DYNC1H1 variant associated with epilepsy: Expanding the phenotypic spectrum

Highlights • DYNC1H1 is a key factor for developmental and epileptic encephalopathies.• Besides developmental and neuromuscular problems, epilepsy may develop in the DYNC1H1 variant.• Brain malformation is common in DYNC1H1 variant-associated epilepsy.• DYNC1H1 variant-associated epilepsy develop in the motor domain by de novo mutations.• Early-onset epilepsy may be an extended phenotype of DYNC1H1-related disorders.

Although DYNC1H1 variants may be associated with brain malformations, which implies possible epileptogenesis, the association between DYNC1H1 variants and epilepsy remains unclear. Here, we report the case of a patient with refractory epilepsy associated with a pathogenic DYNC1H1 variant who presented with intellectual disability (ID), dysmorphism, and brain malformation. In addition, we summarize the clinical, genetic, and neuroimaging characteristics of patients with DYNC1H1 variant-associated epilepsy from the relevant literature.

Case presentation
A 23-year-old woman was born term to nonconsanguineous healthy parents. The patient had a normal birth and had no family history of seizures or neurological disorders. At the age of 11 months, she had experienced episodes of left-side focal to bilateral tonic-clonic seizures with cyanosis after a 2-week course of fever with upper airway symptoms. Electroencephalography (EEG) revealed considerable focal (right side) slow wave activity. Head computed tomography revealed bilateral widening of the sylvian fissure. Although aseptic encephalitis was initially suspected, no laboratory evidence supported this impression. The patient received antiseizure medications, which were discontinued after discharge. No convulsion was noted in the following years. Her medical history revealed a global developmental delay at the age of 11 months, which corresponded to the motor developmental skills of an 8-month-old infant. ID was noted during a follow-up assessment, and the patient attended a special education school. She required assistance with activities of daily living and could not work. After reaching adolescence, she underwent corrective surgeries for bilateral cataracts.
The patient experienced drug-resistant epilepsy at the age of 16 years. It started with a startle response, followed by staring, oral automatisms, and semipurposeful limb movements, such as tearing clothes, kicking, and dashing out suddenly. The episode lasted for approximately 2 min. She received > 5 types of antiseizure medications, namely levetiracetam (2,000 mg/day), topiramate (200 mg/day), lacosamide (100 mg/day), valproic acid (500 mg/day), and clonazepam (1 mg/day). However, she continued to experience approximately-three seizures per week, which were more severe during menstruation and stress.
Physical findings at presentation were as follows: long face, epicanthus, telecanthus, and prominent supraorbital ridges. Neurological examinations revealed an alert but uncooperative attitude, irrelevant speech, and childish behaviors; she had full muscle power, no specific sensory complaints, intact coordination, and no outstanding extrapyramidal signs. Video EEG indicated focal impaired awareness motor seizures. The episodes began with the abrupt onset of oral automatism involving chewing movements, which then turned toward her left side and involved the tonic movement of her left arm and the tonic posturing of all four limbs. Clinical seizures were correlated with the findings on video EEG, which showed trains of sharply contoured sharp waves and spike-and-wave complexes at 5-6 Hz and 30-80 lV, lasting for approximately 150-160 s with an emphasis on the right frontotemporal areas and occasional bilateral synchronization and generalization. Before an episode, occasionally scattered focal blunt or sharply contoured waves were noted at 4-7 Hz and 20-50 lV in the right hemisphere with phase reversal at F8.
The patient's laboratory results, including routine biochemistry, electrolytes, and autoimmune profile, were normal. Brain magnetic resonance imaging (MRI) scan revealed a thin corpus callosum and several nodular heterotopias in the left centrum semiovale. Interictal brain 18 F-fluorodeoxyglucose positron emission tomography indicated reduced metabolism in the right parietotemporal lobes, bilateral frontal lobes, and posterior cingulate (Fig. 1). Ophthalmological fundus examination and retinal photography suggested no degeneration of the retina or optic nerves. Cardiac echo results revealed no intracardiac shunt or septal defect.
Eventually, exome sequencing was performed; the results revealed a heterozygous missense variant NM_001376.5:c.10348 G > A (p.E3450K) in exon 20 of DYNC1H1 (Supplementary Table). In silico analysis, this variant is considered to be responsible for damaging the structure and function of the protein. It is rarely found in the general population; only one case is reported in Clin-Var (variation ID: 453004). The variant was regarded as pathogenic according to the interpretation criteria outlined by the American College of Medical Genetics and Genomics. Single-nucleotide polymorphism arrays revealed no pathogenic copy number variants. The patient was presumed to have had de novo mutations in DYNC1H1 because although the genetic data of her family members were unavailable, they appeared healthy without epilepsy or any other neurological disorders.

