Diagnosis of mitochondrial diseases. Mitochondrial pathology in children

Introduction(features of human mitochondria). A feature of the functioning of mitochondria is the presence of their own mitochondrial genome - circular mitochondrial DNA (mtDNA) containing 37 genes whose products are involved in the process of energy production in the respiratory chain of mitochondria. mtDNA is located in the inner membrane of mitochondria and consists of five conjugated enzyme complexes, which have a total of 86 subunits. They are mainly encoded by nuclear genes (nDNA), but seven subunits of the first enzyme complex (ND1, 2, 3, 4, 4L, 5, 6), one of the third (cytochrome b), three of the fourth (COI, COII, COIII) and two of the fifth (ATPase 6 and 8) are encoded by mtDNA structural genes. Thus, enzyme complexes (i.e., proteins) encoded by both nuclear (nDNA) and mitochondrial genes (mtDNA) are involved in providing diverse biochemical functions of mitochondria.

note! The main biochemical processes that are related to energy metabolism and occur in mitochondria are: tricarboxylic acid cycle (Krebs cycle), beta-oxidation of fatty acids, carnitine cycle, electron transport in the respiratory chain and oxidative phosphorylation. Any of these processes can be disturbed and cause mitochondrial insufficiency.

Cause of mitochondrial disease (hereinafter MB). The main properties of the mitochondrial genome are the cytoplasmic inheritance of genes, the absence of recombinations (i.e., the reorganization of genetic material through the exchange of individual segments, regions, DNA double helixes) and a high mutation rate. The mitochondrial genome is characterized by pronounced instability and a high rate of nucleotide substitutions, on average 10–17 times higher than the mutation rate of nuclear genes, and somatic mutations often occur in it during the life of an individual. The immediate cause of the onset and development of mitochondrial dysfunction lies in defects in the oxidative phosphorylation system, imperfection of repair mechanisms, the absence of histones, and the presence of free radicals oxygen is a by-product of aerobic respiration.

Mutations in the mitochondrial genome are characterized by the phenomenon [ !!! ] heteroplasmy, in which (due to the specificity of mitochondrial inheritance), as a result of cell division, the distribution (which varies widely - from 1 to 99%) of mutant mtDNA between daughter cells occurs randomly and unevenly, as a result of which copies of mtDNA carrying normal and/or mutant allele. At the same time, different tissues of the body or neighboring areas of the same tissue may differ in the degree of heteroplasmy, i.e. according to the degree of presence and ratio in the cells of the body of mitochondria with both mutant and normal mtDNA (in subsequent generations, some cells may have only normal mtDNA, another part only mutant, and a third part - both types of mtDNA). The content of mitochondria with mutant mtDNA increases gradually. Due to this "lag period" (from the English "lag" - delay), future patients often reach sexual maturity (and give offspring, almost always carrying the same mutations in mtDNA). When the number of mutant copies of mtDNA in a cell reaches a certain concentration threshold, the energy metabolism in the cells is significantly impaired and manifests itself in the form of a disease (note: a feature of hereditary MBs is often complete absence any pathological signs at the beginning of the patient's life).

note! Heteroplasmy is characterized by the simultaneous existence of mutant and normal mtDNA in the same cell, tissue, or organ, which determines the severity, nature, and age of MB manifestation. The number of altered mtDNA can also increase with age under the influence of various factors and gradually reach a level that can cause clinical manifestations of the disease.

In accordance with the above features of the double mitochondrial genome, the type of MB inheritance can be different. Since mtDNA in the body is almost exclusively of maternal origin, when a mitochondrial mutation is transmitted to offspring, a maternal type of inheritance takes place in the pedigree - all children of a sick mother get sick. If a mutation occurs in a nuclear gene (nDNA) encoding the synthesis of a mitochondrial protein, the disease is transmitted according to classical Mendelian laws. Sometimes a mtDNA mutation (usually a deletion) occurs de novo at an early stage of ontogeny, and then the disease manifests itself as a sporadic case.

note! Currently, more than 100 point mutations and several hundred mtDNA structural rearrangements are known to be associated with characteristic neuromuscular and other mitochondrial syndromes, ranging from lethal in the neonatal period of life to diseases with a late onset.

