The 6th International Spinal Muscular Atrophy Research Group Meeting held in Schaumburg, Illinois June 21-22, 2002 marked yet another key milestone for Families of SMA. For the first time, the frontier between clinical and fundamental research was truly breached and much of the discussion revolved around the design of clinical trials expected in the near future. Clearly, “near future” means something different if you’re someone with SMA, a parent of a child with SMA, a clinician caring for our “Angels” or a SMA researcher. Researchers notoriously avoid committing to a definitive time-line for fear of raising false expectations; however, this year the air was filled with the hope that clinical trials were a very real possibility. Because past experience has taught us that “first” clinical trials generally fail and a bad trial can cause a serious setback, Families of SMA and the SMA research community have committed themselves to getting it right the first time!

To meet this challenge, we must clearly identify outcome measures that will allow us to test whether a given treatment is effective or not, identify appropriate targets upon which a therapeutic molecule will act upon and test the potential of such molecules in vivo to ensure that SMA clinical trials will be conducted in a rigorous and safe manner. The following will summarize the present gains and future goals pertaining to these issues.

The bottom line of any trial is to demonstrate a “clinical” effect that is “safe”. The identification of outcome measures has been an important preoccupation and variables such as power, function, survival, motor unit count and biochemical markers were discussed. The DCN group headed by Dr. Susan Iannoccone have completed the 2nd part of a 3 phase trial and have assessed strength, motor function and quality of life in 34 SMA patients, from 2 to 18 years of age, distributed among 5 centers. The most reliable outcome measures were identified and the “modus operandi” established to ensure that different evaluators and study centers adhere to precise protocols so that results can be pooled. Dr. Kathy Swoboda has tested a procedure called MUNE to estimate the number of motor units stimulating a given muscle, a procedure that imposes minimal discomfort as confirmed by parents during the Sunday brunch. This technique allows one to repeatedly and non-invasively assess the degree and time course of motor neuron loss and clearly, MUNE will be an important measure of the effect of potential therapeutic agents. In addition, MUNE is currently being done on SMA mice to determine the relationship between motor neuron loss and muscle function. 

Finally, biochemical markers such as the amount of full-length SMN mRNA and protein have been routinely tested in SMA animal models. Dr. Louise Simard’s laboratory is currently developing protocols to measure SMN mRNA and protein from blood samples that may be shipped over long distances to a few molecular laboratories. Coupled with the work from Dr. Glenn Morris’s group, sensitive and reliable quantification of these markers will be possible shortly and these outcome measures may provide very early clues as to the effect of a given therapeutic molecule. Finally, it has become clear that “outcome” will vary depending on whether an individual has type I, type II or type III SMA; therefore, these studies are providing critical baseline information that will allow us to precisely design the appropriate “clinical trial” for each SMA patient.

During the past several years, the most talked about targets have been elements controlling SMN splicing or promoter activity. These targets are being attacked from a number of angles including basic research, candidate proteins and high throughput screens. Work coming from an increasing number of laboratories, too numerous to list here, has resulted in a better understanding of the mechanisms involved in exon 7-recognition and its inclusion/exclusion in mature mRNA. Consequently, some candidate compounds have caught the attention of researchers including histone deacetylases and SR proteins. A number of groups are also involved in high throughput screens for either promoter- or splicing-targeted compounds and the most comprehensive report was given by Dr. Brian Pollock of Aurora Biosciences. Over 550,000 unique compounds have been tested resulting in the identification of 5 types or “classes” of proteins; two increase SMN promoter activity and 3 induce inclusion of exon 7 in mature SMN transcripts. 

Alternative targets include the rescue of motor neurons prior to their loss or retraction from their muscle target or protecting the health and re-innervation capacity of the remaining motor units. These strategies are supported by studies on SMA patients (Swoboda laboratory) as well as SMA mice (Burghes, Melki and Sendtner laboratories) showing that a significant portion of motor neurons are only lost after birth, raising hope that early treatment may retard or halt cell death prior to clinical onset of muscle weakness. Drs. Judith Melki and Douglas Kerr are developing strategies designed to protect motor neuron cells by identifying neuroprotective factors or by stem cell transplantation, respectively. Finally, Drs. Christine DiDonato and Magali Cucchiarini are exploring the use of adenovirus or adeno-associated virus for gene replacement therapy.

