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
nonspeciļ¬c 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.