Solute carrier (SLC) proteins are ubiquitous transmembrane transporters with a crucial role in maintaining the physiological functions of organisms and as medication targets (Pizzagalli et al., 2021; Gan et al., 2022). SLC6A1 [MIM: 137165] is located on the short arm of chromosome 3 (3p25.3) and encodes sodium chloride-dependent GABA transporter type 1 (GAT-1), a voltage-dependent transmembrane protein (Bröer and Gether, 2012; Kahen et al., 2022). GAT-1 activity is essential for GABA reuptake into the presynaptic terminals of neurons and glia (Bain et al., 2022; Gan et al., 2022).

Pathogenic variants of SLC6A1 destabilize the global protein conformation, causing a loss of GAT-1 function that leads to alterations in GABA levels in the synaptic cleft (Kahen et al., 2022). This imbalance may independently affect many pathways involved in both neurological and psychiatric diseases, leading to a wide spectrum of clinical phenotypes, including epilepsy, intellectual disability (ID), and behavioral disorders (Bröer and Gether, 2012; Carvill et al., 2015; Poliquin et al., 2021).

SLC6A1 pathogenic variants were originally detected in patients affected by epilepsy with myoclonic-atonic seizures (EMAtS) (Carvill et al., 2015; Fischer et al., 2022). Later, patients with Lennox–Gastaut Syndrome (LGS) were also reported, expanding the phenotypic spectrum (Cai et al., 2019). Nevertheless, additional authors reported SLC6A1 pathogenic variants in patients suffering from a heterogeneous spectrum of epilepsies, that did not always meeting criteria for a defined epileptic syndrome (Kalvakuntla et al., 2023). Overall, the most frequent seizure types associated with SLC6A1 genetic variants are myoclonic-atonic seizures and absences with eyelid myoclonia, followed by tonic–clonic seizures (TCS) (Johannesen et al., 2018; Goodspeed et al., 2020; Tang et al., 2020; Kahen et al., 2022).

In the last few years, a wide range of neurodevelopmental comorbidities have been associated with SLC6A1-related epilepsy (Fischer et al., 2022; Kahen et al., 2022). Affected subjects often present with language disorder and cognitive impairment that progresses from developmental delay (DD) or developmental regression to intellectual disability (Johannesen et al., 2018; Kahen et al., 2022; Kalvakuntla et al., 2023). Behavioral disorders such as attention-deficit/hyperactivity disorder (ADHD), aggressivity, anxiety, or mood disorders, as well as other more severe psychiatric features, such as autism spectrum disorder (ASD) and schizophrenia, may contribute to the severity of the clinical phenotype (Johannesen et al., 2018; Goodspeed et al., 2020; Rees et al., 2020; Fischer et al., 2022; Kahen et al., 2022; Kalvakuntla et al., 2023). Additional features such as hypotonia, learning disorders, high pain tolerance, sleep disturbances, and movement disorders have also been reported (Poliquin et al., 2021; Kahen et al., 2022; Verhoeven et al., 2022).

In this study, we describe the neuropsychological profile of five new patients carrying SLC6A1 pathogenic variants, detailing the cognitive and behavioral features. The clinical presentation of all subjects was consistent with EMAtS. Moreover, we performed a literature review, focusing on the neuropsychological profiles (cognitive and behavioral details), which includes 204 previously published patients with SLC6A1 pathogenic variants.

We retrospectively reviewed the medical charts of five patients with SLC6A1 pathogenic variants who were followed at Bambino Gesù Children’s Hospital in Rome, Italy. SLC6A1 pathogenic variants were identified in all patients through a trio-based New Generation Sequencing (NGS) epilepsy gene panel. We collected clinical data, including sex, family history, age at seizure onset, seizure semiology and frequency, intellectual disability, behavioral and psychiatric disorders, EEG and brain MRI.

Seizure types and epilepsy syndromes were classified according to the Commission on Classification and Terminology of the International League Against Epilepsy (Fisher et al., 2017; Scheffer et al., 2017; Specchio et al., 2022).