In summary, a number of interesting compounds have been identified and many more are expected; however, there is no evidence at this time to indicate that any of these have a “clinically”measurable effect. Consequently, these molecules/strategies are currently being tested in animal and human cell culture models as well as SMA animals to fully characterize their efficacy and safety in the treatment of SMA. While some of these compounds may be FDA approved, we strongly caution against uncontrolled use of these as therapeutic agents as one cannot monitor for possible adverse side effects that may be specific to SMA individuals outside the context of controlled clinical trials. Even when animal studies have been undertaken, unexpected surprises may arise when a drug is given to human subjects. Again, controlled monitoring is necessary to ensure speedy intervention in the case of health threatening reactions. Key experiments testing potential therapeutic agents in SMA animals, the generation of new animal models to determine when this intervention must be undertaken for measurable rescue of the SMA phenotype and the establishment of precise outcome measures of human trials will each contribute to ensuring that SMA “clinical” trials will have the greatest opportunity to succeed.
 

The 6th Annual international SMA Research and Clinical Meeting brought together researchers and clinicians from all over Europe and North America. The scientific presentations held over two days were broadly divided into  1) basic research findings and 2) clinical studies.

The scientific sessions began with presentations  by Drs. Louise Simard and Glenn Morris who described methods their laboratories have developed to assay SMN protein levels in patients and cells in culture. The latter, in particular will be important in evaluating the effects of small molecules/drugs on increasing SMN protein from the SMN2 gene. Drs. Iannoccone and Swoboda then presented clinical studies demonstrating the use of Gross Motor Function Measures (GMFM), Pulmonary Function Tests (PFT) and Motor Unit Number Estimation (MUNE) as reliable ways of assessing the status of SMA patients. The MUNE studies support two potential strategies in improving outcome of patients with SMA - 1) rescue of motor units before they retract from the muscle and 2) improving the health of surviving motor units and inducing them to re-innervate muscle (connect with the muscle). Dr. Shefner has used MUNE to evaluate the numbers and size of motor units in mouse models of ALS (Amyotrophic Lateral Sclerosis), another common motor neuron disease, and shown it to be very useful in following the disease and monitoring interventions. This technique may thus be extremely useful in assessing the timing of motor neuron loss in mouse models of SMA generated collaboratively by the Burghes and Sendtner laboratories as well as evaluating treatments in these mice.

Another somewhat different mouse model generated by Dr. Judith Melki's group in which SMN is completely depleted in nerves or muscle may be useful for studying the potential of neuro-protective agents (chemicals or proteins that would protect the motor neuron from undergoing destruction). Thus do neuro-protective agents prevent motor neuron death in SMA and does this help the mice? Drs. Wirth talked about the identification of modifying pathways/genes, i.e., genes other than SMN that improve the clinical severity of SMA. The identification  of these genes gives other targets and better understanding of how to develop drugs for the treatment of SMA. Finally Dr Burghes, Sendtner  and Melki talked about mice with SMA. Mice with varying severities of SMA have been made in which treatments can be evaluated and certain genes have been shown to modulate the number of motor neurons and the severity of the SMA. These mice are also being used to determine how deficiency of SMN causes SMA and what particular piece of the motor neuron goes wrong first. Again understanding these mechanisms gives additional targets for intervention as well as better methods for following the effectiveness of therapy. Also both the Melki and Burghes laboratory are attempting to determine the effect of expressing high levels of SMN at particular times in SMA mice. In other words when is it necessary to restore SMN to have an effective treatment? These experiments are being performed in mice and it will be interesting to see the results when they become available.

The discovery that the basic difference between SMN1 (the SMA gene) and SMN2 (the disease modifying gene) is a single nucleotide which causes the latter to produce mostly an unstable form of the protein has prompted many scientists to try to restore the ability of SMN2 to produce sufficient, stable SMN protein. One important way to do this is to modify the way SMN2 is spliced, i.e., to target the single nucleotide difference. Dr. Krainer presented data his laboratory has recently generated which identified a factor that is involved in the splicing of the SMN genes. This has allowed the design of chimeric compounds which will be tested on cells in culture to assess their ability to modify SMN2 splicing. Similarly, Dr Hertel from the University of California (Irvine) and Dr Muntoni (Britain) are using nucleic acid fragments called  antisense oligonucleotides to interfere with/modify SMN2 splicing so as to increase SMN levels in cell culture. These can be looked at as designer drugs which should have the advantage of low toxicity but may be more difficult to deliver to the required tissues.

Results that further elucidate how the SMN genes are spliced and ultimately produce SMN protein were presented by Phil Young ( Dr. Lorson's Laboratory Arizona (congrats Chris on the birth of your son during the meeting I hear you made it back in time) and Dr. Wirth (Germany). Their laboratories independently identified factors, SRp30c and hnRNP-G that form the complex responsible for SMN splicing.

