Studies of Hereditary Neurological Disease: Disease Mechanisms

National Institute Of Neurological Disorders And Stroke

Investigators

Linked publications, trials & patents

Abstract

Recently our research has focused on three hereditary motor neuron diseases: amyotrophic lateral sclerosis type 4 (ALS4) due to mutation in senataxin (SETX), spinal and bulbar muscular atrophy (SBMA) due to mutation in the androgen receptor (AR), and autosomal recessive spinal muscular atrophy (SMA) due to deficiency of the protein SMN. Specific research accomplishments include the following: (1) Studies from our group and others have shown that mutant AR-altered transcriptional activity is key to SBMA pathogenesis. Restoring the transcriptional dysregulation without affecting other AR critical functions holds great promise for the treatment of SBMA and other AR-related conditions; however, how this targeted approach can be achieved and translated into a clinical application remains to be understood. We characterized the role of AR isoform 2, a naturally occurring variant encoding a truncated AR lacking the polyQ-harboring domain, as a regulatory switch of AR genomic functions in androgen-responsive tissues. Delivery of this isoform using a recombinant adeno-associated virus vector type 9 resulted in amelioration of the disease phenotype in SBMA mice by restoring polyQ AR-dysregulated transcriptional activity. (2) SMA is a neuromuscular disorder caused by mutations in the survival motor neuron 1 (SMN1) gene. All patients have at least one copy of a paralog, SMN2, but a C-to-T transition in this gene results in exon 7 skipping in a majority of transcripts. Approved treatment for SMA involves promoting exon 7 inclusion in the SMN2 transcript or increasing the amount of full-length SMN by gene replacement with a viral vector. Increasing the pool of SMN2 transcripts and increasing their translational efficiency can be used to enhance splice correction. We sought to determine whether the 5' untranslated region (5' UTR) of SMN2 contains a repressive feature that can be targeted to increase SMN levels. We found that antisense oligonucleotides (ASOs) complementary to the 5' end of SMN2 increase SMN mRNA and protein levels and that this effect is due to inhibition of SMN2 mRNA decay. Moreover, use of the 5' UTR ASO in combination with a splice-switching oligonucleotide (SSO) increases SMN levels above those attained with the SSO alone. Our results add to the current understanding of SMN mRNA regulation and point toward a new therapeutic target for SMA. (3) We also performed a genome-wide RNAi screen using a luciferase-based activity reporter to identify genes involved in regulating SMN gene expression, RNA processing, and protein stability. We showed that reduced expression of Transcription Export complex components increases SMN levels through the regulation of nuclear/cytoplasmic RNA transport. We also showed that the E3 ligase, Neurl2, works cooperatively with Mib1 to ubiquitinate and promote SMN degradation. Together, our screen uncovers pathways through which SMN protein levels are regulated, potentially revealing additional strategies to treat SMA.