activate them.  Thus, the packaging sequences will not be incorporated into the viral genome and the virus will not reproduce after it has infected the host cell (Adler, Gifford, and Sumner; Naldini, 1996).  Previous HIV vectors used two plasmids as the packaging plasmid contained the viral envelop gene.  However, in the newer, better vectors the packaging plasmid lacks a viral envelop gene because this has been shown to be more desirable in terms of titer (minimum volume needed to cause a particular result in titration), stability, and broad range of target cells (CFAR at UC San Diego).

Packaging Construct pCMVAR9

Envelop Gene of Third Plasmid
    The third plasmid’s envelope gene of a different virus specifies what type of cell to target and infect instead of the T cells (Amado and Chen, 1999).  Normally HIV can infect only helper T-cells because they use their gp120 protein to bind to the CD4 receptor.  However, it is possible to genetically exchange the CD4 receptor-binding protein for another protein that codes for the different cell type on which gene therapy will be performed (Schmidt, HIV as a Vector for Gene Therapy).  This gives the HIV lentiviral vector a broad range of possible target cells.  There are two types of heterologous envelope proteins.  The amphoteric envelop of MLV, another type of vector, is transcribed first followed by the transcription of the G glycoproteins of the vesicular stomatitis virus, known as VSV-G.  Both of these help to provide stability to the vector by bringing together the particles that were made by the packaging plasmid pCMVAR9 (Adler, Gifford, and Sumner; Naldini, 1996).

Envelop Genes in the Third Plasmid

    Scientists are challenged when making efficient packaging lines of HIV gene therapy vectors because expression of the VSV-G envelope and a number of HIV proteins is toxic to cells.  They are dealing with this problem by designing vectors whose expression of the packaging genes and VSV-G can be turned on at will.  Thus, the toxic genes can be turned off to produce more vectors without toxicity.  This cell line can produce high titer vector without generating HIV vectors that can self-replicate and infect the patient with disease (Amado and Chen, 1999).

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How are HIV lentiviral vectors used?     The HIV-based vector can be delivered directly into the body without in vitro manipulations of the patient’s cells (Adler, Gifford, and Sumner).  Additionally, lentiviral vectors have been shown to be superior to murine retroviral vectors.  Ex vivo manipulations that activate stem cells with growth factors to induce cell division must be carried for the retrovirus to be able to enter the stem cells.  However, it has been shown that ex vivo stem cell stimulation is not necessary with lentiviral vectors, so the vectors can be inserted directly into the patient and will find their way to the target cell (Amado and Chen, 1999).
    Previous gene therapy using retroviral vectors required that cells be dividing, limiting therapy to proliferating cells in vivo or ex vivo.  In the ex vivo method, the target cells are removed from the patient's body, treated to stimulate replication and then transduced with the vector before being returned to the patient.  However, with lentiviral vectors there is no need for ex vivo treatment, and the target cells need not be dividing.  The HIV-based vector is simply injected into a patient, upon which it seeks out its target cells based on cell membrane receptor proteins.  Immune responses to the lentiviruses have not been found (Peel, 1998).     Scientists have recently been using the HIV lentiviral vector to repair neurons.  HIV is also being developed as a delivery system to provide successful gene therapy in many diseases such as metabolic diseases, cancer, viral infection, cystic fibrosis, muscular dystrophy, hemophilia, retinitis pigmentosa, and maybe even Alzheimer’s disease (Adler, Gifford, and Sumner; Naldini et al.; Amado and Chen, 1999; Planelles).     There is still concern with using lentiviral vectors for safety reasons.  One concern involves the possibility that the HIV could self-replicate and could be produced during manufacture of the vector in the packaging cell line or in the target cells by a process of recombination.  Thus, the person undergoing gene therapy would also be infected with HIV in addition to the new therapeutic gene.  A self-replicating infectious vector could cause cancer by inserting itself into the host genome and activate a neighboring proto-oncogene, thus causing mutagenesis (Amado and Chen, 1999).

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Current research involving lentiviral vectors
    Because scientists have shown that lentiviruses, such as HIV, are successful and efficient gene delivery vehicles, the field has now turned its attention to producing vectors with built-in safety features to prevent the development of replication competent lentiviruses (RCL).  However, even the earliest studies with HIV lentiviral vectors did not generate RCL in vitro or in vivo (Amado and Chen, 1999), but precautions are still very important.     HIV lentiviral vectors are being produced whose packaging plasmid does not contain the necessary HIV genes.  This does not interfere with efficient vector production and is a great increase in safety because potential RCL’s cannot use the HIV genes necessary for replication of HIV in humans.  The drawback to these vectors is that they cannot transduce macrophages because the accessory gene vpr is needed for HIV infection of this type of cells.  Thus, scientists are showing that the type of lentiviral vector necessary is dependent on the type of cell chosen as target, so the HIV vectors will be made with different accessory genes (Amado and Chen, 1999).

