In animals, diabetes is most commonly encountered in dogs and cats. Middle-aged animals are most commonly affected. Female dogs are twice as likely to be affected as males, while according to some sources, male cats are also more prone than females. In both species, all breeds may be affected, but some small dog breeds are particularly likely to develop diabetes, such as Miniature Poodles.[108] The symptoms may relate to fluid loss and polyuria, but the course may also be insidious. Diabetic animals are more prone to infections. The long-term complications recognized in humans are much rarer in animals. The principles of treatment (weight loss, oral antidiabetics, subcutaneous insulin) and management of emergencies (e.g. ketoacidosis) are similar to those in humans.
Gene therapy is the therapeutic delivery of nucleic acid polymers into a patient’s cells as a drug to treat disease.[1] The first attempt at modifying human DNA was performed in 1980 by Martin Cline, but the first successful and approved[by whom?] nuclear gene transfer in humans was performed in May 1989.[2] The first therapeutic use of gene transfer as well as the first direct insertion of human DNA into the nuclear genome was performed by French Anderson in a trial starting in September 1990.
Between 1989 and February 2016, over 2,300 clinical trials had been conducted, more than half of them in phase I.[3]
It should be noted that not all medical procedures that introduce alterations to a patient’s genetic makeup can be considered gene therapy. Bone marrow transplantation and organ transplants in general have been found to introduce foreign DNA into patients.[4] Gene therapy is defined by the precision of the procedure and the intention of direct therapeutic effects.
Gene therapy was conceptualized in 1972, by authors who urged caution before commencing human gene therapy studies.
The first attempt, an unsuccessful one, at gene therapy (as well as the first case of medical transfer of foreign genes into humans not counting organ transplantation) was performed by Martin Cline on 10 July 1980.[5][6] Cline claimed that one of the genes in his patients was active six months later, though he never published this data or had it verified[7] and even if he is correct, it’s unlikely it produced any significant beneficial effects treating beta-thalassemia.
After extensive research on animals throughout the 1980s and a 1989 bacterial gene tagging trial on humans, the first gene therapy widely accepted as a success was demonstrated in a trial that started on 14 September 1990, when Ashi DeSilva was treated for ADA–SCID.[8]
The first somatic treatment that produced a permanent genetic change was performed in 1993.[9]
This procedure was referred to sensationally and somewhat inaccurately in the media as a “three parent baby”, though mtDNA is not the primary human genome and has little effect on an organism’s individual characteristics beyond powering their cells.
Gene therapy is a way to fix a genetic problem at its source. The polymers are either translated into proteins, interfere with target gene expression, or possibly correct genetic mutations.
The most common form uses DNA that encodes a functional, therapeutic gene to replace a mutated gene. The polymer molecule is packaged within a “vector“, which carries the molecule inside cells.
Early clinical failures led to dismissals of gene therapy. Clinical successes since 2006 regained researchers’ attention, although as of 2014, it was still largely an experimental technique.[10]These include treatment of retinal diseases Leber’s congenital amaurosis[11][12][13][14] and choroideremia,[15] X-linked SCID,[16] ADA-SCID,[17][18] adrenoleukodystrophy,[19] chronic lymphocytic leukemia (CLL),[20] acute lymphocytic leukemia (ALL),[21] multiple myeloma,[22] haemophilia[18] and Parkinson’s disease.[23] Between 2013 and April 2014, US companies invested over $600 million in the field.[24]
The first commercial gene therapy, Gendicine, was approved in China in 2003 for the treatment of certain cancers.[25] In 2011 Neovasculgen was registered in Russia as the first-in-class gene-therapy drug for treatment of peripheral artery disease, including critical limb ischemia.[26] In 2012 Glybera, a treatment for a rare inherited disorder, became the first treatment to be approved for clinical use in either Europe or the United States after its endorsement by the European Commission.
Following early advances in genetic engineering of bacteria, cells, and small animals, scientists started considering how to apply it to medicine. Two main approaches were considered – replacing or disrupting defective genes.[28] Scientists focused on diseases caused by single-gene defects, such as cystic fibrosis, haemophilia, muscular dystrophy, thalassemia and sickle cell anemia. Glybera treats one such disease, caused by a defect in lipoprotein lipase.[27]
DNA must be administered, reach the damaged cells, enter the cell and express/disrupt a protein.[29] Multiple delivery techniques have been explored. The initial approach incorporated DNA into an engineered virus to deliver the DNA into a chromosome.[30][31] Naked DNA approaches have also been explored, especially in the context of vaccine development.[32]
Generally, efforts focused on administering a gene that causes a needed protein to be expressed. More recently, increased understanding of nuclease function has led to more direct DNA editing, using techniques such as zinc finger nucleases and CRISPR. The vector incorporates genes into chromosomes. The expressed nucleases then knock out and replace genes in the chromosome. As of 2014 these approaches involve removing cells from patients, editing a chromosome and returning the transformed cells to patients.[33]
Gene editing is a potential approach to alter the human genome to treat genetic diseases,[34] viral diseases,[35] and cancer.[36] As of 2016 these approaches were still years from being medicine.
