Scrigroup - Documente si articole

Username / Parola inexistente      

Home Documente Upload Resurse Alte limbi doc  


BulgaraCeha slovacaCroataEnglezaEstonaFinlandezaFranceza


Diseases of Genetic Origin


+ Font mai mare | - Font mai mic


Trimite pe Messenger
Drugs Used in the Chemotherapy of Protozoal Infections: Malaria
Cervical Dystonia: Description, Symptoms
Antimicrobial Agents: Sulfonamides, Trimethoprim-Sulfamethoxazole, Quinolones
Homeopathy - The True Classical Homeopathic Approach to Cure
Cancer: Civilization's
Tyrosinase a useful marker for identifying pagetoid invasion in malignant melanoma
Diuretics - Renal Anatomy and Physiology
General Anesthetics
Cervical Plexus Block for Carotid Endarterectomy - Anatomy

Diseases of Genetic Origin

Gene Therapy

Transgenic animals have been used for simulating diseases and testing new therapies, egg. Cardiovascular and neurodegenerative diseases. Animal models provide an opportunity to test methods for the prevention or delay of disease in humans. Some examples:

Genetic alteration

Method of alteration

Human disease equivalent

Introduction of mutant collagen gene into wild type mice

Nuclear microinjection of inducible minigene

Osteogenesis imperfecta

Inactivation of mouse gene encoding hypoxanthine-guanine phosphoribosyl transferase (HPRT)

Insertion of retrovirus into HPRT locus in embryonic stem cells

HPRT deficiency

Mutation at locus for X-linked muscular dystrophy

Male mutagenesis followed by identification of female carriers

X-linked muscular dystrophy

Introduction of activated human ras and c-myc oncogenes

Nuclear microinjection of inducible minigene

Induction of malignancy

Introduction of mutant (Z)allele of human alpha -1-antitrypsin gene

Microinjection of DNA fragment bearing mutant allele

Neonatal hepatitis

Introduction of HIV tat gene

Microinjection of DNA fragment

Kaposi's sarcoma

Introduction of beta-globin sickle gene


Sickle-cell anaemia

Introduction of mouse renin gene



Introduction of (beta)-amyloid protein precursor (APP gene)


Alzheimer's disease

Candidate Diseases for Gene Therapy

We know which gene is mutated in the disorder.

  • We have a normal copy of that gene available.
  • We understand the biology of the disease, including which tissue types are affected and how they are affected.
  • We can predict that adding the normal gene back to the cells that make up the affected tissues will restore a needed function.
  • Now all we have to do is deliver the gene into the proper cells and put it to work. This is not an easy job. Gene delivery is one of the biggest challenges in the field of gene therapy.

    Gene Delivery: The Key to Gene Therapy

    What are some of the hallmarks of successful gene delivery?

    1. TARGETING the right cells. If you want to deliver a gene into cells of the liver, it shouldn't wind up in the big toe. How can you ensure that the gene gets into the correct cells?

    2. ACTIVATING the gene. A gene's journey is not over when it enters the cell. It must go to the cell's nucleus and be 'turned on,' meaning that its transcription and translation are activated to produce the protein product encoded by the gene. For gene delivery to be successful, the protein that is produced must function properly.

    3. INTEGRATING the gene in the cells. You might want the gene to stay put and continue working in the target cells. If so, you need to ensure that the gene integrates into, or becomes part of the host cell's genetic material, or that the gene finds another way to survive in the nucleus without being trashed.

    4. AVOIDING harmful side effects. Anytime you introduce an unfamiliar biological substance into the body, there is a risk that it will be toxic or that the body will mount an immune response against it. Genes are made of DNA. Successful gene delivery requires an efficient way to get the DNA into cells and to make it work. Scientists refer to this DNA delivery 'vehicles' as vectors.

