SMA Support UK at the Cure SMA Conference 2013
08 August 2013
The 2013 annual Cure SMA conference was held in The Disneyland Hotel, Anaheim, California, USA from 13th - 15th of June. More than 1,300 families and researchers from all over the world were in attendance. As usual, two parallel streams of talks and activities were provided, one for the families and one for researchers, with Meet and Greet sessions organised throughout.
Below we provide summaries of a selection of the talks given at the researcher's meeting.
1. Using Stem Cells To Identify New Drugs With Potential To Treat SMA
Mice are the standard animal model for studying human diseases and assessing the efficacy of a potential therapy. Unfortunately, about 9 out of 10 drugs that have proven effective and safe in mice fail in subsequent expensive and time-consuming human clinical trials.
The use of human disease models would be an invaluable addition for reforming the drug discovery and development process, because drugs that have been identified in such models are more likely to work in human patients. Verifying the efficacy of potential drugs in human cells before clinical trials could significantly reduce drug development costs and speed up the process.
Human induced pluripotent stem cell (iPSC) technology may make this idea a reality.
So what are iPSCs?
Almost every cell of the body contains the same genetic information, yet there are clear differences between cell types, for example heart muscle cells and the light-detecting cells of the eye. This is due to each cell producing and responding to distinct, but overlapping sets of signals that drive individual development and growth from a non-specialised state. It was originally believed that once a cell had matured into a particular type, there was no turning back the clock. However, it was recently discovered that skin cells can be reprogrammed to an immature state, i.e. into iPSCs, with the ability to re-develop into almost any cell type (Figure 1).
Importantly for studying disease, the exact genetic makeup of the skin cell donor is captured in the generated cell. This means that disease-causing changes in the DNA known as mutations are retained, allowing the study of disease processes and testing of therapies in the cell type of choice.
Figure 1. Induced pluripotent stem cells. Cells called fibroblasts found in the skin can be biopsied and then reprogrammed to an immature, undifferentiated state by the addition of a small number of genetic factors. Reprogrammed cells are known as induced pluripotent stem cells (iPSCs), and they have the ability to renew themselves and differentiate into almost any cell type found in the human body, for example lower motor neurons.
Maureen Sherry-Lynes (Harvard University, Cambridge, USA) from the laboratory of Lee Rubin presented research that uses iPSC technology to produce the main cell type affected by SMA - the lower motor neurons. A particular set of molecules and proteins is added to the reprogrammed iPSCs to promote their differentiation into the specific neuronal cells.
Lower motor neurons were made from patients with SMA types I, II, and III, and grown in culture. SMA is caused by low levels of the survival motor neuron (SMN) protein. Importantly, the neurons generated from the SMA patients mimicked this critical feature of the disease, i.e. they had significantly reduced SMN levels compared to healthy control cells.
A machine-based system was developed that is able to automatically determine the amount of SMN protein found within these cells. The system was set up so that it could do this in many cells in a short space of time, allowing large-scale experiments to be conducted.
Using this set up, the researchers were able to test a diverse library of drugs for their ability to increase the amount of SMN protein produced by the SMA patient-derived motor neurons. The library included a number of compounds that have already been approved for human use, which could speed their translation to clinical trials if they prove to be useful.
A number of different compounds were identified that boost the levels of SMN protein produced by the patient motor neurons, resulting in improved survival of the cells. How these drugs work can now be determined using the iPSC motor neurons, while their efficacy in SMA mouse models can also be tested.
It is hoped that this work will lead to the development of potential future therapies for SMA, while simultaneously identifying new genes and pathways important for the production of SMN.
2. Disease Stability In SMA Type III
The onset of SMA is often reported as being rather sudden, occurring over a period of days to weeks, especially in more severe forms of the disease. This rapid decline in motor function is then frequently followed by a phase in which disease progression is relatively slow this is often called the “plateau phase” (Figure 2).
Figure 2. Motor function in unaffected and SMA type I children. During the first six months of life, SMA type I children often experience a rapid loss in motor ability. This is then frequently followed by a period of relative stability, which has been referred to as a plateau phase, because there is relatively little change in motor ability. Figure adapted from Swoboda et al. (2012) J Child Neurol 22: 957-66.
