Clinical Features, Neuropathology, and Surgical Outcome in Patients With Refractory Epilepsy and Brain Somatic Variants in the SLC35A2 Gene

Abstract

Background and objectives: The SLC35A2 gene, located at chromosome Xp11.23, encodes for a uridine diphosphate-galactose transporter. We describe clinical, genetic, neuroimaging, EEG, and histopathologic findings and assess possible predictors of postoperative seizure and cognitive outcome in 47 patients with refractory epilepsy and brain somatic SLC35A2 gene variants.

Methods: This is a retrospective multicenter study where we performed a descriptive analysis and classical hypothesis testing. We included the variables of interest significantly associated with the outcomes in the generalized linear models.

Results: Two main phenotypes were associated with brain somatic SLC35A2 variants: (1) early epileptic encephalopathy (EE, 39 patients) with epileptic spasms as the predominant seizure type and moderate to severe intellectual disability and (2) drug-resistant focal epilepsy (DR-FE, 8 patients) associated with normal/borderline cognitive function and specific neuropsychological deficits. Brain MRI was abnormal in all patients with EE and in 50% of those with DR-FE. Histopathology review identified mild malformation of cortical development with oligodendroglial hyperplasia in epilepsy in 44/47 patients and was inconclusive in 3. The 47 patients harbored 42 distinct mosaic SLC35A2 variants, including 14 (33.3%) missense, 13 (30.9%) frameshift, 10 (23.8%) nonsense, 4 (9.5%) in-frame deletions/duplications, and 1 (2.4%) splicing variant. Variant allele frequencies (VAFs) ranged from 1.4% to 52.6% (mean VAF: 17.3 ± 13.5). At last follow-up (35.5 ± 21.5 months), 30 patients (63.8%) were in Engel Class I, of which 26 (55.3%) were in Class IA. Cognitive performances remained unchanged in most patients after surgery. Regression analyses showed that the probability of achieving both Engel Class IA and Class I outcomes, adjusted by age at seizure onset, was lower when the duration of epilepsy increased and higher when postoperative EEG was normal or improved. Lower brain VAF was associated with improved postoperative cognitive outcome in the analysis of associations, but this finding was not confirmed in regression analyses.

Discussion: Brain somatic SLC35A2 gene variants are associated with 2 main clinical phenotypes, EE and DR-FE, and a histopathologic diagnosis of MOGHE. Additional studies will be needed to delineate any possible correlation between specific genetic variants, mutational load in the epileptogenic tissue, and surgical outcomes.

Figure 1. Histopathology in Patients With Brain Somatic SLC35A2 Gene Variants

(A–C) Normal histology findings at the gray-white matter junction of the frontal lobe in a 3-year-old girl with early-onset EE. (A) OLIG2 immunohistochemistry showing a normal cell density; on the left = deep neocortex; on the right = white matter; bound antibodies were colored in brown with methylene blue counterstaining, which applies to all immunostainings shown in this panel with the exception of C and F. (B) MAP2 immunohistochemistry depicting few scattered heterotopic neurons and their neuropil in the deep white matter (<10 neurons/mm²). (C) Nissl–Luxol fast blue representing compact myelination in dark blue; on the left = deep neocortex; on the right = white matter. Scale bar in A = 100 µm, applies also to B, D–E, and G–I. (D–F) Same stainings as those shown above from the same 3-year-old girl and the same surgical sample but taken from a region with MOGHE. (D) OLIG2 immunohistochemistry with an increase of the oligodendroglial cell density >2,200/mm² at the gray-white matter junction, gray matter on the left as above. (E) MAP2 immunohistochemistry depicting many scattered heterotopic neurons and their neuropil in the deep white matter (>30 neurons/mm²). (F) Nissl–Luxol fast blue representing patchy losses of myelination (pale areas on the right). Scale bar in F = 500 µm applies also to C. (G–H) Another hallmark of MOGHE is increased proliferation of oligodendroglia, as indicated by immunohistochemical labeling of the Ki67 epitope. (G) Note the increased proliferation activity in MOGHE, similar to a dysembryoplastic neuroepithelial tumor (H); focal cortical dysplasia ILAE type 1A has a comparatively low proliferation activity (I). EE = epileptic encephalopathy.

Figure 2. MRI Findings in Patients With Brain Somatic SLC35A2 Gene Variants

(A) Five-year-old girl. Axial FLAIR-weighted 3T MRI showing cortical thickening and blurring of the white matter and gray matter–white matter junction in the right frontal lobe (arrows). (B and C) Five-year-old girl. Axial FLAIR-weighted 3T MRI showing cortical thickening and white matter blurring in the left frontobasal area (B) and blurring of the gray matter–white matter junction in the left frontal anterior area (C) (arrows). (D) Thirty-two-year-old woman. Coronal FLAIR-weighted 3T MRI showing extended white matter blurring involving the right frontopolar area (arrow). (E) Three-year-old boy. Axial FLAIR-weighted 3T MRI showing extended white matter blurring (arrows) and mesial simplified gyral pattern (asterisk) involving the left occipito-temporal area. (F) Two-year-old boy. Axial FLAIR-weighted 3T MRI showing cortical thickening, abnormal folding, and white matter blurring in the right frontoparietal area (arrows). (G) Two-year-old girl. Axial FLAIR-weighted 1.5 T MRI showing increased subcortical white matter signal in the left frontomesial area (asterisks). (H) Seven-month-old girl. Axial FLAIR-weighted 1.5 T MRI showing increased subcortical white matter signal in the left frontomesial area (asterisks). (I) Six-year-old girl. Axial FLAIR-weighted 3T MRI showing extended white matter blurring involving the right orbital area (arrowhead). (J and K) Two-year-old girl. Axial (J) and coronal FLAIR-weighted (K) 3T MRI showing left frontal abnormal folding (see arrow) and left hemispheric white matter blurring (asterisk). (L) Two-year-old boy. Axial T2-weighted 1.5 T MRI showing increased and laminar subcortical white matter signal in the right frontal lobe (arrows). (M) Fifty-eight-year-old woman. Axial FLAIR-weighted 3T MRI showing a small white matter blurring involving the R frontolateral blurring (arrowhead).

Figure 3. Membrane Topology Modeling of SLC35A2

Schematic representation of variants included in this study. Novel variants are written in red; variants already described are written in black. Membrane topology was predicted using the Protter online tool (P78381-SLC35A2_HUMAN).

Figure 4. Brain VAF (%) by Seizure Outcome (Engel Class)

The median of brain VAF in patients with Engel Class I was 14.5% (IQR 6%–26.5%), whereas the median of brain VAF in patients with Engel Class IV was 30.5% (IQR 18.5%–41.8%). VAF = variant allele frequency.