3.1 Intrinsic Hemolytic Anemias: Membrane & Metabolic Defects

Intrinsic Hemolytic Anemias: Membrane & Metabolic Defects

Intrinsic hemolytic anemias represent a diverse group of hereditary, intracorpuscular erythrocyte disorders where the underlying defect is structural or metabolic within the red blood cell (RBC) itself. These disorders are broadly divided into membrane cytoskeletal/permeability defects and metabolic enzymopathies of the Embden-Meyerhof or Hexose Monophosphate pathways.

                    INTRINSIC ERYTHROCYTE DISORDERS
                                   |
        +--------------------------+--------------------------+
        |                                                     |
MEMBRANE DEFECTS                                      METABOLIC DEFECTS
        |                                                     |
  +-----+-----+                                         +-----+-----+
  |           |                                         |           |
Vertical   Horizontal  Permeability                HMP Shunt   Glycolysis
Defects     Defects      Defects                    (Oxidative)   (ATP)
  |           |           |                             |           |
  HS       HE / HPP   Stomatocytosis / Xerocytosis    G6PD        PK

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1. Hereditary Spherocytosis (HS)

Molecular Genetics and Pathophysiology

Hereditary Spherocytosis is the most common inherited hemolytic anemia among individuals of Northern European descent (prevalence 1 in 2,000–5,000). It is inherited primarily in an autosomal dominant pattern (~75% of cases), though autosomal recessive and de novo mutations account for the remaining 25%.

HS is characterized by a vertical cytoskeletal defect—a disruption in the perpendicular protein linkages connecting the underlying horizontal spectrin cytoskeleton to the overlying lipid bilayer. The primary molecular mutations involve:

• Ankyrin-1 (ANK1): ~50% of cases; uncouples beta-spectrin from the transmembrane protein Band 3 (Anion Exchanger 1, AE1).
• Band 3 (SLC4A1): ~20% of cases; disrupts both structural anchorage and chloride-bicarbonate exchange.
• $\beta$-Spectrin (SPTB): ~15–20% of cases; impairs binding to ankyrin and protein 4.1R.
• $\alpha$-Spectrin (SPTA1) and Protein 4.2 (EPB42): Less common; recessive forms often involve severe $\alpha$-spectrin mutations.

As the RBC circulates, unanchored lipid bilayer areas form microvesicles that bud off and are lost. Because the lipid membrane is shed while the intracellular volume remains constant, the cell suffers a progressive reduction in surface area-to-volume ratio. The erythrocyte transitions from a biconcave disc to a rigid, spherical microspherocyte.

Vertical Linkage Mutation (ANK1, SLC4A1, SPTB)
  --> Uncoupling of Lipid Bilayer from Cytoskeleton
  --> Progressive Loss of Membrane Microvesicles
  --> Decreased Surface Area-to-Volume Ratio
  --> Rigid Spherocyte Formation
  --> Sequestration & Phagocytosis in Splenic Cords

Splenic Conditioning and Extravascular Hemolysis

Normal erythrocytes deform through the 1–3 $\mu\text{m}$ endothelial slits of the splenic cords of Billroth into the splenic sinusoids. Rigid microspherocytes cannot deform, becoming trapped in the hostile splenic red pulp. Here, they encounter hypoglycemia, lactic acidosis, low pH, and high oxidative stress ("splenic conditioning"). Splenic macrophages recognize and phagocytose these conditioned spherocytes via extravascular hemolysis.

Laboratory Diagnosis

• Complete Blood Count (CBC): Mild-to-moderate normocytic or microcytic anemia. The Mean Corpuscular Hemoglobin Concentration (MCHC) is classically elevated ($>36\text{ g/dL}$, frequently $36\text{--}39\text{ g/dL}$) due to cellular dehydration from $K^+$ and water leakage. The Red Cell Distribution Width (RDW) is elevated ($>15%$), reflecting anisocytosis.
• Peripheral Blood Smear: Abundant microspherocytes (small, darkly stained, perfectly round RBCs lacking central pallor), polychromasia (reticulocytes), and occasional nucleated RBCs.
• Hemolysis Markers: Markedly elevated unconjugated (indirect) bilirubin, elevated serum lactate dehydrogenase (LDH), decreased or absent serum haptoglobin, and elevated reticulocyte count (typically 5–20%). Direct Antiglobulin Test (DAT) is negative, distinguishing HS from immune-mediated spherocytosis (e.g., WAIHA).
• Osmotic Fragility Test (OFT): Erythrocytes are suspended in decreasing concentrations of hypotonic sodium chloride ($0.85%$ down to $0.0%\text{ NaCl}$). Because spherocytes already possess a minimal surface area-to-volume ratio, they cannot accommodate water influx and lyse at higher NaCl concentrations than normal biconcave discs. The osmotic fragility curve is shifted to the right. Fragility is amplified after 24-hour sterile incubation at 37°C.
• Eosin-5-Maleimide (EMA) Binding Flow Cytometry (Gold Standard): EMA is a fluorescent dye that binds covalently to Lys-430 on Band 3 (and associated CD47/glycophorin A complexes). In HS, loss of membrane surface area and reduced Band 3 density cause a statistically significant reduction in mean fluorescence intensity (MFI) compared to control RBCs (sensitivity and specificity $>93%$).
• Acidified Glycerol Lysis Test (AGLT) & Pink Test: Rapid screening methods that measure spectrophotometric rate of lysis in buffered hypotonic glycerol.