Somatic
In somatic cell gene therapy (SCGT), the therapeutic genes are transferred into any cell other than a gamete, germ cell, gametocyte or undifferentiated stem cell. Any such modifications affect the individual patient only, and are not inherited by offspring. Somatic gene therapy represents mainstream basic and clinical research, in which therapeutic DNA (either integrated in the genome or as an external episome or plasmid) is used to treat disease.
Over 600 clinical trials utilizing SCGT are underway in the US. Most focus on severe genetic disorders, including immunodeficiencies, haemophilia, thalassaemia and cystic fibrosis. Such single gene disorders are good candidates for somatic cell therapy. The complete correction of a genetic disorder or the replacement of multiple genes is not yet possible. Only a few of the trials are in the advanced stages.[39]
Germline
In germline gene therapy (GGT), germ cells (sperm or eggs) are modified by the introduction of functional genes into their genomes. Modifying a germ cell causes all the organism’s cells to contain the modified gene. The change is therefore heritable and passed on to later generations. Australia, Canada, Germany, Israel, Switzerland and the Netherlands[40] prohibit GGT for application in human beings, for technical and ethical reasons, including insufficient knowledge about possible risks to future generations[40] and higher risks versus SCGT.[41] The US has no federal controls specifically addressing human genetic modification (beyond FDA regulations for therapies in general).
Vectors
The delivery of DNA into cells can be accomplished by multiple methods. The two major classes are recombinant viruses (sometimes called biological nanoparticles or viral vectors) and naked DNA or DNA complexes (non-viral methods).
Viruses
In order to replicate, viruses introduce their genetic material into the host cell, tricking the host’s cellular machinery into using it as blueprints for viral proteins. Scientists exploit this by substituting a virus’s genetic material with therapeutic DNA. (The term ‘DNA’ may be an oversimplification, as some viruses contain RNA, and gene therapy could take this form as well.) A number of viruses have been used for human gene therapy, including retrovirus, adenovirus, lentivirus, herpes simplex, vaccinia and adeno-associated virus.[3] Like the genetic material (DNA or RNA) in viruses, therapeutic DNA can be designed to simply serve as a temporary blueprint that is degraded naturally or (at least theoretically) to enter the host’s genome, becoming a permanent part of the host’s DNA in infected cells.
Non-viral methods present certain advantages over viral methods, such as large scale production and low host immunogenicity. However, non-viral methods initially produced lower levels of transfection and gene expression, and thus lower therapeutic efficacy. Later technology remedied this deficiency[citation needed].
Methods for non-viral gene therapy include the injection of naked DNA, electroporation, the gene gun, sonoporation, magnetofection, the use of oligonucleotides, lipoplexes, dendrimers, and inorganic nanoparticles.
Some of the unsolved problems include:
- Short-lived nature – Before gene therapy can become a permanent cure for a condition, the therapeutic DNA introduced into target cells must remain functional and the cells containing the therapeutic DNA must be stable. Problems with integrating therapeutic DNA into the genome and the rapidly dividing nature of many cells prevent it from achieving long-term benefits. Patients require multiple treatments.
- Immune response – Any time a foreign object is introduced into human tissues, the immune system is stimulated to attack the invader. Stimulating the immune system in a way that reduces gene therapy effectiveness is possible. The immune system‘s enhanced response to viruses that it has seen before reduces the effectiveness to repeated treatments.
- Problems with viral vectors – Viral vectors carry the risks of toxicity, inflammatory responses, and gene control and targeting issues.
- Multigene disorders – Some commonly occurring disorders, such as heart disease, high blood pressure, Alzheimer’s disease, arthritis, and diabetes, are affected by variations in multiple genes, which complicate gene therapy.
- Some therapies may breach the Weismann barrier (between soma and germ-line) protecting the testes, potentially modifying the germline, falling afoul of regulations in countries that prohibit the latter practice.[45]
- Insertional mutagenesis – If the DNA is integrated in a sensitive spot in the genome, for example in a tumor suppressor gene, the therapy could induce a tumor. This has occurred in clinical trials for X-linked severe combined immunodeficiency (X-SCID) patients, in which hematopoietic stem cells were transduced with a corrective transgene using a retrovirus, and this led to the development of T cell leukemia in 3 of 20 patients.[46][47] One possible solution is to add a functional tumor suppressor gene to the DNA to be integrated. This may be problematic since the longer the DNA is, the harder it is to integrate into cell genomes. CRISPR technology allows researchers to make much more precise genome changes at exact locations.[48]
- Cost – Alipogene tiparvovec or Glybera, for example, at a cost of $1.6 million per patient, was reported in 2013 to be the world’s most expensive drug.