IRF2BPL as a novel causative gene for progressive myoclonus epilepsy

IRF2BPL has recently been described as a novel cause of neurodevelopmental disorders with multisystemic regression, epilepsy, cerebellar symptoms, dysphagia, dystonia, and pyramidal signs. We describe a novel IRF2BPL phenotype consistent with progressive myoclonus epilepsy (PME) in three novel subjects and review the features of the 31 subjects with IRF2BPL-related disorders previously reported. Our three probands, aged 28-40 years, harbored de novo nonsense variants in IRF2BPL (c.370C > T, p.[Gln124*] and c.364C > T; p.[Gln122*], respectively). From late childhood/adolescence, they presented with severe myoclonus epilepsy, stimulus-sensitive myoclonus, and progressive cognitive, speech, and cerebellar impairment, consistent with a typical PME syndrome. The skin biopsy revealed massive intracellular glycogen inclusions in one proband, suggesting a similar pathogenic pathway to other storage disorders. Whereas the two older probands were severely affected, the younger proband had a milder PME phenotype, partially overlapping with some of the previously reported IRF2BPL cases, suggesting that some of them might be unrecognized PME. Interestingly, all three patients harbored protein-truncating variants clustered in a proximal, highly conserved gene region around the "coiled-coil" domain. Our data show that PME can be an additional phenotype within the spectrum of IRF2BPL-related disorders and suggest IRF2BPL as a novel causative gene for PME.