OSMOTIC FRAGILITY CURVE COMPARISON

Lysis (%)
100 |                       / (HS: Shifted Right - Increased Fragility)
    |                      /   / (Normal Range)
 50 |                     /   /   / (Thalassemia/Target Cells: Shifted Left)
    |                    /   /   /
  0 +-------------------/---/---/-------------------
    0.85              0.50  0.40  0.30            0.0 % NaCl

Complications and Therapy

• Aplastic Crisis: Induced by Parvovirus B19 infection, which selectively infects and lyses erythroid progenitor cells (pronormoblasts), halting erythropoiesis for 10–14 days and precipitating sudden, life-threatening anemia.
• Megaloblastic Crisis: Folate depletion driven by chronic compensatory hyper-erythropoiesis.
• Pigment Gallstones: Calcium bilirubinate cholelithiasis occurs in up to 50% of untreated patients.
• Splenectomy: Indicated for moderate-to-severe disease. Splenectomy eliminates the site of extravascular hemolysis, normalizing erythrocyte lifespan and hemoglobin, though spherocytes persist. Postsurgical peripheral smears show Howell-Jolly bodies, target cells, and Pappenheimer bodies. Patients face lifelong risk of Overwhelming Post-Splenectomy Infection (OPSI), primarily from encapsulated organisms (Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae type b).

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2. Hereditary Elliptocytosis (HE) & Hereditary Pyropoikilocytosis (HPP)

Molecular Pathology

Hereditary Elliptocytosis and its severe subtype, Hereditary Pyropoikilocytosis, are caused by horizontal cytoskeletal defects—mutations that destabilize the two-dimensional submembranous spectrin network along the plane of the membrane.

• $\alpha$-Spectrin (SPTA1): Most frequent site of mutation (~65%), disrupting the N-terminal $\alpha\text{I}$ domain.
• $\beta$-Spectrin (SPTB): Mutations in the C-terminal domain impairing $\alpha\beta$ heterodimer self-association into spectrin tetramers.
• Protein 4.1R (EPB41): Impairs formation of the spectrin-actin-protein 4.1 junctional complex.

     NORMAL RBC CYTOSKELETON                HORIZONTAL DEFECT (HE/HPP)
  Spectrin Dimers -> Tetramers         Impaired Dimer-Dimer Association
 [Dimer] ==(Self-Association)==> [Tetramer]   [Dimer] - - X - - [Dimer]
             |                                       |
    Stable Biconcave Shape                Deformation Under Shear Stress
                                            --> Permanent Oval/Elliptocyte

Hereditary Elliptocytosis (HE)

• Inheritance & Clinical Picture: Autosomal dominant. Over 80–90% of individuals are asymptomatic with fully compensated hemolysis.
• Morphology: $>25%$ (often $60\text{--}90%$) elliptical, cigar-shaped, or rod-like erythrocytes (elliptocytes/ovalocytes) on the peripheral blood smear.
• Pathomechanics: Erythrocytes emerge from the bone marrow as biconcave discs. As they circulate through capillaries, shear stress deforms them into elliptical shapes. Defective spectrin tetramer re-assembly prevents them from rebounding to a disc, locking them into permanent elliptocytes.

Hereditary Pyropoikilocytosis (HPP)

• Genetics: A severe, compound heterozygous or homozygous disorder combining an $\alpha$-spectrin defect that prevents tetramer assembly with a second mutant $\alpha$-spectrin allele or partial spectrin deficiency.
• Thermal Instability: Normal red blood cells fragment at $49^\circ\text{C}$. HPP erythrocytes fragment and bud in vitro at $41\text{--}45^\circ\text{C}$.
• Morphology & CBC: Extreme, bizarre poikilocytosis with extensive budding RBCs, micro-spherocytes, triangular schistocyte-like fragments, and elliptocytes. Severe microcytosis with MCV typically $50\text{--}65\text{ fL}$ (frequently flagging analyzer alarms for extreme microcytosis) and low MCHC ($<32\text{ g/dL}$). Severe uncompensated hemolytic anemia requiring transfusion support.

