Thalassemia

Physiology

Background on Human Hemoglobin

  • Hemoglobin A
    • Hemoglobin A is the Major Adult Hemoglobin
    • Hemoglobin A is a Tetramer Composed of One Pair of Alpha Globin Chains and One Pair of Beta Globin Chains
    • Globin Chain Synthesis is Normally Tightly Regulated
      • Ratio of Alpha/Non-Alpha Chains: 1.00 ± 0.05
    • Alpha Globin Chains are Very Insoluble
    • Beta Globin Chains are Soluble: they can assemble to form homotetrameric HbH

Normal Hemoglobin Electrophoresis

  • HbA: 95–98%
  • HbS: 0%
  • HbC: 0%
  • HbF: <1%
  • HbA2: 2.5%

Thalassemias

  • Thalassemias is a Spectrum of Diseases Where There is Decreased or Absent Production of One or More of the Globin Chains, Resulting in an Alteration in the Normal Alpha/Non-Alpha Chain Ratio

Diagnosis

Hemoglobin Analysis

Protein Chemistry Hemoglobin Analysis

  • Hemoglobin Gel Electrophoresis (see Hemoglobin Electrophoresis)
  • Capillary Electrophoresis
  • Cation-Exchange High-Performance Liquid Chromatography (HPLC)
  • Isoelectric Focusing (IEF)

Molecular/DNA-Based Hemoglobin Analysis (Genotyping)

  • Array Comparative Genomic Hybridization (aCGH): also known as Fine Tiling Array
  • DNA Sequencing/Allele-Specific PCR
  • Gap-PCR/Multiplex Ligation-Dependent Amplification (MLPA)
  • Next-Generation Sequencing (NGS)

Point of Care Assays

  • XXX

Other

  • XXX

Alpha Thalassemia Minima (Silent Carrier of Alpha Thalassemia)

Physiology

  • Loss of One of the Four Alpha Globin Genes
    • Heterozygous for Alpha(+) Thalassemia -> aa/a-

Diagnosis

Clinical Manifestations

General Comments

  • Usually Asymptomatic

Hematologic Manifestations

  • Risk of Offspring with HbH Disease
    • Can Occur with Individual Who Carries this Allele and Mates with a Partner Carrying the Alpha Thalassemia-1 Allele

Alpha Thalassemia Minor

Physiology

  • Loss of Two of the Four Alpha Globin Genes
    • Heterozygous for Alpha Thalassemia-1 Trait (Heterozygous for Alpha (0) Thalassemia): both alpha genes on one chromsome have been deleted -> aa/– (“cis” deletion)
      • This Genetic Type Occurs More Commonly in Asians
    • Homozygous for Alpha Thalassemia-2 Trait (Homozygous for Alpha (+) Thalassemia): one alpha gene has been deleted from each chromsome -> a-/a- (“trans” deletion)
      • This is the Most Common Genetic Type in Patients of African Origin (“cis” Deletion is Rare in this Population)
      • Usually Involves the Less Active Alpha Globin Allele, Leading to Milder Disease than that Seen in Asian Populations (Where the “cis” Deletion is More Common)

Diagnosis

Clinical Manifestations

Hematologic Manifestations

  • Mild Microcytic Anemia (see Anemia)
    • Clinically Resembles Mild Beta Thalassemia Trait

Deletional Form of Hemoglobin H Disease (Alpha Thalassemia Intermedia)

Physiology

  • Loss of Three of the Four Alpha Globin Genes
    • Compound Heterozygosity for Both the Alpha Thalassemia-2 Trait and the Alpha Thalassemia-1 Trait -> a-/–

Diagnosis

Clinical Manifestations

General Comments

  • Less Severe than Non-Deletional HbH Disease

Hematologic Manifestations

  • Dramatically Left-Shifted Oxygen Dissociation Curve: due to HbH being a poorly functional oxygen carrier
  • Moderate Microcytic/Hemolytic Anemia (see Anemia)
    • Hemolytic Anemia Throughout Gestation
    • Neonatal Jaundice
    • Hydrops Fetalis: may occur

