Mitochondria: role, symptoms, mitochondrial diseases, causes, treatment

Mitochondrion: Definition, location, origin

What are mitochondria? Where are they located?

THE mitochondria are small structures present in the majority of our cells. “The number of mitochondria per cell varies depending on the cell type and its energy needs.reveals Doctor Erik Bernard (YouTube channel @docteurerik). “Cells with high energy requirements usually have greater numbers of mitochondria”, he emphasizes. Furthermore, as he specifies, “the number of mitochondria in a cell can also change depending on environmental conditions“. Cells can also increase their number of mitochondria (mitochondrial biogenesis) or eliminate defective or unnecessary mitochondria (mitophagy).

Mitochondria, why this name?

Etymologically, the term mitochondrion comes from the ancient Greek “mitos” which means “thread” and “chondros” which can be translated as “grain”. The reason ? “Mitochondria appear under the optical microscope in the form of a thread, a rod or a grain,” explains the Dictionary of the French Academy on its website.

What is the role and function of the mitochondrion?

What are mitochondria for?

Mitochondria are often referred to as the powerhouses or energy factories of the cell. “They are responsible for energy production through the process of cellular respiration”reports Dr Erik Bernard.

Mitochondria: the organ responsible for cellular respiration

The mitochondrion is the place of cellular respirationthat is to say the place where the oxygen is used. “Mitochondria produce a molecule called adenosine triphosphate (ATP) which is the main source of energy for cellular functions”, says Dr. Bernard. In addition to their determining role in cellular energy metabolism, “mitochondria are also involved in communication, differentiation and death of cells, as well as in the control of the cell cycle”he adds.

Note that mitochondria have their own genome (mitochondrial genome), distinct from the DNA contained in the nucleus of the cell. “This mitochondrial DNA is transmitted only by the mother and has many similarities with the genome of bacteria. Mitochondria could arise, during evolution, from the incorporation of bacteria into a primitive cell (endosymbiotic theory)”explains our expert.

Mitochondria, what size?

Mitochondria are often described as rods 0.5 µm in diameter and several µm long as indicated in an article from the University of Lille, are microscopic structures (therefore only visible under a microscope).

Diagram of the structure of a mitochondrion

The University of Lille website offers the diagram below to illustrate a mitochondrion:

Symptoms of mitochondrial diseases

Mitochondrial diseases are a group of rare genetic diseases, often hereditary. In Europe, it is estimated that there are 200 new cases per year (estimated incidence 1/5,000 births). These figures are probably underestimated due to the difficulty in establishing a diagnosis (source: Institut Pasteur). “They can appear at any age, but are often diagnosed during childhood”Erik Bernard informs us.

Mitochondrial diseases: what are the symptoms?

Symptoms of mitochondrial diseases can vary considerably depending on the organs affected and the severity of the damage. Mitochondrial diseases usually manifest as a dysfunction of mitochondria in tissues with high energy demand, i.e. the heart, brain and muscles. Other organs may also be affected, such as eyes, ears, kidneys, liver or digestive system. Some forms can be limited to one organ (e.g. the eye, with Leber's hereditary optic neuropathy) or lead to failure of several organs, often with neurological or muscular function impairment at the forefront. Common symptoms include:

• Fatigue and muscle weakness;
• Neurological disorderssuch as seizures, balance and coordination problems, or loss of hearing or vision;
• Heart problemssuch as rhythm disturbances or heart failure;
• Digestive disorderssuch as nausea, vomiting or abdominal pain;
• Growth and development difficulties in children.

Main mitochondrial maladies:

There are many mitochondrial diseases. Among the most common, we can cite:

• Leigh syndrome (subacute necrotizing encephalomyopathy);
• Le syndrome MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, Stroke-like episodes) ;
• MERRF syndrome (Myoclonus Epilepsy and Red-Ragged-Fibers) ;
• Leber's hereditary optic neuropathy;
• Le syndrome de Kearns-Sayre ;
• Le syndrome NARP (Neuropathy, Ataxia and Retinitis Pigmentosa);
• Alpers syndrome.

“Due to the variety of symptoms, many specialists (neurologistpediatrician, cardiologist, geneticist, etc.) can be involved in the diagnosis and management of these diseases. It is not always possible to make a precise diagnosis.”concedes the general practitioner.

Causes of mitochondrial diseases:

What are the origins of mitochondria? Mitochondrial diseases are caused by genetic mutations that impact energy production and cell function. “The proper functioning of mitochondria depends both on the mitochondrial genome, but also on the genome contained in the cell nucleus (nuclear DNA)”recalls Dr Erik Bernard.

Mitochondrial diseases generally originate from mutations in mitochondrial DNA or nuclear DNA. “The diagnosis of mitochondrial myopathy is often a long process : these diseases are rare and complex, symptoms can vary from one person to another and more than 350 genes have been identified, complicating genetic analysis.he reports.

These genetic mutations can be inherited from one or both parents, or occur spontaneously during a person's life. Among the origins of mitochondria, it also happens, in certain cases, that environmental factors contribute to their development.

Genes necessary for the proper functioning of mitochondria:

In an article dedicated to mitochondrial genetic transmission, AFM Téléthon explains that “the genes necessary for the proper functioning of mitochondria are, for the most part, carried by the chromosomes located in the nucleus of cells (nuclear DNA); mitochondrial DNA actually contains very few genes (less than 50, whereas it exists between 20,000 and 30,000 genes contained in the DNA of chromosomes).

Mitochondrial diseases: what treatment?

According to Dr. Bernard, early diagnosis of patients with mitochondrial diseases is essential to allow therapeutic intervention as early as possible. “The treatment must be done in collaboration with a reference center for mitochondrial diseases and is multidisciplinary.” There is currently no treatment to cure mitochondrial diseases.. A symptomatic management and rehabilitation are always offered and aims to relieve symptoms and improve quality of life of patients.

• The treatment is adapted on a case-by-case basis and can combine: rehabilitation, balneotherapy and massages, to maintain the muscles;
• From physiotherapy, wearing splints and sometimes surgeryto limit muscle retractions and maintain joint flexibility;
• A re-education occupational therapy and or speech therapy when indicated;
• Sessions of respiratory physiotherapy in case of damage to the respiratory chain.
• Of the food supplements such as coenzyme Q10 or certain vitamins.
• Specific management of associated damage: medicines against crises epilepsy where the diabetesmedications and/or pacemaker (pacemaker) for cardiac symptoms, hearing aid for deafness, special glasses or surgery for drooping eyelids, etc.

“A genetic counseling consultation is offered for the patient and his family. Research on mitochondrial diseases is constantly evolving and new therapeutic approaches are currently being studied by scientists. We can cite the genetical therapywhich aims to replace or repair defective genes, and transplant healthy mitochondria into diseased cells.concludes Doctor Erik Bernard.