Genic Therapy

The genic therapy is a therapeutic strategy which consists in making penetrate Gène S in the cells or the fabrics of an individual to treat a Maladie. The genic therapy aims at replacing or complémenter a Allèle defective Mutant by a functional allele or with surexprimer a protein whose activity would have a therapeutic impact.

History

The concept of genic therapy - to repair or modify genetic inheritance to treat a pathology - is really evoked by the scientific community at the end of the years 1960. But if all the theoretical elements are present, the technological level does not make it possible yet to practically carry out this approach. The improvement of knowledge concerning the bonds between certain transferred genes and certain pathologies, the creation of system of transfer of genes starting from made safe viruses, the improvement of technologies of handling of the DNA - in short all the projections of this fabric of concepts and techniques which one calls today the Biotechnologie - make it possible this theoretical idea to be born in the form of a first clinical trial initiated by S. Rosenberg in the USA at the end of the years 1980. The years 1990 and the beginning of XXIe century see hatching a string of clinical trials in very diverse pathologies: cancer, diseases cardiac and vascular, viral infections, hereditary immunodéficiences,… associated with a major passion of the public (in particular thanks to the Telethon) and of the investors. Badly been useful by a communication making little the share of the things between the field reality and the assumptions, vis-a-vis industrial actors or with patients who await immediate positive tests, the genic therapy is quickly confronted with the bitter report emerging from this period: no actual profits are observed for the few 4000 patients enlisted in the 400 to 600 tests carried out during this period. Difficulties of communication between the academic scientific community and that of industry, a progressive disengagement of the capital-risqueurs on the approaches of genic therapy, a mistrust concerning the real potential of this strategy mark the entry of the genic therapy in XXIe century. The therapeutic success of the protocol of Alain Fischer on the treatment of the child-bubbles reached of immunodéficience severe in the years 2000, formally showing the interest of the concept but mitigated by certain serious side effects, does not manage to completely start again the efforts of the various actors on this field (lower cf). Today, in a more mature phase, médiatisée, more considered, more conscious of the many years - even decades - necessary so that this idea falls under a therapeutic routine, many international teams continue to work to make genic therapy an additional tool with the panoply of the hospital treatments.'

All pathologies are potential targets of the genic therapy

Whereas this concept was born on the idea to treat hereditary pathologies, it was quickly directed towards the treatment of all the affections, hereditary or not, in which it was possible to imagine that certain genes were defective or that it was possible to consider a role for new genes. Cancers, the viral infections, the pain, the cardiac affections, the traumatic attacks of the nervous system,… Conceptuellement it is not a pathology which could not profit from an approach of genic therapy, that it is by a strategy of restoration of a failing genetic activity or by the production of an additional activity which can have a therapeutic impact. Practically, since the first clinical trial (which was interested then in the treatment of cancer), one observes that approximately 70% of the tests were focused on the treatment of cancer, approximately 20% on the processing of traditional hereditary diseases, and 10% on various affections like the viral infections. Approximately 1400 clinical trials would have been carried out with the international level. (Source: Newspaper off Gene Medicine, Wiley Intersciences, http://www.wiley.co.uk/genmed/clinical/) If the play of the statistics is trying, it is necessary however to take care not to draw the clear conclusions of the total analysis of these protocols which are interested in very diversified pathologies, technologies and concepts.

To transport a gene: problems of the vectors in genic therapy

Once the gene selected for its therapeutic potential vis-a-vis a pathology, a crucial stage of the genic therapy is to make penetrate new genetic information in the organization of the patient. In this spirit, is described as vector of genic therapy any system allowing the transfer of this gene in a cell. Vectors used in genic therapy, and the cells modified genetically by these vectors are classified like GMO (Genetically Modified Organizations)

Viral vectors

The use of viruses modified to transport a therapeutic gene rests on the report of effectiveness of the viruses to transfer their own genetic material in the human cells. To produce viral vectors, viruses modified genetically, known as protected are used. The principle consists in eliminating the sequences from the virus which codes proteins, in particular those associated with a possible pathogenic behavior of the virus, and preserving only those which are used to build the viral particle and to ensure the cycle of infection. The genome of the virus is rebuilt to carry the sequences of therapeutic gene. The viral proteins which potentially would miss with the formation therapeutic viral particles are provided by cells known as producing or of encapsidation at the time of the phase of production of the vectors.

