Adam Kennedy, PhD  |  Director, Scientific Discovery & Application, Metabolon

Biochemical Phenotyping & Inherited Metabolic Disorders

In this final blog post of our 3-part series on clinical metabolomics, I'd like to summarize a recent paper1 by Cappuccio et al. that highlights how metabolomics can be used to identify novel metabolic abnormalities and potential biomarkers associated with an inherited metabolic disorder called GLUT 1 deficiency syndrome (GLUT1-DS). The study is the first to investigate metabolomics perturbation in GLUT1-DS patients, as well as treatment of the disorder with a ketogenic diet (KD), which is currently the only treatment available.

What is GLUT1-DS?

First, let me tell you a little about GLUT1-DS. It is a rare disease caused by a mutation in the SLC2A1 gene that leads to reduced function of glucose transporter type 1 (GLUT 1), whose function is to transport glucose across cell membranes into cells and tissues. GLUT1-DS is a treatable neurological disorder, but it has a broad clinical presentation that can be confused with other diseases.

According to the NIH, patients may present with following signs and symptoms2, which are typical of the classic type:

  • Recurrent seizures (epilepsy) beginning in the first months of life
  • Microcephaly (unusually small head size) that develops after birth
  • Developmental delay
  • Intellectual disability
  • Speech and language impairment
  • Movement abnormalities (i.e. involuntary eye movements, spasticity, ataxia, dystonia)
  • Behavioral problems

Other symptoms may include headaches, confusion, loss of energy and/or muscle twitches.

While there is no cure for GLUT1-DS, it can be treated with KD, a high fat, moderate protein and low carbohydrate diet that causes the body to utilize ketones for fuel in the absence of optimal levels of glucose. The energy they provide can help lessen some of the symptoms of the disorder and supply better nourishment for the developing brain.3

Screening for GLUT1-DS

Meta IMD baby.jpgWhile babies are routinely screened for several inherited metabolic disorders at birth, GLUT1-DS is not typically included. Symptoms usually develop within the first few months of life. The disorder is diagnosed by measurement of glucose in cerebrospinal fluid (CSF), with the biomarker for disease being a low level of glucose in CSF and a reduced ratio of glucose in CSF compared to blood. Early diagnosis is essential for effective therapy.

Treatment with a Ketogenic Diet and Carnitine for GLUT1-DS patients

Cappuccio et al. set out to understand both the biochemical perturbations in the disease pathogenesis and in treatment with KD. In patients on KD, they observed metabolomic changes that implied that a compensatory neurochemical mechanism was induced by ketone bodies to recover energy homeostasis caused by reduced glucose levels.

In patients not treated with KD, decreased levels of glucose, fructose and mannose, and indeed a decrease in most TCA cycle intermediates, were observed suggesting an impact on mitochondrial physiology. In addition, gross alterations in the lipid super-pathway of metabolism were observed for those on the KD as were changes in the metabolism of carnitine, with increased levels of acylcarnitines linked to fatty acid metabolism and reduced acylcarnitines linked to amino acid metabolism.

Carnitine modulates transport of long-chain fatty acids into mitochondria and regulates energy balance across cell membranes. Free carnitine levels were low in patients on the KD, and the overall metabolomic data was consistent with consumption of carnitine and the metabolism of fatty acids by β oxidation.

These findings resulted in the authors proposing that oral supplementation of carnitine is warranted for GLUT1-DS patients on a KD. This is in line with the idea that depletion of carnitine could impair oxidation of fatty acids and ketone production, thus reducing treatment efficacy.

Biomarkers Identified in Plasma

The authors performed metabolomic analysis of CSF and plasma on nine GLUT1-DS patients before and after KD. They identified multiple dysregulated metabolites and metabolic pathways. The researchers also identified consistent changes in the levels of 3-hydroxybutyrate, 3-hydroxybutyrylcarnitine, 3-methyladipate and N-acetylglycine as potential biomarkers of GLUT1-DS on KD. While KD is the only current treatment option, it is possible that a personalized, optimized diet may be achievable for a patient by monitoring of such plasma biomarkers.

General Takeaways

Metabolomics may provide new understanding of the mechanisms regulating GLUT1 function and may open new avenues of treatment with the collection of additional data from subjects diagnosed with GLUT1-DS or other glucose transporter deficiencies.

In this study, metabolomics revealed multiple dysregulated metabolites in several pathways in CSF, plasma and urine of patients diagnosed with GLUT1-DS. Based on this insight, the authors arrived at a suggestion for improved treatment of GLUT1-DS patients and proposed a biomarker panel to better identify the effect of the ketogenic diet.

This work is one of a growing number of studies where a global metabolomics approach, able to identify up to a thousand metabolites in plasma, could (and should) contribute to a greater knowledge of inherited metabolic disorder pathogenesis, shorten the path to diagnosis, direct therapeutic decisions, and possibly deliver new treatments.

For more information about inherited metabolic diseases and the role of metabolomics in clinical screening, see Miller et al.4 and Kennedy et al.5 or visit our Clinical Applications page.


References
1. G. Cappuccio et al. (2017) Biochemical phenotyping unravels novel metabolic abnormalities and potential biomarkers associated with treatment of GLUT 1 deficiency with ketogenic diet. PloS One 12(9): e0184022
2. https://rarediseases.info.nih.gov/diseases/9265/glut1-deficiency-syndrome
3. http://www.g1dfoundation.org/what-is-glut1-deficiency/about-the-ketogenic-diet/
4. M. Miller et al. (2015) Untargeted metabolomic analysis for the clinical screening of inborn errors of metabolism J Inherit Metabolol Dis 38(6): 1029-1039
5. AD Kennedy et al. (2016) Metabolic-profiling of human urine as a screen for multiple inborn errors of metabolism. Genet Test Mol Biomarkers 20: 485-495

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