JLE

Epileptic Disorders

MENU

Magnetoencephalogram-assisted diagnosis of familial focal epilepsy with variable foci in a Chinese family with a novel DEPDC5 mutation Volume 21, numéro 3, June 2019

Illustrations


  • Figure 1

  • Figure 2

  • Figure 3

Familial focal epilepsy with variable foci (FFEVF) is a disorder characterized by focal seizures arising from different brain lobes in different family members, and typically each individual has only one type of seizure. Currently, the diagnosis of this syndrome mainly depends on the combination of semiology and EEG after exclusion of other types of familial focal epilepsy (Scheffer et al., 1998). Although FFEVF has generally been considered as an epilepsy syndrome without cerebral structural lesions, malformation of cortical development (MCD), especially focal cortical dysplasia (FCD), has been reported recently. Moreover, surgical resection turned out to be a favourable approach for FFEVF patients with suspected FCD (Scheffer et al., 2014; Baulac et al., 2015; Scerri et al., 2015). Pedigree studies have revealed an autosomal dominant inheritance pattern, and mutations in dishevelled, Egl-10, and pleckstrin domain-containing protein 5 (DEPDC5) have been recently identified as a common cause of this syndrome. Variation in severity and incomplete penetration are two striking features of FFEVF (Dibbens et al., 2013; Ishida et al., 2013).

DEPDC5 encodes a ubiquitous protein that plays an important role in brain development from early embryonic development. Together with nitrogen permease regulator-like-2 (NPRL2) and NPRL3, it provides GTPase-activating protein activity towards rags complex 1 (GATOR1), an inhibitor of the mammalian target of rapamycin (mTOR) complex 1 pathway.

This pathway regulates cell growth, proliferation, and apoptosis by acting on the cell cycle, transcription, and translation (Bar-Peled et al., 2013). Abnormal neuronal migration and disturbed neuronal excitability, which may lead to epileptogenesis, were observed in DEPDC5 knock-out mice (Marsan et al., 2016; Ribierre et al., 2018). DEPDC5 is not only a common gene affected in familial focal epilepsy, such as FFEVF, but variants have also been reported in other epilepsy syndromes, such as benign epilepsy of childhood with centro-temporal spikes (BECTS) (Lal et al., 2014), epileptic spasms (Carvill et al., 2015), sudden unexpected death in epilepsy (SUDEP) (Nascimento et al., 2015), and malformation of cortical development (Scheffer et al., 2014; Baulac et al., 2015; Scerri et al., 2015; Cen et al., 2017).

In this study, we report a four-generation FFEVF family with a novel splice site mutation in the DEPDC5 gene. To confirm the diagnosis, we performed magnetoencephalography (MEG) in two patients with refractory epilepsy.

Case study

We studied a Chinese four-generation FFEVF family with nine affected individuals. Electroclinical information was collected from direct interview and medical records. Written informed consent was obtained from all participants and guardians of minors. This study was approved by the ethics committees of Xuanwu Hospital Capital Medical University.

The pedigree of the four-generation family is shown in figure 1. The average onset age of the family was 9.3 years (three months to 20 years), but the onset age of the proband and her daughter (III-7 and IV-5) was earlier.

The proband (III-7) was a 32-year-old female who had seizure onset at three months after birth. She had recurrent impaired awareness seizures with autonomic features, such as flushing and tachycardia. Most seizure attacks were preceded by an aura of déjà-vu. She was diagnosed clinically with temporal epilepsy. EEG confirmed the diagnosis and showed sharp and slow wave complexes predominantly in the right temporal area (Sph-R, Fp2). She was treated with multiple antiepileptic drugs (AEDs), including oxcarbazepine (OXC), valproic acid (VPA), clonazepam (CZP), and lamotrigine (LTG). However, she still had three to five seizure attacks per week.

The proband's daughter (IV-5) was a three-year-old female who had recurrent episodes that included a “strange” smile and transient respiratory arrest from three months after birth. Her clinical presentation was hard to classify in terms of any particular type of epilepsy. Ictal and interictal EEG showed diffuse slow waves at seizure onset, and we could not locate the origin of seizures. She was also refractory to drug treatment which included OXC, VPA, LTG, phenobarbital (PB), and phenytoin (PHT), and she still had 8-10 seizure attacks per day.

The proband's father (II-6) had seizure onset at 20 years old. He had several refractory nocturnal hypermotor seizures per month, which were poorly controlled by multiple drugs (OXC, PB, and PHT). Frontal lobe epilepsy was highly suspected.

The proband's grandfather (I-2), uncles (II-5), aunt (II-8), two cousins (III-2, III-4), and niece (IV-1) all had a history of epilepsy, however, they were free of seizure attacks from middle age and the types of seizure were unclassified. The proband's aunt (II-8) and niece (IV-1) were treated with CBZ, and they stopped their treatment two years after becoming seizure-free. Other members were unable to recall the name of the drug used due to the early termination of treatment.

