The discovery of brain-only mutations is rewriting our understanding of intractable seizures.
Imagine a mysterious form of epilepsy that robs patients of seizure control despite medication, yet leaves doctors baffled—brain scans show no obvious structural cause. For years, these "non-lesional" cases represented a frustrating medical mystery. Then, in 2018, researchers made a breakthrough discovery: somatic mutations in the SLC35A2 gene were lurking in the brain tissue of these patients, absent from their blood samples 1 2 .
This finding not only explained a substantial fraction of previously unexplained epilepsy cases but also revealed an entirely new mechanism of epileptogenesis rooted in impaired glycosylation 1 2 . The investigation into SLC35A2 has since blossomed into a fascinating field, bridging genetics, neurology, and biochemistry to solve one of epilepsy's most perplexing puzzles.
To understand the significance of SLC35A2 discoveries, we must first grasp a fundamental genetic concept: the difference between germline and somatic mutations.
Present from conception, inherited in the egg or sperm, and exist in virtually every cell of the body. These are the mutations typically detected through blood tests.
Occur after conception, during embryonic development or throughout life. They affect only a subset of cells, creating a genetic mosaic within the body .
"When we're talking about epilepsy, specifically, the type of somatic variants we are most interested in are those in neuroprogenitor cells, which interestingly have a much higher mutation rate than embryonic progenitor cells," explains Dr. Christian Bosselmann, an adult neurologist and epileptologist at University Hospital Tübingen .
Affect larger brain areas, potentially causing hemimegalencephaly
Create more focal disruptions like focal cortical dysplasia
In 2018, two independent research teams published landmark studies that would change how we understand non-lesional epilepsy.
Prior to these discoveries, somatic mutations were already known to cause certain forms of lesional epilepsy, particularly those involving visible malformations of cortical development. However, researchers hypothesized that similar mutations might underlie a wider range of focal epilepsy, including cases without radiographic abnormalities 1 .
A multi-center team led by Dr. Melodie Winawer conducted high-depth exome sequencing and ultra-high-depth candidate gene sequencing of DNA from epilepsy surgery specimens and leukocytes from 18 individuals with non-lesional focal epilepsy (NLFE) and 38 with focal malformations of cortical development (MCD) 1 .
The results were striking—somatic mutations in SLC35A2 appeared in 3 of 18 individuals (17%) with NLFE and 2 additional cases in the MCD cohort 1 2 . All mutations were found only in brain tissue, not blood samples, confirming their somatic nature.
The variant allele frequencies (VAFs)—the proportion of cells carrying the mutation—ranged from 2% to 14% in NLFE cases and 19% to 53% in MCD cases 1 . This spectrum of VAFs demonstrated how the same gene could cause different clinical presentations based on mutation burden.
| Patient Group | Cases with SLC35A2 Mutations | Variant Allele Frequency Range | Pathological Findings |
|---|---|---|---|
| Non-lesional Focal Epilepsy (NLFE) | 3/18 (17%) | 2-14% | Focal cortical dysplasia type Ia in 2 cases |
| Focal Cortical Malformations (MCD) | 2/38 | 19-53% | No classical FCD features observed |
One of the most comprehensive investigations into SLC35A2-related epilepsy combined multiple advanced techniques to establish both genetic cause and biochemical consequence 8 .
Conducted on matched brain and blood samples from 13 NLFE patients at mean depth >800x to identify potential somatic mutations.
Performed on an additional 18 patients focusing specifically on SLC35A2 with extreme depth (~1,230x) to detect low-frequency mutations.
Confirmed candidate mutations using ultra-deep sequencing (>100,000x) on a different platform to rule out technical artifacts.
Applied tissue glyco-capture and nano liquid chromatography/mass spectrometry to examine N-glycosylation patterns in affected brain tissues.
The study identified several nonsense and splice-site mutations in SLC35A2, all absent from peripheral tissues 8 . The mutations were primarily loss-of-function variants expected to impair protein function.