    Researchers at the Salk Institute are creating HIV lentiviral vectors that are self-inactivating.  The scientists are working on packaging a defective HIV genome that contains only the necessary elements for gene transduction into a virion that has a broad host range.  HIV normally targets human CD4 (helper T cells) through interactions with membrane-bound target proteins, but to broaden the host cell targets a surrogate targeting molecule (VSV-G) was inserted into the viral membrane.  The HIV genome was modified to produce a minimal construct and the cytomegalovirus promoter and green fluorescence protein as a marker were added (Sikorski and Peters, 1998).  A deletion in the LTR region at the end of the virus genome is also created.  These are unique cis-acting sequences that are essential to the virus life cycle.  The deletion inactivates the LTR promoter and eliminates the production of vector RNA.  The gene to be transferred by the vector is expressed from an exogenous viral or cellular promoter that is inserted into the lentiviral vector.  Inactivation of the promoter activity of the LTR reduces the possibility of insertional mutagenesis as the lentiviral products are integrated into the host genome.  Also, as expression of the vector RNA is eliminated, the potential for RCL production in the target cell is further minimized (Amado and Chen, 1999).

    Other safety methods include using specific internal promoters that regulate gene expression either temporally or with tissue or cell specificity so as to prohibit gene expression that would cause replication of HIV in the gene therapy target cell (Amado and Chen, 1999).

    By using non-human lentiviruses, scientists hope to bypass the issue of host infection by the gene therapy vector.  Researchers are developing non-human lentiviruses such as the feline immunodeficiency virus (FIV)  to be used in gene therapy (Amado and Chen, 1999).  FIV infects two to twenty percent of domestic cats worldwide and causes a disease similar to human AIDS.  While humans have been exposed to this virus through cat bites, humans have never been shown to be infected by the virus.  It has been shown that evolutionarily FIV diverged early on from HIV and other lentiviruses.  Researchers at the University of San Diego, though, have found that while nonprimate lentiviruses may provide safer alternatives to HIV they have highly restricted host range of infection.  However, promoter substitution of FIV enabled an env-deleted, three plasmid, human cell-FIV lentiviral vector system to express high levels of FIV proteins and FIV vectors in human cells.  The researchers replaced the U3 element within the 5’ LTR of FIV with the human cytomegalovirus early gene promoter.  Pseudotyped FIV vectors were shown to be able to efficiently transduced dividing, growth-arrested, and postmitotic human targets.  The researchers also showed that human cells supported mechanisms of the FIV life cycle needed for efficient lentiviral vector transduction.  It is the U3 element in FIV that is the only restriction to the productive phase of FIV replication in human cells.  The researchers concluded that lentivirus-specific properties of FIV vectors are retained in human cells, and they speculate that eventually FIV vector will have advantages in human clinical use.  Additionally, vectors derived from FIV may represent a safer alternative to HIV vectors, even those with deleted nonstructural proteins, because they cannot induce HIV-reactive antibodies in recipients.  Overall, FIV has experimental advantages over HIV (Poeschla, Wong-Staal, and Looney, 1998).

    Researchers at the University of North Carolina at Chapel Hill are working with equine infectious anemia virus (EIAV) to be used as a lentiviral vector in humans.  EIAV is a lentivirus that normally infects horses, donkeys, and mules.  It has been shown to be able to infect mature macrophages, and thus has the potential to infect quiescent cells, and has relatively simple genome organization.  The researchers constructed separate plasmids encoding EIAV proteins, a viral envelop, and an EIAV vector.  They attempted to broaden the host range of the vector to human cells by using non-EIAV enhancer/promoter elements to drive expression and a non-EIAV envelop glycoprotein.  They succeeded in transducing up to about 60 to 70 percent of human CFT1 cells which were placed in a culture dish.  This is still quite a bit lower than the transduction level obtained using murine retroviruses, but more work with EIAV will hopefully increase the efficiency of this procedure.  In addition, the fact that both EIAV-based and HIV vector can mediate gene transfer and expression to non-dividing human cells suggests that nuclear targeting mechanisms of equine and human lentiviruses are functionally conserved (Olsen, 1998).

