An orally available, brain penetrant, small molecule lowers huntingtin levels by enhancing pseudoexon inclusion

Abstract

Huntington’s Disease (HD) is a progressive neurodegenerative disorder caused by CAG trinucleotide repeat expansions in exon 1 of the huntingtin (HTT) gene. The mutant HTT (mHTT) protein causes neuronal dysfunction, causing progressive motor, cognitive and behavioral abnormalities. Current treatments for HD only alleviate symptoms, but cerebral spinal fluid (CSF) or central nervous system (CNS) delivery of antisense oligonucleotides (ASOs) or virus vectors expressing RNA-induced silencing (RNAi) moieties designed to induce mHTT mRNA lowering have progressed to clinical trials. Here, we present an alternative disease modifying therapy the orally available, brain penetrant small molecule branaplam. By promoting inclusion of a pseudoexon in the primary transcript, branaplam lowers mHTT protein levels in HD patient cells, in an HD mouse model and in blood samples from Spinal Muscular Atrophy (SMA) Type I patients dosed orally for SMA (NCT02268552). Our work paves the way for evaluating branaplam’s utility as an HD therapy, leveraging small molecule splicing modulators to reduce expression of dominant disease genes by driving pseudoexon inclusion.

Targeting RNA structures with small molecules

“…NVS-SM2 acts as a molecular glue, as U1 snRNP only bound to SMN2 exon 7 when the small molecule was present 15 . Branaplam was later discovered to reduce mutant huntingtin (mHTT) protein levels by facilitating pseudo-exon inclusion in the HTT mRNA 261 . It is currently in clinical trial for the treatment of both SMA and Huntington disease.…”

Principles and correction of 5’-splice site selection

“…Mechanistically, HTT-D3 has a strong preference for the non-canonical AGA|GUAAG 5ʹss, which is similar to the motif recognized by risdiplam . Consistently, branaplam was shown to be effective in mouse models of HD [ 160 ]. Overall, this shows that HTT-C2 and analogues strengthen the interaction between U1 snRNP and the 5ʹss, thus enabling exon definition by the spliceosome [ 159 , 161 ].…”

A Chemical Biology Perspective to Therapeutic Regulation of RNA Splicing in Spinal Muscular Atrophy (SMA)

“…Altered splicing patters have been identified in multiple diseases such as cancers, genetic and rare disorders, , etc; therefore, the learned lessons from understanding splicing and its correction at the molecular level could be applicable to other targets, assisting in the development of novel drugs. For instance, branaplam has shown promising results in the neurogenerative disorder Huntington’s disease . In conclusion, great times are ahead for the exploration of RNA at the intersection of chemistry, biology, and medicine.…”