Novel STXBP1 Mutations in 2 Patients With Early Infantile Epileptic Encephalopathy

Case Reports

Patient 1 was born uneventfully to nonconsanguineous healthy parents, after a full-term pregnancy, with adequate growth parameters and Apgar score. At 6 weeks of age, he started to have partial and generalized tonic-clonic seizures with a poor response to phenobarbital. At age 3 months, he had tonic spasms and sodium valproate was started. At age 5 months, he began with infantile spasms and psychomotor regression. Electroencephalographic (EEG) recordings showed hypsarrhythmia and burst-suppression during sleep. Brain magnetic resonance imaging (MRI) was unremarkable. Vigabatrin was added and since 6 months of age he remains seizure free. Antiepileptic therapy was stopped at the age of 6 years without recurrence of seizures. Presently he is 8 years old, has no language, has spastic diplegia, generalized ataxia, tremor, and dystonic movements of the upper limbs. STXBP1 sequencing disclosed a not previously reported de novo mutation within intron 16 (c.1462-2A>T), probably resulting in the loss of the acceptor splice-site of intron 16 and a nonfunctional STXBP1 protein.

Patient 2 was born to nonconsanguineous healthy parents, after a full-term pregnancy, with normal growth parameters and Apgar score. At 2 weeks of age he started to have tonic spasms with clustering, partial and myoclonic seizures followed by flexion spasms, all refractory to antiepileptic therapy. Electroencephalographic recordings at age 2 months disclosed burst-suppression pattern during sleep and brain magnetic resonance imaging was normal. Since the age of 10 months, he is seizure free. He is now 2 years old and is severely hypotonic, has intermittent ocular contact, and he has no language. He is medicated with sodium valproate, clonazepam, and levetiracetam. Sequencing of STXBP1 revealed the mutation c.444delC, p.D148DfsX17 in exon 7 in heterozygous state, predicted to truncate the domain-3 region of the STXBP1 protein, which together with domain-1 provides a binding surface for the protein syntaxin-1.

Discussion

We present these 2 case reports for its relevance in the growing knowledge of clinical manifestations of STXBP1 mutations, because fewer than 40 cases have been reported so far. Our cases reports also add further evidence that STXBP1 should be included in the workup diagnosis of patients with early-onset epileptic encephalopathy and no structural brain abnormalities or inborn errors of metabolism. According to published data, STXBP1 is estimated to be mutated in 10% to 33% of these cases,^4,5 being probably the third most common genetic cause of early infantile epileptic encephalopathy, after ARX and CDKL5 mutations.

Both patients had unremarkable familiar and prenatal backgrounds. Both developed tonic spasms, although other seizure types were also present. Similarly to other reports, ⁵ after an initial period of drug-resistant seizures, both patients remained seizure free during the second half of the first year of life. Interictal electroencephalographic findings in our patients were not characteristic of Ohtahara or West syndrome ⁶ : patient 1 at 5 months had a hypsarrhythmia-variant pattern with burst-suppression during sleep and patient 2 at 2 months had a burst-suppression pattern only in sleeping status.

In both patients, cognitive, sensory, and motor development were severely impaired and patient 1 had also dyskinetic movements, a noted feature in other case reports. ⁵

The evolution for a seizure-free dyskinetic steady state in early infantile epileptic encephalopathy type 4, contrary to other types of early infantile epileptic encephalopathy, ⁵ suggests that it might be explained by the effect of other genes involved in the regulation of STXBP1 gene, affecting neurotransmitter docking or priming of synaptic vesicles and neuronal apoptosis. ¹ The clinical variability between individuals with STXBP1 mutations might be explained by specific effects on STXBP1 protein domains or by the presence of other mutated genes involved in epileptogenesis, such as the interaction of SCN9A and SCN1A described in Dravet syndrome. ⁷ A multiple-hit mechanism, involving SHANK2, CHRNA7, and CYFIP1, as recently reported in autism spectrum disorders ⁸ or the existence of polymorphic DNA markers presently unknown could also account for the clinical variability of STXBP1 haploinsufficient patients. Whole-exome sequencing of patients with early infantile epileptic encephalopathy will be an important tool to detect pathogenic mutations or polymorphisms and reveal the extent to which their interactions modulate the phenotypes associated with STXBP1 mutations.

STXBP1 haploinsufficiency results in progressive encephalopathy characterized by intellectual disability and can be accompanied by epilepsy, movement disorders, and autism. ⁹ STXBP1 (or MUNC18-1, paralog in mice) gene is located on chromosome 9q34.11, contains 20 exons and encodes the syntaxin-binding protein 1, which modulates the release of synaptic vesicles through specific interactions with syntaxin 1A and with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex. By changing the conformation of syntaxin 1A, STXBP1 protein regulates neurotransmitter docking and priming of synaptic vesicles. It also interferes in neuronal apoptosis of the inferior pons, changing cortico-subcortical connections, which seems to be the origin of tonic seizures and burst-suppression pattern, as demonstrated in mouse models STXBP1 null. ¹ The discovery of this gene opens a new field for understanding epileptogenesis and for the development of targeted rational drug design, not only for early-onset epileptic encephalopathies but also for other genetic epilepsies ¹⁰ and even for nonsyndromic sporadic intellectual disability. ¹¹

The classification of early-onset epileptic encephalopathies is revolutionized by the growing knowledge on its genetic causes. The designation of early infantile epileptic encephalopathy encompasses the age-dependent electroclinical syndromes Ohtahara and West syndromes, and it is being subclassified according to the correspondent genetic cause. Meanwhile, we are assisting to an expansion of phenotypes associated with each gene involved in early infantile epileptic encephalopathy, including STXBP1. The present cases also illustrate this inconsistency as they are genetically classified as early infantile epileptic encephalopathy type 4 but do not correspond to any of the typical Ohtahara or West syndromes. Accordingly, they should be better classified as early-onset epileptic encephalopathies. ¹² We consider that adopting this last designation to include epileptic encephalopathies that begin in the first 3 to 6 months of age, with a subclassification according to its genetic cause, would be more adequate for diagnostic purposes and more precise for genetic counseling.

The diagnostic workup of early-onset epileptic encephalopathies remains a challenge because of its overlapping etiologies. When a precise genetic etiology is established, it will eliminate the need for further invasive, nonspecific, and costly diagnostic tests and will provide the possibility for genetic counseling.