Red Cell Membrane Disorders Flashcards
What are red cell membrane disorders?
- intrinsic defects of the red cell membrane => premature red cell destruction
- majority are hereditary rather than acquired
RBCs must have deformability in order to squeeze through small spaces. This is based on 3 main cellular properties
- biconcave discoid geometry
- viscoelastic nature (elasticity) of the membrane
- cytoplasmic viscosity
viscoelastic property of red cell membrane is affected by
mutations that alter proteins responsible for membrane structure
cytoplasmic viscosity of red cell may be affected by
numerous mechanisms but membrane disorders are a rare cause and are due to membrane transport protein defects
mutations that alter membrane structure
- hereditary spherocytosis
- hereditary elliptocytosis
- hereditary ovalocytosis
mutations that alter membrane transport proteins
- dehydrated hereditary stomatocytosis
- overhydrated hereditary stomatocytosis
T or F. hemolysis secondary to intrinsic RBS membrane defect is immune and DAT positive
F! non-immune and DAT negative
Lab manifestation of hemolysis
- increased total bilirubin, decreased serum haptoglobin, increased LDH
- reticulocytosis (response by bone marrow)
- if BM can’t keep up = anemia
group of hemolytic anemias caused by defects in proteins that disrupt the vertical interactions between transmembrane proteins and the underlying protein cytoskeleton
hereditary spherocytosis
- usually autosomal dominant inheritance
hereditary spherocytosis mutations
- most common defects are combined ankyrin/spectrin mutations
- band 3 protein (30%)
- sole alpha spectrin, protein 4.2, Rh protein, etc, mutations are rare
What do the defects in vertical membrane protein interactions result in?
separation of the spectrin-phospholipid bilayer
- causes RBCs to lose unsupported lipid membrane via vesicle formation w/out corresponding loss of volume
end result of defects in vertical membrane protein interactions
decreased surface-to-volume ratio = shape becomes spherical = affects deformability
typical lab findings for hereditary spherocytosis
- Hb decreased
- MCHC increased
- RDW increased
- retic count increase
- increase bilirubin, LDH
- decrease haptaglobin
- DAT neg
- spherocytes, polychromasia
other causes of spherocytosis
- immune hemolytic anemia (DAT +)
- burns
- MAHA
- Clostridial sepsis
- hyposplenism (includes normal neonate)
Diagnosis of hereditary spherocytosis: if uncertain, two other tests:
- EMA (eosin-5 maleimide)
- osomotic fragility test
Describe EMA testing
- flow cytometry; incubation with a fluorescent dye (eosin 5-maleimide); binds to transmem proteins - mostly band 3
- even though Hs has various causes (ankyrin mutation), band 3 protein sites are decreased in all forms of HS
- decreased levels of EMA binding sites = decreased fluorescence compared to normal controls
Osmotic fragility test principle
- RBCs put into series of increasingly hypotonic NaCl soln = water entering into RBCs to achieve eqm
- cause swelling; when internal vol gets too great = lysis
- bc spherocytes already have decreased surface area-vol ratio = they lyse in LESS hypotonic solns than normal RBCs
T or F. osmotic fragility is more sensitive and specific than EMA testing
F! reverse
- several mods to soln may increase sensitivity and specificity (glycerol, acidified glycerol and pink test)
Which test identifies membrane protein deficiencies by separating the various proteins via electrophoresis and quantifying via densitometry?
SDS-PAGE
This measures RBC deformability at various osmotic gradients using a laser
Ektacytometry; most specific
hypertonic cryohemolysis
based on HS cells being more sensitive to cooling at 0 degrees C in hypertonic soln than normal cells = this phenomenon is dependent on primary membrane protein defects rather than SA:vol
Clinical manifestations of HS
- rarely severe anemia; majority have well compensated hemolysis w few symptoms
- at risk for:
1. hemolytic crisis: increased rate of hemolysis due to infection or drugs
2. aplastic crisis: BM ability to maintain high compensatory output impaired, most commonly by parvovirus infection or drugs
3. megaloblastic crisis: high BM production outstrips folate supply (pregnancy)
HS treatment
- transfusion as needed
- vitamin supplementation
- if significant chronic anemia or recurrent crises = splenectomy
heterogenous group of membrane disorders; incidence unknown and most are asymptomatic
hereditary elliptocytosis
- more common in African/Med poplns (malarial geography)
these gene mutations result in defective proteins which disrupt horizontal linkages of the protein cytoskeleton
HE!
- alpha spectrin
- beta spectrin
- protein 4.1
when RBCs become ellipticle after repeated exposure to stress in peripheral circulation
- loss of deformability
- severe cases = membrane skeleton weakened to the point of lysis
HE heterozygotes
- RBC lifespan usually normal = asymptomatic (picked up on PBS incidentally)
- may have mild anemia + lab evidence of hemolysis (can worsen with infections, pregnancy, etc.
- proportion of elliptocytes does NOT correlate with severity
other causes of elliptical cells have to be ruled out prior to diagnosing HE
- iron deficiency
- thalassemia
- myelodysplastic syndrome
- primary myelofibrosis
Hereditary Pyropoikilocytosis (HPP)
- HE homozygous or compound heterozygous
- rare
- can have moderate to severe hemolytic anemia
- may or may not have jaundice or splenomegaly
HPP lab findings
- PBS shows extreme poikilocytosis with fragmentation, microspherocytes + elliptocytes
- MCV very low (fragments)
- RDW increased
- increased reticulocytosis
- marked RBC thermal sensitivity
- increased osmotic fragility
- decreased EMA fluorescence (more so than HS)
- may need gel electroph. or genetic testing
AKA Southeast Asian Ovalocytosis
Hereditary Ovalocytosis
HO
- single gene mutation in band 3 protein (tighter binding to ankyrin) = increased rigidity of membrane and decreased elasticity
- autosomal dominant; all cases heterozygous
- hemolysis is mild or absent
- no treatment required
- oval RBCs + 1-2 transverse bars
mutations that alter membrane transport proteins
dehydrated her. stomatocytosis and overhydrated her. stomatocytosis
- affect cytoplasmic viscosity
- rarer than mutations that alter membrane structure
- asymptomatic + not often picked up
this results in loss of ability to regulate cell vol by leakage of cations, resulting in either overhydration or dehydration
mutations that alter membrane transport proteins
T or F. acquired causes of stomatocytosis are much more common
T!
includes: drying artifact on blood smear, alcoholism, cirrhosis, neoplasms, lead intoxication
membrane transport protein mutation resulting in influx of Na+ that exceeds loss of K+ = more water enters cell = swelling
Overhydrated hereditary stomatocytosis
increased permeability to K+ which leaks out but not balanced by increase in Na+ = water loss from cell
mild to moderate anemia and low level of stomatocytosis (dehydrated)
