Repeat Expansions with Small TTTCA Insertions in MARCHF6 Cause Familial Myoclonus without Epilepsy

Abstract

Background: Familial adult myoclonus epilepsy (FAME) is a rare autosomal dominant disorder caused by the same intronic TTTTA/TTTCA repeat expansion in seven distinct genes. TTTTA-only expansions are benign, whereas those containing TTTCA insertions are pathogenic.

Objective: We investigated the genetic basis of dominant cortical myoclonus without seizures in two unrelated families.

Methods: Repeat-primed polymerase chain reaction (PCR), long-range PCR, and nanopore sequencing were used to detect and characterize expansions at known FAME loci.

Results: We identified a novel repeat expansion in MARCHF6, comprising 388 to 454 elongated TTTTA repeats and 5 to 11 TTTCA repeats at the 3'-terminus, segregating with cortical myoclonus in 8 affected individuals. This configuration shows meiotic stability but low-level somatic variability in blood. We observed an inverse correlation between the number of TTTCA repeats and the age at myoclonus onset.

Conclusions: These findings indicate that as little as five TTTCA repeats combined with expanded TTTTA repeats can cause cortical myoclonus without epilepsy, highlighting the potential mechanisms underlying FAME pathophysiology. © 2025 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Keywords: MARCHF6; epilepsy; familial adult myoclonus epilepsy (FAME); myoclonus; repeat expansion.

FIG. 1

(A) Pedigrees of families 5 and 6. Half‐black symbols indicate affected individuals with neurological examination. Squares indicate males, circles indicate females, and diamonds indicate both sexes. Half‐gray symbols indicate affected subjects with a history but no examination. Half‐green symbol: juvenile myoclonic epilepsy; Half‐blue symbols acute symptomatic seizure due to hyponatremia. The numbers in the symbols indicate the number of siblings of the same sex. The numbers below the symbols correspond to the individual ID within the generation (left side of the pedigree). Numbers in red indicate an expansion, and numbers in black indicate its absence. (B) Identification of the expansion at the MARCHF6 locus using repeat‐primed polymerase chain reaction (RP‐PCR). RP‐PCR profiles using TTTCA primer for a control individual (top left, negative control), a patient with a typical expansion in MARCHF6 (top right, positive control), and the 8 affected individuals from families 5 and 6. (C) Gel electrophoresis of long‐range PCR (LR‐PCR) amplicons spanning the MARCHF6 expansion in individuals with myoclonus from families 5 and 6. (D–G) Characterization of repeat expansion sequence and variability using nanopore sequencing. (D) Waterfall plots showing selected nanopore reads after separation of alleles based on the presence of at least two consecutive TTTCA repeats (Methods). Three hundred randomly chosen reads are shown in each graph. Yellow: TTTTA, red: TTTCA, cyan: TTTTG, blue: TTATG, black: other. (E) Median number of TTTTA and TTTCA repeats calculated from nanopore reads for each of the 8 affected individuals from families 5 and 6. (F) Box plots showing the distribution of the number of TTTCA repeats in each affected individual. (G) Box plots showing the distribution of the number of TTTTA repeats in each affected individual. (H–J) Genotype–phenotype correlations. Pearson's correlations of the age at onset (AAO) and the median number of (H) TTTCA repeats, (I) TTTTA repeats, and (J) median size of the expansion including both motifs. (K) Pearson's correlation between the median number of TTTCA and TTTTA repeats.