We report a series of individuals with genetic causes of EOEEs, delineating the phenotypic spectrum and long-term outcome. In the unknown causes of EOEEs, detection of the gene mutation rate was 27.2% (128/470). In the genetic causes of EOEEs, the non-symptomatic EOEEs represent the largest proportion, which is 43.3% (51/118). We find the initial EEG of most patients showing frequent multiple and multifocal discharging. With seizure controlled, EEG discharging gradually decreases. But only a minority of patients’ EEG transform into infrequent discharging or normal EEG. Despite performing several brain MRI, there is no significant change in the later brain MRI. In the long outcome, we find the seizure control rate in the genetic causes of EOEEs is 35.6% (42/118). The death rate is 1.7% (2/118). And we don’t find sudden unexpected death in the genetic causes of EOEEs. Although some patients achieve seizure-free, there is no remarkable improvement in their development.
BS is a common EEG phenomenon in EOEEs, which usually occurs during OS sleep and wakefulness, EME sleep period. There are two different types of BS patterns, namely early BS and late BS[7, 9]. As for the definition of early BS and late BS, it is not very clear at present. Yoshitomi thought that it should be divided according to the age of one month. It is believed that the appearance in the early infancy is related to asymmetric BS pattern, but the appearance in late infancy is related to symmetric BS characteristics. This study did find that BS is not only found in OS, but also in other non-syndromic EOEE. This study provides an in-depth understanding of the genetic factors of EOEE-BS and explains the important role of genetic factors in addition to common causes such as cortical malformations. In this study, pathogenic mutations were identified, accounting for 7.6% (9/118). This study found that the largest genetic subgroup of EOEE-BS is the subgroup with KCNQ2 mutations, accounting for 66.7% (6/9). These 9 patients in this group had various types of seizures. The treatment effect and prognosis were poor. For the early onset of persistent tonic, spasm seizures and other types of intractable seizures, seizures with early EEG-BS performance may suggest the possibility of KCNQ2 pathogenic mutations. However, EOEE-BS is highly heterogeneous in terms of genetic etiology. Except for the largest genetic KCNQ2 subgroup, the second is SCN2A subgroup. For these 2 patients in this study, EEG was temporarily suppressed during sleep at 4 months and 5 months, and disappeared at 7 months. The reason for transient BS in SCN2A subgroup is unknown, which may be related to the immaturity of the central nervous system or gene mutation leading to brain dysfunction at this stage. 1 patient of STXBP1 subgroup was found in the third genetic subgroup. EOEE related to STXBP1 gene mutation has been mostly reported, and the common phenotype is OS. Mutations in the STXBP1 gene can cause abnormal neurotransmitter release, and cause brain stem cell apoptosis and dysfunction, change the excitability of neurons, and cause seizures. This study find that EOEE-BS usually response poorly to AEDS.
Symptoms of dyskinesia include dystonia, chorea, paroxysmal dyskinesia, Parkinson's syndrome, ataxia, tremor and so on. At first, EOEEs with dyskinesia were focused on by Guerrini, who first reported ARX gene mutation associated with dyskinesia. And then STXBP1, FOXG1, CDKL5 related dyskinesia were gradually reported[12–14]. Kobayashi reported 11 cases of infantile dyskinesia associated with EOEE. 9 cases were definitely diagnosed with epilepsy syndrome including WS. In this study, 7 cases of EOEEs with dyskinesia were found. The onset age of dyskinesia ranged from 1 month to 1 year. 3 patients were diagnosed as epilepsy syndrome, namely WS, GLUT1 deficiency syndrome, EME. In this study, the main clinical symptoms of dyskinesia were dystonia, chorea and ataxia. Genetic mutations included CDKL5, SLC2A1, STXBP1, TBC1D24 and GRIA4. Our findings indicate 4 patients wtih dystonia received a good effect with Baclofen. 1 patient of STXBP1 encephalopathy with dystonia showed good response with LEV. This study find that with the control of epileptic seizures, the symptoms of dyskinesia in a few patients were also relieved.
SMC1A mutations can cause early onset epilepsy only in females with cluster seizures. At present, a spectrum of SMC1A gene have been related with Cornelia de Lange syndrome (CdLS), SMC1A-related encephalopathy only with female patients, colorectal carcinomas, bladder cancer and leukemia[15–22]. Consistent with previous clinical reports, our 3 patients have moderate to severe neurological impairment and epilepsy. The seizures usually start in infancy. The presence of cluster seizures is an obvious characteristic. Although a minority of variants have also been found pathogenic, there is no clear relationship between severity of clinical phenotype and mutation types of truncation and missense variants. However, the therapy strategy is still challenging. Among them, most patients show drug-resistant. All our 3 patients became seizure-free when KD was used as add-on therapy. There is evidence to show a relationship between SMC1A-mutated CdLS cell lines and oxidative stress. KD in children with refractory epilepsy has also been demonstrated to improve mitochondrial function and decrease oxidative stress[25, 26]. Therefore, we speculate that KD add-on therapy reduces seizures by down-regulating the level of oxidative stress when combined with AEDS.
