SMA BACKGROUND AND HISTORY
Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disease that results from degeneration of spinal chord motor neurons leading to atrophy of skeletal muscle and overall weakness (ACOG, 2009). Werdnig Hoffman is a form of infantile SMA first brought to light by doctors Guido Werdnig and Johann Hoffman in 1890. Werdnig and Hoffman’s described variation of SMA was the only of its kind, until years later in the 1950’s when doctors Kugelberg and Welander reported a less sever form of SMA. The form of SMA described by Kugelberg and Welander was described to onset in children and even adults rather then infants. Werdnig-Hoffman and Kugelber-Welander were both the first documented reports of Spinal Muscular Atrophy. Both sets of physicians described sever atrophy of muscles reducing motor function and body capabilities. SMA is essentially diagnosed by working from basic to more specific. Generally, physicians will start by looking at the family history and proceed with a physical examine of the patient in question. If there is no clear evidence through simple test, then muscle biopsies and EMG’s are used to examine further.Individuals reported with SMA in the initial cases and those reported with SMA today, lead different lifestyles then normal children and adults. Those diagnosed experience trouble with everyday bodily functions such as walking, talking, and respiratory function. Like any disease SMA can vary in its severity, Clinical manifestations included muscle weakness and paralysis, respiratory failure, and premature death in severe cases (ASK GRAYSEN). Some forms can affect proximal muscles and others may attack the distal muscles depending on severity as well. SMA require major life and medical assistance such as: wheelchairs, arm and leg braces, computer systems to aid in speaking, and even breathing machines to assist with below average respiratory function. While different medical devices can aid those with SMA, often times it is still not enough. Spinal Muscular Atrophy is being researched more, but is still incurable. SMA is often overlooked but should be made aware to more people. SMA affects approximately 1 in 10,000 babies, and about I in every 50 Americans is a genetic carrier (Cure SMA, 2014). SMA shows no racial or gender bias, and is passed recessively. Being that SMA is in autosomal recessive disease both parents must carry the trait and pass it to their offspring. However, this does not mean the disease affects the parents; each parent can be a heterozygous carrier and pass the trait with zero knowledge of doing so. While all known forms of SMA are apparently genetic, they result from defects in different genes and have different inheritance patterns (Muscular Dystrophy Association, 2011). SMA is a dangerous disease and trait that needs to be tested for made, and aware to those who know little about it.
What we know about SMA
Numerous studies on SMA have shown and supported a well-accepted cause. SMA is caused by a mutation in the gene known as the survival motor neuron gene (SMN1), which is responsible for the production of a protein essential to motor neurons (ACOG, 2009). Motor neurons produced by a normal SMN protein are responsible for carrying messages to surrounding body parts and organs. Motor neurons are located near the spinal cord and brainstem, carrying messages away from the central nervous system. Each neuron contains an axon and multiple dendrites. The dendrites receive messages from the axon of other neurons and the axon is what sends the action potentials. Motor neurons also create synapses at the end of their axons, allowing then connection to other cells and message transportation. Each portion of the neuron is vital in relaying messages to the body. The transportation purpose of neurons is to choose if incoming signals should be relayed or stopped. When the SMN1 is functioning properly, neurons can produce and carryout designated functions. However, “Researchers suggest that a shortage of SMN protein leads to the inefficient assembly of the machinery needed to process pre-mRNA. Without mature mRNA, the production of proteins necessary for cell growth and function is disrupted (Genetics Home Reference, 2017). When protein production is disrupted and the SMN is not functioning normally, neurons and cannot properly send and receive nerve impulses. When motor neurons do not properly send messages to muscles, muscles begin to atrophy and grow week due to their inhibitillity to receive messages from the CNS. The SMN1 gene does contain similar copies and phenotypic modifiers. The two SMN copies (SMN1 and SMN2) are highly homologous, displaying only five base-pair differences (Wirth, 1999). Mutations of the SMN1 tend to create a more severe outcome then those of the SMN2. The mutation for SMA is found on chromosome number 5. Gene variations on chromosome 5 allow for different expressivities of SMA. There are four categories that those with SMA usually fall into; however there are two rare genetic variances known as Spinal Muscular Atrophy with respiratory distress (SMARD) and UBE1 a rare x-linked form of SMA. The first of the main four types of SMA is type 1, also known as Werdnig-Hoffman. This type of SMA is characterized by disease on set at infancy. Werdnig-Hoffman patients often struggle in early life doing things such as sitting up, sucking, and swallowing, and usually not living past age two. Type 2 SMA or intermediate SMA also begins early but is usually onset in early childhood, rather then at infancy. Type 2 tends to affect the muscles that are closer to the center of the body faster then those located distal to the center. While many type 2 patients benefit greatly from therapy and medical technology, patients are effected largely by respiratory failure and scoliosis. Type 3 SMA or Kugelberg-Welander (mild SMA) is described as SMA that begins after the age of 18 months. The mild SMA is regularly diagnosed after the effected patient has started walking. Many patients with type 3 SMA will walk until they reach middle age. Much like type 2 SMA, type 3 sees problems with respiration and spine curvature. However, type three patients tend to survive longer and experience more success in adult life. Type 4 SMA is described as adult onset. This type of SMA is much like type 3, in that it is considered mild. Of the four types of SMA, type 4 gives the longest life expectancy. SMA is known to be directly correlated with the highly studied SMN-1 protein; however, new information suggest that additional cell types and neuronal pathways provide additional pathogenesis of SMA.
