Mitochondrial Disease

1. Mitochondrial Disease by Cynthia Tranchemontagne Biochemistry 658; Spring, 2012

2. Mitochondria • Aerobic Respiration generate ATP: the form of energy needed to carry out cellular functions and drive anabolism • Numerous Complex Enzymes crucial to the citric acid cycle and electron transport chain in the aerobic pathways • Contains DNA- mtDNA • takes about 3000 genes to code for a mitochondrion • only 37 genes are located in the mitochondria • the rest reside in nuclear DNA- nDNA

3. MitochondrialDisease Mitochondrial dysfunction • Caused by mutations in mtDNA or the nDNA that code for the mitochondrion - inherited -spontaneous • Connection to age- it is thought that deteriorating mitochondrial function is responsible for many of the aspect s of aging ` -unclear as to which causes which Mito What? • Mystery and lack of awareness • Oversimplification of the function of mitochondria • Catabolic processes intimately linked with anabolic processes • of the 3000 genes that code for a mitochondrion, only about 3% are involved in making ATP!

4. Mitochondrial Disease Hundreds of different Mitochondrial Diseases • Cell differentiation according to function and tissue type- -Same differentiation in mitochondria, giving them specialized processes specific to the tissue type • Broad range of tissue types/organ systems affected by disease • Multitude of symptoms difficult to diagnose and differentiate as mitochondrial dysfunction • Type/location of mutation may affect mitochondrial function in certain tissues, but not in others • Multiple mutations are often involved • Genocopies / Phenocopies

5. Leigh’s Syndrome • Neurometabolic disorder • Develops in infancy or childhood • Onset frequently follows a viral infection • Cause: Pyruvate Dehydrogenase Deficiency • Also caused by Respiratory chain enzyme defects: • Complexes I, II, IV, and V • Original Research Study- • Development of a tool to monitor Pyruvate Dehydrogenase (PDH) activity • Tested on tissue from a patient with Leigh’s Syndrome

6. Pyruvate Dehydrogenase Complex • One of the largest multienzyme complexes in eukaryotic cells • Mitochondrial matrix • Conversion of pyruvate to acetyl-CoA • necessary for entrance to the citric acid cycle for aerobic respiration • Key control point in cellular metabolism • the one control point for the citric acid cycle that exists OUTSIDE the cycle • 5 enzyme complex, compact arrangement: • -3 involved in the conversion: • PDH, dihydrolipoyl transacetylase,dihydrolipoyl dehydrogenase • -2 involved in control of PDH: • pyruvate dehydrogenase kinase (PDK) • pyruvate dehydrogenase phosphatase (PDP)

7. Tight Regulation • through Reversible • Phosphorylation • Control Enzymes • PDK4 isoforms • Phosphorylates PDH • Activated by acetyl-CoA, • NADH, ATP • When activated, inhibits PDH activity • PDP2 isoforms • Hydrolysis of ester linkage: • Dephosphorylation • Activated by pyruvate, ADP • Leads to reactivation of PDH • This regulatory mechanism is currently implicated in varied patterns of metabolic activity in other • conditions, as well, such as: • Pyruvate Dehydrogenase Deficiency Cancer Obesity Insulin Resistance

8. Original Research Study:monitoring phosphorylation of the pyruvate dehydrogenase complex Developed: 3 site-specific antibodies to the 3 phosphorylation sites on the E1α subunit of PDH Purpose: monitor regulatory phosphorylation mechanism for the purpose of understanding its expression and deficiency characteristic to various disorders • Mitochondria isolated from rat kidney tissue • Confirmed that the phosphorylation of PDH is site specific • Different isoforms of PDK & PDP exhibit specificity • Found that different tissue types exhibit varying levels of different isoforms • Demonstrated varied levels of expression of one specific phosphorylation site across different tissue types • Demonstrated use of the site-specific antibodies: • Demonstrated pharmacological inhibition of PDKs- shows promise for the development of small molecule treatments for the regulation of PDH activity • Demonstrated the use of antibodies to monitor and assess this control mechanism in a patient with Leigh’s Syndrome- low levels of phosphorylation were observed

9. Treatments • No cure for Leigh’s Syndrome and other Mitochondrial Diseases • Treatments are geared towards slowing progression of disease and managing symptoms • Avoidance of Physiological Stress- • no strenuous exercise, no large meals • Dietary management and adjustments are key- depends on the pathway(s) that is(are) inhibited • For ex) if lipid metabolism not effected at all- then a high fat, low • carbohydrate diet would utilize metabolic pathways that are functioning • Vitamin / Mineral Supplements • Therapies- PT, Speech Therapy, Respiratory Therapy

10. References Campbell, M. K., & Farrell, S. O. (2012). Biochemistry (7th ed.). Belmont, CA: Brooks/Cole. Rardin, M. J., Wiley, S. E., Naviaux, R. K., Murphy A. N., & Dixon, J. E. (June 15, 2009). Monitoring phosphorylation of the pyruvate dehydrogenase complex. Analytical Biochemistry. Vol. 389 (Issue 2). Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2713743/?tool=pmcentrez. The Mitochondrial Medicine Society. http://www.mitosoc.org. Accessed on: April 20, 2012. United Mitochondrial Disease Foundation. http://www.umdf.org. Accessed on: April 19, 2012.