Most importantly, the glycome analysis revealed aberrant N-glycan structures in brain tissues with SLC35A2 mutations, including elevated N-acetylglucosamine residues 8 . This provided direct evidence that the genetic mutations translated to functional consequences in glycosylation.
| Mutation Type | Functional Consequence | Representative Changes |
|---|---|---|
| Nonsense | Premature stop codon | p.Gln197*, p.Glu254* |
| Splice-site | Disrupted RNA splicing | chrX:48763821C>A |
| Missense | Amino acid substitution | p.Ser304Pro |
| Frameshift | Disrupted reading frame | Various indels |
SLC35A2 encodes a UDP-galactose transporter (UGT) protein that plays a critical role in glycosylation—the process of attaching sugar molecules to proteins and lipids 5 7 . This transporter resides in the Golgi apparatus membrane, where it shuttles UDP-galactose from the cytosol into the Golgi lumen 7 .
SLC35A2 mutation disrupts UDP-galactose transport
Incomplete glycoprotein formation in Golgi
Altered receptors lead to seizure activity
Once inside the Golgi, galactose serves as a essential building block for constructing complex glycans that modify proteins and lipids. These glycans influence everything from cell-cell communication to neuronal excitability 5 .
When SLC35A2 malfunctions due to somatic mutations, the galactosylation process is disrupted, leading to:
The growing interest in SLC35A2 has spurred the development of specialized research tools that enable deeper investigation into disease mechanisms.
| Research Tool | Function/Application | Key Features |
|---|---|---|
| CRISPR-edited hiPSCs | Disease modeling | SLC35A2-S304P (missense) and SLC35A2-KO (knockout) lines |
| Multi-electrode array (MEA) | Network activity assessment | Measures synchronous firing in neuronal cultures |
| MAL-I lectin binding assay | Glycosylation status evaluation | Detects terminal sialic acid residues on glycoproteins |
| UDP-galactose transport assay | Direct transporter function | Measures radiolabeled UDP-galactose uptake in Golgi vesicles |
| Custom targeted sequencing panels | Somatic variant detection | Ultra-deep coverage (>1000x) for low VAF mutation identification |
These tools have enabled researchers to build sophisticated disease models. For instance, cortical neurons derived from SLC35A2-mutant hiPSCs develop synchronous activity earlier than controls, mimicking the hyperexcitability seen in epilepsy patients 4 .
The discovery of SLC35A2's role in epilepsy has transformed clinical practice in several important ways:
SLC35A2 is now recognized as one of the "must-know genes" in somatic epilepsy genetics . Testing for SLC35A2 mutations has become routine in the evaluation of certain epileptogenic lesions, particularly MOGHE (mild malformations of cortical development with oligodendroglial hyperplasia and epilepsy) 3 .
While treatments targeting SLC35A2 dysfunction are still in development, the identification of specific mutations opens doors to precision medicine approaches. Research using hiPSC models is exploring whether galactose supplementation might rescue the glycosylation defects caused by impaired UDP-galactose transport 4 .
For patients undergoing epilepsy surgery, the detection of SLC35A2 mutations in resected brain tissue provides biological validation of the epileptogenic zone and may predict surgical outcomes 8 .
The journey to unravel SLC35A2's role in epilepsy exemplifies how modern neuroscience bridges multiple disciplines—from genetics and molecular biology to neurology and pathology. What began as a mystery of unexplained seizures has evolved into a sophisticated understanding of how somatic mutations disrupt glycosylation to cause neuronal hyperexcitability.
As research continues, the growing toolbox for investigating SLC35A2—from hiPSC models to targeted sequencing—promises to further illuminate this fascinating mechanism. The story of SLC35A2 reminds us that some of medicine's most perplexing mysteries may hide not in our inherited genes, but in the genetic mosaics within our brains.
As Dr. Bosselmann aptly notes, in the world of somatic genetics, "timing is everything" —a principle that continues to guide researchers as they decode the complex relationship between brain development, genetic mutation, and neurological disease.