Evidence for a Dual-Pathway, 2-Hit Genetic Model for Focal Cortical Dysplasia and Epilepsy
Mark F Bennett1, Michael S Hildebrand1, Sayaka Kayumi1
1Population Health and Immunity Division (M.F.B., M.B.), The Walter and Eliza Hall Institute of Medical Research; Department of Medical Biology (M.F.B., M.B.), The University of Melbourne, Parkville; Epilepsy Research Centre (M.F.B., M.S.H., Z.Y., R.B., O.J.H., J.A.D., I.E.S., S.F.B.), Department of Medicine, The University of Melbourne, Austin Health, Heidelberg; Murdoch Children's Research Institute (M.S.H., I.E.S.), Royal Children's Hospital, Parkville; Robinson Research Institute and Adelaide Medical School (S.K., M.A.C., J.G.), The University of Adelaide, South Australia; TY Nelson Department of Neurology and Neurosurgery (S.G.), The Children's Hospital at Westmead; Department of Histopathology (M.K.), The Children's Hospital at Westmead, New South Wales; Victorian Clinical Genetics Services (A.B.), Murdoch Children's Research Institute, Parkville, Victoria; South Australian Health and Medical Research Institute (J.G.), Adelaide, South Australia; Department of Paediatrics (I.E.S.), The University of Melbourne, Royal Children's Hospital; and The Florey Institute (I.E.S.), Parkville, Victoria, Australia.
Background And Objectives:
The 2-hit model of genetic disease is well established in cancer, yet has only recently been reported to cause brain malformations associated with epilepsy. Pathogenic germline and somatic variants in genes in the mechanistic target of rapamycin (mTOR) pathway have been implicated in several malformations of cortical development. We investigated the 2-hit model by performing genetic analysis and searching for germline and somatic variants in genes in the mTOR and related pathways.
Methods:
We searched for germline and somatic pathogenic variants in 2 brothers with drug-resistant focal epilepsy and surgically resected focal cortical dysplasia (FCD) type IIA. Exome sequencing was performed on blood- and brain-derived DNA to identify pathogenic variants, which were validated by droplet digital PCR. In vitro functional assays of a somatic variant were performed.
Results:
Exome analysis revealed a novel, maternally inherited, germline pathogenic truncation variant (c.48delG; p.Ser17Alafs*70) in NPRL3 in both brothers. NPRL3 is a known FCD gene that encodes a negative regulator of the mTOR pathway. Somatic variant calling in brain-derived DNA from both brothers revealed a low allele fraction somatic variant (c.338C>T; p.Ala113Val) in the WNT2 gene in 1 brother, confirmed by droplet digital PCR. In vitro functional studies suggested a loss of WNT2 function as a consequence of this variant. A second somatic variant has not yet been found in the other brother.
Discussion:
We identify a pathogenic germline mTOR pathway variant (NPRL3) and a somatic variant (WNT2) in the intersecting WNT signaling pathway, potentially implicating the WNT2 gene in FCD and supporting a dual-pathway 2-hit model. If confirmed in other cases, this would extend the 2-hit model to pathogenic variants in different genes in critical, intersecting pathways in a malformation of cortical development. Detection of low allele fraction somatic second hits is challenging but promises to unravel the molecular architecture of FCDs.
Insights
The two-hit model explains genetic brain malformations. Researchers found a germline variant in NPRL3 and a somatic variant in WNT2 in brothers with epilepsy, supporting a dual-pathway model for cortical development disorders.
Area of Science:
- Genetics
- Neuroscience
- Molecular Biology
Background:
- The two-hit model is established in cancer genetics but newly applied to brain malformations.
- Mechanistic target of rapamycin (mTOR) pathway gene variants are linked to cortical development disorders.
- Investigating the two-hit model in malformations of cortical development (FCD) is crucial.
Purpose of the Study:
- To investigate the two-hit model in FCD type IIA.
- To identify germline and somatic variants in mTOR and related pathways in patients with epilepsy and FCD.
- To explore the role of intersecting pathways in cortical development.
Main Methods:
- Genetic analysis of germline and somatic variants in two brothers with drug-resistant focal epilepsy and FCD type IIA.
- Whole-exome sequencing on blood and brain DNA.
- Validation of variants using droplet digital PCR and in vitro functional assays.
Main Results:
- A novel germline pathogenic variant in NPRL3 (a negative regulator of the mTOR pathway) was identified in both brothers.
- A low-allele fraction somatic variant in WNT2 (intersecting WNT signaling pathway) was found in one brother.
- In vitro studies suggested loss of WNT2 function due to the somatic variant.
Conclusions:
- Identification of a germline mTOR pathway variant (NPRL3) and a somatic WNT2 variant supports a dual-pathway two-hit model for FCD.
- This finding extends the two-hit model to intersecting pathways in cortical development disorders.
- Detecting low-allele fraction somatic variants is challenging but key to understanding FCD molecular architecture.
Related Concept Videos
01:24Epilepsy and Seizures: Overview
Various factors can trigger epilepsy, including genetic factors, brain damage, metabolic causes, and unknown etiology. Diagnosis of epilepsy involves electroencephalography (EEG), which...
01:27Psychosis: Pathophysiology of Schizophrenia and Other Psychotic Disorders
Researchers have identified genetic factors that increase susceptibility to schizophrenia, underscoring the intricate interplay between genetics and environment in disease development. At the core of schizophrenia's pathophysiology is excessive dopaminergic neurotransmission within...