Non-Deletional Form of Hemoglobin H Disease (Alpha Thalassemia Intermedia)

Physiology

  • Loss of Two of the Four Alpha Globin Genes + Alpha Chain Mutation in One the Remaining Genes
    • Compound Heterozygosity for Both the Alpha Thalassemia-1 Trait and Hemoglobin Constant Spring

Diagnosis

Clinical Manifestations

General Comments

  • More Severe than Deletional HbH Disease

Hematologic Manifestations

  • Dramatically Left-Shifted Oxygen Dissociation Curve: due to HbH being a poorly functional oxygen carrier
  • Microcytic/Hemolytic Anemia (see Anemia)
    • Hemolytic Anemia Throughout Gestation
    • Neonatal Jaundice
    • Hydrops Fetalis: may occur

Acquired Hemoglobin H Disease (Acquired Alpha Thalassemia, Alpha Thalassemia Myelodysplastic Syndrome)

Etiology

  • Myelodysplastic Syndrome (MDS) (see Myelodysplastic Syndrome): occurs in 8% of MDS cases
  • Other Myeloproliferative Disorders: occurs in 2.5% of cases

Physiology

  • Acquired Somatic Mutation of ATRX: ATRX is an X-linked gene encoding a chromatin-associated protein
  • Acquired Deletions of Alpha Globin Loci

Diagnosis

Clinical Manifestations

  • Microcytic Anemia (see Anemia)

Hydrops Fetalis with Hb Barts

Physiology

  • Loss of All Four Alpha Globin Genes
    • Homozygosity for the Alpha Thalassemia-1 Trait: –/–

Diagnosis

Clinical Manifestations

  • Fatal Microcytic Anemia (see Anemia)

Beta Thalassemia Minor (Beta Thalassemia Trait)

Epidemiology

Highest Population Risk Groups

  • General Comments: likely related to selective pressure from Plasmodium Falciparum malaria (over which the beta thalassemia trait is believed to offer some survival advantage)
  • Africa
  • Central Asia
  • Far East/Southeast Asia
  • Indian Subcontinent
  • Mediterranean
    • Cyprus: prevalence is 14%
    • Sardinia: prevalence is 12%
  • Middle East
  • Transcaucasus
    • Armenia
    • Georgia
    • Azerbaijan

Physiology

  • Genetics: Heterozygous State with One Normal Beta Globin Allele and One Beta Globin Thalassemic Allele (Tt)

Diagnosis

  • Complete Blood Count (CBC) and Peripheral Blood Smear (see Complete Blood Count and Peripheral Blood Smear)
    • Mild Microcytic Anemia (see Anemia): Hct usually >30%
    • Mean Corpuscular Volume (MCV) Usually <75 fL: microcytosis is usually more severe (and anemia less severe) than that observed in iron deficiency anemia
      • In Contrast, in Iron Deficiency Anemia, MCV Doesn’t Fall <80 fL Until the Hematocrit Decreases to <30%
    • Hypochromia (see Peripheral Blood Smear)
    • Increased Total RBC Count: often in the polycythemic range
    • Normal Red Cell Distribution Width (RDW): since nearly all cells are microcytic and hypochromic
      • In Contrast, in Iron Deficiency Anemia, RDW is Increased
    • Target Cells (see Peripheral Blood Smear): more dramatic than that seen in all but the most severe cases of iron deficiency anemia
    • Teardrop Cells (Dacrocytes) (see Peripheral Blood Smear): not seen in iron deficiency anemia
  • Hemoglobin Electrophoresis (see Hemoglobin Electrophoresis)
    • HbA: accounts for >90 percent of the hemoglobin
    • HbA2: elevated (usually 3.5–78%) -> however, a normal HbA2 does not rule out beta thalassemia (due to some cases with delta-beta or gamma-delta-beta thalassemia trait or when the beta thalassemia trait is co-inherited with a delta globin gene mutation)
    • HbF: increased in 50% of patients

Clinical Manifestations

General Comments

  • Usually Asymptomatic

Cardiovascular Manifestations

  • Protection Against Arterial Thromboembolic Events: studies indicate that beta thalassemia trait confers a protective effect against arterial cardiovascular and cerebrovascular disease in male patients
    • Likely Due to Low Serum Cholesterol, Slight Anemia, Microcytosis, and a Decrease in Blood Viscosity

