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Terms in this set (22)
what do hbopathies result from?
structural issues or default in synthesis. note that at least 800 variants of Hb have been described. most of these variants are point mutations resulting in single amino acid substitutions and are harmless.
what types of mutations of Hb will cause disease?
-those affecting the heme binding pocket-->methemoglobinemia
-unstable Hbs can cause hemolytic anemias
replacement of proximal histidine with tyrosine, making heme inaccessible to methemoglobin reductase. heterozygotes with this are cyanotic but healthy
describe hemolytic anemic (due to unstable Hb)
-mutations lead to spontaneous denaturation of Hb. the Hb will aggregate forming insoluble Heinz bodies. the abnormal cells are then removed by spleen causing anemia.
what happens if mutations affect interface between the subunits?
-produces Hb wth abnormal O2 affinities. this can cause polycythemia (increased # of RBCs). erythropoiesis happens because increased O2 affinity leads to decreased O2 in tissues and this is body response.
what happens if mutations cause alterations in mRNA processing/degradation of mRNA?
-will lead to anemia if alpha or beta chains are affected. patients will present with thalassemia
describe sickle cell anemia:
-severe hemolytic disease
-beta chain mutation where glutamate is replaced by a valine. this leads to production of Hb S. Note that this removes a negative charge from the beta chain.
-HbS is produced normally and exhibits near normal O2 affinity BUT H2O solubility is reduced.
-only homozygotes have disease, so autosomal recessive mode of inheritance
what is HbA?
what form of HbS is insoluble?
The deoxy form-enough so ppt out of solution and cause a sicking crisis
when do first signs of sickle cell appear?
when fetal Hb has been replaced with adult Hb. note that oxygen depleted vasculature is prone to sickling, which destroys RBCs. then splenic clearance leads to anemia.
describe a sickling cirsis:
sickled cell can block cap. beds and cause infarcts. common in bone, joints and kidneys. patients present with severe pain
what should sickle cell patients avoid?
anything that can lead to hypoxia:
drugs that depress respiration rate
describe sickle cell heterozygotes:
protection vs. malaria: 70% HbA and 30% HbS. Their RBC's wont sickle under normal conditions.
-Hb structure normal, but reduced rate of synthesis.
-all thalassemias lead to anemia
-based on alpha or beta chain deficiency
-heterozygous thalassemias thought to provide protection vs. malaria
-homo trait can cause severe anemia
describe alpha thalassemia:
-caused by large deletions on one/both alpha genes on chromosome 16
-leads to accumulation of excess beta chains
-normal pts have 4 alpha genes--> disease severity dependent on how many of the 4 genes have been disrupted.
where else do we see alpha chains?
in the fetus. complete loss of alpha genes is fatal. this creates abnormal Hb with 10 times more O2 affinity that HbA. called Hb Bart gamma/delta
what is Hb H?
formed from the loss of 3 alpha genes, leading to unstable beta 4 tetramer. note that smaller deletions generally prevalent for thalassemia
what is beta thalassemia?
-caused by different types of mutations
-some caused by large deletions but most from single base deletions. some mutations cause complete loss of beta chains. wide range of clinical severity.
-alpha chains (in excess here) are insoluble so form ppt. these cells are seen as abnormal by macros in bone marrow and spleen-->leads to abortive erythropoiesis and destruction of rbc pecursors in the marrow and premature destruction of RBCs in spleen.
what causes facial deformities in beta thalassemia?
bone marrow response leading to expansion of red bone marrow. eventually you will get extramedually erythropoiesis established in liver and spleen.
how to die from beta thalassemia?
-likely to die of severe anemia or infection in infancy/childhood without tx.
-from iron overload due to regular blood transfusions (~20yo).
what is desferrioxamine?
iron chelator that forms soluble iron complex excreted by the kidneys-prevents iron overload.
major Hb found in homozygous Beta thalassemia?
HbF. severity of disease is inversely related to HbF levels. HbF is protective in all beta chain abnormalities including sickle cell
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