ILAE Genetics Literacy series: Progressive myoclonus epilepsies

CLN6-related continuum phenotype caused by aberrant splicing

Neuronal ceroid lipofuscinoses (NCLs) are genetically heterogeneous neurodegenerative disorders, characterized by progressive cognitive and motor decline, epilepsy, visual impairment, and shortened life-expectancy. CLN6-related NCLs include both late-infantile and adult myoclonic form. We report a 21-year-old patient, with mild developmental delay, who developed occipital seizures at 14 years, and subsequently cognitive decline, cortical myoclonus, and photosensitivity at low and higher frequencies. Overall, the picture suited progressive myoclonus epilepsy. Electroretinogram was normal. A skin biopsy revealed a mixed storage of curvilinear and fingerprint profiles. A brain MRI showed severe cortical atrophy. Performing genetic analyses, two biallelic variants were identified in the CLN6 gene, each inherited from one of the healthy parents, one c.722T>C, p.(Met241Thr) already described in the late-infantile form and the other one c.486+28T>C, intronic and novel, causing aberrant splicing. In the patient, the expression of the allele containing c.722T>C variant was increased, in comparison with the carrier parent. The peculiar genetic pattern observed in the patient could explain a milder clinical picture when compared with late-infantile form, since CLN6 expression was partially preserved. However, the presence of a delay, and the early cognitive decline suggested a continuum phenotype connecting late-infantile and adult CLN6-related forms. PLAIN LANGUAGE SUMMARY: We report a patient with CLN6 disease who developed symptoms at an intermediate age: 9 years for mild intellectual disability and 14 years for occipital seizures and progressive myoclonus epilepsy, without visual impairment. The patient is compound heterozygous for a CLN6 missense variant c.722T>C, p.(Met241Thr) already described in the late-infantile form and for a novel intronic variant c.486+28T>C, causing aberrant splicing. In the patient, the expression of the allele containing c.722T>C variant was increased, compared with the carrier parent. The splice site variant had a milder effect. The peculiar genetic pattern may explain the continuum phenotype between late-infantile and adult forms.

Case series; NUS1 deletions cause a progressive myoclonic epilepsy with ataxia

PURPOSE

Mutations in NUS1 cause a neurological congenital glycosylation disorder which encompasses a spectrum from developmental encephalopathy to musculoskeletal, hearing, and visual abnormalities. Pathogenic variants include both point mutations and genomic deletions. We report an adult phenotype of progressive myoclonus epilepsy (PME) and a review of cases with a complete or partial deletion of NUS1.

METHODS

Our patient, currently age 30, presented with an intellectual disability and developed progressive ataxia with myoclonic tremor, alongside generalised absence and tonic-clonic seizures. At age 28 he was diagnosed with a heterozygous 5.0 Mb deletion of 6q22.1q22.31 involving the NUS1 gene. We are unable to state whether this is a de novo mutation; his mother tested negative for the gene, but his father passed away before any genetic analysis could be performed. Along with the 22 patients reported in published literature, we identified 21 other genetically similar NUS1 deletion variants with sufficient clinical data through ClinVar.

RESULTS

The identification of NUS1 gene deletion disorder does not lead to a change in treatment but predicts a progressive clinical trajectory. Recognition of this helps differentiate neurological progression from the impact of anti-seizure medicine.

CONCLUSION

Copy number variants are an often-overlooked cause of PME. We also describe features of psychosis and spasticity and suggest that these may also be due to the NUS1 deletion, expanding the literature that exists on the phenotype of this very rare genetic disorder.

Progressive Myoclonus Epilepsy and Beyond: A Systematic Review of SEMA6B-related Disorders

Progressive myoclonus epilepsy (PME) is a rare, clinically and genetically heterogeneous epilepsy syndrome, and pathogenic variants in the semaphorin 6B (SEMA6B) gene have recently been reported to be among the causes of PME. Cases with pathogenic variants in the SEMA6B gene are extremely rare, only a limited number of cases have been reported in the literature. In this systematic review, we aimed to present a summary of a PME case in which a heterozygous nonsense variant of c.2086C > T p.(Gln696*) in the SEMA6B gene was detected in the etiology and other cases with SEMA6B pathogenic variant in the literature. Except for our case, 35 cases from 12 studies were included. The main clinical findings in these patients were cognitive problems, seizures, gait and speech disturbances, and cognitive and/or motor regression, and they had a wide spectrum of severity. Response to antiseizure medications was also highly variable, almost half of the patients had pharmacoresistant seizures. Patients were divided into four different phenotypic groups according to their clinical presentations: PME (18/36), developmental and epileptic encephalopathy (13/36), neurodevelopmental disorder (4/36), and epilepsy (1/36), respectively. In conclusion, although SEMA6B has been associated with PME, it may actually cause a much broader phenotypic spectrum. Due to their extreme rarity, our knowledge of SEMA6B-related disorders is limited. As with all other rare diseases, each new SEMA6B-related disorder case could contribute to a better understanding of the disease. A better understanding of the disease may allow the development of specific treatment options in the future.

KCTD7-related progressive myoclonic epilepsy: Report of 42 cases and review of literature

OBJECTIVE

KCTD7-related progressive myoclonic epilepsy (PME) is a rare autosomal-recessive disorder. This study aimed to describe the clinical details and genetic variants in a large international cohort.

METHODS

Families with molecularly confirmed diagnoses of KCTD7-related PME were identified through international collaboration. Furthermore, a systematic review was done to identify previously reported cases. Salient demographic, epilepsy, treatment, genetic testing, electroencephalographic (EEG), and imaging-related variables were collected and summarized.

RESULTS

Forty-two patients (36 families) were included. The median age at first seizure was 14 months (interquartile range = 11.75-22.5). Myoclonic seizures were frequently the first seizure type noted (n = 18, 43.9%). EEG and brain magnetic resonance imaging findings were variable. Many patients exhibited delayed development with subsequent progressive regression (n = 16, 38.1%). Twenty-one cases with genetic testing available (55%) had previously reported variants in KCTD7, and 17 cases (45%) had novel variants in KCTD7 gene. Six patients died in the cohort (age range = 1.5-21 years). The systematic review identified 23 eligible studies and further identified 59 previously reported cases of KCTD7-related disorders from the literature. The phenotype for the majority of the reported cases was consistent with a PME (n = 52, 88%). Other reported phenotypes in the literature included opsoclonus myoclonus ataxia syndrome (n = 2), myoclonus dystonia (n = 2), and neuronal ceroid lipofuscinosis (n = 3). Eight published cases died over time (14%, age range = 3-18 years).

SIGNIFICANCE

This study cohort and systematic review consolidated the phenotypic spectrum and natural history of KCTD7-related disorders. Early onset drug-resistant epilepsy, relentless neuroregression, and severe neurological sequalae were common. Better understanding of the natural history may help future clinical trials.