Development of a rapid functional assay that predicts GLUT1 disease severity

Case for supporting astrocyte energetics in glucose transporter 1 deficiency syndrome

In glucose transporter 1 deficiency syndrome (Glut1DS), glucose transport into brain is reduced due to impaired Glut1 function in endothelial cells at the blood-brain barrier. This can lead to shortages of glucose in brain and is thought to contribute to seizures. Ketogenic diets are the first-line treatment and, among many beneficial effects, provide auxiliary fuel in the form of ketone bodies that are largely metabolized by neurons. However, Glut1 is also the main glucose transporter in astrocytes. Here, we review data indicating that glucose shortage may also impact astrocytes in addition to neurons and discuss the expected negative biochemical consequences of compromised astrocytic glucose transport for neurons. Based on these effects, auxiliary fuels are needed for both cell types and adding medium chain triglycerides (MCTs) to ketogenic diets is a biochemically superior treatment for Glut1DS compared to classical ketogenic diets. MCTs provide medium chain fatty acids (MCFAs), which are largely metabolized by astrocytes and not neurons. MCFAs supply energy and contribute carbons for glutamine and γ-aminobutyric acid synthesis, and decanoic acid can also block α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid glutamate receptors. MCTs do not compete with metabolism of ketone bodies mostly occurring in neurons. Triheptanoin, an anaplerotic but also gluconeogenic uneven MCT, may be another potential addition to ketogenic diets, although maintenance of "ketosis" can be difficult. Gene therapy has also targeted both endothelial cells and astrocytes. Other approaches to increase fuel delivery to the brain currently investigated include exchange of Glut1DS erythrocytes with healthy cells, infusion of lactate, and pharmacological improvement of glucose transport. In conclusion, although it remains difficult to assess impaired astrocytic energy metabolism in vivo, astrocytic energy needs are most likely not met by ketogenic diets in Glut1DS. Thus, we propose prospective studies including monitoring of blood MCFA levels to find optimal doses for add-on MCT to ketogenic diets and assessing of short- and long-term outcomes.

Quantitative determination of SLC2A1 variant functional effects in GLUT1 deficiency syndrome

OBJECTIVE

The goal of this study is to demonstrate the utility of a growth assay to quantify the functional impact of single nucleotide variants (SNVs) in SLC2A1, the gene responsible for Glut1DS.

METHODS

The functional impact of 40 SNVs in SLC2A1 was quantitatively determined in HAP1 cells in which SLC2A1 is required for growth. Donor libraries were introduced into the endogenous SLC2A1 gene in HAP1-Lig4KO cells using CRISPR/Cas9. Cell populations were harvested and sequenced to quantify the effect of variants on growth and generate a functional score. Quantitative functional scores were compared to 3-OMG uptake, SLC2A1 cell surface expression, CADD score, and clinical data, including CSF/blood glucose ratio.

RESULTS

Nonsense variants (N = 3) were reduced in cell culture over time resulting in negative scores (mean score: -1.15 ± 0.17), whereas synonymous variants (N = 10) were not depleted (mean score: 0.25 ± 0.12) (P < 2e-16). Missense variants (N = 27) yielded a range of functional scores including slightly negative scores, supporting a partial function and intermediate phenotype. Several variants with normal results on either cell surface expression (p.N34S and p.W65R) or 3-OMG uptake (p.W65R) had negative functional scores. There is a moderate but significant correlation between our functional scores and CADD scores.

INTERPRETATION

Cell growth is useful to quantitatively determine the functional effects of SLC2A1 variants. Nonsense variants were reliably distinguished from benign variants in this in vitro functional assay. For facilitating early diagnosis and therapeutic intervention, future work is needed to determine the functional effect of every possible variant in SLC2A1.