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3. Hereditary Stomatocytosis & Xerocytosis (Cation Permeability Disorders)

Cation permeability disorders result from genetic alterations in transmembrane ion channels or mechanosensitive channels, disrupting the Donnan equilibrium for sodium ($Na^+$) and potassium ($K^+$).

                  CATION PERMEABILITY DISORDERS
                                |
        +-----------------------+-----------------------+
        |                                               |
OVERHYDRATED STOMATOCYTOSIS                     HEREDITARY XEROCYTOSIS
(Hydrocytosis: Massive Na+ Influx)            (Dehydrated: Net K+/H2O Efflux)
        |                                               |
Cell Swells: High MCV (>110 fL)               Cell Shrinks: High MCHC (>36 g/dL)
Low MCHC (<32 g/dL)                           Target & "Puddle" Cells
Smear: Slit-like Stomatocytes                 SPLENECTOMY CONTRAINDICATED

Overhydrated Hereditary Stomatocytosis (Hydrocytosis)

• Pathophysiology: Massive intracellular influx of $Na^+$ that exceeds the passive loss of $K^+$, with net intracellular water accumulation. Often linked to mutations in RHAG (Rh-associated glycoprotein) or loss of stomatin (band 7.2b).
• Laboratory Findings: MCV is markedly elevated ($110\text{--}130\text{ fL}$), MCHC is decreased ($<32\text{ g/dL}$), and osmotic fragility is markedly increased. Blood smear reveals prominent stomatocytes (RBCs with a slit-like, mouth-shaped central pallor).

Dehydrated Hereditary Stomatocytosis (Hereditary Xerocytosis)

• Pathophysiology: Autosomal dominant gain-of-function mutations in PIEZO1 (mechanosensitive cation channel) or KCNN4 (Gardos channel, calcium-activated potassium channel). Activation causes rapid, excessive efflux of $K^+$ and water exceeding $Na^+$ influx, leaving erythrocytes severely dehydrated.
• Laboratory Findings: Elevated MCHC ($>36\text{ g/dL}$), normal-to-high MCV, and shifted osmotic fragility curve to the left (increased resistance to lysis in hypotonic solutions because cells are pre-shrunk).
• Smear: Target cells, stomatocytes, and characteristic "puddle cells" (desiccated erythrocytes with hemoglobin pooled into one sector or periphery).
• Critical Board Warning: Splenectomy is strictly CONTRAINDICATED in hereditary xerocytosis due to an exceptionally high risk of life-threatening, refractory arterial and venous thromboembolism, deep vein thrombosis, and fatal pulmonary hypertension.

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4. Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency

Metabolic Pathway & Genetics

G6PD deficiency is the most prevalent enzymatic disorder of red blood cells, affecting $>400\text{ million}$ people worldwide. It is inherited as an X-linked recessive disorder (G6PD gene located on Xq28). Because erythrocytes lack nuclei and mitochondria, they cannot synthesize new proteins and rely exclusively on the Hexose Monophosphate (HMP) Shunt (Pentose Phosphate Pathway) for survival.

                      HEXOSE MONOPHOSPHATE SHUNT
                                  |
Glucose-6-Phosphate + NADP+ ----[ G6PD ]----> 6-Phosphogluconate + NADPH
                                                                    |
Glutathione Disulfide (GSSG) <----[ Glutathione Reductase ]<--------+
            |
            v
2 Reduced Glutathione (GSH) + H2O2 ----[ Glutathione Peroxidase ]----> 2 H2O + GSSG
            |
  Protects Hemoglobin Sulfhydryls
  Against Oxidative Denaturation

G6PD catalyzes the first, rate-limiting step of the HMP shunt: converting glucose-6-phosphate to 6-phosphoglucono-$\delta$-lactone while reducing $\text{NADP}^+$ to $\text{NADPH}$. In erythrocytes, NADPH is the sole reducing cofactor required by glutathione reductase to regenerate reduced glutathione (GSH) from oxidized glutathione disulfide (GSSG). GSH is the principal antioxidant defense, neutralizing hydrogen peroxide ($H_2O_2$) and reactive oxygen species (ROS) via glutathione peroxidase.

Variants and Oxidative Triggers

• G6PD A- (African variant): Moderately unstable enzyme; enzyme activity decreases moderately as RBCs age ($t_{1/2}$ shortened), causing mild-to-moderate, self-limiting hemolysis.
• G6PD Mediterranean: Severely unstable enzyme ($<10%$ baseline activity); causes severe, life-threatening favism.
• Oxidative Precipitating Agents:
  1. Drugs: Antimalarials (primaquine, tafenoquine), sulfones (dapsone), sulfonamides (sulfamethoxazole), nitrofurantoin, phenazopyridine, rasburicase, methylene blue.
  2. Foods: Fava beans (Vicia faba, containing vicine, convicine, and isouramil).
  3. Infections & Conditions: Viral hepatitis, pneumonia, typhoid fever, and diabetic ketoacidosis (leukocyte ROS production during phagocytosis).

