Tuesday, June 5, 2018

STXBP1 mutations

Inspired by a patient

Kovacevic J, Maroteaux G, Schut D, Loos M, Dubey M, Pitsch J, Remmelink E, Koopmans B, Crowley J, Cornelisse LN, Sullivan PF, Schoch S, Toonen RF, Stiedl O, Verhage M. Protein instability, haploinsufficiency, and cortical hyper-excitability underlie STXBP1 encephalopathy. Brain. 2018 May 1;141(5):1350-1374.

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. 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.

Liu S, Wang L, Cai XT, Zhou H, Yu D, Wang Z. Therapeutic benefits of ACTH and levetiracetam in STXBP1 encephalopathy with a de novo mutation: A case report and literature review. Medicine (Baltimore). 2018 May;97(18):e0663. doi:10.1097/MD.0000000000010663. Review. PubMed PMID: 29718889.

The case report aims to discuss the clinical symptoms and treatment of encephalopathy caused by a novel syntaxin- binding protein 1 (STXBP1) genetic mutation.

The patient, a girl, was born at 38+4 weeks of gestation. She had frequent spasm attacks accompanied by obvious psychomotor development retardation since the neonatal period. Genetic screening identified a novel STXBP1 genetic mutation.

Early-onset epileptic encephalopathy with STXBP1 mutation.

We adjusted the antiepileptic strategy to oral levetiracetam and topiramate, and intravenous administration of adrenocorticotropic hormone(ACTH) for 2 weeks. Subsequently, prednisone was continued, and gradually reduced and withdrawn over 3 months.

The treatment was effective with complete control of the epileptic seizures and improvements in the electroencephalogram readings. However, the effects on psychomotor ability were slow and limited. A literature review of STXBP1 mutation cases in which ACTH was administered showed that complete seizure control is observed in 60% of cases, 20% are partially affected, and the remaining 20% show no effect.

ACTH and levetiracetam had good therapeutic effects in epilepsy control in this case of de novo STXBP1 mutation. ACTH is an effective drug for early-onset epileptic encephalopathy caused by STXBP1 mutation. However, controlling epilepsy using this therapy does not alter the psychomotor development retardation caused by the STXBP1 mutation.

Yuge K, Iwama K, Yonee C, Matsufuji M, Sano N, Saikusa T, Yae Y, Yamashita Y, Mizuguchi T, Matsumoto N, Matsuishi T. A novel STXBP1 mutation causes typical Rett syndrome in a Japanese girl. Brain Dev. 2018 Jun;40(6):493-497.

Rett syndrome (RTT) is a neurodevelopmental disorder mostly caused by mutations in Methyl-CpG-binding protein 2 (MECP2); however, mutations in various other genes may lead to RTT-like phenotypes. Here, we report the first case of a Japanese girl with RTT caused by a novel syntaxin-binding protein 1 (STXBP1) frameshift mutation (c.60delG, p.Lys21Argfs*16). She showed epilepsy at one year of age, regression of acquired psychomotor abilities thereafter, and exhibited stereotypic hand and limb movements at 3 years of age. Her epilepsy onset was earlier than is typical for RTT patients. However, she fully met the 2010 diagnostic criteria of typical RTT. STXBP1 mutations cause early infantile epileptic encephalopathy (EIEE), various intractable epilepsies, and neurodevelopmental disorders. However, the case described here presented a unique clinical presentation of typical RTT without EIEE and a novel STXBP1 mutation.

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