Loss of Nexmif results in the expression of phenotypic variability and loss of genomic integrity.

We identified two NEXMIF variants in two unrelated individuals with non-autoimmune diabetes and autistic traits, and investigated the expression of Nexmif in mouse and human pancreas and its function in pancreatic beta cells in vitro and in vivo. In insulin-secreting INS-1E cells, Nexmif expression increased strongly in response to oxidative stress. CRISPR Cas9-generated Nexmif knockout mice exhibited a reduced number of proliferating beta cells in pancreatic islets. RNA sequencing of pancreatic islets showed that the downregulated genes in Nexmif mutant islets are involved in stress response and the deposition of epigenetic marks. They include H3f3b, encoding histone H3.3, which is associated with the regulation of beta-cell proliferation and maintains genomic integrity by silencing transposable elements, particularly LINE1 elements. LINE1 activity has been associated with autism and neurodevelopmental disorders in which patients share characteristics with NEXMIF patients, and can cause genomic instability and genetic variation through retrotransposition. Nexmif knockout mice exhibited various other phenotypes. Mortality and phenotypic abnormalities increased in each generation in both Nexmif mutant and non-mutant littermates. In Nexmif mutant mice, LINE1 element expression was upregulated in the pancreas, brain, and testis, possibly inducing genomic instability in Nexmif mutant mice and causing phenotypic variability in their progeny.