CC BY
12Received by the Editor 04.10.2020
IRSTI 76.29.51+76.03.39
CLINICAL ANDGENETIC CHARACTERISTICS OF DUCHENNE MYODYSTROPHY
A. Umurzakova, D. Ayaganov
West Kazakhstan Marat Ospanov Medical University, Aktobe city, Kazakhstan
The clinical practice of neuromuscular diseases is currently undergoing huge changes, directly related to the latest molecular genetic discoveries. Most gene findings in the field of neurological pathology relate to neuromuscular diseases.
A direct consequence of these discoveries is the possibility of accurate diagnosis based on DNA, which gives patients clear prognostic information and allows genetic counseling on the inheritance of pathology.
Key words: neuromuscular desorders, Duchenne myodystrophy, mutations, genetic research, Next Generation Sequensing.
КЛИНИКО-ГЕНЕТИЧЕСКИЕ ХАРАКТЕРИСТИКИ МИОДИСТРОФИИ ДЮШЕННА
А.О. Умурзакова, Д.Н. Аяганов
Западно-Казахстанский медицинский университет имени Марата Оспанова, Актобе, Казахстан
Клиническая практика нервно-мышечных заболеваний в настоящее время претерпевает огромные изменения, непосредственно связанные с последними молекулярно-генетическими открытиями. Большинство генетических находок в области неврологической патологии относятся к нервно-мышечным заболеваниям. Прямым следствием этих открытий является возможность точной диагностики на основе ДНК, что дает пациентам четкую прогностическую информацию и позволяет проводить генетическое консультирование по вопросам наследования патологии.
Ключевые слова: нервно-мышечные заболевания, миодистрофия Дюшенна, мутации генетические исследования, секвенирование нового поколения.
ДЮШЕНН МИОДИСТРОФИЯСЫНЬЩ КЛИНИКАЛЬЩ ЖЭНЕ ГЕНЕТИКАЛЬЩ СИПАТТАМАЛАРЫ
А.О. Умурзакова, Д.Н. АяFанов
Марат Оспанов атындагы Батыс ^азакстан медициналык университет^ Актебе, ^азакстан
ЖYЙке-б¥лшыкет ауруларыныц клиникалык практикасы казiргi уакытта молекулалык-генетикалык ашылуларга байланысты Yлкен езгерютерге ^шырады. Неврологиялык патология саласындагы гендж олжалардыц кепшшп жуйке-б^лшыкет ауруларына жатады.
Б^л жацалыктардыц тiкелей салдары ДНК^-га негiзделген дэл диагностиканыц мYмкiндiгi болып табылады, пациенттерге накты болжамдык акпарат жэне патология т^кым куалау мэселелерi бойынша генетикалык кецес жYргiзуге мYмкiндiк бередi.
Непзп сездер: жYЙке-б¥лшыкет аурулары, Дюшенн миодистрофиясы, мутациялар, генетикалык зерттеулер, жаца буынды секвенирлеу.
Relevance
Neuromuscular disorders are a large heterogeneous group of diseases that usually progress and cause symptoms in a variety of age ranges with varying degrees of severity, and prognostically lead to early disability of patients whose main pathogenetic mechanism of development is a genetically determined lesion of the neuromuscular apparatus[1,2]. According to the World Health Organization (WHO), about 10-15% of newborns have congenital and hereditary diseases (Genomics and Health in the Developing World, 2012; Passarge E, 2013).About half of the cases of early childhood mortality and disability are due to various genetic causes(Passarge Е., 2013). The total prevalence of neuromuscular diseases in the world is 27,2 per 100,000 people, while the "core" of the nosological
spectrum is formed by diseases that occur with a frequency of 1:50,000 or more often (spinal muscular atrophy, myotonia, progressive muscular dystrophy, hereditary motor-sensory neuropathies), the clinical manifestations of this group of diseases are muscle weakness, muscle atrophy, static and locomotion disorders[3-5]. Epidemiological studies report an increase in the prevalence of neuromuscular disorders worldwide[6,7]. The greatest interest has recently been growing in relation to new aspects of pathogenetic treatment, such as dystrophinopathy and spinal muscular atrophy. One of the rapidly progressive forms of dystrophinopathy is known to be Duchenne myodystrophy (DMD), debuting at an early age and leading to early disability, this disease is the most discussed [8]. MDD is an aggressive disease with a fatal outcome, being an X-linked variant, affects only boys[9]. Despite the fact that this disease has been known since 1868, there are still many debatable aspects of the behavior of the mutation in the DMD gene, since there are many mutation variants. The prevalence of the disease in various sources varies from 1:3500 to 1:6300 live-born boys.So, per 100 thousand men in the United States recorded 15.9 cases, in the UK — 1,5[10], in the Republic of Dagestan — 6,64[11], in the Rostov region - 4,8[13], in the Chuvash Republic - 2,95 cases[14]. However, no special studies have been conducted in the Russian Federation as a whole, so the prevalence of DMD can be considered poorly understood.
