fall heme week 2

hemoglobinopathies

genetically determined abnormality of the sturcture(qualitative) or syn(quantitative) of the hgb molecule in globin chains

globin chain struc (qualitative) abnormality clinical expression

can be associated with chronic hemolytic anemia or asymptomatic, depends on class, hemolysis severity, compensatory production of other globin chains, most common in africa, mediterranean and se asia

structural globin chain (qualitative) abnormality

any chain affected, sickle cell anemia, more than 1,200 abnormal hgbs, most mutations in B chain, hgb F (Y chain, not detected after 3-6 months), hgb A2 (delta chain, hgb less than 4%, not sig)

alpha gene mutation

2 copies on each 16 chromosome (4 total), mutation of a single locus= small amount of abnormal hgb, not likely sig

beta chain mutations

single copy on each chromosome 11, clinical phenotype, component of hgb A (major adult hgb)

ID hgb variants

most common involve a single AA sub or deletion, expressed due to hgb molecule altered in function (O2 affinity), stability, solubility

CBC test for hemoglobinopathies

anemia, rdw to distinguish from IDA, rbc indicies to distinguish structural defects from thalassemia (quantity defect)

hgb variant testing test for hemoglobinopathies

separate and quantify hgb fractions, HPLC, capillary and traditional electrophoresis, isoelectric focusing IEF

hgb electrophoresis test

electric charge depends on AA sequence (sub= structural variant), pH of the surrounding medium (ID hgbs)

patterns in beta chain disorders

decreased or absent hgb A (a2B2), increased hgb F and/or increased hgb A2 (a2delta2), band rep mutant hgb may be present

tests for altered physical properties

solubility- normal hgb stays dissolved in cytoplasm, heat precipitation- variant denatures causing precipitates= heinz bodies

molecular tests

PCR and restriction fragment length polymorphism RFLP

mech of altered solubility

a nonpolar AA replaces a polar AA near the molecules surface, Hb S and Hb C

mech of altered function

AA sub near heme pocket or at the a1B1 interface changes how readily the molecule binds and releases oxygen through iron oxidation or altered allosteric behavior

iron oxidation

sub stabilizes heme iron in ferric state causing methemoglobin inability to bind oxygen, hgb M

altered allosteric behavior

sub locks hgb into relaxed or tensed state increasing or decreasing oxygen affinity, Hb Chesapeake= increased affinity

mech of altered stability

reduced stability of hgb tetramer creates unstable hgb, mutations disrupt H bonding and hydrophobic interactions weakening the binding of heme to globin, hgb denatures aggregates and precipitates, congenital heinz body hemolytic anemias

epidemiology of sickle cell anemia

most common symptomatic hemoglobinopathy, tropical africa, mediterranean, ME, india and nepal, migration (african amer 1.3-1.3% sickle cell disease 2 copies of defective Beta gene, 8-10% sickle cell trait 1 copy of defective beta gene)

Hb S structure in sickle cell anemia

B6(A3) Gly→Val, nonpolar valine sub for polar glutamic acid at the 6th position in the A3 helix of the beta chain, decreased O2 affinity of hgb S compared to A

polymerization in sickle cell anemia

solubility in the deoxygenated state is markedly decreased, rigid aggregates, reversible on reoxygenation but time dependent (2-4 min before polymerization and rbc distortion)

deoxygenation→ polymerixation causes

hypoxia (low pO2 in tissue), acidosis (dissociation right shift, Bohr effect), hypertonicity (H2O pulled out raising mchc), temp greater than 37 (increases kinetics)

sickling begins in

spleen, kidney, retina, and bm depending on proportion of non-S hgbs (interfer with polymerization)

polymerization O2 sat

begins when O2 sat is less than 85%, completes when 38%

repeated sickling cycles damage cell membranes

disrupts cation hemostasis causing loss K+ and water from the cel, increased mchc, increased cytoplasmic viscosity, decreased cell deformability, irriversible when 5-50% of rbc in circulation