Pathogenesis of aplastic anemia

According to modern concepts based on numerous cultural, electron microscopic, histological, biochemical, and enzymatic research methods, three main mechanisms are important in the pathogenesis of aplastic anemia: direct damage to pluripotent stem cells (PSCs), changes in the microenvironment of the stem cell and, as a result, inhibition or disruption of its function; and an immunopathological condition.

According to modern concepts, the cause of pancytopenia at the cellular and kinetic level is a significant decrease in the number of PSCs and more mature committed precursors of erythro-, myelo- and thrombocytopoiesis. A certain role is also played by a qualitative defect of residual stem cells, expressed in their inability to produce an adequate number of mature descendants. The defect of PSCs is a primary disorder that manifests itself or intensifies under the influence of various etiologic factors. The primacy of the defect of PSCs, as a leading factor in the pathogenesis of aplastic anemia, is based on the detection of a sharp decrease in the colony-forming capacity of bone marrow cells in patients, which persists even during the period of clinical and hematological remission, and the detection of morphologically defective hematopoietic cells, indicating the functional inferiority of PSCs. It has been established that when the level of PSC decreases by more than 10% from the norm, an imbalance of differentiation and proliferation processes occurs with differentiation predominating, which most likely explains the decrease in the colony-forming capacity of the bone marrow. The primacy of the PSC defect in aplastic anemia is confirmed by the following facts:

• the development of aplastic anemia is possible against the background of taking chloramphenicol (levomycetin), which irreversibly inhibits the incorporation of amino acids into mitochondrial proteins and RNA synthesis in bone marrow precursor cells, which leads to a disruption of their proliferation and differentiation;
• radiation exposure causes the death of part of the PSC and changes developed in the stem system of irradiated individuals can be the cause of aplastic anemia;
• the effectiveness of allogeneic bone marrow transplantation in aplastic anemia has been proven;
• The connection between aplastic anemia and clonal diseases has been confirmed - the transformation of aplastic anemia into paroxysmal nocturnal hemoglobinuria, myelodysplastic syndrome, and acute myeloblastic leukemia is possible.

It is currently believed that the reduction of the hematopoietic progenitor pool is mediated by the mechanism of programmed cell death (apoptosis). The cause of the development of hematopoietic aplasias is probably increased apoptosis of stem cells. Increased susceptibility of stem cells to apoptosis may be congenital (such a mechanism has been postulated for congenital aplasias) or induced by hyperexpression of proapoptotic genes by activated participants of the immune response (idiopathic aplasias, aplasias after infusions of donor lymphocytes) or myelotoxic effects (γ-radiation). It has been established that the rate of reduction of the progenitor pool and specific effector mechanisms of apoptosis differ in different variants of AA.

An important aspect of the pathogenesis of aplastic anemia is the pathology of the hematopoietic microenvironment. A primary defect of the cells of the hematopoietic microenvironment is possible, as evidenced by a decrease in the colony-forming function of bone marrow fibroblasts and a change in the ultrastructural and ultracytochemical indices of bone marrow stromal microenvironment cells. Thus, in patients with aplastic anemia, along with total fatty degeneration, changes common to all stromal cells are noted, regardless of their localization in the bone marrow parenchyma. In addition, an increase in the content of mitochondria, ribosomes and polysomes in the cytoplasm of cells was found. A defect in the function of the bone marrow stroma is possible, which leads to a decrease in the ability of stromal cells to secrete hematopoietic growth factors. Viruses play a significant role in changing the hematopoietic microenvironment. It is known that there is a group of viruses capable of affecting bone marrow cells - these are hepatitis C virus, Dengue virus, Epstein-Barr virus, cytomegalovirus, parvovirus B19, human immunodeficiency virus. Viruses can affect hematopoietic cells both directly and through changes in the hematopoietic microenvironment, as evidenced by the detection of multiple pathological inclusions in the nuclei of almost all stromal cells according to electron microscopy. Persistent viral particles are capable of affecting the genetic apparatus of cells, thereby distorting the adequacy of the transfer of genetic information to other cells and disrupting intercellular interactions, which can be inherited.

Immunological mechanisms of aplastic anemia development are significant. Various immune phenomena have been described that may target hematopoietic tissue: increased activity of T lymphocytes (mainly with the CD 8 phenotype) with increased production of interleukin-2 and suppression of interleukin-1, depression of natural killer activity, impaired maturation of monocytes into macrophages, increased production of interferon, and possibly the presence of antibodies that inhibit the activity of colony-forming cells. Increased expression of DR 2 histocompatibility antigens and elevated levels of tumor necrosis factor, which is a potential inhibitor of hematopoiesis, have been reported. These immunological changes lead to inhibition of hematopoiesis and contribute to the development of hematopoietic aplasia.

Thus, the development of aplastic anemia is based on multifactorial pathological mechanisms.