Vectors Adénoviraux

The adénovirus is a virus with DNA. It shows the characteristic to make penetrate its genetic material in the target cell without awaiting the mitosis (division cellular) and without inserting new genetic information in the genome of the target cell. Although very much used in many clinical trials, the researchers are not still able to date to completely remove it from its genes, thus maintaining a character potentially pathogenic with the built vector.

Vectors Rétroviraux

The retroviruses are used as vector in genic therapy because they make it possible to insert new genetic information in the genome of the target cell. The new gene is then transmitted cells mothers in cells girls in an equal way without " dilution" genetic information in time. The retroviruses have ARN like genetic material, and not of DNA like Adénovirus. The ARN is a molecule of expression of the DNA, which comprises the part of sequence corresponding to a protein. This molecule is synthesized in the core and forwards in the cytoplasm, where the code which it carries of protein is read during the synthesis of protein. The infection by a retrovirus implies a stage of rétrotranscription of the ARN in a fragment of DNA which will be integrated into the chromosomes after transduction in the cellular core. A viral protein combination and proteins of the target cell ensure this stage of transfer of the molecules of DNA of the cellular cytoplasm towards the core and integration in the genome of the host. If the majority of the clinical trials were carried out with vectors derived from retrovirus of mouse, certain clinical trials are currently in hand using vectors derived from virus HIV (traitemnt of the adrenoleucodystophie by the Aubourg team in Paris since 2007, treatment of the infection with HIV in the USA since 2000). This last type of vector, known as vector lentiviral, derived from a human but completely protected virus is a vector particularly in vogue. Indeed, it is able to modify cells at rest genetically, thus opening possibilities of handling neurons, hepatic cells,… a whole range of cellular populations inaccessible to the rétroviraux vectors derived from murine viruses.

Vectors derived from the AAV

The vectors of the type AAV (Adeno Associated Virus) are derived from viruses known as associated with the adénovirus. The viruses have the characteristic to always support an integration of their genome at the same place in chromosome 19. A not controlled insertion being able to involve important disorders in the cellular function, these vectors were strongly developed for their sedentary potential although they are able to only transfer from small genes. Although the construction of the vector starting from the virus eliminates this property from targeting of insertion, the viruses derived from the AAV were used much at the clinical level. Regarded a long time as innofensifs, contrary to the adénoviraux and rétroviraux vectors (see further), their development has been favoured for a few years. But, the recent death during summer 2007 of a patient in a clinical trial of treatment of the polyarthritis rhumatoïde by vectors derived from the AAV also makes it possible from now on to the detractors of this strategy to point the finger on this type of vector.

Vectors derived from other viruses

Beyond these vectors frequently used in private clinic, many attempts at use of vectors starting from virus are described in the literature. One will belong to many work concerning the use of the virus Herpes Simplex (HSV), of the poxvirus (currently under development clinical), of connected animal viruses with HIV, the flu virus,…. These various attempts testify on the one hand to the inexistence of a universal viral vector pushing the scientists to test new ways, and on the other hand of the will for certain industrialists to position in the field with patents owners.

Nonviral vectors

Various strategies were worked out not to resort to the viruses and to use the molecule of DNA directly. These strategies rest on the combination of chemical molecules (polycations,…) with the molecule of DNA in order to facilitate the crossing of the membrane of the cells and the re-entry of the molecules of DNA. These vectors produced by bacteria, easily purifiables, are inert particles and are not potentially pathogenic of the viruses which are at the origin of the viral vectors. Contrary to the viral vectors, they are easier to produce, handle and store and are caractérisables like traditional medicinal products. However, their effectiveness is quite less than that of the viruses to transfer genetic information in a large population from cells, making difficult their use in certain cases (modification of most of the cells of a tumor for example). Moreover, they have only one very reduced capacity to integrate genetic information in the genome, making them thus useless for perennial genetic modifications of cellular populations in active proliferation. This technology can be however perfectly adapted to certain therapeutic strategies resting on the release of a cascade of event starting from some cells modified genetically (activation of the immune system, for example.

Administration of the vector

Many clinical trials of genic therapy used a strategy of protocol known as " ex vivo" , i.e. by taking target cells of the individual and by subjecting them to the vectors of transfer of therapeutic gene apart from the organization. The cells are then reinjected with the patient. That makes it possible to the researchers in certain cases to evaluate the extent of the genetic modification as well on the level of the percentage of cells modified genetically as on the level of the form of therapeutic proteins, or to preselect particular cellular populations (e.g.: blood original cells). Nevertheless, certain strategies, in particular those aiming at eliminating from the tumors, or those aiming at genetically modifying cells which one cannot handle out of the organization, use an approach known as in vivo by directly injecting the vector in targeted fabric, and by letting it act freely.

