Disease-linked mutations in Munc18-1 deplete synaptic Doc2.

Area of Science:

• Neuroscience
• Molecular Biology
• Genetics

Background:

• STXBP1 mutations cause severe neurological disorders known as STXBP1 encephalopathies.
• The primary disease mechanism is haploinsufficiency, but the precise cause of synaptic dysfunction and symptom heterogeneity remains unclear.

Purpose of the Study:

• To investigate the role of Munc18-1 interactors Doc2A and Doc2B in STXBP1 encephalopathies.
• To understand how Munc18-1 dysfunction and mutations affect synaptic function and contribute to disease variability.

Main Methods:

• Biochemical and cell biological analyses using mouse brains, cultured neurons, and heterologous cells.
• Investigation of Munc18-1 interactors' stability, aggregation, and synaptic targeting in heterozygous knockout models and cells expressing disease-associated mutants.

Main Results:

• Synaptic Munc18-1 interactors Doc2A and Doc2B are unstable without Munc18-1 and aggregate with mutant Munc18-1.
• Heterozygous knockout neurons show reduced Doc2A/B levels and impaired synaptic targeting, worsened by the G544D mutation.
• Overexpression of Doc2A/B partially rescued synaptic dysfunction in heterozygous knockout neurons, but not in those with the G544D mutation.

Conclusions:

• STXBP1 encephalopathies involve not only Munc18-1 dysfunction but also that of its binding partners Doc2A and Doc2B.
• The dysfunction of Doc2A/B is exacerbated by specific Munc18-1 missense mutations, potentially explaining the wide spectrum of patient symptoms.