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Heterozygous pathogenic variants in DNM1 cause developmental and epileptic encephalopathy (DEE) as a result of a dominant-negative mechanism impeding vesicular fission. Thus far, pathogenic variants in DNM1 have been studied with a canonical transcript that includes the alternatively spliced exon 10b. However, after performing RNA sequencing in 39 pediatric brain samples, we find the primary transcript expressed in the brain includes the downstream exon 10a instead. Using this information, we evaluated genotype-phenotype correlations of variants affecting exon 10a and identified a cohort of eleven previously unreported individuals. Eight individuals harbor a recurrent de novo splice site variant, c.1197-8G>A (GenBank: NM_001288739.1), which affects exon 10a and leads to DEE consistent with the classical DNM1 phenotype. We find this splice site variant leads to disease through an unexpected dominant-negative mechanism. Functional testing reveals an in-frame upstream splice acceptor causing insertion of two amino acids predicted to impair oligomerization-dependent activity. This is supported by neuropathological samples showing accumulation of enlarged synaptic vesicles adherent to the plasma membrane consistent with impaired vesicular fission. Two additional individuals with missense variants affecting exon 10a, p.Arg399Trp and p.Gly401Asp, had a similar DEE phenotype. In contrast, one individual with a missense variant affecting exon 10b, p.Pro405Leu, which is less expressed in the brain, had a correspondingly less severe presentation. Thus, we implicate variants affecting exon 10a as causing the severe DEE typically associated with DNM1-related disorders. We highlight the importance of considering relevant isoforms for disease-causing variants as well as the possibility of splice site variants acting through a dominant-negative mechanism., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2022 American Society of Human Genetics. Published by Elsevier Inc. All rights reserved.)

Meningioma-1 (MN1) overexpression in AML is associated with poor prognosis, and forced expression of MN1 induces leukemia in mice. We sought to determine how MN1 causes AML. We found that overexpression of MN1 can be induced by translocations that result in hijacking of a downstream enhancer. Structure predictions revealed that the entire MN1 coding frame is disordered. We identified the myeloid progenitor-specific BAF complex as the key interaction partner of MN1. MN1 over-stabilizes BAF on enhancer chromatin, a function directly linked to the presence of a long polyQ-stretch within MN1. BAF over-stabilization at binding sites of transcription factors regulating a hematopoietic stem/progenitor program prevents the developmentally appropriate decommissioning of these enhancers and results in impaired myeloid differentiation and leukemia. Beyond AML, our data detail how the overexpression of a polyQ protein, in the absence of any coding sequence mutation, can be sufficient to cause malignant transformation., Competing Interests: Declaration of interests K.M.B. holds a patent on the use of DOT1L inhibitors for MN1 leukemia. K.M.B. has received research funding from Syndax and has previously consulted for Agios. I.H. received research funding from Bristol-Myers Squibb (BMS), Celgene, Merck Sharpe & Dohme (MSD), Astellas, Otsuka, Ono, Kyowa Kirin, Sanofi, Shionogi, Zenyaku, Daiichi Sankyo, Taiho, Takeda, Chugai, Eli Lilly, Nihon Shinyaku, Novartis, Pfizer, Fujimoto, Tanabe-Mitsubishi, Fukuyu Hospital, and Yamada Yohojo and received honoraria from or holds membership on an entity’s board of directors, speaker’s bureau, or its advisory committees for Celgene, Janssen, Takeda, Ono, BMS, Novartis, Daiichi Sankyo, Kyowa Kirin, Eisai, Nihon-Shinyaku, Pfizer, AbbVie, Otsuka, Shionogi, Mundi, CSL, and MSD. All other authors declare no competing interests., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Single-cell gene expression analyses of mammalian tissues have uncovered profound stage-specific molecular regulatory phenomena that have changed the understanding of unique cell types and signaling pathways critical for lineage determination, morphogenesis, and growth. We discuss here the case for a Pediatric Cell Atlas as part of the Human Cell Atlas consortium to provide single-cell profiles and spatial characterization of gene expression across human tissues and organs. Such data will complement adult and developmentally focused HCA projects to provide a rich cytogenomic framework for understanding not only pediatric health and disease but also environmental and genetic impacts across the human lifespan., (Copyright © 2019 The Authors. Published by Elsevier Inc. All rights reserved.)