Strategies without limits

The genic therapy, like all the approaches of biotechnology, rests on the basic research. Biological mechanisms highlighted, and their subjacent genetic causes, make it possible to imagine strategies of repair or supplementation. The success of these strategies thus depends as much on the capacities to set up adequate techniques (transfer of effective gene, coherent expression of gene,…) that accuracy with which the mechanisms in question are apprehended. Their limit depends only on the imagination of the scientific community and medical. In this spirit, one can distinguish several great approaches.

A disease, a transferred gene, the strategy headlight of the genic therapy

The “repair” of a genetic activity is considered or was tested at the clinical level in many pathologies. Certain immunodéficiences related to deficits in gene coding Adenosine Deaminase, or in that coding the chain gamma-C of the receiver in Interleukine-2 (Fischer protocol, to see below) or the béta-thalassaemias characterized by defects of synthesis of some globines make it possible to imagine their production by genetically modified hematopoietic cells. The hemophilia of the type has and B are respectively associated with defects of production of the factors VIII and IX of the chain of coagulation which could be produced by muscular or hepatic cells releasing these factors in blood. The treatment of the mucoviscidose is considered by the form of gene coding the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) by certain pulmonary cells.

A disease, a genetic context known evil, possible “genes of help”

Certain pathologies are more complex seemingly. Thus, the treatment of the Parkinson's disease is approached in various ways insofar as the bond between the degeneration of the neurons and a genetic mutation is not clearly established. It for example is proposed to express Décarboxylase of the glutamic acid (GAD), Décarboxylase of the aromatic amino-acid (AADC) or Neurturine for at least limiting the degeneration.

Cancer, a disease with the too complex genetics

Cancer is primarily approached through the concept of destruction direct or indirect of the cancer cells. Many clinical protocols were carried out while inserting in the cancer cells of genes coding of proteins sensitizing the cancer cells into drugs. Thus, the gene coding the thymidine kinase of the virus Herpes simplex sensitizes the cells with a normally inoffensive product, to ganciclovir it. The cancer cells are modified directly in the organization by injecting the vectors in vivo and to ganciclovir it is managed in the second time. Resting more on the evolutions of the twenty last years basic research, certain approaches propose use protection mechanisms natural for éradiquer the cancer cells. The stimulation of the immune system by the surexpression of cytokines (GM-CSF, Interferon,…), or the re-establishment of biological chains known as of “cellular death programmed or apoptose (surexpression of p53,…), belong to these strategies.

To block processes by transferring a gene

Many strategies of biological blocking were born in the Nineties primarily to counter the infection by virus HIV. Transferred viral protein expression (lure) interfering with natural proteins of the virus, expression of ARN anti-direction able to inhibit the translation of viral proteins, expression of molecules of natural protection of the cell (let us interfere, proteins of release of the apoptose,…),… all these strategies rest on an interference between the various phases of the cycle of multiplication of the virus and a protein or a ARN whose production is ensured by an exogenic vector transferred in the lymphocytes T from the patient. In another field, many groups work on the protein expression implied in the immunizing mechanisms to block the rejections of Clerc's Office (production of inhibiter of the complement, of cytokines immunosuppressive dérégulant the mechanism of immunizing response, of inhibiters of the interactions between immunizing graft and cell,…). Although little developed certain approaches also is interested in inhibition of the pain by the expression the Preone.

To handle the development

Recent approaches went on the protein expression implied in the development embryonic (NeuroD protein or protein PDX1) to amend of the hepatic cells and to transform them almost into pancreatic cells in order to give again with the patient diabetics of the cells able to produce in manner controlled of insulin.

