Development of genetic therapies for STXBP1 haploinsufficiency

  • Awarded: 2020
  • Award Type: Research
  • Award #: 630236

Pathogenic mutations in STXBP1 cause a developmental and epileptic encephalopathy (DEE) with an occurrence of approximately one in 35,000 children. In fact, estimates suggest that approximately three percent of diagnosed cases of infantile epilepsy harbor de novo STXBP1 mutations, making it the fifth most prevalent known epilepsy risk gene.

STXBP1 plays a crucial role in synaptic transmission via exocytosis of synaptic vesicles. Generally, STXBP1 mutations are germline de novo and encompass the entire mutational spectrum from missense to nonsense, frameshift, deletions and non-synonymous mutations. Multiple studies support protein instability and haploinsufficiency as the primary etiology.

The most common clinical presentations of STXBP1-DEE are global developmental delay, including gross and fine motor changes (100 percent of cases), infantile epilepsy (95 percent) and cognitive impairment (100 percent). Autism-related behaviors are present in approximately 20 percent of cases. The degree of intellectual disability ranges from mild to profound and is classified as severe to profound in 88 percent of cases, making the development of new therapies particularly urgent. Interestingly, developmental delays often occur before seizure onset, and there is no correlation between seizure severity and the degree of cognitive impairment.

Michael Boland and Wayne Frankel hypothesize that brain regional and neuronal subtype effects of STXBP1 haploinsufficiency underlie discrepancies between seizure and cognitive phenotypes. Preliminary studies from their laboratories using behavioral assays, electroencephalogram (EEG), immediate early gene expression assays and multielectrode arrays have identified aberrant brain region-specific activity phenotypes in Stxbp1 heterozygous mice and aberrant firing and synchrony in primary neuronal networks derived from Stxbp1 heterozygous mice.

In the current study, Boland and Frankel plan to use multielectrode arrays and calcium imaging to detect region-specific activity phenotypes in primary neuronal networks derived from Stxbp1 heterozygous mice and in neurons derived from human pluripotent stem cell models of STXBP1 haploinsufficiency as functional readouts with which to assess the efficacy of three genetic therapeutic strategies. Specifically, they plan to test traditional gene replacement as well as CRISPR activation of the endogenous gene and a noncoding RNA strategy to elevate translation of endogenous STXBP1 transcripts.

These studies will provide better insight into the cell subtype requirements for Stxbp1 expression. Furthermore, these findings will not only determine whether genetic therapies are likely to be efficacious for STXBP1 haploinsufficiency but also whether one modality is better than another at correcting functional deficits.

Subscribe to our newsletter and receive SFARI funding announcements and news