History and Metabolic Context

GSD-I Definition:

Glycogen Storage Disease Type I (GSD-I) or Von Gierke’s disease the most common form of glycogen storage disease (GSD) in the world with a prevalence rate of 1:1000,000 individuals with a higher prevalence in Ashkenazi Jews (1:20,000) (Eckstein, 2004). GSD-I is an autosomal recessive disorder of glucose metabolism where the primary defect is in the impaired conversion of glucose-6-phosphate (G6P) to glucose and phosphate during gluconeogenesis. This impairment of glucose production results in a decrease in blood glucose concentrations during fasting conditions. As a result of the increase in G6P, there is increased flux down the glycolytic pathway which generates lactate by anaerobic respiration, increased flux via acetyl CoA which generates lipids, and increased flux via the pentose phosphate shunt to generate 5-carbon glucose molecules. The build-up of 5-carbon glucose molecules eventually results in the formation of nucleic acids, which when catabolized, generate uric acid. It is also observed that GSD-I can result in the excessive buildup of glycogen in body tissues such as the kidneys and liver and can be visible under a microscope. This build-up of glycogen will result in an increased risk of enlarged liver, liver/kidney failure, and liver tumors (benign and malignant).

Figure 1: An example of glycogen accumulation in the liver. Excessively glycogen accumulation will result in enlarged livers and liver tumors. Source: Google Images
Figure 1: An example of glycogen accumulation in the liver. Excessively glycogen accumulation will result in enlarged livers and liver tumors.
Source: Google Images

GSD-I Symptoms:

Early onset of GSD-I include:

  • Hypoglycemia in newborns
  • Increased hunger
  • Stunted growth
  • Hyperuricemia
  • Hyperglyceridemia
  • Fatigue
  • Irritability

Advanced onset of GSD-I include:

  • Increased susceptibility to infections (GSD-Ib only)
  • Enlarged kidenys
  • Gout
  • Kidney failure
  • Seizures and confusion
  • Kidney tumors (benign and malignant)
  • Brain damage
  • Death

GSD-I History

The initial discovery of GSD-I was by Dr. von Gierke in 1929 who was the first to characterize the disease after performing an autopsy of 2 children suffering from chronic nosebleeds prior to their death. Dr. von Gierke discovered that both the children had accumulated excessive glycogen in both their kidneys and their enlarged livers. Then, in 1950’s a similar case was observed with 6 patients by Dr. Cori and Dr. Cori. After performing a through autopsy, Dr. Cori and Dr. Cori discovered that 2 of the children were complete deficient in G6Pase while the other 4 were not (Cori and Cori, 1952). Curious about these cases, Dr. Cori and Dr. Cori homogenized deceased patients’ frozen liver samples, filtered the homogenate through gauzes, and incubated various dilutions of the flow through in G6P to test substrate to enzyme proportionality. These results were then compared to ordinary liver samples with normal G6Pase and Dr. Cori and Dr. Cori successfully demonstrated that hepatic G6Pase was the defective protein resulting in these conditions and linked it back to Dr. von Gierke’s 1929 observations (Cori and Cori, 1952). Unbeknowst to Dr. Cori and Dr. Cori, they had actually discovered GSD-Ia which is a deficient G6Pase protein resulting from a G6PC mutation. Although G6Pase was determined to be the affected enzyme in GSD-I, the mechanism was still unknown despite knowledge that a hydroxylation was performed. It was then in 1960 when Hass and Bryne observed that G6Pase catalyzed an exchange reaction between G6P and glucose that it was confirmed a phosphoenzyme intermediate was being generated (Hass and Bryne, 1960). This observation by Hass and Bryne was consistent with phosphotransferase activity.  Building off of this development, Feldman et al. in 1972 was able to demonstrate through the analysis of isotope exchange reactions and labeling patterns of a radioactive phosphate incubated with G6Pase that a phosphohistidine bond was being generated (Feldman et al., 1972). Although Dr. Cori and Dr. Cori successfully identified the protein responsible for GSD, they both could not figure out why the other 4 children also had the same condition, but demonstrated normal G6Pase function. This observation remained a mystery until 1978 when Narisawa et al.  discovered that a deficiency to the G6Pase translocase protein which transport G6P into the cell resulted in the same conditions observed by Dr. von Gierke, Dr. Cori, and Dr. Cori (Narisawa et al., 1978). The authors performed a G6Pase assay with deceased patients’ liver samples both in the presence of detergent and without detergents. The purpose of the detergents was to destroy the cell membrane and that would tell the authors G6Pase function was consistent both inside and outside the cell. When the assay determined that G6Pase activity was higher with the detergent than without the detergent, that suggests that the complication does not lie with the G6Pase, but with a transport protein. The discovery by Narisawa et al. highlighted the GSD-Ib aspect of GSD-I where a G6Pase translocase defect results in GSD-I symptoms (Narisawa et al., 1978).