Neuroimaging study

Magnetic resonance imaging (MRI) was performed for two patients with refractory epilepsy (the proband and her daughter; III-7 and IV-5). However, no abnormality was observed on brain MRI. We then performed MEG in these two patients and used the single equivalent current dipole (ECD) method and Neuromag software (Elekta Neuromag) to localize the spikes. For the proband (III-7), the dipoles mainly clustered at the bottom of the right frontal lobe (figure 2A). However, in her daughter (IV-5), the dipoles mainly clustered in the left parietal lobe around the angular gyrus (figure 2C).

Genetic study

A novel splice site mutation in DEPDC5 (c.280-1 G>A) was identified in the proband (III-7) using a targeted next-generation sequencing epilepsy panel (figure 3). This mutation was verified by Sanger sequencing in available members and revealed that besides five affected individuals (II-6, II-8, III-2, IV-1, IV-5), two unaffected members (III-5, III-10) also carried this mutation. Four individuals (II-5, III-4, IV-3, IV-4) refused to participate in this study, and their blood samples were unavailable. This mutation was not found in 1000 Genomes, ExAC, or in silico prediction programs (Human Splicing Finder 3.1), and the mutation was suggested to affect splicing due to an alteration in the wild-type acceptor site.

Discussion

We report a novel splice site mutation in a Chinese FFEVF family and applied the non-invasive method of MEG to locate the epileptogenic site, which assisted us in confirming the diagnosis for this family.

In FFEVF, the origins of seizures are different among family members. Currently, the diagnosis of FFEVF is mainly based on the combination of EEG and semiology (Scheffer et al., 1998). However, it is difficult to localize the origins of epileptic discharges, and the clinical manifestation may be atypical. In the present study, we could not locate the origin of seizures in individual IV-5 according to her EEG. MEG is a non-invasive method which can be used to record the undamped magnetic signals generated by small electric currents in neurons. MEG has excellent spatial resolution; sources can be localized with millimetre precision. It is more accurate and less subject to distortion than EEG. The results of MEG in these two drug-resistant patients eliminated any doubt in the diagnosis of FFEVF by showing that the dipoles clustered in different lobes in each individual. Therefore, we propose that MEG can be used to accurately diagnose this disorder.

After the diagnosis of FFEVF, we performed targeted next-generation sequencing in this family and identified a heterozygous novel splice site mutation in DEPDC5 (c.280-1G>A), which was located in exon 6 in nine affected individuals and two unaffected individuals. The incomplete penetration in this pedigree is consistent with former studies. Haploinsufficiency was speculated to be the underlying mechanism of the DEPDC5-related epilepsy syndrome. Based on previous studies, transcribed mRNA with nonsense mutations and a premature stop codon will be digested by the nonsense-mediated decay system (NMD) which leads to haploinsufficiency (Ishida et al., 2013; Picard et al., 2014).

Although DEPDC5-associated FFEVF has been regarded as a non-lesional epilepsy syndrome, a growing number of studies have revealed that this mTOR repressor plays a role in MCD, especially FCD. Histopathological studies of resected brain tissue have revealed FCD IIa in some patients, as well as activation of the mTOR pathway in DEPDC5-related FCD patients (Scerri et al., 2015). The “second-hit” theory was introduced based on secondary effects from genetic and environmental factors in order to explain the formation of FCD and the variation between patients (Scheffer et al., 2014; Baulac et al., 2015). Further research has strengthened this theory based on the discovery of a second brain somatic DEPDC5 mutation with a higher rate of mosaicism in the seizure onset zone (SOZ) than in the surrounding epileptogenic zone (EZ) (Ribierre et al., 2018). Taken together, we propose that for drug-resistant patients carrying a DEPDC5 mutation, MCD should be suspected.

For drug-resistant patients carrying DEPDC5 mutations, surgery proved to be a good approach with favourable outcome. In a previous study, five patients carrying DEPDC5 mutations underwent surgical resection, and all these patients had electroclinical phenotypes highly suggestive of FCD II; three patients became seizure-free and significant improvement was achieved in another (Baulac et al., 2015). Generally, we rely on MRI to identify MCD, however, MRI may be negative in nearly 29% patients. MEG has previously been applied for presurgical localization of epilepsy, and may be particularly important for those with no lesion visible on MRI (RamachandranNair et al., 2007). Consistent with the family reported here, both the proband and her daughter had negative brain MRI. The findings of dipole-clustered areas in these patients not only helped us make the diagnosis, but also revealed the possibility of surgical resection.

To conclude, we identify a novel splice site DEPDC5 mutation in a Chinese FFEVF family. Furthermore, we applied MEG for the first time to confirm the diagnosis of FFEVF by locating the epileptogenic site in two refractory patients with negative MRI, which also provided guidance for possible surgical resection in the future.

Acknowledgements and disclosures

This work was supported by the Ministry of Science and Technology of China (Grant No. 2016YFC0904400) and Beijing Municipal Science & Technology Commission (Grant No. Z161100002616001). None of the authors have any conflict of interest to declare.


* This study has not been previously presented at any meeting.