Hematologic Manifestations

  • Mild Microcytic Anemia (see Anemia)
  • Splenomegaly (see Splenomegaly)
    • Diagnosis: splenic volume (by ultrasound) is 29–67% greater in those with beta thalassemia minor as compared to controls
    • Clinical: however, the spleen is palpable in <20% of patients

Beta Thalassemia Intermedia

Epidemiology

Highest Population Risk Groups

  • General Comments
    • Likely Related to Selective Pressure from Plasmodium Falciparum Malaria (Over Which the Beta Thalassemia Trait is Believed to Offer Some Survival Advantage)
  • Africa
  • Central Asia
  • Far East/Southeast Asia
  • Indian Subcontinent
  • Mediterranean
    • Cyprus: prevalence is 14%
    • Sardinia: prevalence is 12%
  • Middle East
  • Transcaucasus
    • Armenia
    • Georgia
    • Azerbaijan

Physiology

  • Genetics
    • Compound Heterozygote of Two Thalassemia Variants

Diagnosis

  • Complete Blood Count (CBC) and Peripheral Blood Smear (see Complete Blood Count and Peripheral Blood Smear)
  • Hemoglobin Electrophoresis (see Hemoglobin Electrophoresis)
    • HbA2: elevated (usually 3.5–78%) -> however, a normal HbA2 does not rule out beta thalassemia (due to some cases with delta-beta or gamma-delta-beta thalassemia trait or when the beta thalassemia trait is co-inherited with a delta globin gene mutation)
    • HbF: increased in 50% of patients

Clinical Manifestations

General Comments

  • Definition
    • Beta Thalassemia Intermedia is Defined as Patient with Symptomatic Beta Thalassemia Who Does Not Require Transfusion During at Least the First Few Years of Life
    • Beta Thalassemia Intermedia Patients are Generally Able to Survive into the Second Decade of Life without Chronic Hypertransfusion Therapy (ie: They Have Non-Transfusion-Dependent Thalassemia)
    • However, There is Considerable Variability in the Clinical Manifestations

Gastrointestinal/Hepatic Manifestations

  • Hepatomegaly (see Hepatomegaly)
    • Physiology: due to increased red blood cell destruction and hepatic extramedullary hematopoiesis

Hematologic Manifestations

  • Microcytic Anemia (see Anemia)
  • Splenomegaly (see Splenomegaly)
    • Physiology: due to increased red cell destruction and splenic extramedullary hematopoiesis

Rheumatologic/Orthopedic Manifestations

  • Bony Abnormalities
    • “Chipmunk Facies”
    • Frontal Bossing
  • Delayed Skeletal Maturation
  • Osteoporosis (see Osteoporosis)

Treatment

Packed Red Blood Cell (PRBC) Hypertransfusion (see Packed Red Blood Cells)

  • These Patients Usually Only Require Packed Red Blood Cell Transfusion When Red Blood Cell Production is Impaired Due to an Acute Event Which Impairs Erythropoiesis (Such as Infection, etc)

Beta Thalassemia Major (Cooley’s Anemia)

Epidemiology

Highest Population Risk Groups

  • General Comments
    • Population Risk is Likely Related to Selective Pressure from Plasmodium Falciparum Malaria (Over Which the Beta Thalassemia Trait is Believed to Offer Some Survival Advantage) (see Malaria)
  • Africa
  • Central Asia
  • Far East/Southeast Asia
  • Indian Subcontinent
  • Mediterranean
    • Cyprus: prevalence is 14%
    • Sardinia: prevalence is 12%
  • Middle East
  • Transcaucasus
    • Armenia
    • Georgia
    • Azerbaijan

Physiology

Genetics

  • Inheritance of Recessive Thalassemia Trait from Both Parents
    • Beta Globin Gene is a Single Copy on Chromosome 11
      • Inheritance Follows Mendelian Genetics
    • Parents Would Both Be Tt: where T = normal allele, t = thalassemia trait
      • Therefore, the Probability of These Parents Giving Birth to Homozygous Child (tt) with Beta Thalassemia Major Would Be 25%
      • Although Both Parents Would Be Phenotypically “Normal”, They Would Have Normal-Near Normal Hemoglobin Levels with Low MCV (Usually 60–70’s)