Definition. MB can be characterized as diseases caused by genetic and structural-biochemical defects of mitochondria and accompanied by a violation of tissue respiration and, as a result, a systemic defect in energy metabolism, as a result of which the most energy-dependent tissues and target organs are affected in various combinations: the brain, skeletal muscles and myocardium. (mitochondrial encephalomyopathies), pancreas, organ of vision, kidneys, liver. Clinically, violations in these organs can be realized at any age. At the same time, the heterogeneity of symptoms complicates the clinical diagnosis of these diseases. The need to exclude MB arises in the presence of multisystem manifestations that do not fit into the usual pathological process. The frequency of respiratory chain dysfunction is estimated from 1 per 5-10 thousand to 4-5 per 100 thousand newborns.

Semiotics. Neuromuscular pathology in MB is usually represented by dementia, seizures, ataxia, optic neuropathy, retinopathy, sensorineural deafness, peripheral neuropathy, and myopathy. However, about 1/3 of MB patients have normal intelligence and no neuromuscular manifestations. MB includes, in particular, Kearns-Sayre encephalocardiomyopathy (retinitis pigmentosa, external ophthalmoplegia, complete heart block); MERRF syndrome (myoclonus epilepsy, "torn" red fibers); (mitochondrial encephalomyopathy, lactic acidosis, stroke-like episodes); Pearson syndrome (encephalomyopathy, ataxia, dementia, progressive external ophthalmoplegia); NAPR syndrome (neuropathy, ataxia, retinitis pigmentosa); and some forms of ophthalmopathic myopathy. All these forms are united by a myopathic syndrome expressed to one degree or another.

note! The two main clinical signs of MB are the increase over time in the number of organs and tissues involved in the pathological process, as well as the almost inevitable damage to the central nervous system. Polymorphism clinical manifestations, including the defeat of organs, at first glance, physiologically and morphologically unrelated, in combination with different periods of manifestation and the steady progression of the symptoms of the disease with age, and allows one to suspect a [genetic] mtDNA mutation.

note! In clinical practice great importance has the ability to differentiate the clinical picture of MB from more common somatic, autoimmune, endocrine and other pathological conditions, most of which are treatable. It is necessary to conduct a thorough assessment of the family history, data from routine clinical and laboratory-instrumental methods of examination, before assigning specific genetic and biochemical tests to the patient, aimed at finding mitochondrial pathology.

Diagnostics . The algorithm for diagnosing any MB should include the following steps: [ 1 ] identification of a typical clinical picture of the mitochondrial syndrome or an "inexplicable" multisystemic lesion and a hereditary history confirming the maternal type of inheritance; [ 2 ] further diagnostic search should be aimed at detecting common markers of mitochondrial dysfunction: an increase in the level of lactate/pyruvate in the blood serum and cerebrospinal fluid, a violation of carbohydrate, protein, amino acid metabolism, as well as a clinical picture involving at least three of these systems in the pathological process: CNS, cardiovascular system, muscular, endocrine, renal, organs of vision and hearing; [ 3 ] in case of clinical and confirmed laboratory and instrumental signs of mitochondrial pathology, a PCR analysis of blood lymphocytes is performed for a targeted search for mtDNA point mutations; a study that is considered the gold standard for diagnosing MB [cytopathies] - a biopsy of skeletal muscles with histochemical, electron microscopic, immunological and molecular genetic analyzes, characteristic changes in which will be with any MB (see below); [ 5 ] the most sensitive tests for the diagnosis of MB are methods for assessing the level of heteroplasmy of pathological mtDNA in various bodies and tissues: fluorescent PCR, cloning, denaturing high performance liquid chromatography, sequencing, southern blot hybridization, etc.

