A recent publication from Van Hoecke and colleagues (Van Hoecke et al., 2012) suggests a novel therapeutic approach to the treatment of amyotrophic lateral sclerosis (ALS; also called Lou Gehrig’s disease). ALS is a progressive degenerative disorder that affects 1-2/100,000 people per year and results in death, normally by respiratory failure, 3-5 years after onset. It is caused by a loss of the motor neurons that control muscle movement. There is a hereditary component in about 10% of all ALS cases and in these familial ALS subjects, a variety of genes have been implicated (Al Chalabi et al., 2012), including SOD1, TARDBP and FUS and which encode superoxide dismutase 1 (SOD-1; which was the initial gene reported to be associated with familial ALS in 1993) TAR DNA binding protein (TDP-43) and the Fused in Sarcoma protein, respectively. Recently, a hexanucleotide repeat expansions that occurs in the chromosome 9 open-reading frame 72 gene (C9ORF72) has been described to be associated with ALS with frontotemporal dementia (DeJesus-Hernandez et al, 2011; Renton et al, 2011).
The huge advances in our understanding of the genetics underlying the familial form of ALS have yet to result in breakthrough therapies for this disorder and Riluzole remains the only FDA-approved treatment for ALS. It was approved in 1995 on the basis of clinical studies that demonstrated that it increased survival times in patients, yet the effects are relatively modest and there is a clear need for new and improved treatments for ALS. Since Riluzole was approved, there have been over 30 clinical trials of new treatments but for a variety of reasons (including poor clinical trial design and drug delivery or dose selection issues) none have reached the market, although dexpramipexole, which enhances mitochondrial function, is currently undergoing Phase III trials sponsored by Knapp (Cudcowicz et al, 2011).
A key challenge to the development of new drugs based upon the genetic information derived from familial ALS, as well as genes associated with sporadic ALS, is to understand how mutations in the various genes produce a similar clinical and pathological phenotype. In other words, what is the final common pathway by which these genetic mutations produce ALS? Generic explanations such as mitochondrial dysfunction or alterations in protein degradation pathways have been suggested but how these processes are affected by genetic influences remain vague. However, it is not necessary to understand the mechanism if one can develop a screen that rescues the phenotype produced by different mutations, and this is what Van Hoecke and colleagues did. Hence, they screened for different morpholinos (antisense oligos in which ribose or deoxyribose is replaced by a morpholine ring) that rescued a SOD-1 induced axonopathy in zebra fish. The most protective morpolino targeted the zebra fish Rtk2 gene, which has 67% identity to the human EPHA4 gene that encodes for the Epha4 receptor tyrosine kinase that can bind both type A and type B ephrins. Knock down of the Rtk2 gene rescued the phenotype in zebra fish with various SOD1 mutants (A4V, G37R and G93A) and SOD-1-induced axonopathy could also be rescued pharmacologically by inhibition of Epha4 using 2,5-dimethylpyrrolyl benzoic acid. Importantly, knockdown of Rtk1, which is a paralog with 83% identity to human Epha4, was able to rescue the axonopathy induced by either mutant SOD-1, TDP-43 or knockdown of Smn1 in zebra fish, indicating that inhibition of EphA4 is protective against motor neuron degeneration irrespective of the genetic determinant of vulnerability. Having identified Epha4 as a potential modifier of SOD1-mediated pathology, the authors also studied the effects of a deletion of the Epha4 gene in mice overexpressing the G93A mutant SOD1 and were able to show that in heterozygotes, a 50% reduction in Epha4 was able to prolong survival.
As regards ALS itself, EphA4 mRNA expression in total blood was inversely collected to the age of onset such that patients with lower levels of EphA4 expression had an age of onset older than those with higher levels of expression. Suggesting that reduced EphA4 expression is associated with a reduced disease severity. Collectively, these data shed light onto an intriguing pathway in which rescue of the axonopathy is achieved irrespective of the genetic cause. A further understanding of the mechanism by which Epha4 exerts these effects could provide the basis for novel therapeutic approaches to treating ALS.
Al-Chalabi, A., Jones, A., Troakes, C., King, A., Al-Sarraj, S. and van den Berg, L.H. (2012) The genetics and neuropathology of amyotrophic lateral sclerosis. Acta Neuropathol., 124:339-352.
Cudkowicz, M., Bozik, M.E., Ingersoll, E.W., Miller, R., Mitsumoto, H., Shefner, J., Moore, D.H., Schoenfeld, D., Mather, J.L., Archibald, D., Sullivan, M., Amburgey, C., Moritz, J. and Gribkoff, V.K. (2011) The effects of dexpramipexole (KNS-760704) in individuals with amyotrophic lateral sclerosis. Nat. Med., 17:1652-1656.
DeJesus-Hernandez, M., Mackenzie, I.R., Boeve, B.F., et al. (2011) Expanded GGGGCC hexanucleotide repeat in noncoding region of C9ORF72 causes chromosome 9p-linked FTD and ALS. Neuron, 72:245-256.
Renton, A.E., Majounie, E., Waite, A., et al., A hexanucleotide repeat expansion in C9ORF72 is the cause of chromosome 9p21-linked ALS-FTD. Neuron, 72:257-268.
Van Hoecke, A., Schoonaert, L., Lemmens, R., Timmers, M., Staats, K.A., Laird, A.S., Peeters, E., Philips, T., Goris, A., Dubois, B., Andersen, P.M., Al-Chalabi, A., Thijs, V., Turnley, A.M., van Vught, P.W., Veldink, J.H., Hardiman, O., Van Den Bosch, L., Gonzalez-Perez, P., Van Damme, P., Brown, R.H. Jr., van den Berg, L.H. and Robberecht, W. (2012) EPHA4 is a disease modifier of amyotrophic lateral sclerosis in animal models and in humans. Nat. Med., Aug 26. doi: 10.1038/nm.2901. [Epub ahead of print]