Spinal muscular atrophy (SMA) is the most common genetic cause of infant mortality. It is characterized by symmetrical weakness and atrophy of the proximal muscles in the limbs and trunk due to α-motoneuron loss in the spinal cord. Patients with the most frequent and severe form of SMA (Type I) lack the ability to sit or walk, and usually die within the first two years from respiratory failure unless provided with intensive therapy.
SMA is caused by low levels of the full-length Survival Motor Neuron (FL SMN) protein. SMN is transcribed from two nearly identical genes, SMN1 and SMN2. SMA occurs when SMN1 is lost due to deletion or mutation while SMN2 is preserved. The genes are nearly identical, but SMN2 differs by one vital nucleotide that results in approximately 90% of transcripts lacking exon 7 (SMNΔ7). Although SMN is ubiquitous in every cell of the body, motoneurons are specifically sensitive to reduced levels.
A primary objective of spinal muscular atrophy research is to identify therapeutic drugs and determine their mechanism of action. Unlike other vertebrate models, zebrafish are less expensive to maintain and drug screening can be completed in a matter of days. This would significantly decrease the cost of drug development and expedite identification of a therapeutic as non-beneficial or toxic compounds would be removed from study before progressing on to mice or pigs. I have verified that zebrafish can be utilized for screening potentially therapeutic drugs by examining the effects of Trichostatin A (TSA), which has shown to be beneficial in other models of SMA. TSA significantly increases histone 4 acetylation in WT zebrafish within five hours of treatment, which has been shown to promote smn transcription. In concurrence with this, Smn is increased in WT zebrafish after overnight treatment with TSA. In the morphant model, treatment with TSA shows a decreasing trend in the severity of motor axon outgrowth defects. These screening techniques prove that zebrafish models are an efficient tool for examining HDACi and for identifying other compounds capable of increasing levels of Smn.
In addition, I am developing a transgenic zebrafish that is unique from all other SMA animal models available in that it will be used to simultaneously identify and distinguish between drugs that enhance inclusion of exon 7 and those that either promote transcription or stabilize the protein. As quantification of promoter activity and level of FL SMN can be determined in vivo and without harm to the embryo, multiple measurements can be taken throughout development or the course of drug treatment. This unique model will be a valuable tool to expedite the identification of a therapeutic for SMA while decreasing the expense.