POLR3B de novo variants are a rare cause of infantile myoclonic epilepsy

Purpose: To report on a new phenotype in a patient carrying a novel, undescribed de novo variant in POLR3B, affected by generalized myoclonic epilepsy and neurodevelopmental disorder, without neuropathy. It is known that biallelic pathogenic variants in POLR3B cause hypomyelinating leukodystrophy-8, and heterozygous de novo variants are described in association to a phenotype characterized by predominantly demyelinating sensory-motor peripheral neuropathy, ataxia, spasticity, intellectual disability and epilepsy, in which the peripheral neuropathy is often the main clinical presentation. Methods: We collected clinical, electrophysiological and neuroimaging data from the affected subject and performed a Trio-Clinical Exome Sequencing. Results: We detected a de novo novel heterozygous missense variant c.1132A>G in POLR3B (NM_018082.6) that was considered as likely pathogenic following ACMG criteria. We also consulted our custom genomic database of a total of 1485 patients that were genetically analysed from 2018 for epilepsy, and found no other de novo variants in the POLR3B gene. Conclusion: We hypothesize a possible genotype-phenotype correlation, particularly regarding epilepsy. We also provide a review of the literature about the previously described POLR3B heterozygous patients, with particular attention to the epileptic phenotype, underlining the association between POLR3B and early onset myoclonic epilepsy, which can represent the main manifestation of the disease at its onset.