Hemoglobinopathies

Question 1

hemoglobin structural variants are most often caused by …

Answer 1

point mutations in one of the globin genes

• usually single pt mutations => single AA substitution
• changes in AA sequence can alter the charge of the protein and change folding

Question 2

T or F. There can only be point mutations for hemoglobin structural variants

Answer 2

F, there can also be insertions, deletions, and fusions

Question 3

T or F. As with thals, many hemoglobinopathies are common in malaria endemic areas

Answer 3

T!

- confer protection from severe malarial infections

Question 4

Why do delta and gamma variants in hemoglobin have little clinical impact in adults?

Answer 4

low expression

- even if they would otherwise be pathologic

Question 5

Hemoglobin structural variants depends on …

Answer 5

mutation and zygosity

• homozygous: disease
• heterozygous: trait

Question 6

lab techniques for Hb evaluation

Answer 6

• CBC
• blood film
• Hb electrophoresis
• molecular genetic testing (fragment analysis or sequencing)

Question 7

most common and most clinically relevant structural variant

Answer 7

Hb S

Question 8

these homozygotes have sickle cell anemia

Answer 8

Hb SS, beta^S/beta^S

Question 9

beta globin point mutation

Answer 9

HbS

• HBB:c.20A>T
• B6 Glu>Val (E6V) which means 6th AA in beta globin protein is valine instead of glutamate

Question 10

HbS results in a hemoglobin variant with _________ solubility

Answer 10

decreased

• precipitates and polymerizes when in deoxygenated form
• polymerization results in elongated RBCs = ‘sickle cells’

Question 11

rate and extent of Hb S polymerization is dependent on several factors

Answer 11

• high intracell Hb conctn
• presence of other Hb variants (some are protective against sickling while others promote it)
• blood O2 saturation (polymerization occurs with deoxy-Hb S and is reversible in the oxygenated state
• low pH
• low temp
• 2,3- BPG (bisphosphoglyceric acids) levels

Question 12

Hb SS

Answer 12

• sickle cell anemia
• autosomal recessive inheritance
• homozygosity for Hb S mutations leads to clinical symptoms
• clinically heterogeneous, multifactorial pathophysiology
  > chronic inflammatory state with acute exacerbations

Question 13

Hb SS clinical presentations

Answer 13

• vasoocclusive episodes (VOE)
• aplastic crisis
• sequestration crisis
• hyperhemolysis

Question 14

Vasoocclusive episodes (VOE)

Answer 14

• acute exacerbations of underlying chronic pain
• most common manifestation, can occur anywhere in the body, in any tissue (often extremities, back, chest)
• vasooclusion -> tissue hypoxia -> pain
• precipitated by dehydration, infection, cold weather or idiopathic
• can be a med emergency (acute chest syndrome)

Question 15

aplastic crisis

Answer 15

• marked reduction in RBC production, severe anemia

- often triggered by infection (parvovirus B19 most common)

Question 16

sequestration crisis

Answer 16

• sudden massive pooling of RBCs in an organ (usually spleen, sometimes liver) => severe anemia
• usually only occurs in children under 5 y/o (prior to autoinfarction of spleen
• treated with very cautious transfusion

Question 17

hyperhemolysis

Answer 17

• episodes of accelerated rates of hemolysis

- maybe be triggered by VOE or by delayed hemolytic transfusion reactions

Question 18

SCA symptoms

Answer 18

• priapism in males

- pregnancy complications: low birth weight infants, higher risk of maternal death

Question 19

SCA treatment

Answer 19

Supportive:

• pain management
• simple or exchange transfusions
• infection prevention (vaccines, penicillin)

Disease modifying:

• hydroxyurea (increases Hb F)
• allogeneic stem cell transplant

Question 20

Hb AS

Answer 20

• B/B^S
• sickle cell trait
• inheritance of a single Hb S allele
• generally asymptomatic = there is enough Hb A in the cells to prevent sickling, normal lifespan
  = blood film usually normal, sometimes mild microcytosis and a few target cells
• renal pathology, hematuria, acceleration of other renal diseases (PKD)
• higher risk of sudden death (especially related to intense exercise)
• higher risk of UTI (esp in pregnant females)
• associated with renal medullary carcinoma

Question 21

Sickle solubility test

Answer 21

• Hb S present? quick test!
• add blood to reagent and look for turbidity
  > detergent (lyse RBCs)
  > reducing agent 2+ to 3+; deoxygenates the Hb
• deoxy-Hb S polymerizes and precipitates out (sample becomes turbid)
• not 100% specific to HbS; other Hbs can be positive
• false positives = lipemia, excess blood to reagents

Question 22

Hb C

Answer 22

• beta globin pt mutation
  > HBB:c.19G>A
  > B6 Glu>Lys
• second Hb variant to be described after Hb S

Question 23

Hb CC

Answer 23

• homozygous for Hb B^C mutations
• HbC forms crystals inside RBCs -> impaired deformability
  => K+ loss -> water loss -> dehydrated RBCs -> spherocytes, increase MCHC
  = shortened RBC lifespan (40 days)
• mild to moderate splenomegaly (usually no splenic autoinfarction)
• chronic hemolysis => cholelithiasis

Question 24

Lab features of Hb CC

Answer 24

• mild to moderate hemolytic anemia (Hb 100-110 g/L)
• mild microcytosis (MCV 70-75 fL)
• reticulocytosis
• often elevated MCHC
• mildly elevated bilirubin

Question 25

Hb CC blood film

Answer 25

- HbC crystals - targets - microspherocytes - polychromasia

Question 26

sickle cell disease

Answer 26

- Hb SX => X is other beta variant that interacts with Hb S and promotes sickling - most common is Hb SC disease - can also have coinheritance of thalassemias with Hb S > Hb S/B0 > Hb SS with alpha thal

Question 27

Lab features of Hb SC disease

Answer 27

- Hb <80 g/L - MCV around 70 fL - RDW increased - mildly increased reticulocyte count - mild neutrophilia and monocytosis - - SC poikilocytes - boat shaped cells (classic sickle cells less common) - occasional Hb C crystals - target cells

Question 28

Hb E

Answer 28

- beta chain variant; B26 Glu>Lys - abnormal alt splicing between exon 1 and intron 1 - decreased transcription of functional mRNA - quantitative and qualitative defect - B thal-like syndrome - HB EE = mild microcytic anemia w target cells, asymptomatic - HbAW = asymptomatic - HB E/B 0 = can be severe, similar to B thal major

Question 29

Hb D

Answer 29

- various Hb D variants have similar electrophoretic migration - heterozygous is asymptomatic; homo = mild microcytic anemia; Hb S/D-Punjab = sickling disorder - Hb D-Iran is not clinically significant and does not interact with Hb S; clinically like Hb S trait

Question 30

Hb M

Answer 30

- a, B, gamma variants - mutations cause increased tendency towards oxidation to methemoglobin (usually His>Tyr in heme pocket => usually results in pseudocyanosis (lavender-blue or slate grey) - some also have low O2 affinity and cause a degree of true cyanosis (purplish-blue)