By Hai Chen, MD, PhD
Assistant Professor of Clinical Neurology, Weill Cornell Medical College
SYNOSPIS: Eleven somatic variants enriched in the hippocampus were detected in a group of patients with drug-resistant mesial temporal lobe epilepsy (MTLE). Most somatic variants are mutations in the Ras/Raf/mitogen-activated protein kinase (MAPK) pathway. Overactivation of the Ras/Raf/MAPK pathway was detected in MTLE with somatic variants.
SOURCE: Khoshkhoo S, Wang Y, Chahine Y, et al. Contribution of somatic Ras/Raf/mitogen-activated protein kinase variants in the hippocampus in drug-resistant mesial temporal lobe epilepsy. JAMA Neurol 2023;May 1:e230473. doi: 10.1001/jamaneurol.2023.0473. [Online ahead of print].
Somatic variants may lead to focal cortical dysplasia (FCD), which is a cause of focal epilepsy. Mesial temporal lobe epilepsy (MTLE) is the most common form of focal epilepsy and often is resistant to drug treatment. This study investigated the role of somatic variants in drug-resistant MTLE.
The study enrolled 105 drug-resistant MTLE patients who underwent anterior medial temporal lobectomy. Both nonlesional and lesional MTLE subjects were eligible for the study. The clinical histopathologic findings included mesial temporal sclerosis (MTS) (n = 91), MTS with low-grade epilepsy-associated tumors (LEATs) (n = 4), MTS with FCD (n = 2), and non-lesional (n = 8). Patients with non-LEAT primary brain tumors were excluded. Control samples (n = 30) were obtained from neurotypical donors at brain banks.
Surgically resected hippocampal samples were obtained in the MTLE cohort. The hippocampus-derived deoxyribonucleic acid (DNA) was sequenced via whole-exome and gene-panel sequencing methods. Potential pathological somatic variants were detected and then further validated. In the MTLE cohort, pathogenic somatic variants were identified in 11 patients, among whom six patients had the pathological findings of MTS only and the remaining five patients had MTS with LEATs or MTS with FCD. Somatic variants were not observed in samples from the control group. Additionally, in patients with somatic variants, variant allele fraction (VAF) was measured in both hippocampal and paired neocortical temporal samples. The pathogenic variants were either undetectable or less common in the neocortical temporal tissue compared to the hippocampus (mean VAF, 0.3% vs. 1.92%), which suggested that variants were selectively enriched in the hippocampal region.
Interestingly, among the 11 MTLE subjects with somatic variants, 10 patients had mutations in the Ras/Raf/mitogen-activated protein kinase (MAPK) signal pathway, including PTPN11, SOS1, KRAS, BRAF, and NF1. The function of PTPN11 variants was tested in vitro by measuring the degree of Erk1/2 phosphorylation, which is a downstream effector of the Ras/Raf/MAPK pathway. Increased phosphorylated Erk1/2 (pErk1/2) was observed in cells transfected with variant PTPN11, compared with those with the wild-type PTPN11. This finding indicated an increased phosphatase activity and overactivation of the Ras/Raf/MAPK pathway in PTNP11 variants.
Given the enrichment of somatic variants in the hippocampus, the authors further tested whether variant-positive cells play a role in hippocampal epileptogenesis in MTLE. Since overactivation of the Ras/Raf/MAPK pathway was detected in somatic variants, pErk1/2 staining was used to identify variant-positive cells. In MTS patients with somatic variants, colocalization between the density of pErk1/2 and the degree of neuronal loss also was observed in the hippocampus.
Finally, despite the findings in the study, the pathogenic somatic variants were only identified in a small number (11/105) of MTLE patients. Further studies are needed to determine the role of pathogenic variants in MTLE. In addition, the study is limited because of technical factors, i.e., the somatic variant detection was limited to VAFs greater than 1%. Cell loss is a prominent feature of MTS; therefore, the burden of pathogenic somatic variants in MTLE could be underestimated in the study.
COMMENTARY
Enrichment of somatic variants in the hippocampus is found in refractory MTLE subjects, and the majority of variants are involved in the Ras/Raf/MAPK signaling pathway. The density and location of variant-positive cells correlate with neuronal loss and sclerosis in the hippocampus. These findings suggest that somatic variants could contribute to hippocampal epileptogenesis in drug-resistant MTLE.
This study highlights the potential role of molecular testing in the diagnosis and treatment of MTLE. Surgically resected samples were used in this study. However, somatic variants could be detected using less invasive approaches, such as trace brain tissue from stereo-electroencephalography electrodes. Lack of antiepileptogenic properties is a common limitation for anti-seizure medications. Ras/Raf/MAPK pathway activation could play a role in hippocampal epileptogenesis. An agent targeting the Ras/Raf/MAPK pathway may address the underlying mechanism of epilepsy and become an emerging treatment in MTLE with somatic variants.