    Choosing the best vector

    There is no 'perfect vector' that can treat every disorder. Like any type of medical treatment, a gene therapy vector must be customized to address the unique features of the disorder. Part of the challenge in gene therapy is choosing the most suitable vector for treating the disorder. Below, find out more about the most commonly used types of gene therapy vectors.

    Viral vectors

    Mother Nature is a brilliant scientist! Over the last three billion years or so, she's developed an incredibly efficient means of delivering foreign genes into cells: the virus. When faced with the problem of gene delivery, scientists looked to viruses. Why reinvent the wheel if there's a perfectly good one out there? If we can modify viruses to deliver genes without making people sick, we may have a good set of gene therapy tools.

    General advantages of viral vectors:

    • They're very good at targeting and entering cells.
    • Some viral vectors might be engineered to target specific types of cells.
    • They can be modified so that they can't replicate and destroy the cell.

    General drawbacks of viral vectors:

    • A virus can't 'expand' to fit a piece of genetic material larger than it is naturally built to carry. Therefore, some genes may be too big to fit into a certain type of virus.
    • Viruses can cause immune responses in patients, resulting in two potential outcomes:
      • Patients may get sick.
      • A patient's immunity to a virus may prevent him from responding to repeated treatments.

    However, modern viral vectors have been engineered without most of the proteins that would cause an immune response.

    Some of the different types of viruses used as gene therapy vectors:

    • Retroviruses - A class of viruses that can create double-stranded DNA copies of their RNA genomes. These copies of its genome can be integrated into the chromosomes of host cells. Human immunodeficiency virus (HIV) is a retrovirus.
    • Adenoviruses - A class of viruses with double-stranded DNA genomes that cause respiratory, intestinal, and eye infections in humans. The virus that causes the common cold is an adenovirus.
    • Adeno-associated viruses - A class of small, single-stranded DNA viruses that can insert their genetic material at a specific site on chromosome 19.
    • Herpes simplex viruses - A class of double-stranded DNA viruses that infect a particular cell type, neurons. Herpes simplex virus type 1 is a common human pathogen that causes cold sores.


    From the previously listed limitations, it is clear that the use of Ad2- or Ad5-derived vectors for human gene transfer poses a dilemma. On one hand, the therapeutic viral dose must be elevated to overcome the presence of neutralizing antibodies and to compensate for the low efficiency of transduction of the target cells. On the other hand, a high viral dose causes undesired local and systemic toxicity and possibly results in the eradication of the transduced cells by the host immune response. To overcome this dilemma, adenoviral vectors must be generated that are not neutralized by the antiviral antibodies pre-existing in the serum and that are able to specifically and efficiently transduce the tissue of interest and not undesired organs. Several strategies have been explored to develop vectors that specifically target the cell types of interest

    Adeno-associated virus (AAV)-based vectors have recently gained popularity as potential vectors for gene therapy. The major advantages of using AAV are nonpathogenicity, long-term expression, and relatively low immunogenecity. Although AAV was believed to infect both dividing and nondividing cells, recent studies have clearly demonstrated variations in transduction among cell types. The identification of the host cell receptor, coreceptors, and other factors that facilitate AAV entry and intracellular processing, leading to transgene expression, indicates that deficiency in one or more of these molecules can limit a successful transduction. Thus, it is apparent that modifications that will overcome these limitations will not only maximize the application of AAV vectors in gene therapy, but also lead to the development of targeted vectors for transgene delivery to specific cell types.