    Many recent studies with lentiviral vectors have focused on modifying the hematopoietic stem cell which has the capacity to self-renew and to differentiate into all of the mature cells of the blood and immune systems.  Thus, by introducing therapeutic genes into stem cells many diseases that affect these systems could be treated (Amado and Chen, 1999).     Researchers at the Institute for Gene Therapy at the University of Pennsylvania evaluated a replication-deficient vector based on HIV for gene transfer directly into the lung to correct the genetic defects of cystic fibrosis (CF).  They expanded the target range of the vector by adding the vesticular stomatitis virus G protein into the HIV vector envelop.  LacZ was the reporter gene in the HIV-based vector, so the level of transduction was assessed based on the expression of lacZ.  The researchers were successful at transducing nondividing airway epithelial cells in vitro, whereas they were unsuccessful when using murine-based retroviral vectors.  Thus, the vector corrected the CF defect in proliferating airway cells.   There were complications with differentiated epithelial lung cells as the vectors did not effectively transduce these cells.  The blockage appeared to be at the level of entry, the researchers reported.  Further experimentation is being conducted to examine the problems of cell entry into differentiated cells (Goldman, et al., 1997).     Initial research aimed at delivering genes to the liver in vivo with HIV-based lentiviral vectors showed promising results, reported Ganjam Kalpana of Albert Einstein College of Medicine this year.  This scientist developed a crippled version of HIV and used it as a vehicle for in vivo gene therapy on low-density lipoprotein receptor-deficient Watanabe heritable hyperlipidemic rabbits.  A eukaryotic humanized gene fluorescent protein gene was cloned into the transfer vector to act as the reporter gene for successful cell transduction.  The HIV vector was highly superior to previous methods of gene therapy using retroviral vectors which were highly invasive to the patient.  There was also no host mediated cellular immune response to the lentiviral vector (Kalpana, 1999).  This is another application to HIV-based gene therapy vectors that has been shown to be successful.     Retinitis pigmentosa is an inherited genetic disease which causes the retina to degenerate leading to loss of visual field and night blindness.  Genetic defects of photoreceptor cells of the visual system are the cause of this disease.  A vector for gene therapy of retinitis pigmentosa should only target photoreceptor cells, which are located in the outer nuclear layer of the retina.  Miyoshi, Takahashi, Gage, and Verma conducted an experiment using an HIV-based vector with a gfp-gene (green fluorescent protein) as a reporter.  The vector was injected into rat retina.  It was shown that the HIV-based vector did achieve long-term gene expression in the photoreceptor cells when a rhodopsin-promoter was used in the vector.  This is only active in the photoreceptor cells, so the vector only targets these cells and not others in the retina.  Thus, the researchers were successful in performing gene therapy on their rat patients (Schmidt, HIV as a Vector For Gene Therapy).

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Adler, K., J. Gifford, and R. Sumner.  HIV as a Vector in Gene Therapy.  [Online.]
    http://wwwpp.uwrf.edu/%7Ekk00/hivvector/hivvector.htm.  [12-13-99, last date accessed.]

Amado, R. G. and YI. S.. Chen.  1999.  Lentiviral Vectors—the Promise of Gene Therapy Within Reach?  Science.  285
    (5428): 674-76.

CFAR: Center for AIDS Research at UC San Diego.  Last update 10-18-99.  Lentiviral Vector Core.  [Online.]
    http://hsrd.ucsd.edu/Cfar/lenti/lenti.html.  [12-13-99, last date accessed.]

Goldman, M. J., P. Lee, J. Yang, and J. M. Wilson.  1997.  Lentiviral Vectors for Gene Therapy of Cystic Fibrosis.
    Human Gene Therapy.  8: 2261-2268.

Kalpana, G. V.  1999.  Retroviral Vectors for Liver-directed Gene Therapy.  Seminar in Liver Disease.  19 (1): 27-37.

Lasic, D. D.  Liposomes in Gene Delivery.  New York: CRC Press, 1997.

Naldini et al.  1997.  Lentiviral Vectors for in Vivo Gene Delivery.  The International Symposium on Gene Therapy for
    Hemophilia.  [Online.]  http://www.med.unc.edu/wrkunits/3ctrpgm/thromb/naldini.htm.  [12-13-99, last date accessed.]

Naldini et al.  1996.  In Vivo Gene Delivery and Stable Tranduction of Nondividing Cells by a Lentiviral Vector.  Science.
    272: 263-267.

Nichols, E. K.  Human Gene Therapy.  Cambridge, Massachusettes: Harvard University Press, 1998.

Olsen, J. C.  1998.  Gene Transfer Vectors Derived From Equine Infectious Anemia Virus.  Gene Therapy.  5: 1481-1487.