Absent or Significantly Impaired Beta Globin Chain Synthesis with Presence of Excess Alpha Globin Chains

  • Excess Alpha Globin Chains are Unstable and Precipitate Within the Cell
    • Results in Increased Synthesis of Reactive Oxygen Species, Causing Erythroid Membrane Damage, Ineffective Erythropoiesis (with Extramedullary Hematopoiesis), and Hemolysis
    • Results in Iron Overload
    • Degree of Alpha Globin Chain Excess Determines the Severity of Clinical Manifestations
    • Note: the major non-alpha globin produced at the time of birth is gamma globin
      • Therefore, the Major Hemoglobin Synthesized in Early Postnatal Life is Fetal Hemoglobin: it is not until fetal hemoglobin production wanes, in the second 6 months of life, that the symptoms of beta thalassemia occur

Diagnosis

  • Complete Blood Count (CBC) and Peripheral Blood Smear (see Complete Blood Count and Peripheral Blood Smear)
    • Microcytic Anemia
  • Hemoglobin Electrophoresis (see Hemoglobin Electrophoresis)
    • Hemoglobin A: absent-severely decreased (although variable amounts may be present from transfusions)
    • Hemoglobin F: present
    • Hemoglobin A2: present
  • Hyperbilirubinemia (Indirect) (see Hyperbilirubinemia): due to severe hemolytic anemia
    • Often with Other Elevated Liver Function Tests (LFT’s)
  • Hyperferritinemia (see Hyperferritinemia)
    • Due to Iron Overload
  • Hypergammaglobulinemia

Clinical Manifestations

General Comments

  • Timing of Disease Onset
    • Infants are Normal at Birth: as beta globin synthesis is not essential during fetal life or during the immediate perinatal period during which the production of fetal hemoglobin predominates
    • Infants Begin to Manifest Clinical Symptoms During the Second 6 Months of Life: when gamma globin chain production normally decreases and is usually replaced with the production of beta globin to form adult hemoglobin -> however, patients with beta thalassemia major are unable to produce beta globin
  • Variability of Clinical Manifestations
    • Clinical Manifestations are Highly Variable Between Patients (Possibly Related to Differences in Mutations with Regard to Beta Globin Production or Co-Existing Alpha Thalassemia)

Cardiovascular Manifestations

  • Arrhythmias
    • Supraventricular Arrhythmias
    • Ventricular Arrhythmias
  • Cardiac Hemosiderosis
  • High-Output Congestive Heart Failure (CHF) (see Congestive Heart Failure)

Endocrine Manifestations

  • Diabetes Mellitus (see Diabetes Mellitus)
    • Epidemiology
      • Occurs in 13% of Cases
      • Impaired Glucose Tolerance Usually Appears During the Second Decade of Life
    • Physiology
      • Early Onset of Impaired Glucose Tolerance Appears to Be Related More to Insulin Resistance than to Defective Insulin Release
  • Growth Failure/Retardation
    • Epidemiology
      • Occurs in 33% of Cases
  • Hypogonadism
    • Epidemiology
      • Occurs in 40% of Cases
    • Physiology
      • Predicted by Pituitary Iron Overload and Pituitary Volume Loss
  • Hypothyroidism (see Hypothyroidism)
    • Epidemiology
      • Occurs in 10% of Cases
  • Pituitary Iron Overload
    • Epidemiology
      • Occurs During the First Decade of Life

Gastrointestinal/Hepatic Manifestations

  • Bilirubin Gallstone Disease
    • Epidemiology
      • Occurs in 66% of Cases
      • Occurs Before Age 15
  • Biliary Tract Inflammation
  • Cirrhosis (see Cirrhosis)
    • Physiology
      • Due to Iron Overload (Which May Occur Later in the First Decade of Life)
  • Hepatomegaly (see Hepatomegaly)
    • Epidemiology
      • Occurs Early in Course of Disease
    • Physiology
      • Due to Increased Red Blood Cell Destruction and Hepatic Extramedullary Hematopoiesis
  • Hyperbilirubinemia (Indirect)/Jaundice (see Jaundice)
    • Physiology
      • Due to Severe Hemolytic Anemia (with Increased Red Blood Cell Destruction), Resulting in Increased Bilirubin Production