Histochemical study of muscle biopsy specimens of patients, including trichrome staining according to the Gomory method, demonstrates changes characteristic of MB - torn red fibers of myofibrils, which contain a large number of proliferating and damaged mitochondria, forming agglomerates along the periphery of the muscle fiber. In this case, the number of torn red fibers in the biopsy should be ≥ 2%. Enzyme-histochemical analysis shows a deficiency of cytochrome C-oxidase in 2 and 5% of myofibrils (for patients younger than 50 and older than 50 years) of their total number in biopsy specimens. Histochemical analysis of succinate dehydrogenase (SDH) activity demonstrates CDH-positive staining of myofibrils - ragged blue fibers (ragged blue fibers), which, in combination with SDH-positive staining of the walls of arteries supplying the muscles, indicates high degree damage to the mitochondria of myocytes. When conducting electron microscopy of muscle biopsy specimens, pathological inclusions, structural rearrangements of mitochondria, changes in their shape, size and number are determined.

note! Despite significant progress since the discovery of mtDNA genetic mutations, most of the diagnostic methods used in clinical practice have a low degree of specificity for individual MBs. Therefore, the diagnostic criteria for a particular MB, first of all, consist of a combination of specific clinical and morphological patterns.

Principles of treatment . Therapy for MB (cytopathies) is exclusively symptomatic and is aimed at reducing the rate of progression of the disease, as well as improving the quality of life of patients. For this purpose, patients are prescribed a standard combination of drugs, including coenzyme Q10, idebenone - a synthetic analogue of CoQ10, creatine, folic acid, vitamins B2, B6, B12 and others. medicines that improve redox reactions in cells (electron carrier drugs in the respiratory chain and cofactors of enzymatic reactions of energy metabolism). These compounds stimulate the synthesis of ATP molecules and reduce the activity of free radical processes in mitochondria. Meanwhile, according to a systematic review, most of the drugs with antioxidant and metabolic effects used in MB have not been evaluated in large randomized placebo-controlled trials. Therefore, it is difficult to assess the severity of their therapeutic effect and the presence of significant side effects.

Read more about MB in the following sources:

article "Neuromuscular pathology in mitochondrial diseases" L.A. Saykova, V.G. Pustozers; Saint Petersburg Medical Academy postgraduate education Roszdrav (magazine "Bulletin of the St. Petersburg Medical Academy of Postgraduate Education" 2009) [read];

article "Chronic diseases of non-inflammatory genesis and mutations of the human mitochondrial genome" K.Yu. Mitrofanov, A.V. Zhelankin, M.A. Sazonova, I.A. Sobenin, A.Yu. Postnov; Skolkovo Innovation Center. Research Institute of Atherosclerosis, Moscow; GBOU Research Institute of General Pathology and Pathophysiology of the Russian Academy of Medical Sciences, Moscow; Institute of Clinical Cardiology. A.L. Myasnikova FGBU RKNPK of the Ministry of Health and Social Development of the Russian Federation (magazine "Cardiology Bulletin" No. 1, 2012) [read];

article "Mitochondrial DNA and human hereditary pathology" N.S. Prokhorova, L.A. Demidenko; Department of Medical Biology, State Institution "Crimean State Medical University named after I.I. S.I. Georgievsky", Simferopol (magazine "Tauride Medical and Biological Bulletin" No. 4, 2010) [read];

article "Mitochondrial genome and human mitochondrial diseases" I.O. Mazunin, N.V. Volodko, E.B. Starikovskaya, R.I. Sukernik; Institute of Chemical Biology and Fundamental Medicine, Siberian Branch Russian Academy Sciences, Novosibirsk (Journal "Molecular Biology" No. 5, 2010) [read];

article "Prospects for Mitochondrial Medicine" by D.B. Zorov, N.K. Isaev, E.Yu. Plotnikov, D.N. Silachev, L.D. Zorova, I.B. Pevzner, M.A. Morosanova, S.S. Yankauskas, S.D. Zorov, V.A. Babenko; Moscow State University them. M.V. Lomonosov, Institute of Physical and Chemical Biology named after A.I. A.N. Belozersky, Research Institute of Mitoengineering, Laser Science Center, Faculty of Bioengineering and Bioinformatics; Russian national research medical University them. N.I. Pirogov (magazine "Biochemistry" No. 9, 2013) [read];