Peel, David.  1998.  Retroviral Vectors and Lentiviral Vectors.  Department of Microbiology & Immunology, University of
    Leicester.  [Online.]  http://science.uniserve.edu.au/mirror/microbiol/335/peel/peel2.html.  [12-13-99, last date accessed.]

Poeschla, E. M., F. Wong-Staal, and D. J. Looney.  1998.  Efficient Transduction of Nondividing Human Cells by Feline
    Immunodeficiency Virus Lentiviral Vectors.  Nature Medicine.  4 (3): 354-357.

Planelles, V.  1999.  Homepage of Vicente Planelles.  [Online.]
    http://www.urmc.rochester.edu/gebs/faculty/Vicente_Planelles.htm.  [12-13-99, last date accessed.]

Sikorski, R. and R. Peters.  1998.  Gene Therapy: Treating with HIV.  Science.  282 (5393): 1438a.

Schmidt, Uli.  The HIV Page.  [Online.]  http://bioinformatik.biochemtech.uni-halle.de/uli/genetherapy/hiv.htm.  [12-13-99,
    last date accessed.]

_____.  HIV as a Vector for Gene Therapy.  [Online.]
    http://bioinformatik.biochemtech.uni-halle.de/uli/genetherapy/genehiv.htm.  [12-13-99, last date accessed.]

Tighe, R. and J. Fritz.  1996.  Lentiviral Vectors (HIV-based).  [Online.]  http://www.mc.vanderbilt.edu/gcrc/gene/hiv.htm.
    [12-13-99, last date accessed.]

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Adenovirus: another early vector in gene therapy; used when gene transfer into the chromosomes of target cells was not needed

Alzheimer’s disease: a disease marked by progressive loss of mental capacity resulting from degeneration of the brain cells

CD4:  a membrane protein of helper T cells that interacts with membrane proteins of HIV

Cystic fibrosis: a hereditary disease of the exocrine glands, usually developing during early childhood and affecting mainly the pancreas, respiratory system, and sweat glands; characterized by the production of abnormally viscous mucus by the affected glands, usually resulting in chronic respiratory infections and impaired pancreatic function

EIAV: equine infectious anemia virus; a lentivirus that normally infects horses, donkeys, and mules with anemia

Ex vivo: out of the patient’s body

FIV: feline immunodeficiency virus; infects two to twenty percent of domestic cats worldwide and causes a disease similar to human AIDS

Gene therapy: involves the insertion of a functional gene or another molecule that contains and information sequence into a cell to achieve a therapeutic effect

gp120: a membrane protein of HIV that interacts with membrane proteins of its target cell

Helper T cells: components of the human immune system and the target cells of HIV

Hemophilia: several hereditary blood-coagulation disorders in which the blood fails to clot normally because of a deficiency or an abnormality of one of the clotting factors; a recessive trait associated with the X-chromosome so manifested almost exclusively in males

HIV: human immunodeficiency virus; a virus of the human immune system that causes the AIDS disease

In vitro: out of the patient’s body in a test tube or culture dish

In vivo: in the patient’s body

Lentiviral vectors: Lentiviruses are a type of retrovirus that can infect both dividing and nondividing cells because their preintegration complex (virus “shell”) can get through the intact membrane of the nucleus of the target cell

LTR: long terminal repeats; flank the ends of the HIV genome and contain a Psi-sequence near the 5’ end of the RNA-genome

Macrophages: any of the large phagocytic cells of the reticuloendothelial system

Moloney murine leukemia virus: a retrovirus that was used in early gene therapy experiments; used when gene transfer into the chromosomes of target cells was needed

Muscular dystrophy: a group of progressive muscle disorders caused by a defect in one or more genes that control muscle function and characterized by gradual irreversible wasting of skeletal muscle

Mutagenesis: formation or development of a mutation

Proto-oncogene: a normal gene that could develop into one that causes a transformation of normal cells into cancerous tumor cells, especially a viral gene that transforms a host cell into a tumor cell

Psi-sequence: located at the 5’ end of the HIV’s LTR; is necessary for packaging viral RNA into virus capsids to continue the infection of HIV in its host

RCL: a replication competent lentivirus; an HIV lentivirus that can infected its host with the AIDS disease

Retinitis pigmentosis: an inherited genetic disease which causes the retina to degenerate leading to loss of visual field and night blindness; caused by genetic defects of photoreceptor cells of the visual system are the cause of this disease

Retroviruses: a class of viruses

Reporter gene: inserted into a genome along with a new gene to show the position and existence of the new gene

Reverse transcription: the process of converting RNA to DNA