Flashcards in Hemoglobinopathies Deck (32)
Adults have two Hb forms
major form: HbA = α2β2 (97%)
minor form : HbA2 = α2δ2 (2%)
Note that δ level is much lower than β because δ has a weaker promoter
α-cluster: (order of organization on chromosome ____)
Chromosome 16; zeta-alpha2-alpha1 (ζ-α2-α1)
β-cluster: (order of organization on chromosome ____)
Chromosome 11: epsilon-gammaG-gammaA-delta-beta (ε-γG-γA-δ-β)
Deletions of the entire LCR of the beta cluster cause ______, a condition in which zero β-globin synthesis leads to precipitation of the α-globin chains.
Mutations that alter the globin polypeptide properties without affecting its synthesis:
structural variants (qualitative hemoglobinopathies)
Most structural variants are found in the β-globin chain. These mutations may alter O2 binding such as HbKemsey (too tight) and HbKansas (too weak), cause heme loss and denaturation of Hb, or make Hb less soluble such as HbS and HbC. Some can lead to serious red blood cell (RBC) diseases.
Disorders of imbalanced globin levels resulting from markedly reduced/no synthesis of one globin type:
Thalassemias (quantitative hemoglobinopathies).
Can be caused by virtually all types of genetic mutations including deletions, missense and nonsense mutations, and defective transcriptional control.
____ is a group of clinically benign conditions that impair the perinatal switch from _____ to ____ synthesis, leading to continued high-level production of HbF in adults.
Hereditary Persistence of Fetal Hemoglobin (HPFH); γ to β-globin
Sickle cell anemia (HbSS)
Most common among people of African origin, where carrier frequency is ~10%. Single base mutation at codon#6 in the β-globin gene changes glutamate to valine. HbS
is 80% less soluble than HbA when not bound to O2, and polymerizes into long fibers that distort the RBC into a characteristic sickle shape. These sickled cells become
lodged in the micro-capillaries and further exacerbate the sickling crisis
Hemoglobin C disease (HbCC)
A milder form of hemolytic anemia than sickle cell anemia. Caused by a single base mutation at codon#6 of the β-globin gene, changing glutamate to lysine. HbC is less
soluble than HbA and tends to form crystals, reducing the deformability of RBC.
HbS or HbC trait. What do these terms mean?
Both sickle cell anemia and hemoglobin CC disease are of autosomal recessive inheritance. Sickle cell trait (HbS trait) or hemoglobin C trait (HbC trait) describes the conditions expressed in individuals who are heterozygous of HbS/HbA and HbC/HbA, respectively. They are clinically normal except when under severe low pO2 stress.
Hemoglobin SC disease is an example of _____ ? What is the phenotype as compared to HbSS?
Compound heterozygotes (βS/βC) have a milder anemia than sickle cell disease.
A recognition site (CCTNAGG) of the restriction enzyme MstII is destroyed in exon 1 by the _____ missense mutation in the βS mutant allele. The______ gives 1.15 kb + 0.20 kb fragments, whereas the ______ gives one 1.35 kb fragment.
A-to-T (Glu--->Val); normal allele βA; βS mutant allele
Can digestion by MstII distinguish between βA and βS? Between βA and βC? Why or why not?
MstII can distinguish between βA and βS but not between βA and βC because βC mutation is not included in the cut site, so the digestion proceeds equally in both cases.
β4 (precipitates, beta tetramer. no good)
α2β2 (major form 97%)
minor form: α2δ2 (2%)
α-Thalassemias. Affects fetal Hb, Adult Hb, or both?
Mostly caused by deletions of one or both copies of the α-globin gene in the α-cluster. Thus, γ- and β-globin are in excess. Affects the formation of both fetal and adult hemoglobins.
