Pitt–Hopkins syndrome is a rare neurodevelopmental disorder caused by mutation or deletion of the transcription factor 4 gene TCF4 located in chromosomal region 18q21.1. PTHS is characterized by developmental delay, possible breathing problems of episodic hyperventilation and/or breath-holding while awake (55%-60%), recurrent seizures/epilepsy (40%-50%), gastrointestinal issues, lack of speech, and distinctive facial features.
TCF4 encodes a basic helix-loop-helix transcription factor that regulates gene transcription through homodimerization or heterodimerization with other bHLH transcription factors. TCF4 is highly expressed during embryonic and early postnatal development and has particularly high expression in the hippocampus.
Approximately 30% of PTHS individuals have loss-of-function mutations or deletions of TCF4; the remaining 70% of individuals with PTHS have missense mutations, nonsense mutations, or small insertion-deletions that can result in dominant-negative isoforms of TCF4 protein. Therefore, it is critical to identify the common and distinct pathophysiological disruptions that arise across multiple genetic subtypes of PTHS because this will ultimately facilitate therapeutic strategies for PTHS.
Philpot lab has been taking a multidisciplinary approach to tackle four major questions to advance PTHS treatment strategies.
- What is common pathophysiology?
We developed two novel mouse models, one that harbors the highly prevalent R580W (mouse R579W) point mutation and one that has a small in-frame deletion that eliminates three pathogenic arginines, and studied them in parallel with existing PTHS mouse models. Please read our lab’s publication on characterizing common behavioral and physiological phenotypes from these four PTHS mouse models.
2. How can TCF4 be activated? Philpot lab has created a mouse model with luciferase reporter for Tcf4 expression. We can screen thousands of small molecules on Tcf4-Nluc fibroblast cells to find molecules that activate Tcf4 expression.
3. Where is Tcf4 expressed in the mouse brain? TCF4 gene reinstatement strategies, either through gene addition or gene activation, hold a promising opportunity to treat PTHS. However, little is known about the spatial or temporal expression of TCF4. Thus, Philpot lab generated a reporter mouse model with a GFP tag in the Tcf4 allele along with a STOP cassette that disrupts Tcf4 expression. This mouse model allows us to investigate the spatiotemporal expression pattern of TCF4.
4. When is the therapeutic intervention window?
Taking an advantage of Cre-loxp recombination system (LoxP-P2A-EGFP-STOP-LoxP), we can reinstate TCF4 expression at the different age points to determine when Tcf4 should be restored in order to rescue behavioral and physiological deficits associated with PTHS. This strategy will help us establish the critical period for therapeutic intervention windows.