Monoallelic POLR3A variants cause a Pol III-related disorder characterized by peripheral neuropathy

RNA polymerase III (Pol III) is a multi-subunit enzymatic complex essential for the transcription of small noncoding RNAs with structural, translational or regulatory functions. Biallelic pathogenic variants in multiple genes encoding Pol III subunits have been linked to a spectrum of neurological disorders, mainly affecting the central nervous system. To date, pathogenic monoallelic variants have been reported in only one subunit gene (POLR3B) in association with a spectrum of neurodevelopmental disorders, epilepsy and peripheral neuropathy. Here, we describe a novel clinical and genetic Pol III-related entity associated with monoallelic missense variants in POLR3A, and presenting primarily with peripheral neuropathy. We identified eleven patients across eight unrelated families harbouring pathogenic heterozygous missense variants in the POLR3A gene occurring either de novo or segregating in an autosomal dominant fashion. Systematic clinical evaluation revealed the patients present with an early onset, progressive sensorimotor peripheral polyneuropathy with intermediate to demyelinating ranges of nerve conduction slowing and occasionally accompanied with additional neurological or non-neurological features. White matter abnormalities, characteristic for the biallelic Pol III-related disorders, were not observed in the brain magnetic resonance imaging from available individuals. Structural modelling revealed the neuropathy-associated variants cluster in regions critical for the canonical function of the polymerase, and do not overlap with known biallelic disease-causing variants. Follow up transcriptomic studies in patient-derived cells demonstrated impaired Pol III activity, including mis-regulation of individual Pol III targets and global downregulation of tRNA pools. The Pol III dysfunction was not due to impaired POLR3A expression, subcellular localization or subunit interactions. Our findings highlight the importance of recognizing monoallelic POLR3A variants as a cause of peripheral neuropathy. We provide critical insights into the diversity of POLR3A-related allelic disorders offering a framework to understanding their underlying molecular pathomechanisms. This work highlights a central role of RNA Polymerase III dysfunction in human disease and further strengthens the link between tRNA metabolism and peripheral neurodegeneration.