    The eventual development of truly effective methods of in vivo gene delivery for gene therapy will require methods for delivering therapeutic genes to the correct target tissue specifically and efficiently, whether the gene delivery vector is a viral or a nonviral agent. A variety of viral vectors are available for such applications, including retroviral and lentiviral vectors. Retroviral vectors have been effective tools for developing many of our current concepts and techniques for models of gene therapy. They have many advantageous features, especially their ease of manipulation for vector construction and the generally stable transgene expression that results from provirus integration. They also have disadvantages, including their relatively low titers compared with several other viral vector systems, their instability in vivo, and their inability to transduce nonreplicating or slowly replicating cells such as neurons. Several of these deficiencies have been corrected partly by the development of methods to increase titers and alter vector tropism through methods for pseudotyping vectors with surrogate envelope components such as VSV-G protein. The development of the lentiviral vector system has also provided an efficient approach to gene transfer into post mitotic cells that exhibits many of the useful features of other retroviral systems and promises to become important technique in future clinical studies. There are at least two principal difficulties in the use of these and all other viral vectors for therapeutic gene delivery. In the case of retroviruses, our present understanding of virus structure is insufficient to allow the design and production of truly targeted vectors through insertion of potentially cell-specific ligands into the viral capsid. In addition, there is a dearth of rigorous information on the pharmacokinetic and pharmacodynamic properties of these viruses; that is, the in vivo fate of vector particles, their interactions with the vascular endothelium as the first tissue barrier that they encounter, and the mechanisms that define their tissue uptake. In the case of systemic administration of retroviral vectors, as in the preclinical and clinical studies aimed at correction of the factor VIII deficiency of classical haemophilia, very little information is readily available on the distribution of systemically delivered virus particles and the exact identification of the tissue source of the factor VIII protein. In addition, even though the studies involving in vivo delivery of AAV vectors to muscle or liver for treatment of haemophilia B are extremely promising, variable patient response and treatment responses are likely to result from insufficient knowledge of the pharmacological properties of the vectors and variations in the mechanisms of tissue uptake and cell entry.

    Non-Viral Vectors

    Although viruses can effectively deliver genetic material into a patient's cells, they do have some limitations. It is sometimes more efficient to deliver a gene using a non-viral vector, which has fewer size constraints and which won't generate an immune response.

    Delivery of nucleic acids using liposomes holds great promise as a safe and nonimmunogenic approach to gene therapy. Furthermore, gene therapies that use these artificial reagents can be standardized and regulated as drugs rather than as biologics. Much effort has been devoted to the development of nonviral delivery due to the disadvantages of viruses used for gene delivery. The disadvantages of viral delivery include:

    • Generation of immune responses to expressed viral proteins that subsequently kill the target cells required to produce the therapeutic gene product
    • Random integration of some viral vectors into the host chromosome ?Clearance of viral vectors delivered systemically
    • Difficulties in engineering viral envelopes or capsids to achieve specific delivery to cells other than those with natural tropism for the virus
    • Potential recombination of the viral vector with DNA sequences in the host chromosome that generates a replication-competent, infectious virus
    • Inability to administer certain viral vectors more than once
    • High costs for producing large amounts of high-titer viral stocks for use in the clinic
    • Limited size of the nucleic acid that can be packaged and used for viral gene therapy

    The advantages in using liposomes for gene therapy are several and include:

    • Lack of immunogenicity
    • Lack of clearance by complement using improved formulations
    • Unlimited size of nucleic acids that can be delivered (from single nucleotides to large mammalian artificial chromosomes)
    • Ability to perform repeated administrations in vivo without adverse consequences
    • Low cost and relative ease in creating nucleic acid–liposome complexes in large scale for use in the clinic
    • Relative ease in creating targeted complexes for delivery and gene expression in specific cell types, organs, or tissues
    • Greater safety for patients due to few or no viral sequences present in nucleic acids used for delivery, thereby precluding generation of an infectious virus

