Hemoglobinpathies

Hemoglobinopathy

Genetic mutation in one or more genes that affect Hb synthesis
- globin or polypeptide chains

altered amino acid sequence

Structural defect and its function
- qualitative
- Point mutations, deletions, insertions, fusions

Reduced rate of synthesis

- No alteration to amino acid sequence
- quantitative

ζ2ε2

Gower I

α2ε3

Gower II

ζ2γ2

Portland

α2γ2

Hgb F
- 60-90% newborn

α2δ2

Hgb A2

α2β2

Hgb A
- 10-40% newborn

Zygosity

Association between the number of gene mutations and the level of severity

Four outcomes

1. Abnormal Hb= hemolytic anemia
2. Abnormal Hb= methemoglobin
3. Hbs different oxygen affinity
4. Abnormal Hbs= benign

Hemoglobin S genetics

- single point mutation
- β-chain abnormalities
- co-dominant

phenotype of Hgb S

- trait= AS= only one β defected
- homozygous= SS
- heteozygous= Hb SC or Hb S-B-thal

Hgb S chromosome

α2β2-6Glu-Val
- at position 6, glutamic acid is replaced with valine
- produces a +1 charge
- affects oxygen affinity

Oxygenated hgb S

does not produce a hydrophobic pocket for valine
- maintain biconcave disc

Deoxygenated hgb S

- creates a hydrophobic pocket
- form electrostatic bonds between hgb= molecules that elongate in a helical formation

Hgb S RBCs

- Less soluble
- Rigid
- Form tactoids= liquid crystals
- Hg S polymers= sickle

O2 saturation for sickling

- Homozygous <85%
- Heterozygous <40%

Sickle pathophys

- viscous blood= slow flow
- Decrease in O2 tension= low pH, high 2-3 BPG
- blocks capillaries

Reversible sickle cells

- change shape based on oxygen tension
- microvasculature occlusion

Irreversible

- constant shape
- recognized by abnormal spleen

sickle cell anemia

- chronic hemolysis
- hyperplastic BM
- aplastic crisis
- megaloblastic= folate defiency
- cardiac defects
- bacterial infections

Vaso-occlusive crisis

Rigid sickled cells aggregate in the microvasculature
- hypoxia, acidosis

Peripheral Blood Sickle cell

- Normochrom/normocyt
- increased RDW
- Hgb 6 - 10 gm/dL
- Reticulocyte 10 - 25%**
- inclusions
- thrombocytosis
- leukocytosis left shift
- target cells**

Chemistry tests sickle cell

- Increased direct and indirect bilirubin
- Increased LD & uric acid
- Decreased haptoglobin

Diagnosis of SCD

1. insolubility of Hb S= Tactoids crystals
2. HPLC)

Hemoglobin Solubility Test

- adults
- decreased solubility of deoxygenated Hb S= tactoid crystals= turbidity

False positives Hgb solubility

- hyperlipidemia
- Too much blood added

False negatives Hgb solubility

- Infants <6 months
- Low hematocrits

Alkaline hemoglobin electrophoresis

Hb molecules (- charge), migrate towards anode (+ pole)

alkaline pH electrophoresis

Hb S migrates with Hb D and Hb G

acidic pH electrophoresis

Hb S separates from D and G

Plasmodium falciparum

HbS cells with P. falciparum sickle more quickly since the parasite uses the oxygen

- asplenic= fatal

Sickle cell treatment

- stem cell transplant
- Hydroxyurea or butyrate= increases HbF
- supportive care

Sickle Cell Trait

- Heterozygous Hb AS
- Benign and asymptomatic

SCT complication

with hypoxia
- Severe respiratory infections
- un-pressurized aircraft
- Anesthesia
- CHF

SCT test results

- few target cells
- positive Hgb solubility test

Hemoglobin C

- black population
- homozygous CC
- a2b26GluLys
- "bars of gold"
- no vaso-occlusive crisis

Hgb C Lab

- target cells
- slight reticulocytes
- Hgb C cystals
- negative Hgb solubility

Hemoglobin E

- lowd MCV
- Microcytes and target cells
- Differentiate from IDA*
- negative Hgb solubility test

Hemoglobin D

- mild hemolytic anemia*
- splenomegaly
- No treatment is required

Hemoglobin SC

structural defect
- sickle and bar of gold

Hgb SC Lab

- normochrom/normocyt anemia
- hgb 11-13 mg/dL
- retics

Decreased O2 affinity

Shift to right of O2 dissociation curve= quickly release O2 in tissues
- cyanosis, HbKansas

Increased O2 affinity

Shift to left of O2 dissociation curve= Fail to release oxygen on demand
- compensatory erythrocytosis
- Hgb Chesapeake

Hb M variants

AA substitution makes heme iron in ferric form= methemoglobin
- unable to bind oxygen

Unstable hemoglobin disease

Amino acid substitution or deletion have weakened the binding forces that maintain the structure of the molecule
- heinz bodies from denatured hgb