SCA Flashcards
(35 cards)
Hemoglobinopathies are disorders involving abnormal hemoglobin, which can be classified into two main categorized into? ____&____ with example
Qualitative Hemoglobin Disorders:
These result from mutations in the DNA that alter the amino acid sequence of the globin chains, affecting the function of the hemoglobin molecule.
Example: Sickle Cell Disease is a qualitative hemoglobinopathy where a single nucleotide mutation leads to the substitution of valine for glutamic acid at position 6 of the β-globin chain. This causes hemoglobin S (HbS) to polymerize under low oxygen conditions, leading to sickling of red blood cells.
- Quantitative Hemoglobin Disorders:
These disorders are due to imbalances in the production of globin chains. This imbalance leads to an excess of one type of globin chain over another.
Example: Thalassemia results from mutations that reduce or eliminate the production of α or β-globin chains, leading to an excess of the other type and ineffective erythropoiesis.
What are the Clinical Syndromes Produced by Hemoglobin Abnormalities:
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Hemolysis:
- Crystalline Hemoglobins (S, C, D, E): Variants like HbS (sickle hemoglobin) or HbC can form crystals or aggregates that damage red blood cells, leading to hemolysis and anemia.
- Unstable Hemoglobin: Mutations can make hemoglobin unstable, causing it to denature and precipitate, which leads to hemolytic anemia.
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Thalassemia:
- α-Thalassemia and β-Thalassemia: Result from reduced synthesis of α or β-globin chains, causing an imbalance in hemoglobin chain production and ineffective erythropoiesis, leading to anemia and related complications.
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Familial Polycythemia:
- Altered O2 Affinity: Hemoglobins with altered oxygen affinity can cause polycythemia (increased red blood cell count) as the body responds to the lower oxygen delivery by producing more red blood cells.
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Methaemoglobinaemia:
- Failure of Reduction: This condition occurs when hemoglobin is oxidized to methemoglobin, which cannot bind oxygen effectively. This can result from inherited defects or exposure to certain chemicals.
What’s thalassemia
What’s barts hydrops fetalis
Thalassemias are inherited blood disorders characterized by defects in hemoglobin production
What’s alpha thalassemia
What’s coleeys anemia
α-Thalassemia is an inherited disorder which is primarily caused by gene deletions affecting the α-globin chains.
Classify alpha thalassemia
Remember alpha has 4genes
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α-Thalassemia is primarily caused by gene deletions affecting the α-globin chains. The severity of the condition depends on the number of affected α-globin genes:
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Silent α-Thalassemia:
- Genotype: -α/αα
- Description: Only one α-globin gene is deleted. Individuals are asymptomatic and have normal hemoglobin levels.
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α-Thalassemia Trait (α-Thalassemia Minor):
- Genotype: αα/– or -α/-α
- Description: Two α-globin genes are affected, either through one gene deletion on each chromosome or two deletions on one chromosome. This results in mild anemia and usually normal red cell indices.
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Hb H Disease:
- Genotype: –/-α
- Description: Three α-globin genes are deleted. This leads to moderate to severe anemia and the formation of Hb H, an unstable hemoglobin variant. Symptoms include chronic anemia and splenomegaly.
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Hb Bart’s Hydrops Fetalis:
- Genotype: –/–
- Description: All four α-globin genes are deleted. This condition is severe and typically fatal in utero, leading to hydrops fetalis, a condition where the fetus accumulates fluid in multiple body cavities.
Classify beta thalassemia
Remember beta has 2 genes
β-Thalassemia is usually caused by point mutations in the β-globin gene, resulting in reduced or absent β-globin chain production. The severity of the disease is categorized based on the extent of β-globin production:
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β-Thalassemia Trait (Carrier State):
- Genotype: One normal β-globin gene and one mutated gene (-β/+β or -β/-β+)
- Description: Individuals have mild anemia but are carriers. They have increased levels of fetal hemoglobin (HbF) and normal red cell indices.