    The disadvantage of nonviral delivery systems had been the low levels of delivery and gene expression produced by first generation complexes. However, recent advances have been made that dramatically improve transfection efficiencies of nonviral vectors. Delivery of nucleic acid-based therapeutics to specific target tissues, organs, or cells is desirable or required for certain applications. For example, lower amounts of complexes could be administered intravenously if the bulk of the injected material is delivered to target cells that are solely responsible for producing the therapeutic gene product. Injection of lower amounts of complexes would be most cost-effective, would provide another level of safety to the patient, and may produce greater efficacy for the treatment of certain diseases. Furthermore, strategies such as suicide gene approaches that are designed to kill target cells, such as tumour cells, require targeted delivery or gene expression to avoid killing normal cells. Cell or tissue-specific gene expression can also be achieved by creating plasmids containing specific promoters to produce expression exclusively in the target cells. Nonviral delivery vehicles have no target specificity, and therefore, retargeting is not required. Basically, ligands are used to coat nucleic acid–liposome complexes to achieve specific delivery to cell surface receptors. To efficiently coat the surface of complexes, nucleic acids must be encapsulated within the delivery vehicle so that the ligand does not interfere with nucleic acid condensation. In addition, the encapsulated nucleic acid does not prevent the attachment of the ligand on the surface of the complexes.


    Methods used to date for targeted delivery of nonviral delivery vehicles have produced inefficient gene expression in the target cells. Although gene expression is apparently produced primarily in the target cell, the levels of expression are lower in these cells than those produced using the nontargeted vehicle for delivery. Novel approaches involve targeted delivery of liposomes that are optimized for charge on the surface of complexes, cell entry by fusion with the membrane, and penetration across tight barriers in vivo to reach and diffuse through the target tissue /organ efficiently.

    Much effort has gone into developing ways to enhance the specificity of liposomal delivery systems, most commonly by conjugating ligands to the liposome surface that will produce a specific interaction with the target cell. Ligands include vitamins, glycoproteins, peptides and oligonucleotides octamers, in addition to the most commonly used ligands, antibodies or antibody fragments. The first report of Mabs conjugated to liposomes was by Torchilin et al. (1979), who demonstrated that antimyosin-immunoliposome retained its ability to specifically bind to the receptor on the target cells. Various attempts have since been made to improve the efficiency of delivery of the liposome payload including development of pH-sensitive immunoliposomes. However, despite promising in vitro results, the in vivo use of these Mab-immunoliposomes has been hampered by lack of stability in circulation and their rapid clearance by the mononuclear phagocyte system (MPS) (liver and spleen). An example of this is the report by Matzku et al. (1990), who found virtually no uptake of Mab-immunoliposomes in either a human melanoma xenograft model or a syngeneic murine lymphoma model that spontaneously metastasizes to the liver. These investigators speculated that lack of uptake was the result of limited availability of the complex due to only moderate stability in circulation as well as inability of the immunoliposome to extravasate.

    In an effort to sidestep the safety limitations of viral vectors and the cytotoxicity of liposomal carriers, investigators have used receptor-targeted molecular conjugates to direct gene transfer into mammalian cells in vitro. Complexes consisting of DNA, noncovalently bound to a poly-cation polymer that is chemically conjugated to a ligand, can bind a cell surface receptor and be internalized.

    General scheme of receptor targeted DNA complex construction. DNA complexes are formed by mixing plasmid DNA with the molecular conjugate under the proper salt conditions. Molecular conjugates consist of a polycation coupled to a receptor ligand. Polycations modified with enhancers (e.g., PEG or adenovirus) may also be included in the DNA complex.

    How do you know whether a disorder is a good candidate for gene therapy?

    For any candidate disorder, you need to answer the following questions:

