MITOCHONDRIAL DISEASE

by Cynthia TranchemontagneBiochemistry 658; Spring, 2012

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

Mitochondrial Disease

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!

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

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

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 metabolismthe 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)

Tight Regulation through Reversible Phosphorylation

Control Enzymes PDK 4 isoforms

Phosphorylates PDH Activated by acetyl-CoA,

NADH, ATP When activated, inhibits PDH

activity PDP 2 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

Original Research Study:

monitoring phosphorylation of the pyruvate dehydrogenase complex

Developed: 3 site-specific antibodies to the 3 phosphorylation sites on theE1α subunit of PDHPurpose: 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

TreatmentsNo 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) inhibitedFor 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

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.