We retrospectively reviewed all cognitive and behavioral evaluations for all patients performed with the following scales:

This study was performed according to ethical principles for medical research involving human subjects stated in the Declaration of Helsinki and was approved by the local ethics committee.

We also reviewed the literature from January 2012 to February 2023 related to all published patients with SLC6A1 pathogenic variants. We identified a total of 204 patients carrying SLC6A1 genetic variants. The search strategy included the terms “SLC6A1” and/or “Epilepsy” on PubMed. References were also identified manually from relevant articles and by searching through the authors’ files. We limited the review to 50 English-language articles. We excluded patients with microdeletions encompassing SLC6A1 and extended the search to other genes that could be causative of misleading data.

For all reported patients, we collected the genetic variants (including encoded protein, type of mutation and inheritance) and clinical data (including gender, age at the study, seizure types and syndromic classification, cognitive and behavioral features). Supplementary Table S1 shows a summary of the findings. We cannot exclude the presence of overlapping patients within this dataset due to incomplete data.

The clinical phenotype associated with SLC6A1 pathogenic variants includes epilepsy with various seizure types (absences, myoclonic, atonic, and tonic–clonic seizures), neurodevelopmental disorders (i.e., ASD and ADHD), and psychiatric features affecting mood and behavior (i.e., aggressivity and anxiety). To date, few data on neuropsychological features have been reported; however, data on cognition and behavioral disturbances are often missing because of a lack of standardized testing.

Developmental delay has been frequently reported by parents even before epilepsy onset, as in three out of five of our patients (Cai et al., 2019; Goodspeed et al., 2020). Once epileptic seizures appear, those patients are referred to pediatric neurologists, and soon after cognitive evaluations, a wide spectrum of cognitive and behavioral disturbances become evident (Johannesen et al., 2018; Goodspeed et al., 2020).

Data from the literature review report developmental regression in approximately 13% of patients around the age of 3 years (Kalvakuntla et al., 2023). The disability involved several domains, including language, motor, social and adaptive skills (Kalvakuntla et al., 2023). Developmental regression may be underestimated as most of the evaluations were done through unstructured interviews with parents, and standardized tests were rarely reported.

Additionally, the severity of ID was also seldom detailed in the literature, and when reported, it ranged from mild to moderate in most cases.

In general, no significant differences, in terms of cognitive impairment, were reported as related to seizure control (Johannesen et al., 2018). Accordingly, one patient in our series achieved seizure control around the age of 4 years; however, cognitive disability associated with poor autonomy in activities of daily living persisted into adulthood.

Eleven patients with normal IQ were reported in the literature, even if learning disorders were observed in five of them. Six out of 11 patients with normal IQ had epilepsy and interictal epileptiform discharges at EEGs: these findings support that etiology, rather than epilepsy, has a major role in this condition (Johannesen et al., 2018; Kim et al., 2019; Posar and Visconti, 2019; Goodspeed et al., 2020). Most of our patients had speech disorders before seizure onset, making this condition truly developmental and epileptic encephalopathy (DEE), in which epilepsy may worsen cognitive functions, which are already damaged by the genetic variant itself. The spectrum of SLC6A1 genetic variants encompasses wide phenotypic variability, from epilepsy with or without ID to the DEE phenotype.

Our patients received several cognitive and behavioral tests. At the last neuropsychological assessment, three patients had borderline intellectual functioning, one had mild cognitive impairment, and the last patient had moderate cognitive disability. Three out of five had a worsening of cognitive functions over time. Patient 1 moved from borderline functioning to mild cognitive impairment, patient 2 from borderline to moderate cognitive delay, and patient 3 from normal to borderline functioning over 2 years without developing a clear-cut cognitive disability. What was seen in most patients was a delay in acquiring new skills rather than a clear regression. Adult patients achieved personal independence in daily activities, even if they live with their parents. They do not have jobs, and one of them still attends school with a support teacher and an individualized learning plan. They do not practice any rehabilitative or psychotherapeutic therapy. They achieved personal independence in daily activities with minimal supervision.

