GDF-15 as a Biomarker for Mitochondrial Disease

BACKGROUND

Energy insufficiency:

Mitochondrial and metabolic myopathies are inherited diseases compromising cellular energy metabolism, which especially affects skeletal muscle because of its high energy needs. Chemical energy is stored in the body as adenosine triphosphate (ATP), which is derived from different sources including breakdown of carbohydrates, lipids and purine nucleotides. In the respiratory chain in the inner mitochondrial membrane, ATP is released through oxidative phosphorylation.

Any genetic disorder affecting any step in the production of energy, from storage and breakdown of glycogen and lipids to transport and conversion of substrates, can manifest as energy insufficiency in the affected tissues.

Mutations in genes encoding enzymes of the lipid or carbohydrate metabolism result in metabolic myopathies and mutations in the enzyme complexes of the respiratory chain result in mitochondrial disorders.

Mitochondrial disorders:

Mitochondrial disorders are caused by mutations in either mitochondrial DNA (mtDNA) or nuclear DNA (nDNA) which lead to impaired function of the respiratory chain and reduced energy generation. The disorders derived from mtDNA mutations are maternally inherited, while the nDNA mutations are autosomal recessively or dominantly inherited.

Mitochondrial disorders present with a wide range of symptoms and syndromes depending on the mutation and mutation load in tissues. Symptoms usually arise from the brain, nerves, skeletal and cardiac muscle, as these tissues have a high energy demand. The patients may suffer from muscle weakness, exercise intolerance, impaired balance and coordination, seizures, learning deficits, impaired vision, hearing loss and heart defects. Age at disease onset varies and the disease can debut throughout life.

The prevalence is in general 6.2/100.000 births.

Metabolic myopathies:

Metabolic myopathies are either inherited autosomal recessively, dominantly, X-linked or occur spontaneously. Metabolic myopathies can be caused by defective enzymes of the lipid metabolism (Fatty Acid Oxidation Disorders), and glycogen and glucose metabolism (Glycogenoses) with common features of compromised energy generation in the affected tissues, especially in muscle.

Symptoms vary, but patients can suffer from exercise intolerance, muscle contractures, progressive muscle weakness and heart- and respiratory failure. If the symptoms start in childhood, the disease is often more severe and may present with acute metabolic decompensation, hypoketotic hypoglycemia, encephalopathy and risk of coma and death.

The prevalence and incidence are uncertain, since there might be many patients who have not been diagnosed. With an increased awareness and newborn screening programs, more patients are now being diagnosed and survive metabolic decompensation.

Growth Differentiation Factor 15 as a diagnostic tool:

Since the symptoms of mitochondrial disorders and metabolic myopathies are very unspecific, they can easily be mistaken for i.e. cardiopulmonary disease and diagnosis can be difficult. Therefore, it would be useful to have a biomarker that could easily distinguish both disorders from others. A recent study showed that Growth Differentiation Factor 15 (GDF-15) was significantly elevated in blood from patients affected by mitochondrial disorders as compared to healthy individuals, but it is unknown whether this increase is specific for mitochondrial disease. Thus it is unknown how GDF-15 levels are in patients with other muscle disease, including metabolic myopathies in which an energy deficiency, as in mitochondrial diseases, is also present. GDF-15 belongs to the transforming growth factor beta super family of growth factors that regulates inflammation and apoptosis in injured tissue. It is not known why GDF-15 is elevated, but it has been suggested that the oxidative stress, which is a part of the pathophysiology in mitochondrial disorders, increases GDF-15 through activation of P53. Oxidative stress also plays a role in the pathophysiology of some metabolic myopathies [8], and GDF-15 may therefore be elevated in these patients too. To make sure that the GDF-15 is not elevated due to muscle involvement, it will also be measured in a subgroup of muscular dystrophies.

AIM

In this study, we wish to further investigate:

1. if measurement of GDF-15 can be used as a biomarker for mitochondrial myopathy and distinguish these patients from healthy persons.
2. if elevated GDF-15 is also a sign of either metabolic myopathy or muscle dystrophy and not exclusively detects mitochondrial disease.
3. if the GDF-15 concentration in these mitochondrial and metabolic myopathy varies when metabolic demand is increased with exercise.
4. if the GDF-15 concentration correlates with oxidative capacity (VO2max) in patients with mitochondrial myopathy.

We will investigate the concentration of GDF-15 in blood samples in patients affected by mitochondrial disorders and compare it to the concentrations in patients affected by metabolic myopathies, muscular dystrophies and to a group of healthy controls. If there is a significant difference, GDF-15 may be a sensitive biomarker for mitochondrial disorders. We will further investigate the maximal oxidative capacity and the maximal workload capacity to study if and how this relates to the concentration of GDF-15.

METHODS:

30 subjects with mitochondrial disorders, 25 with metabolic myopathy, 25 with muscular dystrophy and 25 healthy controls will be recruited. A blood sample will be taken, and GDF-15 and other muscle markers will be measured. 10-15 subjects with mitochondrial disorders, 10-15 with metabolic myopathy and 10-15 healthy individuals will further be investigated with an exercise test, and blood samples will be taken afterwards.