GSD-I Diagnosis

Fortunately with the advances in modern medicine, it is very easy to diagnose GSD-I. Since GSD-I can result in hypoglycemia, hyperuricemia, and hyperglyceridemia, lactic acidosis, testing a pateint’s blood serum for abnormal blood glucose (<60mg/dL), lactate (>2.5 mmol,L), uric acid (>5.0 mg/dL), and glyceride (>250 mg/dL) concentrations is a reliable method to determine if a patient has GSD-I (Fernandes et al., 1972). A GSD-I diagnosis is further strengthened by a glucagon/epinephrine test (adminstrating glucagon or epinephrine does not increase blood glucose levels, but increases blood lactate levels) and liver histology (surgical dissection and analysis of liver tissue for excessive buildup of glycogen). For definitive tests which confirm GSD-I, then a G6Pase and/or G6Pase translocase activity assay and genetic test will confirm the results. For a G6Pase/G6Pase translocase activity assay, the enzyme’s activity level is tested and compared to standard enzyme activity levels. In GSD-I patients, the G6Pase activity is 10% of the standard enzyme and G6Pase translocase activity is 15% of the standard enzyme.  The genetic test is the most definitive and accurate diagnostic tool available to determine GSD-I because it will allow doctors and scientists to scan a patient’s G6PC and SLC37A4 regions of the DNA for mutations. If there is 1 of the known 80 mutation at either of these two locations, then the patient can accurately be diagnosis with GSD-I.

Figure 2: The gluconeogenesis and glycolysis pathway with enzyme and substrate names. Source: Google Image
Figure 2: The gluconeogenesis and glycolysis pathway with enzyme and substrate names. Source: Google Image

Full Context of GSD-I Disease-Free System

GSD-I is a disease resulting from a deficiency of G6Pase (either from the enzyme itself or the translocase protein). In a disease free system, G6Pase function would be normal and enzymatically activity at standard levels in the liver and kidneys. During fasting periods, glycogen would be broken down and converted via gluconeogenesis into G6P. The G6Pase translocase would transport G6P to the membrane bound G6Pase where G6P is converted into glucose via a hydrolysis reaction and removing a phosphate (Chou et al., 2002). The mechanism of G6Pase begins with a nucleophilic attack on the phosphate by His176 which results in a phosphohistidine bond and the destablization of the carbonyl (Chou et al., 2002). The negatively charged oxygen transfers its electrons in order to restore the carbonyl and eliminating the negatively charged glucose (Chou et al., 2002). The negative charged glucose is then protonated by His119 and generating glucose (Chou et al., 2002). The newly produced glucose is then exported out of the cell via glucose transporter proteins. This conversion of G6P into glucose is a normal process of gluconeogenesis and occurs during fasting periods in order to elevate blood glucose levels and maintain homeostasis. Due to maintained levels of blood glucose, glucose travels to vital areas such as the brain and tissues resulting improved neurological function and growth. Due to the fact that there is no buildup of G6P, excessive amounts of G6P are not being converted into excessive amounts of lactate, uric acid, and glycerides (Chou et al., 2002).

Figure 3: Proposed Mechanism of G6Pase with G6P and active site residues. Created using ChemDraw
Figure 3: Proposed Mechanism of G6Pase with G6P and active site residues.
Created using ChemDraw

6 Replies to “History and Metabolic Context”

  1. Well done Tyler! I appreciated your thorough description of the disease free state. It was well written and made the next section much easier to understand. One change I would make would to make the final figure on the page much larger so it can be viewed within the context of the page as it does a great job illustrating the mechanism.

    You mentioned that there is an increased incidence of infection in patients with GSD-1b only and not in GSD-1a, what is the mechanism behind this?

