THE EPIGENETIC READER PHF21B MODULATES NEUROTRANSMISSION AND SYNAPTIC PLASTICITY-RELATED GENES IN THE HIPPOCAMPUS, AND SOCIAL MEMORY
Authors: Licinio *Julio , Chin Eunice W M , Ma Qi , Ruan Hongyu , Chin Camille , Somasundaram Aditya , Wong Ma-Li 
Published: February 2025

Abstract

Background
The PHD finger proteins are an important class of epigenetic readers of histone post-translational modifications known to associate with chromatin and regulate gene activities. The PHF21B (Plant homeodomain finger protein 21B) in chromosome 22q13.31 in the genomic locus for the Phelan- McDermid syndrome. It is a histone reader that is expressed in several brain regions, such as the frontal cortex and the hippocampus, but its roles in the brain remain unclear.

Aims & Objectives
We have previously reported that Phf21b gene expression in the hippocampus significantly decreases in animals subjected to chronic stress than non-stressed controls. This study characterized the behavior, hippocampal function, and transcriptome changes caused by Phf21b knockdown mice. We aim to develop novel pharmacological targets for social memory treatment strategies.

Method
All animal experiments were performed according to approved protocols by SUNY Upstate Medical University, South Australian Health and Medical Research Institute, Flinders University, and University of Adelaide. Male and female mice were studied. We used the following methods – 1) CRISPR/Cas9 technology (SA Genome Editing Facility University of Adelaide, South Australia, Australia) was used to generate a Phf21b mutant mice in which the exon 4 of the mouse Phf21b gene was mostly deleted by a premature stop codon. 2) Cognitive and emotional behavioral tests were used to determine the effects of Phf21b deficiency (Phf21bΔ4/Δ4). 3) Long-term potentiation in CA1 pyramidal neurons was study with whole-cell patch clamp. 4) Hippocampal transcriptome and differential gene expression analysis were performed and confirmation studies with qRT-PCR. 5) Immunoblotting, immunohistochemistry, and Golgi staining. 6) Histone peptide array.

Results
Phf21bΔ4/Δ4 mice were in general good health, they expressed 60% less PHF21B than WT mice. Combined data of male and females were used for data analyses, as no sex differences were noted. Mice with PHF21B deficiency had social memory deficits. Their interaction time during the habituation trials in the 5-trial social memory test was not decreased. In the 3-chamber social test, the score difference was significantly lower (P&lt;0.05) and increased social novelty (P&lt;0.01). Other cognitive or emotional differences did not appear in these animals. PHF21B deficit was associated with thinner cortex (P&lt;0.001), fewer astrocytes (P&lt;0.0001), and decreased neurogenesis (DCX+ cells, P&lt;0.001). PHF21B deficiency led to decreased synaptic protein expression (fewer PSD95+ clusters/unit area, P&lt;0.001; smaller PSD95+ puncta size, P&lt;0.01), glutamatergic neurotransmission (reduced I/O relationship, P&lt;0.001; impaired LTP), and GluN2B (P&lt;0.05) and Grin2b (P&lt;0.01) levels. Genes involved in neurotransmission were modulated by PHF21B using RNAseq. PHF21B regulates transcription through H3K9ac, H3K9Me2, and CREB, and interacts with H3K36me3.

Discussion & Conclusions
PHF21B is a critical upstream regulator of genes involved in synaptic plasticity. Neurobehavioral disorders may benefit from targeting it as a therapeutic target.