    1. Does the condition result from mutations in one or more genes? For you to even consider gene therapy, the answer must be 'yes.'
    2. Which genes are involved? If you plan to treat a genetic flaw, you need to know which gene(s) to pursue. You must also have a DNA copy of that gene available in your laboratory. The best candidates for gene therapy are the so-called 'single-gene' disorders - which are caused by mutations in only one gene.
    3. What do you know about the biology of the disorder? To design the best possible approach, you need to learn all you can about how the gene factors into the disorder. For example:
      • Which tissues are affected?
      • What role does the protein encoded by the gene play within the cells of that tissue?
      • Exactly how do mutations in the gene affect the protein's function?
    4. Will adding a normal copy of the gene fix the problem in the affected tissue? This may seem like an obvious question, but it's not. What if the mutated gene encodes a protein that prevents the normal protein from doing its job? Mutated genes that function this way are called dominant negative and adding back the normal protein won't fix the problem.
    5. Can you deliver the gene to cells of the affected tissue? The answer will come from several pieces of information, including:
      • How accessible is the tissue? Is it fairly easy (skin, blood or lungs), or more difficult to reach (internal organs)?
      • What is your best mode of delivery? If you can answer 'yes' to Questions 4 and 5, then the disorder may be a good candidate for a gene therapy approach.

    Arguments in Favour of Gene Therapy

    Arguments Against Gene Therapy

    1)                          germ-line gene therapy experiments would involve too much scientific uncertainty and clinical risks, and the long term effects of such therapy are unknown;

    2)                          such gene therapy would open the door to attempts at altering human traits not associated with disease, which could exacerbate problems of social discrimination;

    3)                          as germ-line gene therapy involves research on early embryos and effects their offspring, such research essentially creates generations of unconsenting research subjects;

    4)                          gene therapy is very expensive, and will never be cost effective enough to merit high social priority;

    5)                          germ-line gene therapy would violate the rights of subsequent generations to inherit a genetic endowment that has not been intentionally modified.

    Human Gene Therapy: Present and Future'

    Challenges in Gene Therapy

    Gene therapy is not a new field; it has been evolving for decades. Despite the best efforts of researchers around the world, however, gene therapy has seen only limited success. Why? The answer is that gene therapy poses one of the greatest technical challenges in modern medicine. It is very hard to introduce new genes into cells of the body. Let's look at some of the main technical issues in gene therapy.

    Gene delivery and activation

    Introducing changes into the germ line

    Targeting a gene to the correct cells is crucial to the success of any gene therapy treatment. Just as important, though, is making sure that the gene is not incorporated into the wrong cells. Delivering a gene to the wrong tissue would be inefficient and could cause health problems for the patient.

    For example, improper targeting could incorporate the therapeutic gene into a patient's germ line, or reproductive cells, which ultimately produce sperm and eggs. Should this happen, the patient would pass the introduced gene on to his or her offspring. The consequences would vary, depending on the type of gene introduced.

    Immune response

    Our immune systems are very good at fighting off intruders such as bacteria, viruses and other biological substances. Gene delivery vectors must be able to escape the body's natural surveillance systems. Failure to do so can cause serious illness or even death.

    The story of Jesse Gelsinger illustrates this challenge well. Gelsinger, who had a rare liver disorder, participated in a 1999 gene therapy trial at the University of Pennsylvania. He died of complications from an inflammatory response shortly after receiving a dose of experimental adenovirus vector. His death halted all gene therapy trials in the United States for a time, sparking a much-needed discussion on how best to regulate experimental trials and report health problems in volunteer patients.

    Disrupting important genes in target cells

    The best gene therapy is the one that lasts. Ideally, we would want a gene that is introduced into a group of cells to remain there and continue working. For this to happen, the newly introduced gene must become a permanent part of each cell's genome, usually by integrating, or 'stitching' itself, into the cell's existing DNA. But what happens if the gene stitches itself into an inappropriate location, disrupting another gene?

    This happened recently in a gene therapy trial to treat several children with X-linked Severe Combined Immune Deficiency (SCID). People with this disorder have virtually no immune protection against bacteria and viruses. To escape infections and illnesses, they must live in a completely germ-free environment.