    1. Hello Elaine!
      Thank you for reading about my disease. I am glad to hear that you found it understandable and you enjoyed learning about my project. I have take your suggestion regarding my figure and I have decided to make it larger. Thank you for the suggestion.
      That is a very interesting question. The reason why only GSD-Ib patients are at increased risk for infection is because GSD-Ib also affects neutrophil function. The disruption of G6PT results in the cellular uptake of G6P to be reduced substantially compared to non GSD-Ib patients. This lack of G6P influx causes a decrease in glucose production from G6Pase. Neutrophils require a constant supply of glucose for survival and in a fasting state, glucose production requires gluconeogenesis. Research has also implicated increased generation of ROS species, ER stress, and cellular misfolding as attributing to this observed phenomenon. Current research is currently underway into investigating the moelcular mechanism involved with neutrophils which results into neutrophil dysfunction. If you would like to learn more about this, I would like to direct you to this link (doi: 10.1097/MOH.0b013e328331df85) which is a 2011 article by Dr. Janice Chou who is a prominent researcher into this area that addresses neutrophil dysfunction caused in GSD-Ib patients.
      Thank you again for reading about my disease and for commenting!
      Tyler Florio

  2. Great job Tyler! I felt as though your explanations on the different aspects of GSD-1 were extremely thorough and easy to understand! I’m curious about the difference between the early onset and late onset of the disease. Late onset symptoms tend be much more severe, and I was wondering why this is the case. Is it because early onset is obviously diagnosed earlier, and therefore treatments help to prevent the more severe symptoms? If not, what is the reason for this difference?

    I’m also curious about preventative measures. If one is likely to have GSD-1 due to genetics, are preventative measures besides diet (such as allopurinol) help lessen the severity of symptoms before onset of the disease?

    1. Hello Nicole! Thank you for taking the time to read about my disease and for commenting. I appreciate the feedback and I am glad to hear that you were able to understand the two different mutations which result in GSD-I. Typically, GSD-I diagnoses occurs in infancy because blood glucose tests and failure to reach certain growth benchmarks alert physicians to a problem which then leads them to an eventual diagnosis. With that diagnosis, certain steps and treatments (the cornstarch regimen, allopurinol prescription, vitamin D and calcium regimen, avoidance of sugars, daily small, frequent meals) are then implemented to reduce the symptoms. Only the symptoms can be treated since the disease itself is still incurable. The advance onset of the disease is typically observed in patients who do not receive medical attention or who fail to adhere to the prescribe treatment. The unpalatability of the cornstarch regimen has cause many GSD-I patients to leave the treatment regimen which causes the advanced symptoms demonstrated above. With strict adherence to the established treatment regimen, virtually all of the advanced symptoms can be avoided and the early symptoms mitigated. As for your question regarding preventive measures, there is genetic testing which can be performed to determine if the child will have GSD-I. A GSD-I patient may be prescribed allopurinol to reduce uric acid accumulation and may take increased amount of vitamin D and calcium in order to increase bone mineralization and prevent stunted growth. Aside from those steps, diet is in the main way to treat GSD-I because it revolves around the impairment of the gluconeogenesis pathway and the patient’s inability to form glucose. Current research is tackling RAAV-mediated gene therapy as a potential cure for GSD-I although tests are still in its infancy and there still are some complications with RAAV-human compatibility. Thank you for taking the time to read about my disease and for asking a pair of very important questions.
      Tyler Florio

  3. Really well organized pages and explanations were clear. I found your proposal for future work in gene therapy for GSD-I to be particularly interesting, I was wondering if any attempts have been made to correct a mutant G6PC or SLC37A4 gene using CRISPR/Cas9 system, or is RAAV the only method performed so far? CRISPR doesn’t use a virus and so isn’t affected by the RAAV transduction issue.

    1. Hello Elliott! Thank you for reading about my disease and for the feedback regarding my “Conclusion and Proposals for Future Work” page. I find your question to be very intriguing and I had not located any articles initially regarding CRISPR/cas9 system. After reading your question, I became curious and looked again. After looking through various research databases, I still did not find anything for GSD-I which is unfortunate because I also believe that should be an area of focus as well. RAAV-mediated gene therapy has had its complications transferring into humans, but has also demonstrated success in mouse models in Sun et al. 2002 article. With advances in CRISPR/cas9 system, this also could be a promising area of research. Thank you for taking the time to read about my disease and for a question that really opened my eyes to another possible avenue for gene therapy research.

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