Among the different genetic causes of epileptic encephalopathy, the prevalence of ASD features is not completely known, but it reaches high levels in some specific instances (Additional file
1: Table S1). Out of the disorders where we can calculate prevalence, the lowest prevalence (6%) occurs in
GRIN2A-related disorders, which disrupt the N-methyl-D-aspartate (NMDA) receptor, critical for learning and memory (
http://www.ncbi.nlm.nih.gov/gene/2903), resulting in epilepsy-aphasia spectrum. Out of 36 patients from 16 families with epileptic aphasia spectrum due to
GRIN2A alterations, two non-related individuals had “autistic features” and CSWSS. One of the two had global developmental delay without regression; the other had a normal initial developmental period followed by global regression with predominant effects on language [
26]. In contrast to
GRIN2A-related disorders, epileptic encephalopathies associated with mutations in
CDKL5 (encoding cyclin-dependent kinase-like 5),
SCN1A (encoding sodium voltage-gated channel alpha subunit 1), and
SLC6A1 (encoding GABA transporter 1, which enables GABA re-uptake from the synaptic cleft) have a high co-occurrence of ASD features. In one case series of 10 patients with
CDKL5 mutations, all (100%) patients had “autistic features” in addition to epilepsy, though the cohort was enriched for patients with early-onset epileptic encephalopathy and/or a clinical diagnosis of Rett syndrome with negative
MECP2 sequencing [
27]. Out of 15 patients with DS secondary to mutations in
SCN1A, 11 (or 73%) had a diagnosis of ASD based on Diagnostic and Statistical Manual of Mental Disorders-IV (DSM-IV) criteria, mild-severe intellectual disability (ID), and various seizure types [
28]. In a cohort of 7 patients with
SLC6A1 variants underlying myoclonic-atonic epilepsy, 5 (71%) had “autistic features,” mild-severe ID (along with regression in 2 individuals), and a full spectrum of seizure types [
29]. Other disorders with relatively high rates of ASD features alongside epileptic encephalopathy include
HCN1-related epilepsy (67% [
30]) and
SIK1-related epilepsy (50% [
31]).
HCN1 mutations affect hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels which regulate pacemaker currents in neurons, resulting in an early infantile epileptic encephalopathy with characteristics of DS initially followed later by the emergence of atypical absences, ID, and ASD features [
30]. Defects in salt-inducible kinase 1 (
SIK1), encoding a serine/threonine protein kinase implicated in signal transduction, cause a spectrum of epileptic encephalopathies including early myoclonic encephalopathy, IS, and Ohtahara syndrome (OS) [
31].
Of note, these ASD prevalence numbers are based on descriptive case series comprising at least five patients with variants in the gene of interest who have epileptic encephalopathy. As a result, there is insufficient data to calculate ASD prevalence rates for all the genes in Additional file
1: Table S1. For a number of genes implicated in epileptic encephalopathy, future studies may provide more detailed associations with ASD features. Moreover, these numbers represent estimates rather than true prevalence rates because the primary emphasis of the source studies is not necessarily to characterize the ASD phenotype, which leads to indirect terminology such as “autistic features” rather than detailed accounts of formal ASD evaluations. Methodological limitations notwithstanding, there is ample evidence to suggest that within monogenic epileptic encephalopathies, a substantial percentage of patients may be affected by ASD characteristics.