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Mitochondrial diseases

Mitochondrial diseases are a heterogeneous group of rare genetic disorders caused by dysfunctional mitochondria, the organelles responsible for producing cellular energy (ATP) via the mitochondrial respiratory chain. While individual mitochondrial diseases are rare, collectively they affect up to 1 in 5,000 individuals. Since mitochondria are present in nearly all tissues, these diseases are often multisystemic, primarily affecting high-energy-demand organs such as the brain, skeletal muscle, heart, and sensory organs. The age of onset ranges from the neonatal period to late adulthood.

Mitochondrial diseases present a heterogeneous clinical spectrum with multisystemic involvement. The most frequent manifestations include:

Muscle Involvement

  • Proximal muscle weakness, typically progressive.

  • Exercise intolerance disproportionate to the degree of weakness.

  • Progressive bilateral palpebral ptosis (drooping eyelids).

  • Progressive ophthalmoplegia (paralysis of eye movements).

  • Muscle cramps during or after physical activity.

  • Spontaneous or exertional myalgias.

Neurological Manifestations

  • Stroke-like episodes: Acute neurological deficits resembling ischemic strokes.

  • Epilepsy: Focal and generalized seizures, myoclonus.

  • Progressive cognitive impairment.

  • Migraine and other forms of headache.

  • Cerebellar ataxia with coordination disorders.

  • Parkinsonism.

  • Sensory-motor peripheral neuropathy.

  • Acute or chronic vision loss.

  • Progressive sensorineural hearing loss.

Systemic Involvement

  • Cardiomyopathy: Hypertrophic, dilated, or restrictive.

  • Cardiac conduction defects: AV blocks, arrhythmias.

  • Diabetes mellitus.

  • Gastroparesis and intestinal dysmotility.

  • Renal tubulopathy (De Toni-Debré-Fanconi syndrome).

  • Growth deficiency and delayed puberty.

Specific Clinical Syndromes

These symptoms often combine into specific clinical syndromes, the most frequent of which are:

  • MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes): Characterized by stroke-like episodes, myopathy, epilepsy, migraine, diabetes mellitus, and sensorineural hearing loss. The most common mutation associated with MELAS can also cause only MIDD (Maternally Inherited Diabetes and Deafness).

  • MERRF (Myoclonic Epilepsy with Ragged Red Fibers): Characterized by progressive myoclonic epilepsy, cerebellar ataxia, myopathy, and multiple lipomas.

  • LHON (Leber Hereditary Optic Neuropathy): Characterized by acute episodes of irreversible vision loss.

  • CPEO (Chronic Progressive External Ophthalmoplegia): A relatively benign form presenting with isolated ophthalmoplegia and ptosis.

  • KSS (Kearns-Sayre Syndrome): Defined by the triad of CPEO, pigmentary retinopathy, and onset before age 20, often associated with cardiac block and short stature.

  • Leigh Syndrome: A severe acute encephalopathy of childhood characterized by psychomotor regression, hypotonia, movement disorders, breathing abnormalities, and typical bilateral lesions of the basal ganglia and brainstem on MRI. It can be caused by either nDNA or mtDNA mutations (MILS).

  • NARP (Neuropathy, Ataxia, and Retinitis Pigmentosa).

Note: Clinical expression is highly variable; not all patients affected by these syndromes will develop every listed manifestation.

The clinical spectrum is vast. Key manifestations include:

  • Muscular: Proximal weakness, exercise intolerance, progressive external ophthalmoplegia (PEO), ptosis, and cramps.

  • Neurological: Stroke-like episodes, epilepsy, ataxia, migraine, and sensorineural hearing loss.

  • Systemic: Cardiomyopathy, diabetes mellitus, intestinal dysmotility, and renal tubulopathy.

    • Laboratory Tests: Elevated serum lactate (at rest or post-exercise), altered lactate/pyruvate ratio, and elevated alanine.

    • Specialized Imaging: * Brain MRI: Stroke-like lesions (not respecting vascular territories), basal ganglia calcifications.

