hereditary methemoglobinemia
Introduction
Introduction to hereditary methemoglobinemia Methoglobin (MetHb) is a genetic factor or a toxic compound that is produced by red blood cells. If it exceeds a certain level, it can cause methemoglobinemia, which is divided into acquired and hereditary. basic knowledge The proportion of illness: 60% (familial genetic disease, the incidence of family history is as high as 60%) Susceptible people: no special people Mode of infection: non-infectious complication:
Cause
Causes of hereditary methemoglobinemia
(1) Causes of the disease
Hereditary methemoglobinemia is due to the lack of cytochrome b5 reductase (b5R), b5R is a flavoprotein with membrane-bound and soluble, and two b5R are the expression products of the same gene. The chromosome, 31 kb in length, has been found to have at least 11 variants of b5R. In addition, five b5R variants have been found, and their electrophoretic behavior is abnormal, but the enzyme activity is normal and belongs to the polymorphism of the enzyme.
The disease is autosomal recessive, divided into 2 types:
Type I: Also known as simple red blood cell type, the patient has a hair loss since birth, and the blood MetHb content accounts for 8% to 50% of the total amount of Hb.
Type II: Also known as systemic type, all kinds of cells in the body lack b5R activity, including membrane-bound and soluble, accounting for about 10%. This type of b5R abnormality and fatty acid destruction and extension, cholesterol synthesis and certain drugs Related to metabolism.
In other cases, b5R-deficient heterozygotes have no increase in blood MetHb, but when exposed to drugs that produce MetHb, MetHb is produced much higher than normal.
(two) pathogenesis
The pathogenesis of hereditary metabolic defects is the same as other molecular diseases (such as hemoglobinopathy), mainly due to:
The 1-point mutation causes the first-order structural change of the protein (enzyme) encoded by the synthesis, the spatial structure is unstable, and the content caused by the degradation is reduced;
2 The spatial structure change of the enzyme not only affects the stability of the enzyme, but also changes or loses the enzyme activity;
3 Due to point mutations leading to errors in transcription, splicing, etc., it is impossible to synthesize or synthesize an enzyme protein with a large fragment deletion.
The replacement of an amino acid in the primary structure of a protein (enzyme) will affect the stability of its secondary and tertiary structures. Experiments have shown that several types of b5R variants listed above show reduced thermal stability. In the mature red blood cells, the nucleus and organelles have been lost. Unlike other tissue cells, the required enzymes can no longer be re-synthesized. Therefore, if the main defect is only the decrease in stability, only the b5R decrease and the activity decrease in the red blood cells are prominently manifested. It is characterized by type I MetHbemia. If the blood is centrifuged, it will be found that the underlying red blood cells (old red blood cells) have lower b5R activity and higher MetHb content than the upper layer of young red blood cells.
If the amino acid of some key parts of the b5R molecule is replaced, it not only affects the stability of the enzyme, but also affects its biological activity, such as the affinity with the substrate NADH is significantly reduced (the Km value increases), the result will seriously affect its Catalytic efficiency, the synthesis of enzymes that have lost activity, thus not only affecting red blood cells, but also the loss of b5R activity in various tissue cells, affecting important substance metabolism, such as cerebroside and ganglioside in neuromyelin Lipid biosynthesis disorders can cause nervous system dysplasia. Such mutations are clinically characterized by type II MetHb. Using genetic engineering and site-directed mutagenesis techniques, various mutant enzymes can be artificially synthesized in the laboratory for biochemistry. Characterization studies have also confirmed that all of the mutant enzymes that cause type II MetHbemia have a severe loss of catalytic efficiency.
In addition, as listed in the table, if the gene mutation affects the correct expression, such as mRNA splicing errors, early termination, large fragment omission, etc., it will also manifest as type II MHb.
Prevention
Hereditary methemoglobinemia prevention
There is no effective preventive measure for this disease. Early detection and early diagnosis are the key to the prevention and treatment of this disease.
Complication
Hereditary methemoglobinemia complications Complication
Generally no complications.
Symptom
Hereditary symptoms of methemoglobinemia Common symptoms Breathing shortness of breath Shortness of heart palpitations Mental retardation
Type I patients have cyanosis since birth. Because MetHb is brown, its hair loss is more obvious than that of general Hb hypoxia. MetHb accounts for 5% to 50% of total Hb. The general case is asymptomatic, while a few cases are MetHb. When the total amount of Hb is 40% or higher, the patient feels guilty, short of breath, and even more obvious dyspnea. He is qualified for general physical labor. Some patients have white blood cells in addition to red blood cells, b5R enzyme activity in platelets and fibroblasts. reduce.