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β-Thalassemia Intermedia:
- Description: A more severe form than β-thalassemia trait but less severe than β-thalassemia major. There is a partial reduction in β-globin synthesis, leading to moderate anemia and possible need for occasional blood transfusions.
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β-Thalassemia Major (Cooley’s Anemia):
- Description: This severe form results from the absence of β-globin production (β0/β0). Patients require regular blood transfusions for survival and face serious complications such as iron overload and bone deformities. Symptoms start early in childhood and include severe anemia, failure to thrive, and skeletal deformities.
Differentiate between SCA and SC Dxs
SCA=SS
SCD=Sickle Cell Disease (SCD) refers to a group of genetic disorders that are characterized by the presence of hemoglobin S (HbS), a variant of the normal hemoglobin molecule (HbA). SCD occurs when the sickle cell mutation is inherited alongside another mutation in the β-globin gene that either reduces or abolishes normal β-globin production.
Note SCA is a form of SCD
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Sickle Cell Trait (HbAS):
- Genotype: Heterozygous inheritance of one sickle cell gene (HbS) and one normal β-globin gene (HbA).
- Description: Individuals with the sickle cell trait typically do not exhibit symptoms of the disease because they produce both normal hemoglobin (HbA) and some sickle hemoglobin (HbS). This condition provides some protection against malaria but does not lead to the severe symptoms seen in sickle cell anemia.
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Sickle Cell Anemia (SCA):
- Genotype: Homozygous inheritance of the sickle cell gene (HbSS).
- Description: Sickle cell anemia is the most common and severe form of SCD, characterized by the presence of sickle-shaped red blood cells. These abnormal cells can cause various clinical manifestations, including painful vaso-occlusive crises, chronic hemolysis, and organ damage. The condition is inherited when both parents carry the sickle cell trait (HbAS), giving a 25% chance that a child will inherit sickle cell anemia.
What’s the Inheritance Patterns
If both parents have the sickle cell trait (HbAS):
- 25% chance that a child will have sickle cell anemia (HbSS).
- 50% chance that a child will inherit the sickle cell trait (HbAS).
- 25% chance that a child will have normal hemoglobin (HbAA).
What’s the position of mutation clinical severity of Sca
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HbSS (Sickle Cell Anemia):
- Mutation: Glutamic acid (Glu) is replaced by valine (Val) at position 6 of the β-globin chain.
- Phenotype: This genotype is associated with a severe or moderately severe disease course, with significant clinical symptoms including painful crises, anemia, and organ damage.
What’s the position of mutation clinical severity of HbSC
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HbSC:
- Mutation: Combination of HbS (Glu → Val at position 6) and HbC (Glu → Lys at position 6).
- Phenotype: The clinical severity is intermediate between HbSS and HbAS. Patients may experience vaso-occlusive episodes and mild to moderate anemia, but the overall course is usually less severe than in HbSS.
What’s the mutation and clinical severity of 3. HbS/β° Thalassemia:
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HbS/β° Thalassemia:
- Genotype: Combination of HbS with a β-thalassemia mutation that leads to no β-globin production.
- Phenotype: Clinically almost indistinguishable from sickle cell anemia, presenting with a similar severity of symptoms.
What’s the mutation and clinical severity of . HbS/β+ Thalassemia:
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HbS/β+ Thalassemia:
- Genotype: Combination of HbS with a β-thalassemia mutation that results in reduced β-globin production.
- Phenotype: The disease severity varies, but it is generally milder than HbSS. The clinical course can differ across different ethnic groups.
What’s the mutation and clinical severity of HbS/HPFH (Hereditary Persistence of Fetal Hemoglobin):
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HbS/HPFH (Hereditary Persistence of Fetal Hemoglobin):
- Phenotype: This combination results in a mild phenotype or may be completely asymptomatic, as high levels of fetal hemoglobin (HbF) mitigate the effects of HbS.