Hematologic Manifestations

  • Aplastic Crisis
    • Infection with Parvovirus B19 (see Parvovirus B19): may have a disproportionate impact on these patients (due to shortened red blood cell survival)
  • Hemolytic Anemia with Ineffective Erythropoiesis (see Hemolytic Anemia)
    • Clinical
      • Associated with Expansion and Invasion of Erythroid Bone Marrow with Widening of Marrow Spaces and Attenuation of Cortex of the Bone
  • Iron Overload (see Iron Overload)
  • Microcytic Anemia (see Anemia)
  • Splenomegaly (see Splenomegaly)
    • Epidemiology
      • Occurs Early in the Course of Disease: due to increased red cell destruction and splenic extramedullary hematopoiesis
    • Clinical
      • May Be Massive

Infectious Manifestations

  • Increased Risk of Infection (Especially Encapsulated Organisms)
    • Physiology
      • Due to Loss of Splenic Immune Function

Neurologic Manifestations

  • Chronic Pain

Pulmonary Manifestations

  • Abnormal Pulmonary Function Tests (PFT’s)/Exercise Testing: for unclear reasons -> these defects correct with transfusion and do not correlate with the iron burden, blood counts, or degree of hemolysis
  • Pulmonary Hypertension (see Pulmonary Hypertension)
    • Risk Factors
    • Physiology: unclear etiology

Renal Manifestations

  • Dilated Renal Tubules
  • Enlarged Kidneys
    • Physiology
      • Due to Extramedullary Hematopoiesis
  • Hemosiderinuria (see Hemosiderinuria)
  • Increased Urinary Oxalate/Urate/Uric Acid

Rheumatologic/Orthopedic Manifestations

  • Bony Abnormalities
    • “Chipmunk Facies”
    • Frontal Bossing
  • Delayed Skeletal Maturation
  • Osteoporosis (see Osteoporosis)

Other Manifestations

Prenatal Screening

  • XXXXXXXXXXX

Treatment

Packed Red Blood Cell (PRBC) Hypertransfusion (see Packed Red Blood Cells)

  • Indicated for Anemia

Iron Chelation Therapy

  • General Comments
    • Iron Chelation Should Be Started Around 2 Years of Age (After About 1.5 Years of Transfusions)
  • Deferasirox (Exjade, Jadenu) (see Deferasirox)
  • Deferoxamine (Desferal, DFO) (see Deferoxamine)
  • Deferiprone (Ferriprox) (see Deferiprone)

Splenectomy (see Splenectomy)

  • Used as a Last Resort in Most Developed Countries

Hematopoietic Stem Cell Transplant (HSCT) (see Hematopoietic Stem Cell Transplant)

  • Preferred Donor: HLA-matched sibling marrow or cord blood stem cells
  • Allogeneic BMT: being studied

Pharmacologic Manipulation of Fetal Hemoglobin Levels

  • Experimental

Gene Therapy

  • Experimental

Prognosis

  • Mortality in Untreated Cases: 80% of untreated children will die within the first 5 years of life (usually due to severe anemia, high output congestive heart failure, failure to thrive, and unsusual susceptibility to infection)
  • Mortality in Treated Cases: survival into 50’s and beyond can be achieved with appropriate transfusion and iron chelation treatment

References

  • The beta-thalassemias. N Engl J Med. 1999;341(2):99 [MEDLINE]
  • The definition and epidemiology of non-transfusion-dependent thalassemia. Blood Rev. 2012;26 Suppl 1:S3 [MEDLINE]
  • Thalassemias. Pediatr Clin North Am. 2013;60(6):1383 [MEDLINE]
  • Thalassaemia. Lancet. 2018;391(10116):155 [MEDLINE]