Pathogenesis. The dystrophin gene (DMD) is mapped in the region of the short arm of the X chromosome (loci Xp21.1-P21.2)[15]. The coding composition of the gene is divided into 78 exons and 78 introns, the non-coding part. Dystrophin is a protein from the class of spectrin-actin membrane proteins and is part of the dystrophin-associated protein complex (DAPC).The protein plays a significant structural role in muscle fibers, ensuring their integrity by stabilizing the cell membrane. In the absence of dystrophin, the muscle fiber is not formed, but is replaced by connective tissue cells, thus progressing muscular dystrophy. However, dystrophin, being a protein present mainly in muscle cells, is found in other non-muscle tissues[16,17].Isoforms of dystrophin are also found in cells of the central nervous system (CNS), cardiomyocytes, in cells of the spleen, liver, and retina. The main role of the dystrophin isoform represented in neurons is considered to ensure the functioning of potential-dependent ducts and stimulate the formation of synapses. As a result of mutations in the dystrophin gene, the structure and functioning of the DAPC complex is disrupted, which leads to the development of symptoms affecting the tissues expressing this protein: progressive muscle weakness, cardiomyopathy, and mental retardation.The above-described pathogenetic mechanism of violation of the muscle framework leads to progressive muscle weakness, which is expressed in each patient differently at the age of debut, depending on one or another form of mutation in the DMD gene.A common age for the onset of muscle weakness is the age of 5 years, increasing in dynamics, muscle weakness leads up to immobility at the age of 10-12 years, referred to as the non-ambulatory period [18,19]. One of the key and significant signs of Duchenne muscular dystrophy is a severe progressive pathology of the muscular system caused by violation of ability to decrease muscle fibers, reflected progressive muscle weakness, with subsequent reduction of active movements, decreased tone, atrophy of muscles according to a certain muscle formula . Most patients are characterized by a delay in early motor development. At the beginning of walking (at the age of 14 months and older), there are frequent falls, awkwardness in movements, and rapid fatigue. Early development of Achilles tendon retraction- walking on socks. One of the main symptoms is also the features of getting up from the floor or climbing stairs, helping yourself, the patient makes various auxiliary movements, called as a symptom of Govers (reception of Govers) [20,21].As the disease progresses, namely, with the replacement of connective muscle tissue, especially noticeable in the calf muscles causes pseudohypertrophy of these muscles. Sometimes they note the large size of the calf muscles and a change in gait (waddling gait — Duchenne gait)[22]. All gluteal muscles are affected (strength less than 3 points), then the quadriceps and adductor muscles of the thighs. Relatively preserved function (muscle strength 3 points or higher) of the iliopsoas, posterior thigh muscles and fully preserved leg muscles (at an early stage of the disease). Weakness (and atrophy) capture the same muscles of the right and left sides, that is, there is a symmetry of the lesion, usually characteristic of hereditary myopathies. Deep reflexes on the legs disappear early. There is an upward spread of muscle lesions. As the disease develops, there are secondary deformities of the spine (lumbar hyperlordosis, kyphosis,
scoliosis), the chest, which becomes saddle-shaped or keel-shaped. Formed "wasp waist", pterygoid shoulder blades, a symptom of " free shoulder pads»[23,24]. Patients cannot raise their arms above the horizontal level, while the volume of movement in the elbow and wrist joints and muscle strength remain preserved for a long time. The muscles of the distal parts of the arms and legs retain trophism and strength (including the calf muscles in the early phase of the disease). Disappear deep reflexes of the arms. When examining the patient, pseudohypertrophy of the calf and deltoid muscles catches the eye, creating a false impression of an athletic build. To the touch these muscles have an unusual consistency: «dough»[25]. As the disease progresses, pseudohypertrophy can be replaced by muscle atrophy. If the pelvic girdle muscles are affected, it is difficult to climb the stairs or get up from a sitting position. At the same time, the patient helps himself, leaning on foreign objects, gets up in several stages using the Govers technique ("climbing by yourself1', "climbing a ladder") [26]. Lifting the patient and changing the position in bed occur in stages. To change the position, e.g. supine to sitting, the patient should turn back up, down buttocks on your heels, then straighten torso, leaning his arms on the bed, finally free from under her feet. To get out of bed, the patient has to turn his back up with a series of roundabout movements and grips, then lower one leg after the other to the floor, and then proceed to the most difficult maneuver - the extension of the trunk. For this, a patient finds the highest point of support of: table, headboard, etc., and uses it to lift the torso as you can, after that, pushing away his arm, and abdominal muscles the one hand, he reaches that the body describes an arc and is bent to the side[27]. When the torso thus comes into the same frontal plane with the lower limbs, a slight strain of the muscles is enough to deflect it posteriorly.The back assumes the only position in which the patient can stand without support and walk. With this form of myodystrophy, the heart muscle can be affected, at least half of the patients have dilated cardiomyopathy, myocardial hypertrophy, and myocardiodystrophy [28]. Due to significant violations in cortical architectonics in conjunction with dendrite abnormalities in neurons that usually express dystrophin, children with Duchenne disease may experience a decrease in intelligence. It is believed that intellectual disabilities are more likely to affect verbal than non-verbal skills[29]. During the course of the disease, it is advisable to consider several consecutive stages: 1) asymptomatic stage; 2) early stage of preserving the ability to move independently; 3) late stage of preserving the ability to move independently; 4) early stage of violation of the ability to move independently; 5) late stage of violation of the ability to move independently[29,30]. The disease progresses quickly, invalidizes the patient early, so that by the age of 10-15, he becomes a "wheelchair" and cannot stand and move independently. Death occurs in 20-25 years from pneumonia (due to weakness of the respiratory muscles) or heart failure[31].
