Evidence for Mitochondrial Dysfunction in Down Syndrome

George T. Capone, M.D.
Research Scientist, Kennedy Krieger Institute Assistant Professor of Pediatrics at the Johns Hopkins University School of Medicine - Baltimore, MD
Trisomy 21 Research Conference
September 13-14, 2003. New Orleans, LA

Reprinted with the permission of the author.

What are Mitochondria?

Cellular organelle
Energy production center
Aerobic respiration (OXPHOS) results in ATP synthesis (stored energy)
ATP high-energy molecule currency of all biochemical reactions

Mitochondrial DNA

Human mtDNA 16,569-nucleotides, circular
Codes 13 genes (20%), essential for OXPHOS (remaining 80% coded by nDNA)
Independent machinery for gene expression
Unique evolutionary heritage (prokaryotic)
Symbiotic relationship with cell

Mitochondrial Genetics

Maternally inherited

Ovum contains > 100,000 mtDNAs

Sperm contains < 500 mtDNAs

Mitochondria Energy Production

Mitochondrial ROS Production

Reactive Oxygen

Reactive Oxygen and Aging

Free-radical theory (Harman 1956)
observed effects of ionizing radiation and cell damage
Free radicals produced during OXPHOS cause cumulative oxidative damage which results in cellular aging & death

Living to Die

Rate of living hypothesis (Pearl 1928)
observed that species with high metabolic rate also have a shorter maximum life-span...they age faster
Rate of energy consumption (per se) is responsible for cellular aging

Down Syndrome and Aging

Longevity 50s-60s - common
Age-related health conditions — precocious
Autoimmunity, immune dysfunction*, SN hearing loss, osteoporosis, skin & hair, periodontal disease, cataracts*, Alzheimer's pathology*(amyloidosis, neuronal impairment, seizures, dementia)

Biomarkers for Oxidative Damage

Lipids- unsaturated FA attack by OH radicals
Malondialdehyde (MDA)
Thiobarbituric Acid Reactive Substance (TBARS)
4-Hydroxynonenal (HNE)
F2-Isoprostanes

Biomarkers for Oxidative Damage

: DNA- oxidation and modification of nucleotide bases resulting in point mutation; 8-Hydroxy-2-deoxyguanosine (8-OHdG); Excision-repair cross complementing proteins (ERCC)

Evidence for Oxidative Damage in Living Persons with Down Syndrome

Lipids
MDA (plasma) lower (Gromadzinska 1988)
TBARS (urine) increased (Jovanovic 1998)
F2-Isoprostanes (urine) increased (Pratico 2000)
MDA (serum) increased (Muchova 2001)
DNA
8-OHdG (urine) increased (Jovanovic 1998)

Evidence for Oxidative Damage in Down Syndrome Brain

Fetal cerebral cortex
TBARS up 36% (Brooksbank 1984)
TBARS up 100%, HNE up 32% (Odetti 1998)
Adult (> 45 yr) cerebral cortex
MDA no change (Hayn 1996)
Fetal cerebrocortical neurons (tissue-culture)
CPA flourescence up 400% (Busciglio 1995)

Sources of ROS in Down Syndrome

Mitochondria (OXPHOS)
SOD-1 catalyzed H₂0₂ .... + Fe⁺² .... ·OH
Brain specific
APP processing to ABeta peptide 42
Activated microglia (inflammation)

Kennedy Krieger Study

GROUP	N	Age (mo)	Gender	IQ
Down syndrome	46	48.1 (24.5)	M=26 F=20	52 (13.4)
Cognitively impaired	30	66.7* (17.4)	M=20 F=10	50 (19/7)
Controls	43	49.3 (25.6)	M=22 F=21	103** (12/7)

Research Protocol

All subjects 1-8 years old
DS group — trisomy or translocation
CI group — unknown etiology
Psychological testing
Blood draw 5-10 ml

Mitochondrial Superoxide Detection

Superoxide Production Group Comparison

Biomarkers of Superoxide & Lipid Peroxidation

	LDCL peak	Lipid peroxides	TBARS	IQ
Down syndrome	0.74* (0.52)	5.19 (9.3)	0.20 (0.13)	52.1 (13.5)
Cognitively impaired	0.54 (0.48)	7.28 (11.34)	0.18 (0.15)	49.6 (19.7)
Controls	0.47 (0.41)	3.7 (4.83)	0.16 (0.10)	103.4* (12.4)

Mitochondrial Dysfunction in Down Syndrome?

Impaired cAMP production (Tam 1980)
Lower resting metabolic rate (Luke 1994)
Lower rCMRglucose (Azari 1994)
Decreased repair mtDNA following ROS induced breakage (Druzhyna 1998)
Decreased Complex I subunits (ETS) (Kim 2001)
Increased citric acid-cycle (mitochondrial) enzymes (Bago 2002)

The Vulnerable Brain

High metabolic demands > 20% O₂ consumption
High unsaturated fats
Large iron stores
Low antioxidant capacity
Non-dividing neuronal population

Lipid Peroxidation of Neuronal Membranes

Implications for Cognitive & Mental Health

Impaired chemical signaling between neurons (receptor mediated) and within neurons (intracellular signaling)
Impaired information processing: learning, memory, emotions, sensations?

Possible Therapeutics for Mitochodrial Energy Dysfunction

Coenzyme Q10 (antioxidant & ETS)
Lipoic acid (antioxidant & possible Mt co-factor)
Carnitine (mitochondrial FA transport)
Nicotinamide (supplies e- for Complex I)

Future Directions

Further delineation of mitochondrial impairment
Chrs 21 localized genes: ATP₅₀ subunit
Biochemical correlation: cell culture, animal models & humans
Impact: on CNS development, aging, cognition, mental health
Clinical trials