Discussion
DYNC1H1 consists of several major domains, including the stem domain (amino acid [aa] residues 1-1,867), motor domain (aa residues 1,868-3,168 and 3,553-4,221), and a stalk/microtubulebinding domain (stalk/ MTBD; aa residues 3,189-3,500). A motor domain comprises a ring of siX aAA + domains, which help localize ATP bindings [2]. Each domain of DYNC1H1 exhibits a genotypephenotype correlation in DYNC1H1-related disorders [1,6]. NMDassociated DYNC1H1 variants are located in the stem domain, whereas those associated with brain involvement and neurodevelopmental disorders are located in the motor or stalk/MTBD domain [5,6]. Mutations in the stalk domain, which is formed by two MTBD-linked coiled-coil domains [1], reduce the frequency, distance, and speed of the movement of the dynein complex, thus weakening the interaction between the MTBD and microtubules [2] and subsequently interfering with neurodevelopment.
The two possible pathophysiologies of epilepsy patients with DYNC1H1 variants are as follows. First, most mutations occur in the motor and stalk/MTBD domains of DYNC1H1, affecting brain development; these mutations result in abnormal brain morphologies, including posterior predominant pachygyria, dysgyria, polymicrogyria, microcephaly, nodular heterotopia, and corpus callosum hypoplasia [4,7], most of which are associated with epilepsy or may serve as epileptic foci. Second, DYNC1H1 variants (without MCDs) may lead to epilepsy [13]. In an earlier study, DYNC1H1 was coexpressed with 46 associated epilepsy genes; 15 genes were considered high-confidence genes in the EpilepsyGene database [20]. The findings indicate that DYNC1H1 is a potential factor associated with epileptic encephalopathies. Together, the aforementioned observations emphasize the importance of DYNC1H1 in brain development and its epileptogenicity. The other characteristics, including facial dysmorphism and ophthalmic abnormalities, were similar across the patients. Thus, mutations in the motor domain affect not only the brain but also the lens and facial development, resulting in cataract and facial dysmorphism.
Notably, the cases reviewed in the present study reported muscle weakness with lower limb predominance; some patients with this condition presented with axonal neuropathy or motor neuronopathy (assessed through electrophysiological examinations),        which may be a manifestation of CMT or SMALED-which are wellknown phenotypes of DYNC1H1 variants. In these cases, DYNC1H1 variants may not always be located in the stem domain. The genetic variants of MCDs or epilepsy phenotypes are not confined to the motor domain. These findings highlight the overlapping of the clinical phenotypes of various DYNC1H1-related disorders.

Conclusions
We report the case of a patient with a pathogenic DYNC1H1 variant who presented with rare drug-resistant focal epilepsy. In addition, we summarized the characteristics of patients with DYNC1H1 variant-related epilepsy by reviewing the relevant literature. To the best of our knowledge, this is the most comprehensive study to explore the association between epilepsy and DYNC1H1 variants to date. This report expands the phenotypic spectrum of DYNC1H1-related disorders to early-onset epilepsy, which is frequently associated with MCDs, neurodevelopmental delay, and multi-systemic involvement. Genetically, they have common mutation sites in the motor domain and mostly occur through de novo mutations. Overall, DYNC1H1 is a key factor for patients wtih developmental and epileptic encephalopathies.

Consent for publication
The written informed consent was obtained from the patient's relatives for scientific publication of the case presentation.

Funding
The study was supported by the Ministry of Science and Technology (MOST 107-2314-B-002-070), Ministry of Science and Technology (MOST 109-2634-F-002-029) and National Taiwan University Hospital (NTUH 109-N4657 and 110-A158). The funder had no role in the design, analyses, collection or interpretation of the data or the decision to submit for publication.

Authors contributions
CTC drafted the manuscript for intellectual content. TJ and NCL designed of the study and revised the manuscript for intellectual content. CTC, TJ, NCL and CHC was responsible for acquisition of the data. CTC, TJ, NCL, SPF, MZH and YHL was responsible for analysis and/or interpretation of data.

Ethics approval and consent to participate
Written informed consent was obtained from patient's relatives for the inclusion of deidentified clinical data in a scientific publication, in accordance with the Declaration of Helsinki.
The study was approved by the Research Ethics Committee of National Taiwan University Hospital (NTUH-REC). The committee's reference number: (NTUH-REC No: 201505135RINA) and (NTUH-REC No: 201902064RINC).

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.