A Novel Mitochondrial Metabolic Myopathy (mtMM) Caused by Variant

Mitochondrial DNAMichael M. Rothkopf, MD, FACN, Lisa Haverstick, RD, Eleni Pellazgu, MSN, APN

Metabolic Medicine Centerand

Darius Adams, MD, Personalized Genomic MedicineGenetics and Metabolism, Goryeb Children's Hospital

Atlantic Health System, Morristown, NJNational Board of Physician Nutrition Specialists

Case Report: AR – 26 year old male

• Referred from PCP for episodic rhabdomyolysis with persistent creatine kinase (CK) elevation

• Has experienced collapse with severe exertion• On no meds or supplements• Does not drink or use recreation drugs• Otherwise fit and asymptomatic• Physical exam completely normal

Laboratory and Imaging Studies

• Mild hepatic dysfunction (AST = 72/ALT = 71)• Mild renal dysfunction (Creat = 1.3; eGFR = 76)• CK = 1712 U/L (nl 24-204), 100% CK-MM• Serum myoglobin = 277 ng/ml (nl 28-72)• HbA1C = 5.8% (nl <5.7)• Carnitine and micronutrient levels normal • MRI of both thighs – no myositis or atrophy.

Metabolic Myopathies (MM)• A group of hereditary muscle disorders caused by specific

enzymatic defects. • Most considered primary inborn errors of metabolism (IEOM)

and are associated with disturbances of intracellular energy metabolism.

• MM diseases are grouped into abnormalities of glycogen, lipid, purine, or mitochondrial biochemistry.

• Also grouped by symptomatology: dynamic vs static• Metabolic myopathies are rare but potentially treatable

disorders. They are sometimes misdiagnosed as muscular dystrophies or inflammatory myopathies.

Rhabdomyolysis-associated MM• Disorders of Carbohydrate Metabolism

– Myophosphorylase deficiency (GSD V;McArdle’s)– Phorphorylase kinase deficiency (GSD IX)– Phosphofructokinase deficiency (GSD VII: Tauri)– acid alpha-glucosidase (GSD II; Pompe)– Phosphoglycerate kinase deficiency – Phosphoglycerate mutase deficiency– Lactate dehydrogenase deficiency– Aldolase A (GSD XII)– β-enolase (GSD XIII)

• Disorders of Lipid Metabolism– Carnitine Palmitoyltransferase Deficiency (CPT I/II)– Carnitine deficiency– Defects of beta-oxidation enzymes– Neutral Lipid Storage Disease– Lipin-1 deficiency

• Disorders of Purine Metabolism– Myoadenylate deaminase (MADA) deficiency; erythrocyte, liver, muscle subtypes

• Other Defects– Malignant hyperthermia– Alpha-methylacyl-CoQ recemase (AMACR) deficiency– Calcium adenosine triphosphates deficiency (Brody)

• Mitochondrial Disorders

Differential diagnosis

Symptomatic Classification of MMs

Mitochondrial Metabolic Myopthy - mtMM

• Mitochondrial DNA disease phenotypes are typically multisystemic

• Muscle, CNS, PNS, hepatic, GI, other tissues.

• Myopathy often involves key enzymes of energy metabolism

• Can cause disease early in life • Mild cases may go unrecognized into

adulthood

Mitochondrial Structure

ATP Production from Substrate

Oxidative Phosphorylation

• Defects in the enzymes of oxidative phosphorylation effect all types of substrate utilization (glucose, fatty acids, amino acids)

Mitochondrial DNA (mtDNA)

• Only organelle other than nucleus with its own DNA• Different structure than nuclear DNA - circular

Genes & Functions• mtDNA encodes for 37 genes

– Peptides • Encodes 13 of mitochondrial peptide subunits• All 13 peptides are in mitochondrial respiratory-chain complex (OXPHOS)• Remaining > 67 OXPHOS subunits are nuclear encoded

– rRNAs: 2 – tRNAs: 22; Located between every 2 rRNA or Protein coding genes – Non coding region: Triple stranded (D) displacement-loop

• Produced from additional synthesis of a piece of mitochondrial DNA, 7s DNA

• Contains promoter region – Origins of replication for H and L strand replication– Contains elements for initiation of leading strand replication

Mitochondrial DNA variation

– Homoplasmy; All copies of mtDNA are identical within cells– Heteroplasmy: Cells contain varied mtDNA populations

• Occurs with some mtDNA mutations • Due to presence of multiple mitochondria in one cell, each

containing several mtDNA copies • Produces tissue variation • Post-mitotic tissues

– Usually contain highest levels of mutated mtDNA – Neurons; Skeletal & Cardiac muscle; Endocrine tissue – Mutations in mtDNA

» % vs normal in mtDNA can vary among tissues in an individual» Mutational loads may change over time» Tissues are differentially sensitive to levels of mtDNA mutations: ?

Related to oxidative energy requirements

Inheritance of mtDNA

• Maternal (ovum)• Paternal (sperm)

mtDNA actively degraded

• Maternal mtDNA mutations can be amplified by the “bottleneck effect” of primary oocytes

Case AR – Nuclear DNA Studies• Otherwise healthy 26 yo male with mild

hepatic and renal dysfunction; persistent ↑CK/myoglobin

• Rhabdomyolysis genetic sequencing screen (Baylor/Miraca) - no known pathologic variants in 27 MM nuclear genes

Case AR – mtDNA Genome • Mitochondrial genome submitted and

compared to the MITOMAP database • Homoplasmic variant in cytochrome c

oxidase (COX) subunit III (mt-CO3) gene found at position 9696 in which cytosine was substituted for thymine

• Pathologic variant encodes a defective COX III protein - phenylalanine substituted leucine at position 164

• Patient’s mother shares the variant, without apparent symptoms

Cytochrome c Oxidase (COX) subunit III

COX III Abnormality at Position 164 (L164F) - Phenylalanine Substitutes Leucine

https://www.ebi.ac.uk/pdbe/entry/pdb/3abk/protein/3

Assessment

• We suspected MM clinically on the basis of persistent CK and myoglobin elevations

• We used a diagnostic algorithm and nuclear DNA testing to eliminate abnormalities of glycogen, lipid and purine biochemistry

• We then proceeded with mitochondrial DNA sequencing to reveal a genetic defect in COXIII

Management and Follow-up

• Complex carbohydrate, high protein diet • Careful attention to fluid intake to reduce the

impact of myoglobin on renal function• Patient instructed to avoid heavy exertion• CK, myoglobin, liver enzymes and renal function

have improved with this approach• Supplemental coQ10, creatine and alpha lipoic

acid supplementation may be considered

Discussion/Conclusions• The patient’s rhabdomyolysis is apparently due to

defect in OXPHOS caused by a pathologic variant in the gene encoding his COXIII.

• He is clinically stable unless he undergoes extreme exertion.

• He has no evidence of neurological, ophthalmologic or cardiac dysfunction but hepatic, renal and endocrine systems may be compromised.

• This condition appears to represent a novel MM due to a maternally inherited pathologic variant in mtDNA.

Thank You for Your Kind Attention!