ASTROCYTES INFLUENCE THE SEVERITY OF SMA
In a research study conducted by Hansjorg Rindt and colleagues published in April of 2015, the influence of astrocytes on Spinal Muscular Atrophy was the primary focus. Prior to conducting research, Rindt and associates we aware that SMA was considered motor neuron-autonomous (Rindt, 2015). However new findings suggested that other tissues and pathways play a significant role in SMA severity. The research conducted hypothesized that astrocyte functions are disrupted in SMA, exacerbating disease progression (Rindt, 2015). In order to test the hypothesis, the research team used viral restoration that was tailored directly towards astrocytes and injected in to mice. Initial steps in the research looked at prior effects of drivers such as: olig2-cre driver, Hb9-cre transgenic driver, ChAT-cre driver, and a pan-neuronal driver. The results related to these drivers prominently resulted in little to no changes in lifespan of mice. However the pan-neuronal driver (PrP protein promoter) showed prominent results in wild-type levels of mice with sever SMA. With that being said, the PrP promoter is also leaky in a variety of tissues, including astrocytes (Rindt, 2015). Being that PrP had a prominent effect and leaks to areas such as astrocytes, adds to the validity of Rindt’s hypothesis that astrocytes play a role if the pathogenesis of SMA. Astrocytes function in helping blood flow to areas of the CNS.
The opening question was focused on whether or not restoration of SMN within astrocytes would have a positive effect on the severity of SMA. In order to answer this question an expression cassette was both created and cloned. The cloned expression cassette was cloned to generate scAAV-SMN^gfap. This clone is a viral based restoration tailored to determine the significance of astrocytes. The scAAV-SMN^gfap was pseudotyped with the AAV stereotype 9 capsid (Rindt, 2015). The reason for pseudotyping with the AAV is so that the generated the scAAV-SMN^gfap could cross the blood brain enabling it to act in the Central Nervous System. In order to make sure that the system was set up correctly Rindt and fellow researchers also created a reporter virus (scAAV-GFP^gfap). The reporter was set to be green and fluorescent and easily detectable in desired areas. The reason for the reporter protein was to test and see if traces showed in motor neurons. After using the reporter protein, test compared accurately with prior reports confirming the experimental set up was valid. After validating the use of the scAAV-SMN^gfap SMN7 mice were able to be injected. The SMN7 mouse is a widely used model and recapulates features of SMA, including loss of lower motor neurons, paralysis and premature death before weaning age (Rindt, 2015). After injection, results showed that the treated mice live almost twice as long as untreated SMN7 mice. Not only did the SMN7 show increased life span, they also showed increased motor function and weight gain. The opening results displayed prominent success in the SMN7 mice; however, the SMN7 are compared closer in correlation only to those with severe SMA. In continuation of the experiment, the scAAV-SMN^gfap was injected in to SMN^2b/- mice. The SMN^2b/- minus are use in comparison those who have less sever variations of SMA. While the SMN7 mice had a base life expectancy of 16 days, the SMN^2b/- mice base life span was between 30 – 35 days. After injection the SMN^2b/- mice showed results similar to the first study extending the mice life span to 100 days.
