Enhanced calcium activity and transcriptomic alterations in iPSC-derived neurons from BAFME patients with repeat expansions

Benign adult familial myoclonus epilepsy (BAFME) is caused by intronic TTTCA and TTTTA repeat expansions in SAMD12 and other genes; the neuronal basis of cortical hyperexcitability, however, remains unclear. We generated induced pluripotent stem cell (iPSC)-derived glutamatergic and GABAergic neurons from three BAFME1 patients and examined functional and transcriptomic phenotypes. Patient-derived neurons retained the pathogenic repeat expansions and showed a tendency toward upstream intronic RNA accumulation. Calcium imaging revealed increased spontaneous Ca^{2 +} transient frequency in both neuronal subtypes, indicating heightened activity. Pharmacological profiling demonstrated attenuated responses to calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor (CP-AMPAR) blockade and GABA_A receptor antagonism in GABAergic neurons, suggesting altered inhibitory signaling. RNA sequencing revealed transcriptomic alterations without differential expression of ion channels and neurotransmitter receptors. In glutamatergic neurons, ATF4-regulated genes, including SLC7A5 encoding LAT1, a Kv1.2 channel modulator, were downregulated. Reduced SLC7A5 expression was validated at both mRNA and protein levels. In GABAergic neurons, synapse-associated genes PTPRD and GPC6 were upregulated. TCERG1L and NLRP2 were suppressed across both neuronal subtypes. These findings suggest subtype-specific alterations may contribute to neuronal hyperexcitability in BAFME and provide a platform for mechanistic studies of repeat expansion-associated epilepsies.