Inherited Muscular diseases, such as Muscular Dystrophy, occur when there is a defect in the code of a person’s genes, whose translation is required for normal muscle structure and function[1]. The expression of these defected genes are very varied, but were categorised in 1967 by Emery and Walton. They fall into three categories, as shown in the figure below. These categories were decided through the way of the inheritance [2].
The two most common are the X-linked Muscular Dystrophy diseases, Duchenne Muscular Dystrophy and Becker Muscular Dystrophy. These diseases occur when the mutation of the recessive gene happens on the X chromosome therefore it is more likely to occur with boys than girls, as they only have the X chromosome from their paternal mother [3]. Then the two Autosomal categories are where the genetic defect occurs in the loci of those remaining chromosomes that do not decide sex, either dominant or recessive depending on whether the defect is on both copies of the chromosome (recessive) or just one (dominant) [4].
Therefore, treatments to these diseases are as varied as the diseases themselves. This means that the amount and diversity of the research will continue to expand and explore new avenues to get the best possible treatment for each disease. There are several different methods that are being researched presently and that show good potential going forward.
The first path into finding treatment for inherited muscle disease is Pluripotent Stem Cell Therapy. Pluripotent Stem cells is a term combining two main types of stem cell. These are Embryonic Stem Cells, derived from embryos, and Induced Pluripotent Cells, or iPSCs, where a normal matured cell can be ‘reprogrammed’ by the introduction of several genes to allow it to become pluripotent. [5]
For the treatment of inherited muscle diseases, it is the second source of pluripotent stem cells, the iPSCs, which are used. This occurs by taking mature cells, such as fibroblasts and myoblasts, and reprogramming them to create iPSCs. Protocols are created to cause these iPSCs to develop into the required differentiated cells, which can be seen through the presence of the specific gene and cell surface markers of the cell that is trying to be reproduced. These iPSC-derived cells will then be genetically corrected and checked, then transplanted into the patient, with the aim of causing a strong therapeutic effect that will help treat the disease [6].
An example of where iPSCs were used was in a study for mice with Limb-Girdle Muscular Dystrophy, an autosomal recessive type of muscular dystrophy, which ‘causes the mice to have a reduced number of mesoangioblasts that are used in autologous cell therapy’. The iPSCs were derived from the fibroblast tissue of the mice, used to form iPSC-derived mesoangioblasts, and then transplanted into these affected mice, causing an increase in the progenitors that were depleted due to the disease. This shows that transplanting these iPSCs could be useful in the treatment of this disease and maybe other forms of muscular dystrophy. [7]
As with any treatment, there are always both advantages and disadvantages to using this method. The advantages of iPSCs are quite clear; first, it eliminates the ethical issues that surround the use of embryonic stem cells, such as foetal rights and religious views. This method of taking adult cells and changing them will cause as much issue as donating an organ or giving blood [8].
Another advantage of this approach is the ability of tailoring the iPSCs so specifically, creating the required cells that have been reduced by the disease, whether it is the mesoangioblasts depleted in Limb-Girdle [7], or synthesis of Dystrophin that has been lost due to Duchenne Muscle Dystrophy [6].
The final advantage of iPSCs in the treatment of inherited muscle disease is the much more reduced chances of immunorejection. As the original cells are taken from the patient’s own body, when the iPSCs-derived cells are implanted, they will not cause an immunological reaction and an antigen specific secondary immune response should not be occur [9].
Whilst this is an innovative and seemingly promising treatment, there are some disadvantages to it. So far, the data we have on has come from animal testing. Therefore, sometimes the same genetic mutations that will cause the disease in human patients will not do so in the test subject, using it as a treatment of all muscle dystrophy disease may not be viable. [10]
As well as this, protocols that are required for the creation of complex iPSCs-derived cells may not have been created or shared; as such, it means that this treatment could only work for specific diseases that have established protocols. [10]
Whilst this method has eliminated the ethical issues brought by embryonic stem cells, and iPSCs are also able to differentiate, they do so at a slower rate than ESCs and with a higher rate of cell death as reported by several studies [11].
With both the main disadvantages and advantages shown, it is clear that this avenue of research must continue as it does show some major promise, not to fix all the problems of the disease but a great treatment option that could maybe be complimentary with ESCs to try to attain the best possible treatment for those with inherited muscular diseases.