(- -). Common in Southeast Asia. Caused by deletion of both copies of α-globin genes in the α-cluster. Homozygous state (- -/- -) results in "hydrops fetalis" (stillborn). Most fetal hemoglobin is γ4 (Hb Bart’s) although there is enough ζ2γ2 (Hb Portland) to sustain fetal development. Heterozygotes (αα/--) have mild anemia, a.k.a. α-thalassemia-1 trait
(α -). Common in Africa, Mediterranean, and Asia. Deletion of one of the two α-globin genes in the α-cluster. 50% decrease in α-globin synthesis. No disease phenotype in heterozygote (αα/α-, silent carrier). Mild anemia in homozygotes (α-/α-), a.k.a. α-thalassemia-2 trait
(α -/- -). Compound heterozygous individuals with only 25% of normal α-globin level. Severe anemia. a.k.a. HbH disease. About 5-30% of their hemoglobin is β4 (HbH), which precipitates.
Show a wide range of severity and can be cause by virtually every possible type of mutation in the β-globin gene. High allelic heterogeneity means most patients with β-thalassemia are compound heterozygotes carrying two different mutant alleles of the β-globin gene. Categorization of β-thalassemias: Based on clinical conditions – thalassemia major vs thalassemia minor
Characterized by severe anemia, in which most RBCs are destroyed before being released into the circulation. Thinning bone cortex, enlarged liver and spleen resulted from massive effort of blood production at these sites. Treat temporarily with blood transfusions; however, iron accumulation from repeated transfusion leads to
organ failure. Iron chelation therapy (e.g. desferrioxamine) is used to reduce the complications of iron overload. (CLINICAL conditions define major/minor)
Clinically normal, carriers of one β-thalassemia allele. Based on CLINICAL conditions.
Caused by mutations or deletions that impair the production of the β-globin chain alone. Other genes in the β-globin cluster unaffected. (Based on the molecular nature of the mutation – simple β-thalassemia vs complex thalassemia)
Caused by large deletions that remove the β-globin gene plus other genes in the β-cluster, or the LCR. Note that some deletions within β-cluster cause HPFH instead of thalassemia (Based on the molecular nature of the mutation – simple β-thalassemia vs complex thalassemia)
Most common form of β-thalassemia (90% of the cases). Some β-globin is made so that some HbA is present. Decrease in β-globin synthesis can be caused by
mutations affecting transcription, RNA processing, or protein stability.
Zero β-globin synthesis so that no HbA is present. Caused by deletion of the β-globin gene, nonsense or frameshift mutations at the 5’ of the coding region that lead
to an early stop codon, or mutations that result in no RNA synthesis. [Hb] is ~5% of normal level (of which 95% is α2γ2 with 5% α2δ2), which is not enough for good survival.
No δ or β synthesis due to deletion of the coding sequences for both δ- and β-globin. Milder
clinical phenotype than β0-thalassemia because the remaining γ gene(s) is still active after birth instead of switching off as would normally occur. Therefore, HbF (α2γ2) compensates for the absence of HbA (α2β2), about 5-18% of normal level of total hemoglobin production.
Note: People with deletions that remove the δ and β genes (γ is intact) may have either δβ0-thalassemia or HPFH, depending on the range of the deletion.
HPFH (hereditary persistent fetal hemoglobin)
No δ or β synthesis because of deletions of both genes. Increased γ-globin expression caused by either of the two following mechanisms:
(1) extended deletion of additional downstream sequences, which likely brings a cis-acting
enhancer element closer to the γ-globin gene, or
(2) mutations in the promoter region of one of the two γ-globin genes that destroy the binding site of a repressor, thereby relieving postnatal repression of γ.
HPFH individuals are disease free, since adequate levels of γ chains are still made due to the disruption of the perinatal globin switch from γ to β. 100% of hemoglobin is HbF (α2γ2), which is about 17-35% of normal level of total hemoglobin production. HPFH individuals have higher HbF (17-35%) level than δβ0-thalassemia individuals (5-18%).
Significance of HPFH: Understanding the mechanism of HPFH may make it possible to express HbF at high levels
postnatally to treat patients of β-thalassemia and sickle cell anemia.