In order to determine how SMA type III progresses over time, a group of clinicians and scientists throughout Europe (Belgium, Germany, Italy, the Netherlands, Spain, and the UK) performed a longitudinal study over 12 months looking at the ability of ambulant SMA type III patients to perform the six minute walk test.
The six minute walk test has previously been shown to be suitable for assessing mobility in type III SMA patients, by determining how far an individual is able to walk on a hard, flat surface within six minutes. The goal is for the individual to walk as far as they can within the allotted time.
Eugenio Mercuri (Catholic University, Rome, Italy) presented work from the long-term study. 38 ambulant children and adults with SMA type III performed the test at the start of the study and then again 12 months later, in order to determine whether there was improvement, decline, or stability in their scores over time.
At the start of the study, the distance achieved in the six minutes ranged from 75 to 510 metres (average of 294.9 metres). After 12 months, the distance covered was between 50 and 611 metres (average 293.4 metres).
Some patients managed to walk further at 12 months, some walked less far, and some showed no change. Overall it was found that there was very little difference between the scores at the start and end of the study - there was an average decrease of 1.5 metres.
The researchers were able to show that age and the initial test score had little effect on the distance walked at 12 months. However, younger patients reaching puberty had a relatively higher chance of walking less far by 30 metres or more at the 12 month time-point compared to older patients.
This study suggests that on average ambulant patients with SMA type III are relatively stable over a 12 month period. This fits in with the hypothesis previously reported on for the Jennifer Trust that there appears to be a critical window during the early stages of life in which the requirement for SMN protein is greatest.
Six minute walk test information
3. Using SMA Patient Natural Histories To Help Improve Clinical Trials
Understanding how a disease progresses and how it affects each individual patient can provide important information that could guide the design and development of future clinical trials and therapies. This information, known as the natural history of a patient, is generated from visits to a clinician, but also directly from the personal experience and knowledge of patients and their families.
Adele D’Amico (Children's Hospital Bambino Gesù, Rome, Italy) presented the statistical results from a questionnaire administered in Italy, for parents of children with SMA type I. It included 38 questions covering a range of issues and was designed to gather information about the natural history of SMA type I. The workshop did not cover the 'quality of life' information.
The couples were asked about their reproductive histories, the outcome of their pregnancies, the age of their child at SMA diagnosis, the age of respiratory and swallowing problem onset, the timing and choice of mechanical ventilation, body weights over time, and the survival of their child. 59 families with children born after 2001 completed the questionnaire.
The average age at SMA type I diagnosis was reported as approximately 3 months, with no difference between the sexes. One in three patients were able to raise their head unaided, and this ability was linked to a slightly older age at diagnosis (4.7 months compared to 2.5 months for children unable to lift their head).
Body weight data with at least two recordings (including birth weight) were available for 39 children. Birth weight was in the normal range for both males and females. However, in the first few weeks after birth, a progressive loss in weight was reported. This occurred before any clinical signs, and was more severe in males than in females.
Half of the families in the Italian study chose no pro-active respiratory intervention at all, less than one in ten chose a tracheostomy, and the remaining families chose non-invasive ventilation such as the use of a breathing mask. The survey found that ventilation support was used at an average age of 7.9 months, with no differences between the sexes.
Swallowing difficulties manifested at an average age of 5.3 months across both sexes, but at 3.8 months in males and 6.5 months in females.
The average survival time without ventilation support was 5 months for males and 7.6 months for females. This was extended to about 12 months for both sexes when non-invasive ventilation was provided.
Overall, the survey suggests that males may be more severely affected by SMA type I than females, with earlier development of symptoms and more pronounced weight loss, although it was pointed out that this should be confirmed in a larger sample of patients.
It is hoped that the data generated from this survey will help to identify the most suitable measures of success for future clinical trials for SMA. For instance, body weight increase could be used to measure the effectiveness of a potential treatment; however, differences between the sexes will need to be accounted for.
4. Identifying Genetic Modifiers In SMA Mice
On average, the severity of SMA depends on the amount of available SMN protein. The SMN2 gene is able to produce a small amount of functional SMN (Figure 3.). SMN2 is therefore able to modify the severity of SMA, because the more copies that you have, the more SMN protein that you are able to produce. For this reason, SMN2 has been called a genetic modifier of SMA; the presence or absence of SMN2 affects, or modifies, the severity of disease.