    In the late 1990s, researchers tested a gene therapy treatment that would restore the function of a crucial gene, gamma c, to cells of the immune system. This treatment appeared very successful, restoring immune function to most of the children who received it. But later, two of these children developed leukemia. Researchers found that the leukemia occurred because the newly transferred gamma c gene had stitched itself into the wrong place, interrupting the function of a gene that normally helps regulate the rate at which cells divide. As a result, the cells began to divide out of control, causing the blood cancer leukemia. Although doctors have treated the children successfully with chemotherapy, the fact that they developed leukemia during treatment raises another important safety-related issue that gene therapy researchers must address.

    New Approaches to Gene Therapy

    Dominant NegativeThere are times, though, when adding a 'good' copy of the gene won't solve the problem. For example, if the mutated gene encodes a protein that prevents the normal protein from doing its job, adding back the normal gene won't help. Mutated genes that function this way are called dominant negative.

    How to deal with a dominant negative?

    To address this situation, you could either repair the mutated gene's product, or you could get rid of it altogether. Here are some of the newest methods that scientists are developing as potential approaches to gene therapy. Each of these techniques also requires a specific and efficient means of delivering the gene to the target cell.

    A technique for repairing mutations:

    SMaRT™ The term SMaRT™ stands for 'Spliceosome-Mediated RNA Trans-splicing.' This technique targets and repairs the messenger RNA (mRNA) transcripts copied from the mutated gene. Instead of attempting to replace the entire gene, this technique repairs just the section of the mRNA transcript that contains the mutation. After a gene is copied into mRNA, the cell uses RNA-based machinery called spliceosomes to cut out the non-coding introns and splice the exons together.

    SMaRT™ involves three steps:

    1.                  Delivery of an RNA strand that pairs specifically with the intron next to the mutated segment of mRNA. Once bound, this RNA strand prevents spliceosomes from including the mutated segment in the final, spliced RNA product.

    2.                  Simultaneous delivery of a correct version of the segment to replace the mutated piece in the final mRNA product

    3.                  Translation of the repaired mRNA to produce the normal, functional protein

    SMaRT™ is a trademark of Intronn, Inc.

    Techniques to prevent the production of a mutated protein:

    ·         Triple-helix-forming oligonucleotidesTriple-helix-forming oligonucleotides

    Triple-helix-forming oligonucleotide gene therapy targets the DNA sequence of a mutated gene to prevent its transcription. This technique involves the delivery of short, single-stranded pieces of DNA, called oligonucleotides that bind specifically in the groove between the double strands of the mutated gene's DNA. Binding produces a triple-helix structure that prevents that segment of DNA from being transcribed into mRNA.


    Antisense gene therapy aims to turn off a mutated gene in a cell by targeting the mRNA transcripts copied from the gene. Genes are made up of two paired DNA strands. During transcription, the sequence of one strand is copied into a single strand of mRNA. This mRNA is called the 'sense' strand because it contains the code that will be read by the cell as it makes a protein. The opposite strand is the 'antisense' strand.

    Antisense gene therapy involves the following steps:

      1. Delivery of an RNA strand containing the antisense code of a mutated gene
      2. Binding of the antisense RNA strands to the mutated sense mRNA strands, preventing the mRNA from being translated into a mutated protein

    ·         Ribozymes

    Like antisense, ribozyme gene therapy aims to turn off a mutated gene in a cell by targeting the mRNA transcripts copied from the gene. This approach prevents the production of the mutated protein.

    Ribozymes are RNA molecules that act as enzymes. Most often, they act as molecular scissors that cut RNA. For example, spliceosomes (described above) are believed to be a type of ribozyme

    Ribozyme gene therapy involves the following steps:

      1. Delivery of RNA strands engineered to function as ribozymes.
      2. Specific binding of the ribozyme RNA to mRNA encoded by the mutated gene
      3. Cleavage of the target mRNA, preventing it from being translated into a protein


    RNA Interference

    RNAi is present in a wide variety of eukaryotic organisms including mammals. For the most part, it is believed that RNAi serves as an antiviral defence mechanism although there is preliminary evidence that it also plays a role in the formation and maintenance of heterochromatin during mitosis and meiosis.