Genetics
MDD is inherited by a recessive, X-linked type. Boys get sick, women transmit the disease. This is confirmed by the analysis of pedigrees in which the disease is manifested only in males, the appearance of sick boys with DMD or Becker's myodystrophy (MDB), born to the same mothers, but different fathers[31]. The most common marriage situation is the marriage of a heterozygous woman with a healthy man. The risk of having sick sons in such a marriage is 50% of each new pregnancy, 50% of daughters are hidden carriers of the gene. In extremely rare, exceptional cases, MDD has been reported in women. All cases described so far in women fall into several categories[32].
The first is the presence of a phenotypic woman with only one X chromosome. This is possible with Shereshevsky-Turner syndrome- a chromosomal disorder associated with monosomy on the X chromosome (45X) or with a combination of two hereditary syndromes: MDD/MDB and testicular fenimization syndrome, in which a child with a female phenotype is born with karyotype 46XY[33]. If a single X chromosome has a mutation in the dystrophin gene, the woman will have MDD/MDB. The second category, the disease can manifest itself in a female carrier of dystrophin mutations-unequal lyonization of the X chromosome. The third category is if bothX chromosomes are damaged by the DMD gene. This combination is possible in the daughters of MDB patients or carriers of mutations in the DMD gene[34,35].
The most frequent mutations in DMD are large deletions of one or more exons, the frequency of occurrence is 60-65%, duplications are 6-10% of different gene regions, most often the lesion of
exons is 45-55, in the remaining 35% of cases - point mutations and microdeletions that violate the information reading frame, which leads to the termination of the synthesis of the normal dystrophin protein[36].
«Reading frame» - a sequence of nucleotides that sets the position of the first base. When the reading frame is shifted, there is a very high probability of accidental formation of a stop codon, which will result in premature termination of translation, a shortened protein is formed that is not protected from proteolytic enzymes, and complete degradation of this protein is possible at the time of synthesis or after it[37,38]. According to the Monaco rule, deletions (within the Central domain) that violate the reading frame lead to a heavy MDD phenotype, while deletions that preserve the reading frame lead to a milder MDB phenotype[39,40].
Most often, genetic studies such as multiplex polymerase chain reaction (PCR), multiplex ligase amplification of the probe (MLPA) are used to identify mutations in the DMD gene[41].Multiplex PCR is widely available and least expensive, but it only detects deletions and does not analyze the entire gene, so it is not always possible to characterize the detected deletions [42]. MLPA can detect both deletions and duplications and can analyze all exons. If the deletion/duplication could not be detected , it is necessary to re-sequence the gene in order to search for point mutations or small deletions (inserts). In cases after a genetic study, but the identification of the mutation was not successful, while a high concentration of creatine kinase and signs characteristic of DMD are determined, the next diagnostic step is a muscle biopsy. This also applies to cases where the suspicion of a DMD diagnosis is supported by a family history, but the presence of a family mutation remains unknown [42,43].
The above circumstances aim at a targeted study of this issue in the context of the Republic.
Conclusion
Knowledge of the molecular and genetic basis of pathogenesis of dystrophin synthesis disorders is the basis of pathogenetic therapy.Various approaches to mutation correction or frame reading therapies offered on the market are based on accurate mutation data, the correction of which leads to the continuation of protein synthesis, which can preserve muscle strength. All these measures will be effective if early diagnosis and interventions are made. The issue of medical and genetic counseling (MGC) remains very important, since many cases of lesions of all boys in the same family are described, with untimely MGC, which is the basis for secondary prevention of hereditary diseases. Further searches for new mutations in the DMD gene will allow a deeper understanding of the pathomechanism and the possibility of alternative restoration of protein synthesis in the gene itself.
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Corresponding author: Ainur Umurzakova, e-mail: umurzakova.aa@mail.ru