      • MR Spectroscopy: Identification of a lactate peak in the brain.

    • Genetic Testing (Gold Standard):

      • mtDNA analysis: Targeted for common mutations or full mtDNA sequencing. Some mutations (e.g., for CPEO/KSS) must be sought in muscle or urine as they may be undetectable in blood.

      • nDNA analysis: At our center, we utilize Next-Generation Sequencing (Clinical Exome) with virtual panels for simultaneous analysis of all known mitochondrial genes.

    • Muscle Biopsy: * Histochemistry: Identification of Ragged Red Fibers (RRF) and COX-negative fibers.

      • Electron Microscopy: Structural mitochondrial abnormalities and paracrystalline inclusions.

      • Biochemical Assay: Measurement of residual activity in respiratory chain complexes (I-V).

 

Coinvolgimento sistemico

  • Cardiomiopatia: ipertrofica, dilatativa o restrittiva
  • Disturbi conduzione cardiaca: blocchi AV, aritmie
  • Diabete mellito
  • Gastroparesi e dismotilità intestinale
  • Tubulopatia renale (sindrome De Toni-Debré-Fanconi)
  • Deficit crescita e ritardo pubertà

 

Questi sintomi si combinano in alcune sindromi cliniche specifiche, tra cui le più frequenti sono:

  • MELAS (encefalomiopatia mitocondriale con acidosi lattica ed episodi stroke-like): episodi stroke-like, miopatia, epilessia, emicrania, diabete mellito, ipoacusia neurosensoriale. La mutazione più frequentemente associata alla MELAS può determinare solo diabete e sordità ad ereditarietà materna (MIDD).
  • MERRF (epilessia mioclonica con fibre rosse sfilacciate): epilessia mioclonica progressiva, atassia cerebellare, miopatia, lipomi multipli.
  • LHON (neuropatia ottica ereditaria di Leber): episodi acuti di perdita visiva irreversibile.
  • CPEO (oftalmoplegia esterna cronica progressiva): forma benigna con oftalmoplegia e ptosi isolate.
  • KSS (sindrome di Kearns-Sayre): CPEO, blocco cardiaco, retinopatia, bassa statura.
  • Sindrome di Leigh: grave encefalopatia acuta ad esordio infantile caratterizzata da regressione psicomotoria, ipotonia, disturbi del movimento, alterazioni del respiro e tipiche lesioni bilaterali dei gangli della base e del tronco encefalico alla risonanza magnetica. Può essere dovuta a mutazioni del nDNA o del mtDNA (MILS).
  • NARP (neuropatia, atassia, retinopatia pigmentosa)

Non tutti i pazienti affetti da queste sindromi sviluppano tutte le manifestazioni cliniche.

  • Laboratory Tests: Elevated serum lactate (at rest or post-exercise), altered lactate/pyruvate ratio, and elevated alanine.

  • Specialized Imaging: * Brain MRI: Stroke-like lesions (not respecting vascular territories), basal ganglia calcifications.

    • MR Spectroscopy: Identification of a lactate peak in the brain.

  • Genetic Testing (Gold Standard):

    • mtDNA analysis: Targeted for common mutations or full mtDNA sequencing. Some mutations (e.g., for CPEO/KSS) must be sought in muscle or urine as they may be undetectable in blood.

    • nDNA analysis: At our center, we utilize Next-Generation Sequencing (Clinical Exome) with virtual panels for simultaneous analysis of all known mitochondrial genes.

  • Muscle Biopsy: * Histochemistry: Identification of Ragged Red Fibers (RRF) and COX-negative fibers.

    • Electron Microscopy: Structural mitochondrial abnormalities and paracrystalline inclusions.

    • Biochemical Assay: Measurement of residual activity in respiratory chain complexes (I-V).

Available therapies

Currently, there are no specific curative therapies for mitochondrial diseases. Management focuses on supporting mitochondrial function, alleviating symptoms, and preventing complications.

