Cortical hyperexcitability, protein instability, and haploinsufficiency are disease mechanisms underlying the disorder STXBP1 encephalopathy with epilepsy, new research from a group in the Netherlands indicates.
STXBP1 encephalopathy with epilepsy is a condition characterized by recurrent seizures (epilepsy), abnormal brain function (encephalopathy), and intellectual disability, including autism spectrum disorders. It is caused by mutations in the STXBP1 gene. This gene provides instructions for making syntaxin-binding protein 1.
In neurons, syntaxin-binding protein 1 helps regulate the release of chemical messengers called neurotransmitters from compartments known as synaptic vesicles. The release of neurotransmitters relays signals between neurons and is critical for normal brain function.
Prof. dr. Matthijs Verhage, of the Center for Neurogenomics and Cognitive Research, and colleagues investigated the cellular deficits of seven different STXBP1 mutations. They also developed four mouse models to represent abnormal EEG activity and cognitive aspects of human STXBP1-encephalopathy.
“De novo heterozygous mutations in STXBP1/Munc18-1 cause early infantile epileptic encephalopathies (EIEE4, OMIM #612164) characterized by infantile epilepsy, developmental delay, intellectual disability, and can include autistic features. We characterized the cellular deficits for an allelic series of seven STXBP1 mutations and developed four mouse models that recapitulate the abnormal EEG activity and cognitive aspects of human STXBP1-encephalopathy.
Disease-causing STXBP1 variants supported synaptic transmission to a variable extent on a null background, but had no effect when overexpressed on a heterozygous background. All disease variants had severely decreased protein levels. Together, these cellular studies suggest that impaired protein stability and STXBP1 haploinsufficiency explain STXBP1-encephalopathy and that, therefore, Stxbp1+/− mice provide a valid mouse model. Simultaneous video and EEG recordings revealed that Stxbp1+/− mice with different genomic backgrounds recapitulate the seizure/spasm phenotype observed in humans, characterized by myoclonic jerks and spike-wave discharges that were suppressed by the antiepileptic drug levetiracetam.
Learning and memory in Stxbp1+/− mice in classical spatial paradigms and recently developed automated tasks in the PhenoTyper. (A and B) Latency and distance travelled to find the escape hole in the Barnes maze during the learning phase for congenic BL6 Stxbp1+/− mice. Congenic BL6 Stxbp1+/− (HZ BL6) showed longer latency to find new escape hole during the learning phase (P = 0.026) and during R1 (P < 0.001) and travelled longer distance to the new escape hole (R1-R3: P = 0.024) compared to their controls (WT BL6). (C) HZ BL6 mice showed narrower distribution of holes visit around the target hole during the first probe trial (P1) compared to wild-type BL6. (D) Probability of hole visits in the old target octant during the P1 and P2 tended to be higher for HZ BL6 mice compared to their controls (P = 0.086 and P = 0.060, respectively) and there were no differences in the probability of hole visits in the new target octant during the P2. (E and F) Latency and distance travelled to find the escape hole in the Barnes maze during the learning phase were similar for Stxbp1cre/+ mice (HZ cond) and their controls (wild-type cond). (G and H) Escape latency and distance travelled to the platform during the training in the Morris water maze was similar for reverse 129Sv Stxbp1+/− mice and control mice. (I) Time spent per quadrant during the probe trial was similar for reverse 129Sv Stxbp1+/− mice and control mice. (J) Schematic overview of the CognitionWall DL/RL task. (K and L) Kaplan-Meier survival curves shows the fraction of congenic BL6 and conditional Stxbp1 mice that reached the 80% criterion as a function of hole entries during the DL and RL phases. (M) Average number of entries made per group to reach 80% criterion during the DL and RL phases. HZ BL6 mice reached the 80% criterion during RL with lower number of entries compared to control (P = 0.004). (N) Schematic overview of the Shelter task protocol in the PhenoTyper to assess avoidance learning. (O and P) The preference index during the dark phases of the avoidance learning task was similar for HZ BL6 and HZ cond mice and their controls. Insets represent the learning effect on the preference index (D5/D6/D7-D4). The insets in graph O shows that congenic BL6 Stxbp1+/− mice showed a stronger learning effect compared to their controls (P = 0.012). (Q and R) The aversion index during the dark phases of avoidance learning task showed trend toward lower values for HZ BL6 mice compared to their controls (P = 0.101). The aversion index was similar between HZ cond mice and their controls. *P < 0.05; **P < 0.01; ***P < 0.001 compared to respective control.
Mice heterozygous for Stxbp1 in GABAergic neurons only, showed impaired viability, 50% died within 2–3 weeks, and the rest showed stronger epileptic activity. c-Fos staining implicated neocortical areas, but not other brain regions, as the seizure foci. Stxbp1+/− mice showed impaired cognitive performance, hyperactivity and anxiety-like behaviour, without altered social behaviour.
Taken together, these data demonstrate the construct, face and predictive validity of Stxbp1+/− mice and point to protein instability, haploinsufficiency and imbalanced excitation in neocortex, as the underlying mechanism of STXBP1-encephalopathy. The mouse models reported here are valid models for development of therapeutic interventions targeting STXBP1-encephalopathy.”
Early-infantile Epileptic Encephalopathy 4
The signs and symptoms of STXBP1 encephalopathy with epilepsy, also known as early-infantile epileptic encephalopathy 4 (EIEE4), typically begin in infancy but can first appear later in childhood or early adulthood. In many affected individuals, seizures stop in early childhood with the other neurological problems continuing throughout life. However, some people with STXBP1 encephalopathy with epilepsy have seizures that persist.
STXBP1 gene mutations reduce the amount of functional protein produced from the gene, which impairs the release of neurotransmitters from neurons.
A change in neurotransmitter levels can lead to uncontrolled activation (excitation) of neurons, which causes seizures. This altered neuronal activity does not appear to impair the development or survival of neurons; the cause of the encephalopathy and other neurological problems in this condition is unclear.
The work was supported by Agentschap NL, the Netherlands Organization for Scientific Research, and by the European Union ERC Advanced Grant.