INTRODUCTION
Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterised by social and communication deficits with repetitive, restricted behaviours and interests. Genetic aetiology of ASD is significantly influenced by rare de novo and common inherited variants (Michaelson et al., 2012; Krumm et al., 2014). Several studies accumulated strong evidences on the genetic burden of ASD, leading to the identification of recurrently mutated high-risk-conferring ASD genes. One such gene- with among the highest de novoloss-of-function (LoF) mutation rates in ASD encodes theChromodomain Helicase DNA-binding protein 8 (CHD8 ) protein which regulates gene expression through chromatin remodelling (Guo et al., 2018; Wade et al., 2019; Satterstrom et al., 2020). Mutations inCHD8 produced a broad range of phenotypes, including ASD, macrocephaly, facial deformities, Intellectual Disability (ID), gastrointestinal (GI) disorders and cancers (Barnard et al., 2015).
Chromatin remodeling enzymes are crucial for the accurate organization of genomic DNA within chromatin. There are two classes of enzymes: ones that mediated post-translational histone modifications and others that utilize the energy derived from ATP hydrolysis to alter the histone-DNA contacts within the nucleosome (Marfella & Imbalzano, 2007). The family of ATP-dependent chromatin remodellers is characterised by two signature sequence motifs: the tandem chromodomains in the N-terminal end that enables histone binding (Wade et al., 2019) and Sucrose NonFermentable2 (SNF2)-like ATP dependant helicase (ATPase) domain (Micucci et al., 2015). CHD8 belongs to subfamily III (CHD6-CHD9) with additional functional motifs- BRK (Brahma and Kismet) domains, a SANT-like (switching-defective protein 3, adaptor 2, nuclear receptor co-repressor, transcription factor IIIB) domain, Helicase-C-terminal and a CHD7-binding motif (Marfella & Imbalzano, 2007). The DNA-binding SANT and SLIDE domain functions as a histone-binding module, confers nonspecific DNA binding, particularly to the linker DNA between nucleosomes (Micucci et al., 2015).
Expression studies revealed that CHD8 mutations indirectly down-regulated gene expression in pathways involving neurodevelopment (Sugathan et al., 2014). Mouse knockdown models of CHD8 resulted in defective Neuronal Progenitor Cells (NPC) proliferation and differentiation, causing abnormal neuronal morphology and behaviours in adult mice. CHD8 disrupted expression of key transducers in Wnt signaling pathway- crucial for the correct balance between NPC proliferation and differentiation (Durak et al., 2016). CHD8 was highly expressed in neurons, but at low levels in glial cells of humans and mice, playing an essential role in dendritic and axon development and migration of cortical neurons (Xu et al., 2018). Reduced CHD8expression led to profound alterations to both excitatory and inhibitory synaptic transmission resulting in a reduced excitatory:inhibitory balance (Ellingford et al., 2020). Thus, these multi-layered pieces of evidence have rightly prompted the categorisation of CHD8 as a master regulator of the foundational pathways in neurodevelopment and ASD (Barnard et al., 2015).
To date, only one study by (An et al., 2020) described the mutational landscape of CHD8 with respect to its domains across three different populations- ASD, cancer and general population. However, they relied on just one parameter for variant prioritisation, i.e., effect prediction score. Considering the immense genetic burden appended byCHD8 on ASD manifestation, we performed a more comprehensive mutational burden analysis with emphasis on deciphering the specific roles of ASD-associated CHD8 variations.