Repeat expansions with small TTTCA insertions in MARCHF6 cause Familial Adult Myoclonus without Epilepsy

Familial Adult Myoclonic Epilepsy (FAME) is a rare autosomal dominant disorder characterized by cortical tremulous myoclonus and generalized tonic clonic (GTCS) or myoclonic seizures. Intrafamilial variability regarding the prevalence of seizure has been reported. Despite apparent genetic heterogeneity, FAME is caused by the same TTTTA/TTTCA pentanucleotide repeat expansion in introns of seven different genes with no common function. Expansions containing only TTTTA repeats are benign, and only expansions including a TTTCA expansion insertion, which can be located 3' to the TTTTA expansion or in the middle of the TTTTA repeats, are pathogenic. We aimed to identify the underlying genetic cause of dominant cortical myoclonus without GTCS in two unrelated families. We used a combination of repeat-primed PCR, long-range PCR and nanopore sequencing to detect and characterize expansions at known FAME loci. We report a novel configuration of the FAME3 repeat expansion in MARCHF6 segregating with cortical myoclonus without epilepsy GTCS in both families. This expansion predominantly comprises elongated TTTTA repeats, ranging from 388 to 454 repeats, with the presence of only 5-11 TTTCA repeats at its 3' terminus (i.e., 5’-(TTTTA)~388-454(TTTCA)5-11-3’). This configuration exhibits greater stability upon meiotic transmission but somatic variability in TTTCA repeat number was detected at low level in blood. We observed an inverse correlation between the number of TTTCA repeats and the age at myoclonus onset. This study shows that as little as five TTTCA juxtaposed with expanded TTTTA expansions is sufficient to cause cortical myoclonus without GTCS. This finding has implications regarding possible pathophysiological mechanisms leading to FAME.

• Type: Gene Regulation Study
• Archiver: European Genome-Phenome Archive (EGA)