Another pathway for the treatment of inherited muscular diseases is the use of Gene therapy, which is not a new concept in the treatment of genetic inherited diseases but there is still research and development that can be done, especially in the case of muscular diseases, which is what I will be going into.
The two most common diseases as mentioned before is Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD), and these are the main targets of the Gene Therapy specific to muscular dystrophy. These diseases are caused by the lack of production of the protein Dystrophin, BMD has milder symptoms due to the fact there are smaller Dystrophin proteins present in BMD but none at all in DMD. Dystrophin’s gene that expresses it sits on the X chromosome, showing how they are X-linked muscular diseases. Therefore the most direct way to fix this problem is to restore the Dystrophin production [12]. Dystrophin is used by the body to act as a kind of shock absorber for when the muscle fibres contract.
One of the key way to transfer these genes is by the use of vectors, created from the adeno-associated virus, also known as AAV. This is the most effective transport vehicle for the large gene and protein replacement as it will need to replace them all over the body, therefore the transport vehicle must be able to access all the body for an effective delivery of the replacement gene. Unfortunately, Dystrophin is just too big for the small AAV to carry therefore mini-gene cassettes had to be produced that would both allow AAV to transport it and, when transported, are able to produce a therapeutic effect of a functional protein [13], these mini cassettes are known as ‘micro-Dystrophin’ [12].
There are several major advantages of Gene Therapy, the first being the directness of the approach of treatment. Trying to replace the Dystrophin and it’s gene that is reduced, due to the faulty gene expression of the protein, seems to be showing great results in either restoring or maintaining muscle function [12].
Another advantage of this Gene therapy method is that the genes can be delivered to both skeletal and cardiac muscles throughout the body whereas other forms of transport vehicle, such as adenoviruses, are unable to transfer the gene quite as effectively as AAV. AAV also is able to stay in the cells for years, which is of great importance however, it is unable to maintain its high effectiveness forever, and so ‘booster’ shots will be required to keep the treatment at an effective level. [13]
One final advantage is the ‘micro-Dystrophin’ itself, and it’s effectiveness in producing an efficiently high therapeutic effect that is able to halt muscle degeneration when it is just under 4x smaller than the original muscular isoform of Dystrophin [13].
There are some disadvantages as well to this Gene therapy approach, namely that AAV in too large a dose will cause an immune response due to it being a foreign body and you are in fact ‘infecting’ the patient to transfer the gene across. As such, the body’s defences will activate, however it is not a severe immune reaction and depends on the place of delivery. Most places in the body will accept an AAV delivered protein/gene without consequence, however there are some locations in the body that, without a muscle delivery promoter, will cause a rejection of the transported protein [12].
Another problem is one that was mentioned earlier, the need for ‘booster’ applications to maintain the required high level of treatment. This appears to be due to AAV not being able to integrate sufficiently with the cell, so whilst it does last for a good few years, depending on the patient, it is not a permanent cure [12].
The research and development of the drugs that are used to help treat inherited muscular diseases are many and varied, not only due to trying to treat the various diseases but also as they are in the market against one another, which promotes competition and innovation to find the next best drug, and therefore the best treatment possible.
However, for the drugs on the market right now, it is more of a case of treating the symptoms of the diseases, not treating the causes. For instance, the NHS uses a group of Corticosteroids called Glucocorticoids, such as Prednisone, to help treat the muscle weakness and wastage, caused by DMD, as it helps increase muscle strength and the respiratory system [14]. Whilst these do give beneficial treatment to the patient, it is not treating the cause.
As well as this, it is a steroid, and therefore it has a host of side effects including weight gain, abnormal heart growth and mood swings. If the development of this course of treatment continues then the development of a drug with the similar therapeutic effects as prednisone but with less severe or no side effects will be required.
One drug study that showed potential in the areas I looked at was a drug developed by ‘The Therapeutics for Rare and Neglected Diseases program’ in America, called VB15, a tissue selective steroid-derivative that would not cause the side effects experienced with the continued use of Prednisone but would provide the required treatment [15]. It unfortunately did not pass a critical pre-clinic safety test and the project was discontinued [16].
When a muscular dystrophy disorder occurs, the tissue starts to become liable to necrosis. A theory behind why this happens is due to the severe calcium imbalances that occur due to the stress the remaining muscle fibres are under [17]. Therefore, a drug that can help repair this imbalance will decrease the level of weakness and deterioration that is observed in the muscles.
