NUS1 and Epilepsy-myoclonus-ataxia Syndrome: An Under-recognized Entity?

Case report: Novel NUS1 variant in a Chinese patient with tremors and intellectual disability

Introduction

Nuclear undecaprenyl pyrophosphate synthase 1 (NUS1) gene variants are associated with a range of phenotypes, including epilepsy, intellectual disability, cerebellar ataxia, Parkinson's disease, dystonia, and congenital disorders of glycosylation. Additionally, cases describing genotypes and clinical features are rare.

Case Presentation

Herein, we report the case of a 23-year-old Chinese female patient who presented with tremors, intellectual disability, and epilepsy. A history of carbon monoxide exposure, brain trauma, or encephalitis was not present in this case. Trio whole-exome sequencing analysis revealed a de novo pathogenic variant of c.750del in exon 4, leading to p.Leu251* amino acid substitution. Genetic analysis failed to identify the identical mutations in the remaining family members who underwent screening. The patient was diagnosed with a rare congenital disease, "congenital glycosylation disorder, type 1aa, autosomal dominant, type 55, with seizures (MRD-55)."

Conclusion

We provide further evidence for the role of variants in NUS1 in the development of tremors, epilepsy, and intellectual disabilities. These findings expand our understanding of the clinical phenotypes of NUS1 variants.

Progressive Myoclonus Epilepsy: A Scoping Review of Diagnostic, Phenotypic and Therapeutic Advances

The progressive myoclonus epilepsies (PME) are a diverse group of disorders that feature both myoclonus and seizures that worsen gradually over a variable timeframe. While each of the disorders is individually rare, they collectively make up a non-trivial portion of the complex epilepsy and myoclonus cases that are seen in tertiary care centers. The last decade has seen substantial progress in our understanding of the pathophysiology, diagnosis, prognosis, and, in select disorders, therapies of these diseases. In this scoping review, we examine English language publications from the past decade that address diagnostic, phenotypic, and therapeutic advances in all PMEs. We then highlight the major lessons that have been learned and point out avenues for future investigation that seem promising.

DHDDS and NUS1: A Converging Pathway and Common Phenotype

BACKGROUND

Variants in dehydrodolichol diphosphate synthetase (DHDDS) and nuclear undecaprenyl pyrophosphate synthase 1 (NUS1) cause a neurodevelopmental disorder, classically with prominent epilepsy. Recent reports suggest a complex movement disorder and an overlapping phenotype has been postulated due to their combined role in dolichol synthesis.

CASES

We describe three patients with heterozygous variants in DHDDS and five with variants affecting NUS1. They bear a remarkably similar phenotype of a movement disorder dominated by multifocal myoclonus. Diagnostic clues include myoclonus exacerbated by action and facial involvement, and slowly progressive or stable, gait ataxia with disproportionately impaired tandem gait. Myoclonus is confirmed with neurophysiology, including EMG of facial muscles.

LITERATURE REVIEW

Ninety-eight reports of heterozygous variants in DHDDS, NUS1 and chromosome 6q22.1 structural alterations spanning NUS1, confirm the convergent phenotype of hypotonia at birth, developmental delay, multifocal myoclonus, ataxia, dystonia and later parkinsonism with or without generalized epilepsy. Other features include periodic exacerbations, stereotypies, anxiety, and dysmorphisms. Although their gene products contribute to dolichol biosynthesis, a key step in N-glycosylation, transferrin isoform profiles are typically normal. Imaging is normal or non-specific.

CONCLUSIONS

Recognition of their shared phenotype may expedite diagnosis through chromosomal microarray and by including DHDDS/NUS1 in movement disorder gene panels.

Novel NUS1 variant in a Chinese patient with progressive myoclonus epilepsy: a case report and systematic review

BACKGROUND

Variants of the NUS1 gene have been associated with an extensive spectrum of phenotypes, including epilepsy, intellectual disability, cerebellar ataxia, Parkinson's disease, dystonia, and congenital disorder of glycosylation. It is rarely reported in progressive myoclonus epilepsy (PME).

METHODS AND RESULTS

Herein, we report the case of PME caused by a novel de novo NUS1 missense variant (c.302T>A, p.Met101Lys). In addition, we reviewed the current literature of NUS1-associated PME. At present, five patients with NUS1 variants and PME have been reported in the literature. Due to limited cases reported, the relationship between NUS1 variants and PME is not well-established.

CONCLUSIONS

Our case provides further evidence of the role of NUS1 variants in PME. These findings expand the clinical phenotypes of NUS1 variants, which should be included in the PME genetic screening panel.

Case report: splicing effect of a novel heterozygous variant of the NUS1 gene in a child with epilepsy

NUS1 is responsible for encoding of the Nogo-B receptor (NgBR), which is a subunit of cis-prenyltransferase. Over 25 variants in NUS1 have been reported, and these variants have been found to be associated with various phenotypes, such as congenital disorders of glycosylation (CDG) and developmental and epileptic encephalopathy (DEE). We report on the case of a patient who presented with language and motor retardation, epilepsy, and electroencephalogram abnormalities. Upon conducting whole-exome sequencing, we discovered a novel pathogenic variant (chr6:118024873, NM_138459.5: c.791 + 6T>G) in NUS1, which was shown to cause Exon 4 to be skipped, resulting in a loss of 56 amino acids. Our findings strongly suggest that this novel variant of NUS1 is responsible for the development of neurological disorders, including epilepsy. It is believed that the truncation of Nogo-B receptor results in the loss of cis-prenyltransferase activity, which may be the underlying cause of the disease.

Rhythmic cortical myoclonus in patients with 6Q22.1 deletion

DNA deletions involving 6q22.1 region result in developmental encephalopathy (DE), often associated with movement disorders and epilepsy. The phenotype is attributed to the loss of the NUS1 gene included in the deleted region. Here we report three patients with 6q22.1 deletions of variable length all showing developmental delay, and rhythmic cortical myoclonus. Two patients had generalized seizures beginning in infancy. Myoclonic jerks had polygraphic features consistent with a cortical origin, also supported by cortico-muscular coherence analysis displaying a significant peak around 20 Hz contralateral to activated segment. Deletions in 6q22.1 region, similarly to NUS1 loss-of-function mutations, give rise to DE and cortical myoclonus via a haploinsufficiency mechanism. A phenotype of progressive myoclonic epilepsy (PME) may also occur.