Further experimental procedures were used to test to see if neuromuscular circuitry was improved by the injection of the scAAV-SMN^gfap. Certain muscles along with NMJ’s can be used to test the pathogenesis of SMA. Taking this in to account the affected and unaffected mice were compared in multiple areas. The prominently SMA affected muscles in the treated mice were compared with those of the completely unaffected mice. Results show evidence that the injections reduced the denervated NMJ’s and also rescued muscle growth to a minor extent. Interestingly, NMJ morphology improvements were accompanied by an increase in the number of vGLUT1+ synapses (Rindt, 2015). This suggest again the role of astrocytes. The increase in vGLUT1 can be attributed to astrocyte mediation. Research was also done to see if the scAAV-SMN normalized astrocyte activation. In order to closely examine astrocyte function many things were taken in to account, such as: white matter, grey matter, astroglia, and glial cells. Together, these results identify astrocytes as an important cell type involved in the pathogenesis of SMA (Rindt, 2015). The significance of this experiment was established by comparing two-tailed t tests, and Kaplan-Meier survival curves. In both tests used to show the experimental significance a p-value less than 0.05 was considered statistically significant. In comparing life spans of the treated and untreated mice statistical significance was found (see fig 2.). Results for the NMJ’s showed significance between all parties involved as well (see fig.3). However Figure 3 also shows significance between atleast two of three parties involved in graphs E,F, and H. Graphs E, F,H show motor neuron numbers, ventral root axons, and vGLUT1+ respectively.
Overall the study conducted on astrocyte influence showed positive results and research. Rindt and associates took many precautionary measures to make sure their experiment was valid before continuing. This was clear in the injection of the reporter protein. The injection of the reporter protein tested for extensive motor neuron involvement, which could have potentially voided the stated hypothesis. The research also explored both severe and moderate variations of SMA. Along inspecting both forms of SMA, the methodology was clearly explained. The methodology included: how test subjects we obtained, how samples were isolated, the production of viral solutions, and clear explanation of terms. While there were many strong points to the research of SMA, specific numerical values should have been stated. In the research it is not clearly stated how many subjects were tested. Without a clear numerical value to associate with statistical values, that statement of statistical significance loses authority. Overall the article provided insight to the study on non-motor neuron influences involved in the pathogenesis of SMA. There were no declared conflicts of interest in the research conducted.
SUMMARY
Spinal Muscular Atrophy is more common the most are aware of. We can visible see the taxing effects of SMA on the body. Individuals with SMA can have a variety of symptoms caused by SMA. Symptoms of SMA range from paralysis all the way to respiratory failure. These symptoms cause those effected by SMA to live with vast medical help. Medical is a necessity to those diagnosed with SMA. Without help from medical professionals SMA patients life expectancy would be reduced further then the already low rate. With SMA being the leading cause of infantile death, more time should not only be put towards research, but also spreading awareness. The majority of individuals that carry the gene for SMA have no idea, being that is autosomal recessive and can be carried with no visible effects. Individuals may also have no prior knowledge of the families proband. The proband is simply just the first person in the family to phenotypically display the mutation for a specific gene. With raised awareness families are more likely to determine if SMA is possible in future offspring. Pre-determining the possibility of SMA can allow medical professionals to collaborate with families and prepare for lifestyle adjustments. Though lifestyle changes can be made to live with SMA, there is no cure. Studies of SMA widely accepts the SMN1 gene as the causative agent. The SMN gene is responisible for producing healthy motor neurons. Healthy motor neurons allow messages to be sent to the muscles and received from the muscles. Sending and receiving messages from the Central Nervous System via motor neurons is vital in contracting muscles. Motor neurons not only allow us to contract, but keep other muscles relaxed as the target area contracts. So when an individual reacts to something hot or painful, he or she is able to pull or move away because of functioning motor neurons. While there is no arguing that nonfunctional motor neurons created by the SMN1 gene variation cause muscle atrophy. Recent studies such as the one conducted by Hansjorg Rindt and colleagues suggest that astrocytes and other physiological aspects effect the pathogenesis of SMA. By effecting the pathogenesis of SMA these factors play a role and how expressive the SMA will be in a given individual. Studies such as this do not contain a cure for SMA, but lay ground work for being able to regulate the severity of the disease. The results in the study conducted by Rindt suggest that with research in to the pathogenesis of SMA rather than distinct cause, it could be possible to extend the life span of those diagnosed with SMA. The results from the study also suggest the possibility of regulating muscle mass. Suggested information form studies such as this propose that with further research and understanding of SMA we may be able to allow those with SMA to live and cope with the disease. This does not mean an easy life, but could potentially mean the ability to experience life events that previously seemed impossible. Spinal Muscular atrophy takes on any shape and size. SMA is a disease relevant to all and shows no gender or racial bias.
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