FSMA and scientists have spent a considerable amount of time and effort pursuing high-throughput drug screens in the search for synthetic  or natural molecules which will increase SMN levels. In the final session on the first day of the scientific meeting, a number of presentations were made outlining the progress on the high-throughput drug screens. Drs. Fischbeck's (NINDS) laboratory and Dr Brahe's (Italy) laboratory presented data showing that two compounds, increase SMN levels in cell culture. These are in addition to previously identified compounds  which increase SMN from the SMN2 gene in cell culture. An update on the screens at Vertex (Aurora) Biosciences was made by Dr. Pollok. Three distinct classes of compounds that alter SMN2 mRNA in SMA cells in culture have been identified. 

Although this is a promising result, it will be important to "optimize" these compounds using medicinal chemistry to increase their safety and efficacy. Dr Brent Stockwell presented his strategies for high through put screens to identify both positive and negative molecules that effect SMA in addition they have also identified positive hits in screens. The use of different types of compound libraries as well as chemical biological approaches to dissect out how SMN deficiency cause SMA are very exciting approaches for understanding and developing treatments for SMA. The clear message from this section is compounds (chemicals) that can alter the production of SMN from the SMN2 gene  can be identified. In addition, some of these appear not to have severe toxic side effects. The big challenge now is to identify the compounds that work in a mouse ( or other whole animal) to alter the SMN2 gene to produce more SMN. This will in all likelihood require the chemicals to be manipulated and modified in various ways to get optimal activity and to work in the whole animal. A factor that often helps in these manipulations is determining how and on what these chemicals act.

Alternative therapeutic strategies in SMA were discussed by Drs. DiDonato (Canada), Terwilliger ( Boston) and Kerr (Baltimore). The first is designing gene therapy vectors which will be used to carry the full length SMN1 gene into cells in culture and then into animal models of  SMA. The difficulties being can we get enough vector with SMN delivered to enough motor neurons to change the SMA phenotype in the mice and eventually in humans. At present this is far from clear and question like how many motor neurons need to express SMN to have a clinical impact will need to be looked at. Dr. Kerr has developed an elegant system to deliver stem cells into the spinal cord of rats and shown partial recovery in previously paralyzed animals. He intends to use a similar procedure to deliver stem cells into a type III mouse model of SMA developed by the Burghes /Sendtner labs. The stem cell approach is very exciting but in all likelihood will require time to develop cells with the required regeneration potential and the ability to redeveloped the neurons required. The question that does arise is what is the impact of molecules that help nerve cells and are made and secreted by stem cells on SMA motor neurons. 

As part of the scientific session on the 22nd, studies were presented which give further insight into the basic function of the SMN protein. These studies are very important as they can tell us what goes wrong when we have low SMN levels and will also identify additional therapeutic targets for intervention. Dr. Dreyfuss (Philadelphia) presented data describing the SMN complex and its constituents. He also briefly presented a different form of high through put screen, which, screens for a function of SMN by rescuing a cell that is deficient in SMN. Whether these molecules restore an SMN function or induce  molecules that rescue the cell phenotype will be interesting to determine as will the identity of these molecules. The motor neuron specificity of the SMA phenotype is also the subject of intense study. Three separate studies by groups in Germany (Sendtner), Italy (Battaglia) and Canada (Simard) provide evidence that SMN localizes to specific regions of the motor neuron, namely the neuritic extensions. Further studies by Dr. Sendtner shed light on what may be a motor neuron specific function of SMN. Based on the interaction of the SMN protein and two novel proteins, hnRNP-Q and hnRNP-R, Dr. Sendtner proposed that SMN may be involved in mRNA editing and transport along the axon and proximal dendrites of neurons. Low levels of SMN compromise these functions resulting in the degeneration of the motor neurons. 

In the final session of the scientific meeting a study involving the mapping of an X-linked form of SMA was presented. Of the 100 genes in the candidate region of Xp11.3-q11.2, a number presumed to be important in muscle and nerves are being analyzed. Dr Zerres (Germany) presented an interesting study on attitudes to treatment in SMA. SMA research is at an exciting juncture. We have molecules that induce SMN in cells in culture but can we get them to work in whole animals? Can we get the SMN gene delivered to sufficient numbers of motor neurons? When does SMN need to be reintroduced? Why does SMN deficiency affect motor neurons so profoundly? Is functional replacement of SMN possible? What are the best measures in a clinical trial? A number of these questions can be addressed, making it very exciting. However, we do not yet know the answers to these questions making it hard if not impossible to know when we will have treatment options. I hope that over the next year answers to these questions will come and the path towards treatment of SMA become clearer. I will reiterate that this is a very exciting time as research has suggested approaches towards treatment. The question that continues to remain is whether we can translate these into reality? The transitions from identification of a gene in a genetic disorder to therapy has not been easy. I leave with the hope that SMA becomes one of the first disorders to make the transition from gene identification to a viable therapeutic option. Next year it will be interesting to see where we stand as we push into unknown territory.
 

 
From FSMA Direction, Summer 2002
Posted August  2002