article "Strokes in mitochondrial diseases" N.V. Pizov; Department of Nervous Diseases with courses in neurosurgery and medical genetics, SBEI HPE "Yaroslavl State Medical Academy" (journal "Neurology, Neuropsychiatry, Psychosomatics" No. 2, 2012) [read];

article "Diagnosis and prevention of nuclear-encoded mitochondrial diseases in children" E.A. Nikolaev; Research Clinical Institute of Pediatrics, Moscow (journal "Russian Bulletin of Perinatology and Pediatrics" No. 2, 2014) [read];

article "Epilepsy in children with mitochondrial diseases: features of diagnosis and treatment" Zavadenko N.N., Kholin A.A.; GBOU VPO Russian National Research Medical University. N.I. Pirogov of the Ministry of Health and Social Development of Russia, Moscow (journal "Epilepsy and paroxysmal conditions" No. 2, 2012) [read];

article " Mitochondrial pathology and problems of the pathogenesis of mental disorders” V.S. Sukhorukov; Moscow Research Institute of Pediatrics and Pediatric Surgery of Rosmedtekhnologii (Journal of Neurology and Psychiatry, No. 6, 2008) [read];

article "Algorithm for the diagnosis of mitochondrial encephalomyopathies" S.N. Illarioshkin (Nervous Diseases magazine No. 3, 2007) [read];

article "Actual issues of treatment of mitochondrial disorders" by V.S. Sukhorukov; Federal State Budgetary Institution "Moscow Research Institute of Pediatrics and Pediatric Surgery" of the Ministry of Health of Russia (journal "Effective Pharmacotherapy. Pediatrics" No. 4, 2012 [read];

article "Leukoencephalopathy with a predominant lesion of the brainstem, spinal cord and increased lactate in MR spectroscopy (clinical observation)" V.I. Guzeva, E. A. Efet, O. M. Nikolaeva; St. Petersburg Pediatric Medical University, St. Petersburg, Russia (journal "Neurosurgery and neurology of childhood" No. 1, 2013) [read];

teaching aid for third-year students of the medical diagnostic faculty of medical universities "Hereditary mitochondrial diseases" T.S. Ugolnik, I. V. Manaenkova; Educational Institution "Gomel State Medical University", Department of Pathological Physiology, 2012 [read];

fast: Mitochondrial diseases(neurodegeneration) - to the site with 17 links to sources (articles, presentations, etc.).

© Laesus De Liro

Mitochondrial diseases, and in particular the mitochondrial syndrome, which can be manifested by lesions of the central nervous system, heart, and skeletal muscle pathologies, are today one of the most important sections of neuropediatrics.

Mitochondria - what is it?

As many remember from school course biology, the mitochondrion is one of the cellular organelles, whose main function is the formation of the ATP molecule during cellular respiration. In addition, the cycle of tricarboxylic acids and many other processes take place in it. Studies conducted at the end of the 20th century revealed the key role of mitochondria in such processes as drug sensitivity and aging (programmed cell death). Accordingly, a violation of their functions leads to a lack of energy exchange, and as a result, damage and death of the cell. These disorders are especially pronounced in the cells of the nervous system and skeletal muscles.

Mitochondriology

Genetic studies have made it possible to determine that mitochondria have their own genome, different from the genome of the cell nucleus, and disturbances in its functioning are most often associated with mutations occurring there. All this made it possible to single out a scientific direction that studies diseases associated with impaired mitochondrial functions - mitochondrial cytopathies. They can be both sporadic and congenital, inherited through the mother.

Symptoms

Mitochondrial syndrome can manifest itself in various human systems, but the most pronounced manifestations are neurological symptoms. This is due to the fact that the nervous tissue is most strongly affected by hypoxia. Characteristic features hypotension, inability to adequately tolerate physical activity, various myopathies, ophthalmoparesis (ptosis paralysis. From the nervous system, there may be stroke-like manifestations, convulsions, pyramidal disorders, mental disorders. As a rule, mitochondrial syndrome in a child is always manifested by a developmental delay or loss of already acquired skills, psychomotor disorders.On the part of the endocrine system, the development of diabetes, dysfunction of the thyroid and pancreas, growth retardation, puberty is not excluded.Heart lesions can develop both against the background of pathologies of other organs, and in isolation The mitochondrial syndrome in this case is represented by cardiomyopathy.

