Heat shock factor 1 signaling: A novel pathway implicated in Rett syndrome pathophysiology

Rett syndrome (RTT) is a neurodevelopmental disorder that is associated with loss-of-function mutations in the methyl CpG binding protein 2 (MECP2) gene. MECP2 regulates transcription both locally and globally, making it challenging to distinguish between genes that are pathogenic and those that constitute transcriptional noise. A rare subpopulation of patients lack MECP2 mutations despite presenting with sufficient symptoms to warrant a clinical diagnosis of RTT. These patients are classified as having atypical and MECP2 mutation-negative forms of the disorder. We hypothesized that identifying pathways with conserved disruption between typical and atypical forms of RTT would be a viable mechanism to reduce transcriptional noise and identify the genes that are most critical to their shared clinical presentation. To test this theory, we conducted differential RNA sequencing using 5 atypical RTT, 6 typical RTT (R255X), and 9 neurotypical control temporal cortex autopsy samples. Pathways associated with heat shock factor 1 (HSF1) signaling were among the most enriched in both RTT populations. Validation studies using 37 patient temporal cortex samples showed that increased HSF1 signaling was enriched in those with classically severe MECP2 mutations. To investigate whether increased HSF1 signaling is compensatory or pathogenic, we conducted in vivo hyperthermia experiments complemented by cellular stress array analyses. These experiments established that RTT model mice exhibit faster and larger induction of cellular stress-associated proteins. Pharmacological induction of HSF1 in Mecp2^+/- mice was consistent with hyperthermia experiments, showing seizure-like phenotypes and lethality. Conversely, chronic inhibition of HSF1 signaling improved RTT-like phenotypes in domains of motor learning and general health. Together, these data suggest that promiscuous HSF1 signaling is likely a pathogenic amplifier of severe phenotypes and provide a rationale that inhibiting this pathology may hold therapeutic potential in RTT and related disorders. SIGNIFICANCE STATEMENT: Rett syndrome is a devastating neurodevelopmental disorder with limited therapeutic options. This manuscript identifies heat shock factor 1-signaling as a novel therapeutic target and proposes a molecular mechanism by which cellular stress responses are regulated in Rett syndrome.