Genetic Mechanisms and Clinical Features of Myoclonic Epilepsies

Myoclonic epilepsies encompass a spectrum of hereditary disorders characterised by sudden, brief involuntary muscle jerks often accompanied by seizures of varying severity. At their core, many familial adult myoclonic epilepsy (FAME) syndromes arise from non-coding pentanucleotide repeat expansions—typically TTTTA and TTTCA or ATTTC motifs—within intronic regions of at least six distinct genes. These expansions display remarkable somatic instability, producing heterogeneity in repeat length and structure between tissues and even among cells, which correlates with age at onset and clinical severity. Pathophysiologically, myoclonic epilepsies reflect abnormal sensorimotor cortical hyperexcitability, impaired inhibitory circuits and dysregulated cerebellar–thalamic–cortical loops, with altered subcortical connectivity contributing to tremor and seizures. Clinically, affected individuals present in adolescence or adulthood with cortical tremor, action-induced myoclonus and infrequent generalised tonic-clonic seizures. Diagnosis hinges on electrophysiological biomarkers such as giant somatosensory evoked potentials, cortico-muscular coherence and long-latency reflexes, alongside molecular assays capable of detecting dynamic repeat expansions. Advances in long-read sequencing, repeat-primed PCR and neurophysiological mapping have improved diagnostic accuracy, while emerging RNA-targeted and gene-silencing therapies hold promise for tailored interventions.

Research from Nature Portfolio

Recent studies have pinpointed intronic TTTTA/TTTCA repeat expansions in MARCH6, revealing that these expansions can extend from a few kilobases to more than 14 kb with high somatic variability. Single-molecule analyses demonstrated multiple expansion configurations within individual patients and uncovered micro-rearrangements proximal to the expanded repeats. This work underscores the complexity of repeat instability as a driver of FAME pathology and highlights the need for sensitive molecular techniques to capture expansion heterogeneity. In parallel, investigations into the ATTTC repeat within STARD7 clarified its role in FAME2, showing complete segregation of the pentanucleotide expansion in affected families worldwide. Transcript analyses indicated that gene expression remains unaltered, suggesting that pathogenesis arises from RNA-mediated toxicity rather than protein loss. Together, these findings expand the catalogue of pathogenic loci and illuminate common molecular mechanisms across genetically diverse myoclonic epilepsy syndromes.

Genetic Mechanisms and Clinical Features of Myoclonic Epilepsies publication trend

The graph below shows the total number of articles in genetic mechanisms and clinical features of myoclonic epilepsies across all publications each year (not limited to Nature Index journals).

Technical terms

Pentanucleotide repeat expansion: A genetic mutation in which a five-nucleotide sequence is abnormally repeated in non-coding regions, leading to disease through RNA toxicity or structural genome disruption.

Somatic instability: Variability in repeat length and sequence structure of expanded alleles across different tissues or cell types within the same individual.

Cortical hyperexcitability: An increase in the propensity of sensorimotor cortical neurons to fire abnormal, synchronized discharges, underlying myoclonic jerks.

Cerebellar–thalamic–cortical loop: A neural circuit linking the cerebellum, thalamus and cortex, critical for motor coordination and inhibitory control, whose dysfunction contributes to tremor and seizures.

Repeat-primed PCR: A molecular technique designed to detect and size expanded repeat regions by priming within the repeat motif, producing a characteristic ladder pattern on electrophoresis.

References

1. Genetics of familial adult myoclonus epilepsy: From linkage studies to noncoding repeat expansions. Epilepsia (2023).
2. Familial Adult Myoclonus Epilepsy: A Non-Coding Repeat Expansion Disorder of Cerebellar–Thalamic–Cortical Loop. Cells (2023).
3. Unstable TTTTA/TTTCA expansions in MARCH6 are associated with Familial Adult Myoclonic Epilepsy type 3. Nature Communications (2019).
4. Intronic ATTTC repeat expansions in STARD7 in familial adult myoclonic epilepsy linked to chromosome 2. Nature Communications (2019).
5. Abnormal sensorimotor cortex and thalamo-cortical networks in familial adult myoclonic epilepsy type 2: pathophysiology and diagnostic implications. Brain Communications (2022).