    SiRNAs have a well defined structure: a short (usually 21-nt) double-strand of RNA (dsRNA) with 2-nt 3' overhangs on either end:


    Each strand has a 5' phosphate group and a 3' hydroxyl (-OH) group. This structure is the result of Dicer processing, an enzyme that converts either long dsRNAs or hairpin RNAs into siRNAs. SiRNAs can also be exogenously (artificially) introduced into cells by various transfection methods to bring about the specific knockdown of a gene of interest. Essentially any gene of which the sequence is known can thus be targeted based on sequence complementarity with an appropriately tailored siRNA. This has made siRNAs an important tool for gene function and drug target validation studies in the post-genomic era.

    Mechanism of RNA interference (RNAi)

    Preliminary studies in mammalian systems using long dsRNAs to initiate the RNAi response failed because they led to the induction of a non-specific Type I interferon response that produced extensive changes in protein expression and eventually resulted in cell death. Subsequent studies, however, using synthetic, short double-stranded RNAs that mimic the siRNAs produced by the enzyme dicer, sequence specific gene silencing could be achieved in mammalian cells without inducing the interferon response. siRNA technology is now extensively recognized as a powerful tool for the specific suppression of gene expression and is presently being used by researchers in a wide range of disciplines for the assessment of gene function.

    Challenges: Avoiding non-specific effects

    RNAi intersects with a number of other pathways, so it is not surprising that on occasion non-specific effects are triggered by the experimental introduction of an siRNA. When a mammalian cell encounters a double-stranded RNA such as an siRNA, it may mistake it as a viral by-product and mount an immune response. Furthermore, since structurally related microRNAs modulate gene expression largely via incomplete complementarity with a target mRNA, unintended off-targeting may be effected by the introduction of an siRNA.

    Innate Immunity

    Introduction of too much siRNA can result in non-specific events due to activation of innate immune responses. Most papers suggest that this is probably due to activation of the dsRNA sensor PKR, although retinoic acid inducible Gene I (RIG-I) may also be involved. One promising method of reducing the non-specific effects is to convert the shRNA into a microRNA. MicroRNAs occur naturally, and by harnessing this endogenous pathway it should be possible to achieve similar gene knockdown at comparatively low concentrations of resulting siRNAs. This should minimise non-specific effects.

    The Future

    Opinion 1 -Given the ability to knockdown essentially any gene of interest, RNAi via siRNAs has generated a great deal of interest in both basic and applied biology. There is an increasing number of large-scale RNAi screens that are designed to identify the important genes in various biological pathways. As disease processes also depend on the activity of multiple genes, it is expected that by turning off their activity with siRNAs or their biosynthetic precursors, therapeutic benefit can be derived directly via RNAi. Indeed, phase I results of the first two therapeutic RNAi trials (indicated for age-related macular degeneration, aka AMD) reported at the end of 2005, demonstrate that siRNAs are well tolerated and have suitable pharmacokinetic properties. SiRNAs and related RNAi induction methods therefore stand to become an important new class of drugs in the foreseeable future.

    Opinion 2 - Using siRNA's/shRNA's to knockdown specific genes is certainly a valuable tool in the laboratory. However, there are a great deal of challenges when it comes to taking a laboratory technique and applying it to living animals, especially humans. Firstly, siRNA's show different effectiveness in different cell types, apparently indiscriminately - some cells respond well to siRNA's and show a robust knockdown, others show no such knockdown (even despite efficient transfection). Secondly, and most importantly, the non-specific responses of si/shRNA's are still relatively poorly understood. Until these responses can be understood and overcome, the chances of using si/shRNA's outside of the lab, e.g. as an effective new class of drug, remain slim.