Diagnostics

Mitochondrial diseases are most often detected in or during the first years of a child's life. According to foreign studies, this pathology is diagnosed in one newborn out of 5 thousand. For diagnosis, a comprehensive clinical, genetic, instrumental, biochemical and molecular examination is carried out. To date, there are a number of methods to determine this pathology.

1. Electromyography - with normal results against the background of pronounced muscle weakness in a patient, it is possible to suspect mitochondrial pathologies.
2. Lactic acidosis very often accompanies mitochondrial diseases. Of course, its presence alone is not enough to make a diagnosis, but measuring the level of lactic acid in the blood after exercise can be very informative.
3. Biopsy and histochemical examination of the obtained biopsy is the most informative.
4. Good results are shown by the simultaneous use of light and electron microscopy of skeletal muscles.

One of the most common childhood diseases associated with genetic changes in mitochondria is Leigh's syndrome, first described in 1951. The first signs appear at the age of one to three years, but earlier manifestations are possible - in the first month of life or, on the contrary, after seven years. The first manifestations are developmental delay, weight loss, loss of appetite, repeated vomiting. Over time, neurological symptoms join - a violation of muscle tone (hypotension, dystonia, hypertonicity), convulsions, impaired coordination.

The disease affects the organs of vision: degeneration of the retina develops, oculomotor disorders. In most children, the disease gradually progresses, signs of pyramidal disorders increase, swallowing and respiratory function disorders appear.

One of the children suffering from such a pathology was Efim Pugachev, who was diagnosed with mitochondrial syndrome in 2014. His mother, Elena, asks for help from all caring people.

The prognosis, unfortunately, today is most often disappointing. This is due both to the late diagnosis of the disease, the lack of detailed information about the pathogenesis, the severe condition of patients associated with the multisystemic lesions, and the lack of a single criterion for assessing the effectiveness of therapy.

Thus, the treatment of such diseases is still under development. As a rule, it comes down to symptomatic and supportive therapy.

The occurrence of these diseases is associated with changes in the DNA of mitochondria. The mitochondrial DNA genome has been completely deciphered. It contains ribosomal RNA genes, 22 tRNAs, and 13 polypeptides involved in oxidative phosphorylation reactions. Most mitochondrial proteins are encoded by nuclear DNA genes, are translated in the cytoplasm, and then enter the mitochondria. Mitochondrial DNA is maternally inherited. The cytoplasm of the egg contains thousands of mitochondria, while the mitochondria of the sperm do not end up in the zygote. Therefore, males inherit mtDNA from their mothers but do not pass it on to their offspring.

Each mitochondria contains 10 or more DNA molecules. Usually, all copies of mtDNA are identical. Sometimes, however, mutations occur in mtDNA that can be transmitted to both daughter mitochondria and daughter cells.

Clinically, mutations can manifest themselves in the form of various symptoms in any organ or tissue and at any age. The most energy-dependent, and therefore vulnerable, are the brain, heart, skeletal muscles, endocrine system, liver. Lesions of the nervous system are usually accompanied by convulsions, impaired coordination (ataxia), decreased intelligence, neurosensory deafness.

Examples of hereditary diseases: Leber's optic disc atrophy (acute loss of central vision, can occur at any age), mitochondrial encephalomyopathy, myoclonic epilepsy syndrome and torn muscle fibers.

Multifactorial diseases

They occur in individuals with an appropriate combination of predisposing alleles, there is a polymorphism of clinical signs, diseases manifest themselves at any age, any system or organ can be involved in the pathological process. Examples: hypertension, atherosclerosis, peptic ulcer, schizophrenia, epilepsy, glaucoma, psoriasis, bronchial asthma, etc.

Peculiarities:

High frequency of occurrence in the population

Existence of various clinical forms

The dependence of the degree of risk for the relatives of the patient:

The rarer the disease in the population, the higher the risk for relatives of the proband

The more pronounced the disease in the proband, the higher the risk of the disease in his relative

The risk for relatives of the proband will be higher if there is another sick blood relative.

