Transition of allele-specific DNA hydroxymethylation at regulatory loci is associated with phenotypic variation in monozygotic twins discordant for psychiatric disorders

Major psychiatric disorders such as schizophrenia (SCZ) and bipolar disorder (BPD) are complex genetic mental illnesses. Their non-Mendelian features such as monozygotic twins discordant for SCZ or BPD are likely complicated by environmental modifiers of genetic effects. 5-hydroxymethylcytosine (5hmC) is an important epigenetic marker in gene regulation and whether its links with genetic variants contribute to the non-Mendelian features remain largely unexplored. Here, we performed hydroxymethylome and genome analyses of blood DNA from psychiatric disorder-discordant monozygotic twins to study how allele-specific hydroxymethylation (AShM) mediates phenotypic variations. We identified thousands of genetic variants with AShM imbalances who exhibit phenotypic variation-associated AShM transition at regulatory loci. These AShMs have plausible causal associations with psychiatric disorders through effects on interactions between transcription factors, DNA methylations, or other epigenomic markers and then contribute to dysregulated gene expression, which eventually increases disease susceptibility. We then validated that competitive binding of POU3F2 on the alternative allele of psyAShM site rs4558409 (G/T) in PLLP can enhance the PLLP expression, while hydroxymethylated alternative allele alleviating the transcription factor binding activity at rs4558409 site might be associated with downregulated PLLP expression observed in BPD or SCZ. Moreover, disruption of rs4558409 induces gain of PLLP function and promotes neural development and vesicle trafficking. Our study provides a powerful strategy for prioritizing regulatory risk variants and contributes to our understanding of the interplay between genetic and epigenetic factors in mediating complex disease susceptibility. To validate the inhibitory role of rs4558409-containg region in PLLP expression, we employed the Cas9/sgRNA editing to disrupt the sequences around rs4558409 in SK-N-SH cells.