Mono and biallelic variants inHCN2cause severe neurodevelopmental disorders
Authors: Houdayer Clara , Phillips A Marie , Chabbert Marie , Bourreau Jennifer , Maroofian Reza , Houlden Henry , Richards Kay , Saadi Nebal Waill , Dad’ová Eliška , Van Bogaert Patrick , Rupin Mailys , Keren Boris , Charles Perrine , Smol Thomas , Riquet Audrey , Pais Lynn , O’Donnell-Luria Anne , VanNoy Grace E. , Bayat Allan , Møller Rikke S , Olofsson Kern , Abou Jamra Rami , Syrbe Steffen , Dasouki Majed , Seaver Laurie H , Sullivan Jennifer A , Shashi Vandana , Alkuraya Fowzan S , Poss Alexis F , Spence J Edward , Schnur Rhonda E , Forster Ian C , Mckenzie Chaseley E , Simons Cas , Wang Min , Snell Penny , Kothur Kavitha , Buckley Michael , Roscioli Tony , Elserafy Noha , Dauriat Benjamin , Procaccio Vincent , Henrion Daniel , Lenaers Guy , Colin Estelle , Verbeek Nienke E. , Van Gassen Koen L. , Legendre Claire , Bonneau Dominique , Reid Christopher A , Howell Katherine B , Ziegler Alban , Legros Christian 
Published: March 2024

Hyperpolarization activated Cyclic Nucleotide (HCN) gated channels are crucial for various neurophysiological functions, including learning and sensory functions, and their dysfunction are responsible for brain disorders, such as epilepsy. To date,HCN2variants have only been associated with mild epilepsy and recently, one monoallelic missense variant has been linked to developmental and epileptic encephalopathy. Here, we expand the phenotypic spectrum ofHCN2-related disorders by describing twenty-one additional individuals from fifteen unrelated families carryingHCN2variants. Seventeen individuals had developmental delay/intellectual disability (DD/ID), two had borderline DD/ID, and one had borderline DD. Ten individuals had epilepsy with DD/ID, with median age of onset of 10 months, and one had epilepsy with normal development. Molecular diagnosis identified thirteen different pathogenicHCN2variants, including eleven missense variants affecting highly conserved amino acids, one frameshift variant, and one in-frame deletion. Seven variants were monoallelic of which five occurredde novo,one was not maternally inherited, one was inherited from a father with mild learning disabilities, and one was of unknown inheritance. The remaining six variants were biallelic, with four homozygous and two compound heterozygous variants. Functional studies using two-electrode voltage-clamp recordings inXenopus laevisoocytes were performed on three monoallelic variants, p.(Arg324His), p.(Ala363Val), and p.(Met374Leu), and three biallelic variants, p.(Leu377His), p.(Pro493Leu) and p.(Gly587Asp). The p.(Arg324His) variant induced a strong increase of HCN2 conductance, while p.(Ala363Val) and p.(Met374Leu) displayed dominant negative effects, leading to a partial loss of HCN2 channel function. By confocal imaging, we found that the p.(Leu377His), p.(Pro493Leu) and p.(Gly587Asp) pathogenic variants impaired membrane trafficking, resulting in a complete loss of HCN2 elicited currents inXenopusoocytes. Structural 3D-analysis in depolarized and hyperpolarized states of HCN2 channels, revealed that the pathogenic variants p.(His205Gln), p.(Ser409Leu), p.(Arg324Cys), p.(Asn369Ser) and p.(Gly460Asp) modify molecular interactions altering HCN2 function. Taken together, our data broadens the clinical spectrum associated withHCN2variants, and disclose thatHCN2is involved in developmental encephalopathy with or without epilepsy.