Medical genetic counseling

This is one of the types of specialized medical care for the population. Geneticists, as well as other specialists (obstetricians, pediatricians, endocrinologists, neuropathologists) work in the consultation. The main tasks of the consultation:

Assisting doctors in diagnosing a hereditary disease

Determining the probability of having a child with a hereditary pathology

Explanations to parents about the meaning of genetic risk

Stages of counseling:

1. Examination of the patient and diagnosis of a hereditary disease. Various methods are used for this: cytogenetic, biochemical, DNA diagnostics. Indications for counseling are:

Established or suspected hereditary disease in the family

Birth of a child with malformations

Repeated spontaneous abortions, stillbirths, infertility

Lagging children in mental and physical development

Violation of sexual development

consanguineous marriages

Possible exposure to teratogens in the first 3 months of pregnancy

2. Determining the risk of having a sick child. When determining risk, the following situations are possible:

a) in case of monogenically inherited diseases, the calculation of risk is based on the laws of G. Mendel. This takes into account the genotype of the parents and the features of the expression of the gene (penetrance and expressivity).

b) for polygenically inherited diseases (diseases with a hereditary predisposition), special tables are used to calculate the risk, and the following features are taken into account:

The rarer the disease in the population, the higher the risk for the relatives of the proband

The more pronounced the disease in the proband, the higher the risk of the disease in his relatives.

The risk for relatives of the proband will be higher if there is another sick blood relative

c) sporadic cases of the disease: a sick child is born to phenotypically healthy parents, while there are no data in a similar pathology in relatives. The reasons:

Generative mutations in one of the parents or somatic mutations in the early stages of embryonic development

The transition of a recessive gene to a homozygous state

Concealment by one of the parents of family pathology.

3. Conclusion of consultation and advice to parents. A genetic risk of up to 5% is considered low and is not a contraindication for childbearing. The risk is from 6 to 20% - is defined as medium and is regarded as a contraindication to conception or as an indication for termination of pregnancy. Regardless of the degree of risk, prenatal diagnosis is advisable.

Prenatal (prenatal) diagnosis.

Many diseases can be detected even before the birth of a child. If serious diseases are detected in the fetus, the doctor offers the family an artificial termination of pregnancy. The final decision on this issue must be made by the family. Prenatal diagnostic methods include:

1. Biopsy of chorionic villi. Produced at 7-9 weeks of pregnancy. It serves to detect chromosomal defects, enzyme activity in order to diagnose hereditary metabolic diseases and DNA diagnostics.

2. Amniocentesis (taking amniotic fluid with cells contained in it). Produced from 12-14 weeks of pregnancy.

3. Cordocentesis (blood sampling from the umbilical vessels) is performed at 20-25 weeks of gestation and is used for the same purposes.

4. Maternal blood test. Detection of α-fetoprotein (a protein that is produced by the liver of the fetus and penetrates through the placental barrier into the mother's blood). An increase in it several times at the 16th week of pregnancy may indicate neural tube defects. A decrease in its concentration in relation to the norm may indicate Down syndrome.

5. An ultrasound examination of the fetus is performed at all stages of pregnancy. Ultrasound examination is the main method of visual determination of fetal malformations and the state of the placenta. Ultrasound examination is recommended for all women at least 2 times during pregnancy.

Mitochondrial diseases are a large heterogeneous group of hereditary diseases and pathological conditions caused by disorders in the structure and functions of mitochondria and tissue respiration. According to foreign researchers, the frequency of these diseases in newborns is 1:5000.

ICD-10 code

Metabolic disorders, class IV, E70-E90.