As a result of the damaging effect, the bone marrow of patients with aplastic anemia undergoes a number of significant changes. Inevitably, the content of proliferating hematopoietic cells decreases, which leads to a varying degree of decrease in the cellularity (nucleation) of the bone marrow, as well as to the replacement of bone marrow with fatty tissue (fatty infiltration), an increase in the number of lymphoid elements and stromal cells. In severe cases, there is an almost complete disappearance of hematopoietic tissue. It is known that the lifespan of erythrocytes in aplastic anemia is shortened, which is usually due to a decrease in the activity of individual erythroid enzymes, while during an exacerbation of the disease, an increase in the level of fetal hemoglobin is noted. In addition, it has been established that intramedullary destruction of erythroid cells occurs.

Pathology of leukopoiesis is manifested by a decrease in the number of granulocytes and a violation of their function, there are structural changes in the lymphoid pool in combination with a violation of the kinetics of lymphocytes. Reduced indicators of humoral immunity (concentration of immunoglobins G and A) and non-specific defense factors (beta-lysines, lysozyme). Disruption of thrombopoiesis is expressed in thrombocytopenia, a sharp decrease in the number of megakaryocytes in the bone marrow, various morphological changes. The lifespan of platelets is moderately shortened.

In the pathogenesis of hereditary aplastic anemias, great importance is attached to genetic defects and the influence of unfavorable effects at the early stages of embryogenesis. At present, it has been established that the occurrence of hereditary aplastic anemias is associated with an increased congenital tendency of the PSC to apoptosis. Fanconi anemia may be inherited in an autosomal recessive manner; about 10-20% of patients are born from consanguineous marriages. Cytogenetic studies conducted in children with Fanconi anemia revealed distinct changes in the chromosome structure in the form of various chromosomal aberrations (chromatid breaks, gaps, rearrangements, exchanges, endoreduplications) caused by changes in chromosomes 1 and 7 (complete or partial deletion or transformation). Previously, it was believed that the pathogenesis of Fanconi anemia is based on a defect in DNA repair, since many agents called clastogens are used to diagnose Fanconi anemia, suggesting the above-mentioned mechanism. These agents (mitomycin C, diepoxybutane, nitrogen mustard) damage DNA by causing interstrand crosslinks, intrastrand crosslinks, and breaks. Currently, an alternative hypothesis is that the increased sensitivity of Fanconi anemia cells to mitomycin C is due to damage caused by oxygen radicals, rather than abnormalities in DNA crosslinks. Oxygen free radicals include superoxide anion, hydrogen peroxide, and hydroxyl radical. They are mutagens, and hydroxyl ion in particular can cause chromosomal abnormalities and DNA breaks. Various detoxification mechanisms exist to remove oxygen free radicals and protect cells from damage. These include the enzymatic systems superoxide dismutase (SOD) and catalase. Addition of SOD or catalase to lymphocytes of patients with Fanconi anemia reduces chromosome damage. Clinical studies using recombinant SOD have shown that its administration in some cases reduces the number of breaks. The data obtained served as the basis for reconsidering the role of oxygen free radicals in the existence of increased sensitivity of cells of patients with Fanconi anemia to mitomycin C and for studying the role of apoptosis in this situation. Mitomycin C exists in an inactivated state and as an oxide. Many enzymes in the cell can catalyze the loss of one electron in the mitomycin C molecule, which becomes highly active. At low oxygen concentrations, which exist in cells of hypoxic cell lines, mitomycin C reacts with DNA and leads to the formation of cross-links. However, at high oxygen concentrations, which are typical for normal cell culture, mitomycin C is overoxidized by oxygen to form oxygen free radicals, and its ability to cross-link DNA is significantly reduced. Apoptosis studies using special research systems have shown thatthat at low (5%) oxygen concentrations there are no differences in the severity of apoptosis in normal cells and cells of patients with Fanconi anemia. However, at high oxygen concentrations (20%), which promote the formation of free radicals under the influence of mitomycin C, apoptosis in cells of patients with Fanconi anemia is more pronounced and qualitatively different than in normal cells.

In Blackfan-Diamond anemia, it has been established that the disease is not associated with either a loss of the ability of the microenvironment to support erythropoiesis or with an immune response against erythroid precursors (studies supporting this hypothesis have shown transfusion-dependent alloimmunization). The most likely hypothesis for the development of Blackfan-Diamond anemia is an intracellular defect in signal transduction mechanisms or transcription factors at the stage of early hematopoiesis (the earliest erythroid precursor or pluripotent stem cell). Such changes can lead to increased sensitivity of erythroid cells to apoptosis: when cultured in vitro without erythropoietin, such cells enter programmed cell death faster than normal cells from control group individuals.

Genetics of Blackfan-Diamond anemia: more than 75% of cases are sporadic, 25% of patients have a mutation in the gene located on chromosomes 19ql3, encoding ribosomal protein S19. The consequence of this mutation is the development of Blackfan-Diamond anemia. The gene mutation was found in both sporadic and familial cases of anemia, when several patients with this anemia are observed in one family. Familial cases include a clear dominant inheritance of anemia in the proband and in one of the parents or the occurrence of anomalies in siblings born one after another; the possibility of autosomal recessive and X-linked inheritance types cannot be excluded. Random anomalies were found in most patients with Blackfan-Diamond anemia, for example, anomalies of chromosomes 1 and 16.

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