Understanding small molecule modulation of splicing for Huntington's disease therapy

Summary: Small molecule splicing modulators have emerged recently as exciting new approaches to efficiently modulate protein levels in genetic diseases that affected the CNS. One such compound, risdiplam, is orally bioavailable with excellent CNS distribution and has been approved by the FDA in 2020 for the treatment of Spinal Muscular Atrophy (SMA). This approval has generated tremendous excitement about using small molecule splicing modulators as a new way to reduce huntingtin (HTT) protein levels in Huntington’s disease (HD) by inducing the inclusion of a “poison” exon in HTT, leading to non-sense mediated decay of its mRNA. Although branaplam, a splicing modulator developed by Novartis, has recently been suspended in phase 2 clinical trial for HD due to peripheral neuropathy, PTC Therapeutics was able to identify a different class of splicing modulator that induces the same “poison” exon inclusion but enters the CNS much better, potentially avoiding peripheral neuropathy. An analog in this class is currently in phase 2 clinical trial. These results highlight the potential of splicing modulators for HD therapy, as well as the critical need for understanding the mechanism of action of these modulators to facilitate compound optimization and overcome the many hurdles in the journey of these compounds in becoming an FDA-approved drug. Despite the spectacular success of these splicing modulators in treating CNS disease such as SMA, the mechanism of action of these compounds remains unknown. We propose to evaluate the feasibility of determining the mechanism of action of these compounds using a combination of cyro-EM structural determination and biochemical approaches. Support from this R21 grant will allow us to work out the challenges and develop a tool to understand the mechanism of action of these modulators. This will not only be a huge facilitator for the development of these modulators into HD therapeutics, but also will potentially benefit the development of these compounds for many other neurological diseases in the future.