The study of the nature of these pathological conditions began in 1962, when a group of researchers described a 30-year-old patient with non-thyroidal hypermetabolism, muscle weakness, and a high level of basal metabolic rate. It has been suggested that these changes are associated with a violation of the processes of oxidative phosphorylation in the mitochondria of muscle tissue. In 1988, other scientists first reported the discovery of a mutation in mitochondrial DNA (mtDNA) in patients with myopathy and optic neuropathy. Ten years later, mutations in the nuclear genes encoding the respiratory chain complexes in young children were found. Thus, a new direction has been formed in the structure of childhood diseases - mitochondrial pathology, mitochondrial myopathies, mitochondrial encephalomyopathies.

Mitochondria are intracellular organelles that are present in the form of several hundred copies in all cells (except erythrocytes) and produce ATP. The length of the mitochondria is 1.5 µm, the width is 0.5 µm. Their renewal occurs continuously throughout the entire cell cycle. The organelle has 2 membranes - external and internal. From the inner membrane, folds called cristae extend inward. The inner space is filled with the matrix - the main homogeneous or fine-grained substance of the cell. It contains a circular DNA molecule, specific RNA, granules of calcium and magnesium salts. Enzymes involved in oxidative phosphorylation (complex of cytochromes b, c, a and a3) and electron transfer are fixed on the inner membrane. This is an energy-converting membrane that converts the chemical energy of substrate oxidation into energy that accumulates in the form of ATP, creatine phosphate, etc. Enzymes involved in the transport and oxidation of fatty acids are concentrated on the outer membrane. Mitochondria are capable of self-replication.

The main function of mitochondria is aerobic biological oxidation (tissue respiration using oxygen by the cell) - an energy use system organic matter with its gradual release in the cell. In the process of tissue respiration, there is a sequential transfer of hydrogen ions (protons) and electrons through various compounds (acceptors and donors) to oxygen.

In the process of catabolism of amino acids, carbohydrates, fats, glycerol, carbon dioxide, water, acetyl-coenzyme A, pyruvate, oxaloacetate, ketoglutarate are formed, which then enter the Krebs cycle. The resulting hydrogen ions are accepted by adenine nucleotides - adenine (NAD +) and flavin (FAD +) nucleotides. The reduced coenzymes NADH and FADH are oxidized in the respiratory chain, which is represented by 5 respiratory complexes.

In the process of electron transfer, energy is accumulated in the form of ATP, creatine phosphate and other macroergic compounds.

The respiratory chain is represented by 5 protein complexes that carry out the entire complex process of biological oxidation (Table 10-1):

• 1st complex - NADH-ubiquinone reductase (this complex consists of 25 polypeptides, the synthesis of 6 of which is encoded by mtDNA);
• 2nd complex - succinate-ubiquinone oxidoreductase (consists of 5-6 polypeptides, including succinate dehydrogenase, encoded only by mtDNA);
• 3rd complex - cytochrome C-oxidoreductase (transfers electrons from coenzyme Q to complex 4, consists of 9-10 proteins, the synthesis of one of them is encoded by mtDNA);
• 4th complex - cytochrome oxidase [consists of 2 cytochromes (a and a3), encoded by mtDNA];
• 5th complex - mitochondrial H + -ATPase (consists of 12-14 subunits, carries out ATP synthesis).

In addition, the electrons of the 4 fatty acids undergoing beta-oxidation are carried by an electron-carrying protein.

In mitochondria, another important process is carried out - beta-oxidation of fatty acids, which results in the formation of acetyl-CoA and carnitine esters. In each fatty acid oxidation cycle, 4 enzymatic reactions occur.

The first stage is provided by acyl-CoA dehydrogenases (short, medium and long chain) and 2 electron carriers.

In 1963, it was found that mitochondria have their own unique maternally inherited genome. It is represented by a single small circular chromosome 16,569 bp long, encoding 2 ribosomal RNAs, 22 transfer RNAs, and 13 subunits of enzyme complexes of the electron transport chain (seven of them belong to complex 1, one to complex 3, three to complex 4, two - to complex 5). Most of the mitochondrial proteins involved in the processes of oxidative phosphorylation (about 70) are encoded by nuclear DNA, and only 2% (13 polypeptides) are synthesized in the mitochondrial matrix under the control of structural genes.