HNRNPU locating at chromosome 1, encodes heteronuclear ribonucleoprotein u. It is expressed in adult brain, heart, kidney and liver, especially in cerebellum. Firstly, it was reported related to 1q43-q44 deletion syndrome. Later, a variety of clinical phenotypes related to HNRNPU mutation were reported, mainly including early-onset epilepsy with severe mental retardation, WS, EOEE, Lennox Gastaut syndrome and craniofacial deformity[28–30]. Durkin thought that HNRNPU gene mutation related disease is more likely to be a kind of neurodevelopmental syndrome. Durkin reported 21 cases of children, of which 3 cases were onset with febrile convulsion. Combined with 1 case in this study, we think that EOEEs onset with febrile convulsion is a special phenotype of HNRNPU related neurodevelopmental syndrome, similar to DS.
These responsible genes and their functions are mainly classified as: genes responsible for ion channels, genes responsible for the synapsis, neurotransmitters, and receptors, genes responsible for signal transduction, genes regulating DNA and RNA, genes responsible for the organelles and cell membrane, genes responsible for the development and growth of the neurons. In this study, we find the ion channel gene mutations are the most common, representing the largest proportion (66/118, 55.9%). Among them, sodium channel gene mutations represent the largest proportion (47/66, 71.2%). In WS, we detect SCN3A, SCN2A, SCN8A, CACNA1H, DEPDC5, MECP2, DYNC1H1, CDKL5, ALG11, CCDC88C, GABAA1, IL1RAPL1, RNASEH2B, SLC19A3, STXBP1, RARS2, COL4A2 mutations. In addition to common gene mutations, we report rare possible pathogenic genes: CCDC88C, IL1RAPL1, RNASEH2B and COL4A2 in WS. In non-syndromic genetic causes of EOEEs, we detect rare possible pathogenic genes: SETBP1, DPYD, CSNK2B and H3F3A. As for genetic modes, denovo heterozygous mutations account for the largest proportion, 88.1% (104/118). Among these types of mutations, missense mutations represent the largest proportion, 68.6%(81/118). As expected, some of the genes are included in more than one group of the classification, as they have multiple functions.
Generally, genetic causes of EOEEs response poorly to AEDS treatment. However, we find that some gene mutation related EOEEs receive a good effect on specific AEDS. Besides the effect of KD is sure on the treatment of SMC1A encephalopathy. We also find that VPA added treatment shows a good effect on KCNB1 and PACS2 encephalopathy, LEV added treatment shows a good effect on STXBP1 encephalopathy, OXC added treatment shows a good effect on SCN8A encephalopathy.
VPA is a broad-spectrum antiepileptic drug, which exerts its anticonvulsant effect through a variety of mechanisms. VPA promotes the synthesis and release of gamma aminobutyric acid (GABA) through presynaptic and postsynaptic mechanisms, thereby increasing GABA mediated inhibition[32, 33]. VPA can regulate the expression of endoplasmic reticulum stress proteins (GRP78, GRP94 and calreticulin). It proves that VPA can inhibit excessive endoplasmic reticulum stress, reduce neuronal apoptosis and play a neuroprotective role in acute epileptic seizures[34–36]. VPA can also regulate the level of intracellular Ca2+ by increasing the expression of endoplasmic reticulum stress protein, improve the calcium binding ability of endoplasmic reticulum, and enable cells to adapt to the cellular stress caused by the imbalance of intracellular Ca2+ homeostasis. PACS2 plays an important role in controlling endoplasmic reticulum (ER) - mitochondrial communication, including the connection between mitochondria and ER and the homeostasis of ER. PACS2 is necessary for effective Ca2+ transfer between endoplasmic reticulum and mitochondria, while GRP78 is involved in Ca2+ transport from endoplasmic reticulum to mitochondria[37–39]. Both of them play an important role in maintaining endoplasmic reticulum mitochondrial Ca2+ homeostasis. Therefore, we speculate that VPA may not only increase the concentration of GABA neurotransmitter and inhibit the voltage-gated Na+ channel, but also play a role by enhancing Akt phosphorylation, inhibiting endoplasmic reticulum stress and regulating intracellular Ca2+ level in children with PACS2 encephalopathy.
Certainly, knowing the pathophysiology of the underlying gene defect will help to pave the way for possible future individualized treatments. The limitations of our study are the small number of rare genes. Further research should include a larger cohort to validate our observations. We will continue to study and explore the detailed mechanism between rare gene mutation and seizure outcome.