Re-analysis of whole-exome sequencing data reveals a novel splicing variant in the SLC2A1 in a patient with GLUT1 Deficiency Syndrome 1 accompanied by hemangioma: a case report

BACKGROUND

GLUT1 Deficiency Syndrome 1 (GLUT1DS1) is a neurological disorder caused by either heterozygous or homozygous mutations in the Solute Carrier Family 2, Member 1 (SLC2A1) gene. SLC2A1 encodes Glucose transporter type 1 (GLUT1) protein, which is the primary glucose transporter at the blood-brain barrier. A ketogenic diet (KD) provides an alternative fuel for brain metabolism to treat impaired glucose transport. By reanalyzing exome data, we identified a de novo heterozygous SLC2A1 variant in a girl with epilepsy. After reversed phenotyping with neurometabolic tests, she was diagnosed with GLUT1DS1 and started on a KD. The patient's symptoms responded to the diet. Here, we report a patient with GLUT1DS1 with a novel SLC2A1 mutation. She also has a hemangioma which has not been reported in association with this syndrome before.

CASE PRESENTATION

A 5-year 8-month girl with global developmental delay, spasticity, intellectual disability, dysarthric speech, abnormal eye movements, and hemangioma. The electroencephalography (EEG) result revealed that she had epilepsy. Magnetic resonance imaging (MRI) showed that non-specific white matter abnormalities. Whole Exome Sequencing (WES) was previously performed, but the case remained unsolved. The re-analysis of WES data revealed a heterozygous splicing variant in the SLC2A1 gene. Segregation analysis with parental DNA samples indicated that the variant occurred de novo. Lumbar puncture (LP) confirmed the diagnosis, and the patient started on a KD. Her seizures responded to the KD. She has been seizure-free since shortly after the initiation of the diet. She also had decreased involuntary movements, her speech became more understandable, and her vocabulary increased after the diet.

CONCLUSIONS

We identified a novel de novo variant in the SLC2A1 gene in a patient who previously had a negative WES result. The patient has been diagnosed with GLUT1DS1. The syndrome is a treatable condition, but the differential diagnosis is not an easy process due to showing a wide range of phenotypic spectrum and the overlapping symptoms with other neurological diseases. The diagnosis necessitates a genomic testing approach. Our findings also highlight the importance of re-analysis to undiagnosed cases after initial WES to reveal disease-causing variants.

Genetics of Paroxysmal Dyskinesia: Novel Variants Corroborate the Role of KCNA1 in Paroxysmal Dyskinesia and Highlight the Diverse Phenotypic Spectrum of KCNA1- and SLC2A1-Related Disorders

Paroxysmal dyskinesias (PxD) are rare movement disorders with characteristic episodes of involuntary mixed hyperkinetic movements. Scientific efforts and technical advances in molecular genetics have led to the discovery of a variety of genes associated with PxD; however, clinical and genetic information of rarely affected genes or infrequent variants is often limited. In our case series, we present two individuals with PxD including one with classical paroxysmal kinesigenic dyskinesia, who carry new likely pathogenic de novo variants in KCNA1 (p.Gly396Val and p.Gly396Arg). The gene has only recently been discovered to be causative for familial paroxysmal kinesigenic dyskinesia. We also provide genetic evidence for pathogenicity of two newly identified disease-causing variants in SLC2A1 (p.Met96Thr and p.Leu231Pro) leading to paroxysmal exercise-induced dyskinesia. Since clinical information of carriers of variants in known disease-causing genes is often scarce, we encourage to share clinical data of individuals with rare or novel (likely) pathogenic variants to improve disease understanding.

Usefulness of diagnostic tools in a GLUT1 deficiency syndrome patient with 2 inherited mutations

In some patients with GLUT1 deficiency syndrome (GLUT1-DS), the diagnosis can be difficult to reach. We report a child with 2 inherited mutations suggesting an autosomal recessive transmission of SLC2A1 mutations.

METHODS

The child and her parents were explored with erythrocyte 3-O-methyl-d-Glucose uptake, glucose uptake in oocytes expressing GLUT1 with the gene mutations and measure of the expression of GLUT1 at the surface of the circulating red blood cells by flow cytometry (METAglut1™ test).

RESULTS

Both erythrocyte glucose uptake and glucose uptake in oocyte with the patient's mutations did not support the diagnosis of a mild GLUT1-DS phenotype with autosomal recessive transmission of SLC2A1 mutations. Instead, GLUT-1 expression at the surface of the erythrocytes appeared to better correlate with the clinical phenotypes in this family.

CONCLUSION

The diagnostic value of these functional/expression tools need to be further studied with a focus on mild phenotype of GLUT1-DS.