What’s the mutation and clinical severity of HbS/HbE
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HbS/HbE:
- Mutation: Glutamic acid (Glu) is replaced by lysine (Lys) at position 26.
- Phenotype: This is a rare combination, generally leading to a mild clinical course.
Other Rare Combinations:
- HbS with HbD Punjab, HbO Arab, G-Philadelphia, etc.:
- These combinations are rare and may result in varying clinical severities, ranging from mild to moderate, depending on the specific hemoglobin variant involved.
- Protection Against Malaria: Individuals who carry the sickle cell trait (HbAS) — meaning they have one normal hemoglobin gene (HbA) and one sickle cell gene (HbS) — have a selective advantage in these regions. The presence of HbS in red blood cells provides protection against P. falciparum malaria. This protection occurs because the parasite has difficulty surviving and replicating in the sickle-shaped red blood cells. The sickling process, especially under low oxygen conditions, creates an inhospitable environment for the parasite.
Biochemistry of sickle Cell anemia
Sickle Cell Anemia is caused by a single point mutation in the β-globin gene, located on chromosome 11.
Glu to Val
What’s the pathology of sickle cell anemia?
Pathology
The pathology of SCA is characterized by the sickling of red blood cells, which leads to vaso-occlusion (blockage of blood vessels), hemolysis (destruction of red blood cells), and chronic anemia. These sickled cells are less flexible and can get stuck in small blood vessels, causing painful episodes called vaso-occlusive crises, along with organ damage and other severe complications like stroke and acute chest syndrome
Explain how SCA shows balance polymorphism
SCA is a prime example of how natural selection operates. The sickle cell trait (heterozygous state, HbAS) provides a protective advantage against Plasmodium falciparum malaria. This has led to the high prevalence of the HbS allele in regions where malaria is endemic, as individuals with the trait are more likely to survive and reproduce in these areas. This phenomenon is an example of balanced polymorphism, where both alleles are maintained in the population due to selective pressures.
Gene expression studies in SCA focus on the regulation of the β-globin gene and how it interacts with other genes in the hemoglobin production pathway. Understanding the expression of globin genes, particularly the switch from fetal hemoglobin (HbF) to adult hemoglobin (HbA and HbS), is crucial for developing therapeutic strategies. Genomic studies also explore the variations in the β-globin gene cluster and their impact on disease severity.
Globin haplotypes refer to specific combinations of genetic markers within the β-globin gene cluster on chromosome 11. These haplotypes are defined by a series of polymorphisms (variations) identified using restriction endonucleases (enzymes that cut DNA at specific sequences).
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African Haplotypes: In individuals of African descent, the βs-globin gene associated with sickle cell disease is found on three major haplotypes, each localized to different geographical regions in Africa. These haplotypes are:
- Benin Haplotype: Predominantly found in West Africa, particularly in countries like Nigeria and Benin.
- Central African Republic (Bantu) Haplotype: Found in Central Africa, including regions like the Democratic Republic of the Congo.
- Senegal Haplotype: Common in West Africa, especially in Senegal and surrounding regions.
Each haplotype has distinct genetic variations that may influence the clinical severity of SCA, the response to treatment, and the overall prognosis. For example, the Senegal haplotype is often associated with higher levels of fetal hemoglobin (HbF), which can ameliorate the severity of the disease by inhibiting sickling.
Sickle Cell Anemia is not just a disease but a focal point where various biological disciplines intersect. The study of SCA provides insights into how genetic mutations can affect biochemical processes, lead to specific pathologies, and influence population dynamics through natural selection. Additionally, the exploration of globin haplotypes within the context of genomics underscores the importance of genetic diversity in understanding disease mechanisms and developing targeted therapies.
Explain the genetic basis of sickle cell anemia. How does it differ from sickle cell trait?