The clinical manifestations of type II patients are severe mental and developmental disorders, neuropsychiatric abnormalities such as small head, angular arch reversal, hand and foot tremor, and generalized muscle tone loss.
Examine
Examination of hereditary methemoglobinemia
In most patients, red blood cells do not increase. A few patients have red blood cell hyperplasia, red blood cells in the blood, increased reticulocytes, but normal MCV, MCH, MCHC, and normal red blood cell salt fragility, indicating that the red blood cell thickness is normal, the red blood cell life is normal, and a few cases have Those who have combined jaundice (hemolytic).
1. Visual observation of heparin anticoagulation in the medium test tube, the blood is brownish brown, the color does not change after shaking for 1 min in the air, or take 1 drop of peripheral blood on the filter paper, the color is still visible after shaking for 30s in the air. Brown, if necessary, with normal blood control, the above test can rule out hypoxic hair loss caused by respiratory or circulatory failure.
2. Absorption spectrum of MHb Blood is diluted 5 to 20 times with distilled water, and directly observed with a spectroscope. There is one dark band in the red zone, and 10 drops of potassium cyanide (sodium) or dithionke is added. The band disappeared, or the wavelength of the recording spectrophotometer was used to scan, and the change of the absorption spectrum around 630 nm before and after the addition of potassium cyanide was observed. The test should have a normal blood control.
3. Quantification of MHb The MetHb content was determined by Evelyn and Malloy spectrophotometry.
4. MHb reduction test The patient or normal human erythrocytes are treated with nitrite and incubated in lactic acid monophosphate buffer for several hours (or overnight). The color of red blood cells of normal and toxic MetHb is changed from chocolate to bright red. The color of red blood cells of congenital MetHb is still chocolate, and the heterozygous red blood cells are brownish red.
5. Enzyme activity determination NADH-MetHb reductase activity was determined by using cyanmethemoglobin as substrate, or diaphorase activity was determined by using dichlorophenol phenol as substrate, or b5R activity was determined by using cytochrome b5 as substrate. The results measured by the method, although not completely parallel, were significantly reduced compared with normal people, and the heterozygotes were centered. It should be emphasized that due to the different mechanisms of action of different genetic variants, in the test tube (high substrate concentration) The results of the assay do not accurately reflect the extent of catalytic efficiency reduction in living cells (at low substrate concentrations), because if the molecular structure of the enzyme changes its affinity to the substrate NADH (Km value increases), Under physiological conditions, the concentration of NADH in the cells is very low, and the enzyme activity is almost completely lost. However, when the enzyme activity is measured in a test tube, the added NADH is equivalent to several tens of times or even hundreds of times the physiological concentration, and the activity of the enzyme can be completely measured. If the enzyme activity is measured at a low substrate concentration, the instantaneous initial velocity must be recorded by time scanning, otherwise the error is too large, and other genetic enzyme deficiency diseases have similar problems.
6. Determination of enzyme antigen The amount of b5R antigen can be determined by Western blot or ELISA double antibody sandwich method. The general enzyme antigen amount is mostly low, but some b5R variants, the enzyme activity is reduced, not necessarily accompanied by the decrease of enzyme antigen amount. Not completely parallel, but simultaneous determination of the amount of enzyme antigen is important for identifying different variants and explaining their pathogenesis.
7. Molecular biology experimental techniques to further determine the variant, detect heterozygotes, conduct family surveys and prenatal diagnosis, and choose various molecular biology experimental techniques.
According to clinical manifestations, symptoms, signs, choose to do ECG, chest X-ray, B-ultrasound, and other tests.
Diagnosis
Diagnosis and differentiation of hereditary methemoglobinemia
diagnosis
According to the clinical manifestations and the reduction of reduced Hb (detailed examination of cardiopulmonary abnormalities), except for hemoglobinemia M (with specific absorption spectrum), simultaneous confirmation of b5R activity and enzyme antigenicity determination can be confirmed.
Differential diagnosis
1. Anoxic hair loss.
2. Congenital interventricular septal defect.
3. Abnormal hemoglobin disease has found several hemoglobin M. Due to some mutations in the globin peptide chain close to the heme prosthetic group, Fe2 in the heme prosthetic group can be changed to Fe3. The characteristic of hemoglobin M is characteristic. Absorption spectrum, and can not be reduced, in addition, some abnormal Hb affects the affinity with oxygen, more oxygen release, will lead to a significant increase in deoxygenated Hb content in the blood, there will also be familial hairpin, unstable Hb will also lead to MetHb The content of the abnormal Hb disease is dominant, which is different from the genetic law of congenital MetHb.