Figure 3. Two different genes make the SMN protein. The majority of the population possesses two genes that produce SMN protein: SMN1 and SMN2. SMN1 produces full-length, functional SMN protein, whereas SMN2 mainly produces a shorter, non-functional form of the protein. In SMA patients, SMN1 is either missing or mutated, leading to a large decrease in the available SMN protein.
SMN2 is not the only gene that has been identified as being able to affect SMA development and progression. We have previously reported on a gene called Plastin 3, the levels of which have been shown to modify the severity of SMA in females. A third gene, denoted MOD2, has also been highlighted, which was reported in last year’s Families of SMA conference report.
Plastin 3 and MOD2 were both identified by Brunhilde Wirth and colleagues by comparing gene levels in SMA-discordant siblings. SMA-discordant siblings are siblings that possess comparable levels of the SMN protein (through having the same SMN1 mutations, and the same SMN2 copy number), yet display different degrees of SMA progression and severity. By looking at the levels of all the genes in the genome, and how these levels differ between the siblings, it is possible to identify what may be causing the discordance.
Umrao Monani (Columbia University, New York, USA) presented research that is attempting in a similar way to identify genetic modifiers in SMA mice.
Mouse models of disease are inbred such that they have essentially identical genetic backgrounds. That means that when researchers are studying the effects of a mutation in a single gene, they can be sure that what they are observing is caused by the expected mutation and not an aberration in a different gene.
In the scientific community a number of different genetic backgrounds have been produced and maintained. This means that the effects of a mutation, for example in the mouse SMN gene, can be assessed on number of different genetic backgrounds.
Interestingly, SMA model mice show differing severities depending on which background the mutations are found. For instance, SMA mice can live for two weeks on one background, but die before birth on a different background. This is similar to the phenomenon of SMA-discordance between siblings, in that subtle differences in the levels of non-SMN protein producing genes can affect the severity of the disease.
The Monani group is attempting to uncover the genetic cause of discrepancies in disease severity between the different backgrounds. Genes identified by this study are likely to play a similar role in humans, and may therefore provide additional targets for future potential therapies for SMA.
5. Increased Chondrolectin Levels Improve Motor Neuron Defects In SMA Zebrafish
It has previously been shown in SMA mouse spinal cord that chondrolectin levels are reduced very early on in disease before symptoms occur. Since chondrolectin is found in the lower motor neurons of a number of vertebrates including humans, it has been suggested that this alteration in Chodl may be playing an integral role in SMA progression.
SMA Support UK has previously reported on work from the laboratory of Catherina Becker indicating that the fine-tuning of chondrolectin levels is very important for the development of the zebrafish nervous system (Figure 4). Too much chondrolectin results in defective growth and development of the motor nerves, and so does not having enough chondrolectin.
Figure 4. The zebrafish is a model organism that is often used in the laboratory to study biology and disease. Zebrafish reproduce quickly and in large numbers, share many genes with humans, and have a nervous system that is similar to ours in the way it develops and functions. Like mice, zebrafish have been used to study and model SMA. In the middle picture of a fish with normal SMN levels, there are two healthy motor neurons (in green) with long thin extensions growing downwards (out of the spinal cord). However, artificially reducing SMN levels affects the growth and development of these nerves, such that the extensions do not extend out in nice organised pathways like in the healthy fish.
James Sleigh (University of Oxford, Oxford, UK) from Kevin Talbot’s laboratory presented follow up work on the chondrolectin protein in SMA.
When neuronal cells in culture have less SMN protein, they produce fewer processes and cell-to-cell connections. Furthermore, the processes that they do produce are much shorter when compared to cells with normal SMN levels.
Chondrolectin levels were experimentally increased in cultured motor neuron-like cells with normal or artificially reduced SMN levels, with the later mimicking the situation seen in SMA. This resulted in a significant improvement in SMN-depleted cells in both the number of processes and their average length.
Similar to cells in culture, reducing SMN levels in zebrafish affects the growth and normal development of motor neurons (Figure 4). The researchers therefore decided to see whether providing extra chondrolectin to SMN-depleted zebrafish could improve the defects observed in the nervous system.
Interestingly, they found that when they increased the amount of available chondrolectin in SMA zebrafish, there was a significant improvement in the survival, growth, and development of the motor neurons.
This work suggests that the reduced levels of chondrolectin in the spinal cord of SMA mice may be contributing to early stages of disease, and that chondrolectin may serve as a target for future SMA therapies.