Supportive Therapies

  • Vitamin Supplementation (“Mitochondrial Cocktail”):

    • Coenzyme Q10: 100–300 mg/day

    • Riboflavin (Vitamin B2): 100–400 mg/day

    • Thiamine (Vitamin B1): 100–300 mg/day

    • Creatine Monohydrate: 5–10 g/day

  • Antioxidants: Alpha-lipoic acid, Vitamin C, Vitamin E, and N-acetylcysteine.

  • Idebenone: Specifically indicated for LHON (900 mg/day).

  • L-Arginine: Used for the prevention and acute management of stroke-like episodes (5–10 g/day).

Management of Complications

  • Cardiomyopathy: Annual echocardiogram, ACE inhibitors, beta-blockers, and pacemaker implantation for AV blocks.

  • Epilepsy: Avoid Valproate (due to mitochondrial hepatotoxicity); preferred options include Levetiracetam or Carbamazepine.

  • Respiratory Insufficiency: Non-invasive ventilation (BiPAP) and respiratory physiotherapy.

  • Metabolic Crises: Avoid prolonged fasting, manage metabolic stress, and maintain adequate hydration.

Multidisciplinary Rehabilitative Approach

  • Physiotherapy: Low-intensity aerobic exercise, stretching, and respiratory re-education.

  • Occupational Therapy: Aids for autonomy, environmental adaptations, and energy conservation techniques.

  • Nutritional Support: Frequent small meals, avoidance of fasting, and a balanced diet.

Prevention of Triggering Factors

  • Avoid Mitotoxic Drugs: Strictly avoid aminoglycoside antibiotics, valproate, and use statins with caution.

  • Infection Prevention: Proactive management of infections and metabolic stress.

  • Anesthetic Precautions: Specific protocols are required for patients undergoing general anesthesia to avoid mitochondrial suppression.

Research in progress

Current research is focused on overcoming the unique challenges of the mitochondrial genome and improving the quality of life for patients through several innovative pathways:

Development of Innovative Therapies

  • Mitochondrial Gene Editing: Cutting-edge research into the correction of mtDNA mutations using specialized tools (such as mitoTALENs or base editors) designed to bypass the mitochondrial double membrane.

  • Pharmacological Modulators of Mitochondrial Biogenesis: Development of compounds that stimulate cells to produce new, healthy mitochondria (e.g., PPAR agonists, AMPK activators).

  • Next-Generation Antioxidants: Targeted antioxidants designed to accumulate specifically within the mitochondria to neutralize reactive oxygen species (ROS) more effectively.

  • Neuroprotective Therapies: Specific strategies aimed at preserving neuronal function and preventing cell death in the central nervous system.

Biomarker Identification

  • Non-invasive Progression Markers: Research into serum or imaging-based markers to track the disease course without the need for repeated biopsies.

  • Therapeutic Response Indicators: Identifying physiological or biochemical signals that demonstrate whether a treatment is effectively reaching its target.

  • Liquid Biopsy Biomarkers: Utilizing cell-free DNA (cfDNA) or specific metabolites for real-time monitoring of mitochondrial dysfunction.

Natural History Studies

  • International Patient Registries: Participation in global databases to aggregate data on these rare conditions, facilitating larger clinical trials.

  • Longitudinal Phenotypic Characterization: Detailed long-term observation of patient symptoms to define the clinical trajectory of different syndromes.

  • Genotype-Phenotype Correlations: Deepening the understanding of how specific genetic mutations (and heteroplasmy levels) translate into diverse clinical outcomes.

Reference laboratory

Contacts and informations

Email/Telefono: malattieneuromuscolari@policlinico.mi.it

Associazioni pazienti:

  • UILDM:  Tel. 049 8021001
  • MITOCON: Associazione Italiana Malattie Mitocondriali

Risorse specialistiche:

  • TREAT-NMD: Network internazionale malattie neuromuscolari
  • EURO-MITO: Network europeo malattie mitocondriali
  • World Muscle Society: worldmusclesociety.org
  • MitoMap: – Database mutazioni mtDNA – A human mitochondrial genome database
  • gov: trials clinici attivi per malattie mitocondriali specifiche
  • Orphanet: Database malattie rare