TCF4 Mediates PHF6 Transcriptional Regulation of Neural Stem Cells in the Developing Brain, a Mechanism Disrupted in Börjeson Forssman Lehmann Syndrome (BFLS)

Abstract

Transcriptional hierarchies govern neural stem cell (NSC) fate decisions during neurodevelopment, and disruption of these regulatory networks underlies neurodevelopmental disorders. Borjeson-Forssman-Lehmann syndrome (BFLS) is an X-linked intellectual disability (XLID) caused by mutations in a chromatin-associated transcriptional regulator, the plant homeodomain zinc finger protein 6 (PHF6). Although BFLS mouse models harboring PHF6 patient mutations recapitulate neurogenic and cognitive deficits, the molecular mechanisms linking PHF6 dysfunction to these phenotypes remain poorly understood. Here, we identify transcription factor 4 (TCF4), a basic helix-loop-helix transcription factor implicated in neurodevelopmental and psychiatric disorders, as a critical downstream effector of PHF6 in NSC fate regulation. Tcf4 expression is reduced in BFLS mouse models harbouring R342X and C99F-m mutations and in a conditional Phf6 knockout driven by Nestin-Cre. Mechanistically, PHF6 binds the Tcf4 gene regulatory element upstream of its transcriptional start site (TSS) and promotes Tcf4 expression. Tcf4 depletion in embryonic NSCs enhances self-renewal and stemness, increasing neurosphere formation and expression of stem cell markers, Nestin and Sox2. Conversely, TCF4 restoration rescues NSC defects caused by PHF6 loss of function in BFLS and Phf6/Nestin-Cre mice. This regulatory relationship persists in adult NSCs where TCF4 similarly restricts stem cell expansion. Importantly, restoration of TCF4 expression in embryonic brain ameliorates behavioral and cognitive deficits in BFLS mice. Together, these findings establish a PHF6-Tcf4 transcriptional pathway that restricts NSC self-renewal and links disrupted transcriptional control of NSC fate to the neurodevelopmental and behavioral abnormalities of BFLS.