    The term miRNA was first introduced in a set of three articles in Science (26 October 2001).

    miRNA (micro-RNA) is a form of single-stranded RNA which is typically 20-25 nucleotides long, and is thought to regulate the expression of other genes. miRNAs are RNA genes which are transcribed from DNA, but are not translated into protein. The DNA sequence that codes for an miRNA gene is longer than the miRNA. This DNA sequence includes the miRNA sequence and an approximate reverse complement. When this DNA sequence is transcribed into a single-stranded RNA molecule, the miRNA sequence and its reverse-complement base pair to form a double stranded RNA hairpin loop; this forms a primary miRNA structure (pri-miRNA). In animals, the nuclear enzyme Drosha cleaves the base of the hairpin to form pre-miRNA. The pre-miRNA molecule is then actively transported out of the nucleus into the cytoplasm by Exportin 5, a carrier protein. The Dicer enzyme then cuts 20-25 nucleotides from the base of the hairpin to release the mature miRNA. In plants, which lack Drosha homologues, pri- and pre-miRNA processing by Dicer probably takes place in the nucleus, and mature miRNA duplexes are exported to the cytosol by Exportin 5.

    The function of miRNAs appears to be in gene regulation. For that purpose, a miRNA is complementary to a part of one or more messenger RNAs (mRNAs). Animal miRNAs are usually complementary to a site in the 3' UTR whereas plant miRNAs are usually complementary to coding regions of mRNAs. The annealing of the miRNA to the mRNA then inhibits protein translation, but sometimes facilitates cleavage of the mRNA. This is thought to be the primary mode of action of plant miRNAs. In such cases, the formation of the double-stranded RNA through the binding of the miRNA triggers the degradation of the mRNA transcript through a process similar to RNA interference (RNAi), though in other cases it is believed that the miRNA complex blocks the protein translation machinery or otherwise prevents protein translation without causing the mRNA to be degraded. miRNAs may also target methylation of genomic sites which correspond to targeted mRNAs. miRNAs function in association with a complement of proteins collectively termed the miRNP.

    This effect was first described for the worm Caenorhabditis elegans in 1993 by R. C. Lee of Harvard University. As of 2002, miRNAs have been confirmed in various plants and animals, including C. elegans, human and the plant Arabidopsis thaliana. Genes have been found in bacteria that are similar in the sense that they control mRNA abundance or translation by binding an mRNA by base pairing, however they are not generally considered to be miRNAs because the Dicer enzyme is not involved.

    miRNA and cancer

    miRNA has been found to have links with some types of cancer.

    A study of mice altered to produce excess c-myc — a protein implicated in several cancers — shows that miRNA has an effect on the development of cancer. Mice that were engineered to produce a surplus of types of miRNA found in lymphoma cells developed the disease within 50 days and died two weeks later. In contrast, mice without the surplus miRNA lived over 100 days.

    Another study found that two types of miRNA inhibit the E2F1 protein, which regulates cell proliferation. miRNA appears to bind to messenger RNA before it can be translated to proteins that switch genes on and off.

    By measuring activity among 217 genes encoding miRNA, patterns of gene activity that can distinguish types of cancers can be discerned. miRNA signatures may enable classification of cancer. This will allow doctors to determine the original tissue type which spawned a cancer and to be able to target a treatment course based on the original tissue type. miRNA profiling has already been able to determine whether patients with chronic lymphocytic leukemia had slow growing or aggressive forms of the cancer.

    illustration of DNA and the units that compose it (cell, nucleus, chromosomes) and illustration of a vector, target cell, and repaired cell

    illustration of vectors with SCID-repaired genes being joined with bone marrow cells and the resulting repaired cells

    Politica de confidentialitate



    Vizualizari: 3083
    Importanta: rank

    Comenteaza documentul:

    Te rugam sa te autentifici sau sa iti faci cont pentru a putea comenta

    Creaza cont nou

    Termeni si conditii de utilizare | Contact
    © SCRIGROUP 2019 . All rights reserved

    Distribuie URL

    Adauga cod HTML in site