Reprogramming of the epigenome in neurodevelopmental disorders

Abstract

The etiology of neurodevelopmental disorders (NDDs) remains a challenge for researchers. Human brain development is tightly regulated and sensitive to cellular alterations caused by endogenous or exogenous factors. Intriguingly, the surge of clinical sequencing studies has revealed that many of these disorders are monogenic and monoallelic. Notably, chromatin regulation has emerged as highly dysregulated in NDDs, with many syndromes demonstrating phenotypic overlap, such as intellectual disabilities, with one another. Here we discuss epigenetic writers, erasers, readers, remodelers, and even histones mutated in NDD patients, predicted to affect gene regulation. Moreover, this review focuses on disorders associated with mutations in enzymes involved in histone acetylation and methylation, and it highlights syndromes involving chromatin remodeling complexes. Finally, we explore recently discovered histone germline mutations and their pathogenic outcome on neurological function. Epigenetic regulators are mutated at every level of chromatin organization. Throughout this review, we discuss mechanistic investigations, as well as various animal and iPSC models of these disorders and their usefulness in determining pathomechanism and potential therapeutics. Understanding the mechanism of these mutations will illuminate common pathways between disorders. Ultimately, classifying these disorders based on their effects on the epigenome will not only aid in prognosis in patients but will aid in understanding the role of epigenetic machinery throughout neurodevelopment.

Keywords: Chromatin; de novo mutations; epigenetics; histone; intellectual disability; monogenic disorders; neurodevelopmental disorders.

FIGURE 1:

Chromatin-modifying enzymes and the complexes they belong. Proteins mutated in NDDs are shown in color.

FIGURE 2:

Histone post-translational modifications are classified as activating or repressing depending on the modification site and the degree of modifications. Histone acetylation is primarily activating whereas methylation can play both roles. The HATs and KMTs which are mutated in NDD, as well as their targets, are shown above.

FIGURE 3:

Spatial expression of epigenetic factors dysregulated in NDD. (left) Graphical representation of brain regions in which most genes are highly expressed and their respective functions. (right) Expression in these three regions according to Consensus normalized expression (NX) levels on Protein atlas. This dataset was created by combining two transcriptomic datasets (GTEx and FANTOM5).