Genetic variants in RNU4-2, which encodes U4, a key non-coding small nuclear RNA (snRNA) component of the major spliceosome, were recently shown to cause a prevalent neurodevelopmental disorder (NDD) called ReNU syndrome. These variants, which almost exclusively arise de novo, act in a dominant fashion and are clustered within 18 nucleotides (nt) in the centre of RNU4-2. Here we describe a novel recessive NDD associated with homozygous and compound heterozygous variants in RNU4-2. We identified 32 individuals with biallelic variants outside of the 18 nt ReNU syndrome region, that cluster within other functionally important elements of U4, including the Stem II region, the k-turn motif, and the Sm protein binding site. We characterise the clinical phenotype in 27 of these individuals, demonstrating that the recessive disorder is clinically distinct from dominant ReNU syndrome and is associated with distinctive white matter abnormalities, including enlarged perivascular spaces. Together, these findings expand the genotypic and phenotypic spectrum of RNU4-2-associated NDDs.

N.W. receives research funding from Novo Nordisk and BioMarin Pharmaceutical. D.G.M. is a paid consultant for GlaxoSmithKline, Insitro and Overtone Therapeutics and receives research support from Microsoft. D.P. provides consulting service to Ionis Pharmaceuticals, Acadia Pharmaceuticals and M2DS Therapeutics. S.J.S. receives research funding from BioMarin Pharmaceutical. A.OD.-L. is on the scientific advisory board for Congenica, was a paid consultant for Tome Biosciences, Ono Pharma USA Inc. and at present for Addition Therapeutics, and received reagents from PacBio to support rare disease research. Y.C. has a PhD studentship funded by Novo Nordisk. All other authors declare no competing interests.

N.W. is supported by a Wellcome Career Development Award (grant no. 305292/Z/23/Z), a Lister Institute research prize, and grant funding from Novo Nordisk. Y.C. is supported by a studentship from Novo Nordisk. The Francis Crick Institute receives its core funding (G.M.F.) from Cancer Research UK (CC2190), the UK Medical Research Council (CC2190), and the Wellcome Trust (CC2190). A.B. is supported by a Wellcome PhD Training Fellowship for Clinicians and the 4Ward North PhD Programme for Health Professionals (223521/Z/21/Z). Analysis was supported by the Centre for Population Genomics (Garvan Institute of Medical Research and Murdoch Childrens Research Institute) and was funded in part by a National Health and Medical Research Council investigator grant (2009982) and the Medical Research Future Fund (MRFF) Genomics Health Futures Mission (2032931). Massimos Mission acknowledges funding support from the Australian Government Department of Health and Aged Care (EPCD000034). O.M. is supported by the Hazem Ben-Gacem Tunisia Medical Fellowship Fund. D.G.C. was supported by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health under award number K12NS098482. This study was supported by the National Institute for Health and Care Research (NIHR) Manchester Biomedical Research Centre (NIHR203308) and funded in part by US National Institutes of Health Genomics Research Elucidates Genetics of Rare, GREGoR, Program (HG011758). Sequencing and analysis of Individual 24 were provided by the Broad Institute Center for Mendelian Genomics (Broad CMG) and were funded by the National Human Genome Research Institute (NHGRI) grants U01HG011755 (GREGoR consortium), R01HG009141, and in part by the Chan Zuckerberg Initiative Donor-Advised Fund at the Silicon Valley Community Foundation (funder DOI 10.13039/100014989) grants 2020-224274, 2022-309464, 2022-316726, and 2022-316726 (https://doi.org/10.37921/236582yuakxy). Research reported in this publication was supported by the National Institute Of Neurological Disorders And Stroke of the National Institutes of Health under Award Number U01NS134358. The content is solely the responsibility of the authors and does not necessarily represent the official views of the funding agencies.

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