STXBP1 encephalopathy (STXBP1-E) is a devastating neurodevelopmental disorder that often begins in infancy. Intellectual disability is a core feature, often severe to profound. Nearly all have epilepsy (95% in the largest series). The epilepsy is clinically heterogeneous, and may present as a well-defined epilepsy syndrome (e.g., early infantile epileptic encephalopathy, infantile spasms, epilepsy of infancy migrating focal seizure, or Dravet syndrome) or as non-syndromic epilepsy. Seizures are refractory to medications in one third. Affected individuals may have autistic features (1 in 5), low tone, movement disorders (including ataxia and bruxism), abnormal EEGs (> 60% with focal or multifocal epileptiform discharges), and/or abnormal MRI brain imaging (atrophy, thin corpus callosum, delayed myelination). The clinical spectrum is broad -- some individuals are profoundly impaired with seizures that begin in the first days of life; whereas others may have a few seizures in late infancy and mild learning difficulties.~STXBP1 knockout mice show normal early brain assembly with subsequent degeneration, decreased neurite outgrowth, and completely abolished neurotransmitter release These mice die shortly after birth. Heterozygous STXBP1 mice are similar to wild-type, except they have abnormal behaviors during sleep (twitches and jumps) and EEG abnormalities. Thus, heterozygous STXBP1 mice recapitulate some aspects of the human disease, though they have neither seizures nor overt behavioral abnormalities. Human embryonic stem cell-derived neurons engineered for STXBP1 loss of function exhibit normal initial synaptogenesis, synapse size, and soma size; however, heterozygotes show decreased neurotransmitter release corresponding to decreased STXBP1 levels, while homozygous loss of function causes significant neural degeneration. Published experiments on neuronal lines derived from affected patients show decreased STXBP1 protein, STXBP1 protein mislocalization, and decreased neurite outgrowth. Early work in heterologous cell lines demonstrated STXBP1 mutations cause protein misfolding that leads to aggregation of the mutant protein with wild-type STXBP1.~In laboratory settings, stabilizing protein folding of the STXBP1 protein product with chemical chaperones rescued molecular and functional deficits in all tested models, using any of three chemical chaperones: sorbitol, trehalose, and 4-phenylbutyrate. Sorbitol and trehalose are sugars, and would be metabolized in the gut. 4-phenylbutyrate, however, is available as an FDA approved medication, either via sodium phenylbutyrate or glycerol phenylbutyrate. The glycerol formulation is better tolerated, thus this trial.~SLC6A1-related neurodevelopmental disorder (SLC6A1-NDD) begins in early childhood and is characterized by epilepsy (~91%, typically generalized) and developmental delay (~82%). The epilepsy is typically generalized (absence, atonic, myoclonic, generalized tonic-clonic) though is sometimes focal. Substantial minorities have an autism spectrum disorder, movement disorder, or problems with attention or aggression.~The protein product of SLC6A1 is GABA transporter protein type 1 (GAT-1), which is important for GABA homeostasis in the brain. Pathogenic mutations in SLC6A1 lead to loss of function and haploinsufficiency. Preliminary data suggests a dramatic impairment in GABA uptake in cells with homozygous variants in the GAT-1 protein, which improves with administration of phenylbutyrate.~The investigators are starting the investigation of phenylbutyrate for STXBP1-E and SLC6A1-NDD with this pilot study (i.e. Phase 1 study) in order to (a) understand safety and tolerability of the medication in children with STXBP1-E and SLC6A1-NDD, (b) understand the peak plasma concentrations in order to estimate CSF levels, and (c) generate exploratory information about clinical outcomes as a means to estimate effect sizes and pilot a battery of clinical testing for STXBP1-E and SLC6A1-NDD for future trials.
STXBP1 Encephalopathy is a severe disease that can cause seizures and developmental delays in infants and children. It occurs when one copy of the STXBP1 gene has a mutation that causes the gene's protein to not work properly.~SLC6A1 neurodevelopmental disorder is characterized by developmental delay and often epilepsy. It occurs when one copy of the SLC6A1 gene has a mutation that causes the gene's protein to not work properly.~Both STXBP1 encephalopathy and SLC6A1 neurodevelopmental disorder cause symptom because there are not enough working proteins made by these genes. It is possible that a medication called phenylbutyrate may help the the remaining proteins work better. This study is to test if phenylbutyrate is safe and well tolerated in children with STXBP1 encephalopathy and SLC6A1 neurodevelopmental disorder.