Sickle cell anemia (SCA) is a hereditary blood disorder caused by a mutation in the gene that encodes the beta-globin chain of hemoglobin (Hb). The mutation occurs in the HBB gene on chromosome 11, where a single nucleotide substitution changes glutamic acid to valine at the sixth position of the beta-globin chain. This creates an abnormal form of hemoglobin known as hemoglobin S (HbS).
- Inheritance pattern: Sickle cell anemia follows an autosomal recessive inheritance. For a person to have sickle cell anemia, they must inherit two copies of the mutated gene (HbS) from each parent. This leads to the production of HbS, which polymerizes under low-oxygen conditions, causing red blood cells to become rigid and adopt a sickle shape.
- Sickle cell trait: Individuals with sickle cell trait inherit one normal gene (HbA) and one HbS gene (heterozygous). They are usually asymptomatic because HbA is sufficient to prevent significant sickling under normal conditions. However, they may experience complications under extreme conditions like severe hypoxia or dehydration.
The key difference between sickle cell anemia and sickle cell trait is that individuals with sickle cell anemia (homozygous HbSS) are symptomatic and prone to complications, while those with the trait (heterozygous HbAS) are typically asymptomatic carriers
- Describe the pathophysiology of sickle cell disease. What are the consequences of red blood cell sickling?
In sickle cell disease (SCD), the polymerization of HbS under deoxygenated conditions triggers the sickling of red blood cells (RBCs). This leads to several pathological consequences:
- Sickling of RBCs: Under low oxygen tension (e.g., during exercise, infection, or dehydration), HbS polymerizes and causes RBCs to assume a crescent or sickle shape. These sickled cells are rigid and less flexible, which makes them prone to occluding small blood vessels, leading to ischemia.
- Vaso-occlusion: The sickled cells can clog capillaries and small vessels, leading to vaso-occlusive crises, which cause ischemia, pain, and end-organ damage. Common sites affected include bones (bone pain), lungs (acute chest syndrome), and the spleen (splenic sequestration).
- Hemolysis: Sickled cells are fragile and have a shortened lifespan (~10-20 days compared to the normal 120 days). This leads to chronic hemolysis, causing anemia and increased bilirubin levels, which can result in jaundice and gallstones.
- End-organ damage: Chronic vaso-occlusion and hemolysis contribute to cumulative damage to multiple organs, including the kidneys (renal failure), brain (strokes), and lungs (pulmonary hypertension).
In summary, the key consequences of sickling in SCD are vaso-occlusive crises, hemolytic anemia, and chronic organ damage, all of which contribute to the morbidity and mortality associated with the disease
- What are the clinical manifestations of sickle cell disease? How do these symptoms relate to the underlying pathology?
The clinical manifestations of sickle cell disease result from the ongoing processes of vaso-occlusion and hemolysis:
- Vaso-occlusive crises (pain crises): These are the hallmark of SCD and occur when sickled cells block blood flow, causing ischemia and severe pain. Commonly affected areas include the long bones, chest, and abdomen. Episodes may be triggered by dehydration, infection, or stress.
- Acute chest syndrome: This is a life-threatening condition that presents with chest pain, fever, and respiratory symptoms, often due to pulmonary vaso-occlusion. It can be precipitated by infections, fat embolism from bone marrow necrosis, or direct lung involvement.
- Stroke: Occlusion of cerebral blood vessels can lead to ischemic strokes, which are more common in children with SCD. This complication can result in long-term neurological deficits.
- Splenic sequestration: The spleen becomes progressively infarcted due to repeated vaso-occlusion, leading to functional asplenia (loss of spleen function) and increased susceptibility to infections, particularly from encapsulated bacteria like Streptococcus pneumoniae.
- Hemolytic anemia: The chronic destruction of sickled RBCs leads to symptoms of anemia, including fatigue, pallor, and jaundice. Chronic hemolysis also increases the risk of gallstones (cholelithiasis).
Each clinical manifestation can be traced back to the two central pathological processes: vaso-occlusion (leading to ischemia, pain, and organ damage) and hemolysis (leading to anemia and its associated symptoms).
