An attempt to explain what intrinsically disordered TCF4 does in its spare time when PTHS-related mutations prevent it from doing its job

Abstract

Pitt-Hopkins Syndrome (PTHS) is a rare neurodevelopmental disorder caused by mutations in the TCF4 gene (18q21.2), encoding the transcription factor 4 (TCF4). This protein is critical for central nervous system development and neuronal maturation. Mutations in TCF4, which range from point mutations to large deletions, result in varying clinical severity, including intellectual disability (ID), motor impairments, and autistic features. Despite its rarity, PTHS has gained increasing attention due to advances in understanding the genetic and molecular mechanisms underlying TCF4 function. Recent research has enhanced diagnostic approaches, including genetic testing techniques like genome sequencing, enabling more accurate identification of the disorder. Despite the evident enhancement in the PTHS management from a medical standpoint, the molecular underpinnings of the disorder progression remain puzzling. This is particularly the case where the disease is caused by point mutations. This review summarizes the latest findings on TCF4 function in PTHS, discusses the variability in mutation effects, highlights current diagnostic and therapeutic advancements, and attempts to explain the molecular bases of mutated TCF4 malfunctionality.

Keywords: Genetic disorder; Neurodevelopment; Pitt-Hopkins syndrome (PTHS); Protein aggregation; Rare disease; Transcription factor 4 (TCF4).

Fig. 1

Schematic representation of TCF4 canonical isoform (B ^–) and its mutations (point substitutions) in PTHS. AD1 (1–99) and AD2 (325–432) are activation domain 1 [55, 56] and 2 [31, 55], respectively; CE (171–185) is conserved element [65] (marked in yellow); Rep (511–540) is repression domain [66] (marked in blue), bHLH (564–617) is basic helix-loop-helix domain [108] (b marked in red and HLH marked in green); NLS1 (156–178) and NLS2 (564–602) are nuclear localization signal 1 [34] and 2 [68], respectively (black dashed areas); NES1 (585–594) and NES2 (600–614) are nuclear export signals [68]. PTHS-related substitutions in TCF4 appear mostly in the basic, DNA-binding region of bHLH. Reported cases include: G358V [7, 46]; D535G [14, 46]; R565W [9]; R572G [14]; R572Q [9]; R574H [7]; R574P [7]; R574C [25]; R576Q [17]; R576W [17, 28]; R578P [18]; N581D [25]; A583P [9]; T602A [109]; K603E [109]; A610V [14]; V613I [109]

Fig. 2

Mutations associated with PTHS affect the aggregation rate of TCF4. (A) Amino acid sequence of TCF4 bHLH with PTHS-related mutations sites marked red, with numbering corresponding to B ^– isoform. Arrows correspond to residues observed to interact with DNA (black – both 6OD3 and 6OD4, gray – only 6OD3 [112]). (B) PTHS-related variants and correlation of their ability to form nuclear puncta with propensity to aggregation. + indicates that nuclear puncta were observed, - that nuclear puncta were not observed (localization of TCF4 variant was the same as wild type), ND indicates no data available. AggreRATE-Pred Change in Aggregation Rate is , where is apparent aggregation rate constant, in units of hour ^− 1, and and are aggregation rates of pathogenic variant and wild type protein, respectively. indicates that mutation leads to a higher propensity for aggregation (↑), whereas indicates that pathogenic variant has lower propensity for aggregation than wild type protein (↓)