RAS pathway: The new frontier of brain mosaicism in epilepsy

As cells divide during development, errors in DNA replication and repair lead to somatic mosaicism – a phenomenon in which different cell lineages harbor unique constellations of genetic variants. Over the past decade, somatic variants that disrupt mTOR signaling, protein glycosylation, and other functions during brain development have been linked to cortical malformations and focal epilepsy. More recently, emerging evidence points to a role for Ras pathway mosaicism in epilepsy. The Ras family of proteins is a critical driver of MAPK signaling. Disruption of the Ras pathway is most known for its association with tumorigenesis; however, developmental disorders known as RASopathies commonly have a neurological component that sometimes includes epilepsy, offering evidence for Ras involvement in brain development and epileptogenesis. Brain somatic variants affecting the Ras pathway (e.g., KRAS, PTPN11, BRAF) are now strongly associated with focal epilepsy through genotype-phenotype association studies as well as mechanistic evidence. This review summarizes the Ras pathway and its involvement in epilepsy and neurodevelopmental disorders, focusing on new evidence regarding Ras pathway mosaicism and the potential future clinical implications

Lamin B1 decline underlies age-related loss of adult hippocampal neurogenesis

Neurogenesis in the adult hippocampus declines with age, a process that has been implicated in cognitive and emotional impairments. However, the mechanisms underlying this decline have remained elusive. Here, we show that the age-dependent downregulation of lamin B1, one of the nuclear lamins in adult neural stem/progenitor cells (ANSPCs), underlies age-related alterations in adult hippocampal neurogenesis. Our results indicate that higher levels of lamin B1 in ANSPCs safeguard against premature differentiation and regulate the maintenance of ANSPCs. However, the level of lamin B1 in ANSPCs declines during aging. Precocious loss of lamin B1 in ANSPCs transiently promotes neurogenesis but eventually depletes it. Furthermore, the reduction of lamin B1 in ANSPCs recapitulates age-related anxiety-like behavior in mice. Our results indicate that the decline in lamin B1 underlies stem cell aging and impacts the homeostasis of adult neurogenesis and mood regulation.Peer reviewe

The Rhox gene cluster suppresses germline LINE1 transposition

Transposable elements (TEs) are mobile sequences that engender widespread mutations and thus are a major hazard that must be silenced. The most abundant active class of TEs in mammalian genomes is long interspersed element class 1 (LINE1). Here, we report that LINE1 transposition is suppressed in the male germline by transcription factors encoded by a rapidly evolving X-linked homeobox gene cluster. LINE1 transposition is repressed by many members of this RHOX transcription factor family, including those with different patterns of expression during spermatogenesis. One family member-RHOX10-suppresses LINE1 transposition during fetal development in vivo when the germline would otherwise be susceptible to LINE1 activation because of epigenetic reprogramming. We provide evidence that RHOX10 suppresses LINE transposition by inducing Piwil2, which encodes a key component in the Piwi-interacting RNA pathway that protects against TEs. The ability of RHOX transcription factors to suppress LINE1 is conserved in humans but is lost in RHOXF2 mutants from several infertile human patients, raising the possibility that loss of RHOXF2 causes human infertility by allowing uncontrolled LINE1 expression in the germline. Together, our results support a model in which the Rhox gene cluster is in an evolutionary arms race with TEs, resulting in expansion of the Rhox gene cluster to suppress TEs in different biological contexts

Single-nuclei transcriptomics enable detection of somatic variants in patient brain tissue

Abstract Somatic variants are a major cause of human disease, including neurological disorders like focal epilepsies, but can be challenging to study due to their mosaicism in bulk tissue biopsies. Coupling single-cell genotype and transcriptomic data has potential to provide insight into the role somatic variants play in disease etiology, such as by determining what cell types are affected or how the mutations affect gene expression. Here, we asked whether commonly used single-nucleus 3’- or 5’-RNA-sequencing assays can be used to derive single-nucleus genotype data for a priori known variants that are located near to either end of a transcript. To that end, we compared performance of commercially available single-nuclei 3’- and 5’- gene expression kits using resected brain samples from three pediatric patients with focal epilepsy. We quantified the ability to detect genetic variants in single-nucleus datasets depending on distance from the transcript end. Finally, we demonstrated the ability to identify affected cell types in a patient with a RHEB somatic variant causing an epilepsy-associated cortical malformation. Our results demonstrate that single-nuclei 3’ or 5’-RNA-sequencing data can be used to identify known somatic variants in single-nuclei when they are expressed within proximity to a transcript end