Morphology & Heinz Body Formation

Oxidative stress oxidizes the sulfhydryl ($-SH$) groups of hemoglobin tetramers. Denatured hemoglobin precipitates into insoluble intraerythrocytic aggregates termed Heinz bodies.

• Staining Rule: Heinz bodies are invisible on routine Romanowsky / Wright-Giemsa stains because the methanol fixation dissolves the refractive differential. They require supravital stains (brilliant cresyl blue, new methylene blue, or crystal violet).
• Pitting & Remodeling: As Heinz-body-laden RBCs pass through the spleen, splenic macrophages "pit" or excise the rigid inclusions from the membrane, producing degmacytes (bite cells) and blister cells (eccentrocytes) (where hemoglobin has retracted away from the cell membrane, leaving an empty pouch).

Oxidative Stress --> Depleted GSH --> Denatured Hemoglobin Precipitates
  --> Heinz Bodies (Supravital Stain Only)
  --> Splenic Macrophage Pitting
  --> Degmacytes (Bite Cells) & Blister Cells (Eccentrocytes)
  --> Acute Intravascular & Extravascular Hemolysis

Diagnostic Pitfall: Testing Window

During or immediately following an acute hemolytic crisis, the oldest erythrocytes (which possess the lowest G6PD levels) have already hemolyzed. The bone marrow responds with robust reticulocytosis. Because young reticulocytes have recently left the marrow and possess normal-to-high G6PD enzyme activity, a quantitative G6PD assay performed during the acute crisis can yield a false-normal result. Testing must be repeated 2 to 3 months after the hemolytic episode resolves.

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5. Pyruvate Kinase (PK) Deficiency

Pathophysiology

Pyruvate Kinase (PK) deficiency is the most common enzymopathy of the Embden-Meyerhof glycolytic pathway and the second most common red cell enzymopathy overall. It is inherited as an autosomal recessive trait caused by mutations in the PKLR gene.

PK catalyzes the final, irreversible substrate-level phosphorylation in glycolysis: converting phosphoenolpyruvate (PEP) to pyruvate, yielding one molecule of ATP per triose (two ATP molecules net per glucose molecule).

                  EMBDEN-MEYERHOF GLYCOLYTIC DEFECT

1,3-Bisphosphoglycerate <======================> 2,3-Bisphosphoglycerate (2,3-BPG)
          |                                        (Markedly Elevated: Shifts
Phosphoenolpyruvate (PEP)                            ODC Right, Releases O2)
          |
   [ PYRUVATE KINASE ]  <-- DEFICIENT IN PK DEFICIENCY
          |
          v
       Pyruvate  +  [ ATP GENERATION BLOCKED ]

In PK deficiency, RBCs suffer severe intracellular ATP depletion. Without ATP:

1. Membrane $Na^+/K^+$ ATPase pumps fail $\to$ intracellular $K^+$ and water are lost $\to$ cell dehydration.
2. Membrane deformability is lost, transforming erythrocytes into rigid, spiny echinocytes (crenated cells) and acanthocytes/prickle cells.
3. Rigid cells are destroyed via extravascular hemolysis in the spleen and liver.

2,3-Bisphosphoglycerate Elevation & Oxygen Affinity

The enzymatic blockage at the PK step leads to an accumulation of upstream glycolytic intermediates, notably 2,3-bisphosphoglycerate (2,3-BPG / 2,3-DPG) (often 2–3 times normal). Elevated 2,3-BPG binds to the central cavity of deoxygenated hemoglobin, stabilizing the T (taut) state and shifting the oxygen-dissociation curve (ODC) to the right ($P_{50}$ increased). This enhances oxygen unloading to peripheral tissues, allowing patients with PK deficiency to tolerate severe anemia ($Hb\text{ 6--8 g/dL}$) with surprisingly mild clinical symptoms.

Laboratory Diagnosis

• CBC & Smear: Normocytic, normochromic anemia with marked polychromasia (reticulocytes typically 15–30%), nucleated RBCs, anisocytosis, poikilocytosis, and crenated/echinocytic cells (especially post-splenectomy).
• Enzyme Assay: Quantitative spectrophotometric assay measuring the rate of NADH oxidation in a coupled reaction (PEP $\to$ Pyruvate $\to$ Lactate via LDH). Prior to assaying, leukocytes must be rigorously removed via filtration, as WBCs contain normal M2-isozyme pyruvate kinase and can cause a massive false-normal result.
• Differentiation: Direct antiglobulin test is negative; G6PD enzyme activity is normal or elevated; Heinz bodies are absent.

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6. Comprehensive Summary Table of Intrinsic RBC Defects