Progressive Myoclonus Epilepsy Caused by a Homozygous Splicing Variant of SLC7A6OS

Exome sequencing was performed in 2 unrelated families with progressive myoclonus epilepsy. Affected individuals from both families shared a rare, homozygous c.191A > G variant affecting a splice site in SLC7A6OS. Analysis of cDNA from lymphoblastoid cells demonstrated partial splice site abolition and the creation of an abnormal isoform. Quantitative reverse transcriptase polymerase chain reaction and Western blot showed a marked reduction of protein expression. Haplotype analysis identified a ~0.85cM shared genomic region on chromosome 16q encompassing the c.191A > G variant, consistent with a distant ancestor common to both families. Our results suggest that biallelic loss-of-function variants in SLC7A6OS are a novel genetic cause of progressive myoclonus epilepsy. ANN NEUROL 2021;89:402-407.