Such a drug is in Phase 1 clinical stage of development, created by ARMGO® Pharma, Inc. It is called ARM210 and has been developed specifically for treating DMD. The drug works by correcting a calcium ion channel found in the muscles, called “ryanodine receptor calcium channel complex” or RyR for short. This then restores the calcium balance to that of a healthy person. When the animal trials occurred, the drug showed great potential, as the animals had increased activity and strength, enough to cause The Muscular Dystrophy Association to award the pharma company $1 million to continue to develop the research, who’s initial human trials should be finishing near the end of this year [18].
The advantages of using the avenue of drug treatment for these conditions are the same as any drug treatment. These include quick delivery in whatever form they come, precise dosage and control, already a huge logistic set up for buying/taking drugs in the wider pharmaceutical market and so on. Those drugs that are used now, such as the corticosteroids, have a proven use to help treat the symptoms of the diseases.
However, the disadvantages are the same. The side effects that can occur, the potential abuse of the drug, the cost of it to obtain for someone if there is no free health service and unable to pay for insurance.
There are drugs that are being developed which are beginning to treat the actual issues of the patient, instead of just trying to relieve the effects of the symptoms, which is a step in the right direction for the actual treatment of the diseases.
The final avenue of potential research that is being developed to help in the treatment and management of inherited muscular diseases is the use of biomedical interventions to treat and improve the patient’s standard of living.
These interventions include braces, mobility aids and breathing assistance devices. There are two types of braces currently on the market, Ankle-Foot orthoses (AFO) and then there is the Knee-Ankle-Foot orthoses (KAFO) [19]. Children with DMD wear these to reduce contractures and to ensure that the foot does not begin to point downwards, as this will then tighten the Achilles and cause the child even more distress. Wearing these in both night-time and daytime appears to improve gait of children [20], ensuring that they are not having to compensate for their disability as badly and therefore their entire posture is better, increasing their standards of living and level of discomfort immensely.
Development of these is not a high priority, with the R&D going into making these braces more comfortable and lighter so that they effect the child’s sleep pattern as little as possible [19, 21]. This is good for both the child, and the carers/parents, who otherwise will hardly be able to get any sleep due to having to care for the child at night when the contractures occur.
For mobility aids, there are so many variations and choices on the market, from how severe the condition is to the age of the person affected. Devices that can help both children and adults with such tasks as sitting and standing as well as therapeutic tools and even designed beds to allow for a better night sleep.
As patients, especially children, with Muscular dystrophy deteriorate, it is vital they try to maintain a good posture and muscular strength to ensure the body is not worsened more than it already is [22]. These sitting and standing devices also allow patients to be more independent and live a better quality of life.
The final biomedical intervention device examined is the breathing assistive apparatus that is used when the disease is in its worst stage and the muscles around the chest cannot even contract enough by themselves to get enough oxygen in.
There are both invasive and non-invasive measures, however non-invasive the more recommended solution due to the risk of infection from the invasive procedure. This technique of non-invasive breathing assistance was proven to help prolong the patient’s life by keeping a high enough oxyhaemoglobin saturation percentage using an intermittent positive pressure ventilation device. Of those, in a study, to not have this assistance available, the vast majority died due to respiratory causes [23].
The advantages of using and then developing these devices into the future means that the quality of life for those effected will continue to increase. These treatment options, as like most of those of the drug examples, are being used to help manage the disease however they can, but not working towards fixing it. Whilst the use of the IPPV device increases life span, development in that area will be leaning to make it as compact and as user friendly as possible to allow the patient the longest possible time for movement and whatever quality of life they can get.
For the mobility aids and braces, research of more comfortable and easy-to-manage devices that try to maintain and help development of those with the disease will be how the future is looking, with more attachments for each device to increase their versatility.
In conclusion, there are many ways of helping to cure and treat inherited muscular diseases, which is good as they are complicated diseases. In the future, work on Gene therapy will be one of the main avenues of research as it appears to bring the most advantages and actually seems to have the potential for a cure to at least some of the diseases told at the beginning.
However, the development of treatment such as the biomedical devices as a way to help improve the life of those affected by these diseases and their families and carers around them should always be continuing as whilst the other treatment options seem more viable and research should be taking place, the devices truly help those effected in leading normal lives.
Bibliography
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