The structure and function of mtDNA differs from that of the nuclear genome. First, it does not contain introns, which provides a high gene density compared to nuclear DNA. Second, most mRNAs do not contain 5'-3' untranslated sequences. Third, mtDNA has a D-loop, which is its regulatory region. Replication is a two-step process. Differences between the mtDNA genetic code and the nuclear one were also revealed. Of particular note is that there is big number copies of the first. Each mitochondria contains 2 to 10 copies or more. Considering the fact that cells can contain hundreds and thousands of mitochondria, up to 10,000 copies of mtDNA may exist. It is very sensitive to mutations and currently 3 types of such changes have been identified: point mutations of proteins encoding mtDNA genes (mit- mutations), point mutations in mtDNA-tRNA genes (sy/7-mutations), and large mtDNA rearrangements (p-mutations).

Normally, the entire cellular genotype of the mitochondrial genome is identical (homoplasmy), however, when mutations occur, part of the genome remains identical, while the other part remains changed. This phenomenon is called heteroplasmy. The manifestation of a mutant gene occurs when the number of mutations reaches a certain critical level (threshold), after which there is a violation of the processes of cellular bioenergetics. This explains the fact that with minimal disturbances, the most energy-dependent organs and tissues (nervous system, brain, eyes, muscles) will suffer first of all.

Charlie Guard, a terminally ill baby in the UK, is receiving international attention as his parents seek an experimental treatment they hope can help their son, whose rare type of "DNA depletion" usually results in death in the first few months of life. But what causes this condition and why does it have such a devastating effect on the body?

Charlie Guard with his parents

Charlie was born on August 4, 2016 and has been admitted to Great Ormond Street Hospital in London since October, according to The New York Times. The 11-month-old baby is reported to be unable to breathe on her own, has convulsions, is blind and deaf. His parents want to take him to the United States for an experimental treatment, but the doctors disagree, stating that the treatment will not help and will only prolong Charlie's suffering. Instead, the hospital concluded that the most humane solution would be euthanasia.

This case reopened the debate about the rights of parents to treat their children. Several British courts sided with the hospital. However, his parents said the hospital delayed the euthanasia to give them more time to say goodbye to their child.

Encephalomyopathic Mitochondrial DNA Depletion Syndrome is caused by mutations in genes that help maintain the DNA found inside the mitochondria (the "powerhouses" of cells) that convert nutrients into energy and have their own set of DNA.

In Charlie's case, the mutation is in a gene called RRM2B, which is involved in making this mitochondrial DNA. The mutation leads to a decrease in the amount of mitochondrial DNA and prevents the normal functioning of mitochondria.

The disease affects many organs of the body, but especially the muscles, brain and kidneys, which have high energy requirements. This can cause muscle weakness, microcephaly (smaller-than-normal head size), kidney problems, seizures, and hearing loss. Weakness in the muscles used for breathing can lead to serious breathing problems and, in Charlie's case, ventilation was required.

The disease is extremely rare. Prior to Charlie's case, only about 15 babies worldwide had this particular form of mitochondrial DNA depletion syndrome.

Symptoms usually start very early. According to The Times, Charlie started showing signs when he was just a few weeks old. And children with this condition usually do not survive beyond infancy. In a 2008 review of the cases of seven children with mitochondrial DNA wasting syndrome due to mutations in the RRM2B gene, all died before they reached 4 months of age.

There is no cure, only symptom management, such as providing nutritional support or ventilation to help with breathing, according to a University of Washington review.

Charlie's parents said they want their son to have an experimental treatment called nucleoside therapy, an unproven treatment that targets DNA materials that his cells cannot produce. This treatment has previously been used for patients with a less severe form of mitochondrial DNA depletion known as a TK2 mutation, according to the Times. However, the therapy has never been used for RRM2B mutation. And even the doctor who initially agreed to help the Gards with this treatment later admitted that therapy was unlikely to help Charlie because the child was in the advanced stages of the disease.

Recently, the Bambino Ges hospital in Italy asked if the baby could be moved to them, but Great Ormond Street refused to move him, citing legal reasons, according to The Washington Post. British Foreign Secretary Boris Johnson also said the "decision continues to be guided by expert medical advice upheld by the courts."