Mysm1 mutations in meander tail mice cause anterior-selective cerebellum malformation

Mouse meander tail (mea) mutations produce kinked tails and selective malformation of the cerebellum anterior compartment. The anterior cerebellum defects are cell autonomous with respect to granule cell precursors, but the molecular basis has not been known. Myb-like, SWIRM, and MPN domain containing protein 1 (MYSM1) is a chromatin-associated deubiquitinase that promotes gene expression by removing monoubiquitin from histone H2A, among other targets. Loss of MYSM1 function in mice or humans results in bone marrow failure with defective maturation of B cell lineages. Here we show that extant mea alleles have mutations in Mysm1 and cause both neurological and hematological phenotypes, as do new non-complementing endonuclease-mediated mutations. Multimodal single-nucleus assays show Mysm1 effects on gene expression in several lineages and on the proportion of granule cell precursors by E14.5. Intriguingly, Mysm1 orthologs have been independently lost in several animal and fungal lineages, including yeast, flies, and nematodes. These results unite previously disconnected literature and demonstrate a requirement for MYSM1 activity in compartment-specific development of the cerebellum and suggest potential for compensatory pathways. Significance StatementPerturbations to core regulatory machinery often produce pleiotropic effects and even intensively studied systems can have significant phenotypic effects that were not assessed in models developed for a different purpose. Here we show that classical meander tail mice, characterized by ankylosing spondylitis in tail vertebrae and a cerebellum malformation that defined the anterior-posterior compartment boundary, have mutations in Mysm1, encoding a histone 2A deubiquitinase. We show pigmentation defects and hematopoietic abnormalities that model human disease. While Mysm1 mutations change gene expression patterns in many cerebellar cell types, they selectively decrease the proportion of granule cell lineages. Recurrent loss of Mysm1 orthologs across fungal and animal phylogenies suggests the potential for bypass mechanisms.