These findings underline the importance of longitudinal assessments in patients with epilepsy over time, especially in children of developing ages. Accordingly, periodic cognitive evaluations with standardized testing, jointly with structured behavioral assessments, might be useful to adapt over time for a patient-specific rehabilitation program to identify the best cognitive functioning and reach good daily life autonomies during adulthood.

Regarding behavioral disorders, data from the literature may be misleading. Few authors have properly analyzed these features, reported as aggressive or oppositional behaviors with psychiatric traits (Rauch et al., 2012; Lucariello et al., 2016; Snoeijen-Schouwenaars et al., 2018; Kim et al., 2019).

Consistent with literature data (Poliquin et al., 2021), we found an attention deficit disorder in all analyzed patients together with high levels of anxiety, social and disruptive behavior disorder with a pattern of oppositionality, emotional instability, and heteroaggressiveness. No pharmacological therapy was prescribed for such symptoms. Worthy of mention is the possible role of some ASMs in worsening cognitive abilities and behavioral disturbances (Strzelczyk and Schubert-Bast, 2022). Indeed, among them, drugs such as benzodiazepines may have a negative impact on attention and behavior, provoking irritability, as occurred in one of our patients.

Several studies have reported that SLC6A1 patients with autism, from traits to ASD, are frequently related to developmental regression (Kalvakuntla et al., 2023). In particular, autism was reported in approximately one-third of patients (31.4%); in our series, one patient presented with autistic traits. We did not test patients with ASD-specific scales; therefore, the prevalence of ASD in these patients may be underestimated.

The main limitation is the small size of the sample and the lack of longitudinal data. Regarding the literature review, the unavailability of a single common database for all published cases and the lack of many details may have introduced some bias in the evaluation. In addition, considering the retrospective nature of this study, our data are heterogeneous in terms of age, timing/frequency and type of testing. We used different tests depending upon age and language barriers. It should be considered that RPM and Leiter- R, being two nonverbal cognitive tests, may overestimate patients’ IQ, unlike the Wechsler scales, which provide full IQ.

Despite the limited number of patients and the few neuropsychological data reported in the literature, the overall data suggest the need for a specific assessment for ADHD, ASD and cognitive development in patients with epilepsy related to SLC6A1 pathogenic variants.

The high presence of behavioral disturbances and reduced cognitive performance underline the need for a targeted rehabilitation program, including strengthening attention skills.

Finally, early integrated intervention, including speech, psychological and psychomotor therapies during the early years of life, may be crucial to improve cognitive outcomes and support patients’ daily life autonomy in adulthood.

The deep phenotyping of SLC6A1-related disorders, including cognitive and behavioral comorbidities, is also important considering the promising pharmacological perspectives as hypothesized in vivo and in vitro studies, using chemical and pharmacological chaperones as possible treatments in deficiency of GAT-1 expression and function (Kasture et al., 2023).

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary material.

The studies involving human participants were reviewed and approved by the Ethic Committee of Bambino Gesù Children’s Hospital. Written informed consent to participate in this study was provided by the participants’ legal guardian/next of kin. Written informed consent was obtained from the individual(s), and minor(s)’ legal guardian/next of kin, for the publication of any potentially identifiable images or data included in this article.

MT, AB, AD, and CC collected and analyzed clinical data. AD and AN provided and analyzed genetic data. CQ and SC collected and analyzed neuropsychological data. MT, AB, and CQ wrote the first draft of the manuscript. SC, NS, FV, and AN supervised the study and reviewed the last version of the manuscript. All authors read and approved the submitted version.

This work was supported by #NEXTGENERATIONEU (NGEU) and funded by the Ministry of University and Research (MUR), National Recovery and Resilience Plan (NRRP), project MNESYS (PE0000006)—a multiscale integrated approach to the study of the nervous system in health and disease (DN. 1553 11.10.2022). This work was supported by the Italian Ministry of Health with Current Research funds.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.