Non-coding repeat expansions associated with familial adult myoclonic epilepsy: a new paradigm of gene-independent monogenic disorders

Abstract Familial adult myoclonic epilepsy (FAME) is a rare autosomal dominant disorder characterized by cortical myoclonic tremor and seizures. FAME has been mapped to chromosomes (chr) 2, 3, 5 and 8, but the cause has remained elusive for more than a decade. An expansion of intronic TTTTA and TTTCA repeats in SAMD12 was identified as the cause of FAME1 in Japanese families linked to chr 8 in 2018. This discovery triggered the identification of identical repeat expansions at five additional loci (FAME2: STARD7; FAME3: MARCHF6; FAME4: YEATS2; FAME6: TNRC6A and FAME7: RAPGEF2). These genes encode proteins with different functions and subcellular localizations and their expression is unaltered in available peripheral tissues, suggesting that the expansion is pathogenic independently of the gene itself. The pathophysiological mechanisms are not yet known but possibly include toxicity at the RNA level or translation of toxic polypeptides from the repeats, a mechanism known as repeat-associated non-AUG (RAN) translation. FAME is a paradigm of human genetic disorder caused by a non-coding expansion unrelated to the gene where it occurs.