A mitigated success, many failures, first side effects

Majority, if not the totality of the clinical trials of genic therapy since the beginning them years 1990 can be regarded as failures insofar as they only have very seldom, and that briefly, improved the clinical state of the patients, and they never have leads to the installation of therapeutic recognized and used on the level international. Currently only one strategy, that employed by Alain Fischer and Marina Cavazzana-Calvo, aiming at treating the incapacity to develop new immunizing responses born reached severe immunodéficience (SCID, Severe Combined Immunodeficiency) can be regarded as a success with however a complex situation because this strategy is sometimes at the origin of serious side effects. In 1998-99, very young children reached of the suffering SCID-X of a immunodéficience (of the " baby bulles") received a treatment aiming at making active their lymphocytes T defective. More precisely, the therapy consisted in inserting a functional gene restoring the functionality of a receiver of the interleukine 2. The change of certain proteins of this receiver prevents these patients from having an effective immune reaction making them sensitive to all the opportunist infections. In the first time, the company proved to be a total success with the cure of the patients: the babies could leave their bubbles and live normally during two years. However, three of these miraculés on the 9 treaties developed a Leucémie after one to two years. All data convergent to think that the type of vector used was integrated in a significant area of the genome of certain cells of the patients and led to the deregulation of a gene, the proto-oncogene LMO2, frequently found activated in natural lymphomas. One can correlate this integration of the vector to the anarchistic multiplication of the still undifferentiated white globules at the origin of leukemia, it would thus act well of an ascribable direct side effect to the strategy itself. Doesn't the majority of the patients (approximately about thirty) implied in the various clinical trials of this type carried out by the world have (still?) developed this type of leukemia.

This clinical trial " phare" genic therapy had several repercussions. It first of all showed that the concept of genic therapy was valid and that a strategy of handling of the genome could have a therapeutic impact. But it also highlighted the need for improving the strategies (use of new vectors limiting genotoxic insertions, reduction of the quantity of cells exposed to the vector then reinjected with the patient, in order to limit the risk of touching another gene,…) and the risks which could be associated with this strategy. Lastly, and in manner very innoportune, it also strongly slowed down the development of a genic therapy which started to suffer from a bad reputation being given the little of successes observed in the protocols private clinic. It should be noted that these events returned in synergy with the death of a patient to the USA in 1999, Jerry Jelsinger, during the injection of strong amounts of a vector derived from a adénovirus, which shook the scientific community and medical, and that more recent from a patient treated by a vector derived from AAV.

Genic therapy and company

Whereas the genic therapy functions well in the animal model (mouse, dogs,…), it is generally ineffective at the man because of the combination of several parameters: the inefficiency of the vectors with transduire an important percentage of cells, the difficulty in creating vectors which make it possible to reproduce the complex kinetics of form of genes, sometimes the use of inadequate therapeutic genes because of conceptual errors concerning the mechanisms of the disease, the health condition of certain patients for whom the genic therapy could in any event nothing bring,… This inefficiency makes acuter the consideration ethics, sociological, and sedentary,… with a subjacent question: let us must stop? Problems involved in the risk of diffusion of the virus vector in the population, as that of a germinal transmission (which would result in transmitting to the child of the patient new genes at the time of fecundation) are currently practically non-existent, and the side effects, if they remain dramatic at the human level, are overall very rare and do not justify a stop of the efforts of R & D. the various authorities implied in the control of the tests in genic therapy (the AFSSAPS in France, the RAC in the USA) start to adopt regulation frameworks allowing an optimal protection of the patient and his entourage, and one can consider today that the genic therapyis " not more risquée" that other experimental therapeutic approaches.

A sociological and ethical, traditional problem of any medical approach resting on biotechnology, is that of the cost and the financial effort which the company grants the development of the genic therapy. Still non-existent from a commercial point of view, the cost of the genic therapy is currently ensured by the caritative or governmental public agencies, and especially by industry. Regarded as therapeutic of rich countries, not being able médicalement to make state of a very positive assessment neither nor commercially, and vis-a-vis the difficulties of financing of the scientific research of many voices rise to ask a redistribution of the money allocated the genic therapy, and to stop the investigations. The context is not in the facts not also Manichean whom one could imagine. For example, if the development of the genic therapy is the fact of rich countries, it should be noted that certain studies base their concept on the use of vectors of the naked type DNA (nonviral) which could be easily produced, stored, sent and of relatively low cost thus allowing poor countries to reach treatments which today rests on heavy medicamentous approaches at the financial level. And especially, vis-a-vis all the interrogations no argument today makes it possible to answer without ambiguity this question: is this the moment to stop a therapeutic approach which will appear finally not very interesting or did not do yet enough efforts for a treatment with a future?

See too

Vector (biology)
genetic HIV
Engineering
DNA

References

Random links: