While Munc18-1 interacts with Syntaxin1 and controls the formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) complex to regulate presynaptic vesicle fusion in developed neurons, this molecule is likely to be involved in brain development since its gene abnormalities cause early infantile epileptic encephalopathy with suppression-burst (Ohtahara syndrome), neonatal epileptic encephalopathy and other neurodevelopmental disorders.
Mutations in the STXBP1 gene (MUNC18-1) were first described to cause Ohtahara syndrome (Early infantile epileptic encephalopathy, EIEE)(12-14) characterized by very early infantile epileptic encephalopathy with frequent tonic spasms and a suppression-burst pattern on electroencephalogram.
This previously unreported STXBP1 mutation in a subject with Ohtahara syndrome and non-lesional magnetic resonance imaging (MRI) broadens the mutational spectrum associated with this devastating epileptic syndrome.
Specific mutations in at least four genes (whose protein products are essential in lower brain's neuronal and interneuronal functions, including mitochondrial respiratory chains have been identified in unrelated individuals with EIEE and include: (a) the ARX (aristaless-related) homeobox gene at Xp22.13 (EIEE-1 variant); (b) the CDKL5 (SYK9) gene at Xp22 (EIEE-2 variant); (c) the SLC25A22 (GC1) gene at 11p15.5 (EIEE-3 variant); and (d) the Stxbp1 (MUNC18-1) gene at 9q34-1 (EIEE-4 variant).
We confirm that STXBP1 is a major gene to screen in cases of Ohtahara syndrome, since it is mutated in >10% of the Ohtahara patients within our cohort.
We report the identification of a novel exon, which is referred to as exon 16b, within the cyclin-dependent kinase (CDK)-like 5 (CDKL5) gene that is implicated in the X-linked infantile spasm syndrome and the early-onset seizure variant of Rett syndrome.
Mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene have been identified in patients with Rett syndrome, West syndrome, and X-linked infantile spasms sharing the common features of generally intractable early seizures and mental retardation.
To further characterise CDKL5-related disorder, previously classified as an early-onset seizure variant of Rett syndrome, which is currently considered a specific and independent early-infantile epileptic encephalopathy.
Mutations in the CDKL5 (cyclin-dependent kinase-like 5) gene are associated with a severe epileptic encephalopathy (early infantile epileptic encephalopathy type 2, EIEE2) characterized by early-onset intractable seizures, infantile spasms, severe developmental delay, intellectual disability, and Rett syndrome (RTT)-like features.
Mutations of the syntaxin binding protein 1 (STXBP1) have been associated with severe infantile epileptic encephalopathies (Ohtahara syndrome and West syndrome), but also with moderate to severe cognitive impairment and nonsyndromic epilepsy.
The authors suggest that in spite of the rarity of STXBP1 mutations, molecular analysis of STXBP1 gene should be performed in patients with early infantile epileptic encephalopathy, after exclusion of ARX mutations in male patients and CDKL5 mutations in female patients.
De novo missense mutations in STXBP1 were recently reported in patients with Ohtahara syndrome, a form of encephalopathy with severe early-onset epilepsy.
Although haploinsufficiency of STXBP1 was involved in early infantile epileptic encephalopathy in a previous different cohort study (group B), no mutations of STXBP1 were found in two of the remaining three subjects of group A (one was unavailable).
For infants presenting with Ohtahara syndrome with responsiveness to folinic acid and negative antiquitin deficiency analyses, genetic testing for other possible causative genes such as STXBP1 mutation is recommended.
Somatic mosaicism, including germline, has been described in several epileptic encephalopathies such as Dravet syndrome, KCNQ2 neonatal epileptic encephalopathy, SCN8A epileptic encephalopathy and STXBP1 related Ohtahara syndrome.
This disorder is allelic with X-linked infantile spasms (ISSX; MIM 308350) where polyalanine tract expansions are the commonly observed molecular defect.
Cyclin-dependent kinase-like 5 (<i>CDKL5</i>) gene mutations have mainly been found in females with early infantile epileptic encephalopathy (EIEE), severe intellectual disability, and Rett-like features.
This study shows that mutations in STXBP1 are not limited to patients with Ohtahara syndrome, but are also present in 10% (5/49) of patients with an early-onset epileptic encephalopathy that does not fit into either Ohtahara or West syndrome and rarely in typical West syndrome.
Heterozygous mutations in the syntaxin-binding protein 1 (STXBP1) gene, which encodes Munc18-1, a core component of the presynaptic membrane-fusion machinery, cause infantile early epileptic encephalopathy (Ohtahara syndrome), but it is unclear how a partial loss of Munc18-1 produces this severe clinical presentation.