Transcript Sickle-Cell Anemia - College of Charleston

Malaria and Sickle-Cell Anemia:
Genetic, Physiological, and Cultural
Adaptations
Revised by Brad R. Huber, 7/17/2015 8:00:37 AM
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Malaria and sickle cell anemia are an
example of a balanced polymorphism
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An equilibrium mixture of homozygotes
and heterozygotes maintained by natural
selection against both homozygotes
Two opposing selective forces:
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Malaria selects against
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Sickle-Cell Anemia selects against
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Homozygous Dominant (HbA, HbA)
Homozygous Recessive (HbS, HbS)
(HbS, HbA) Heterozygous (Sickle Cell Trait)
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No anemia & less susceptible to malaria
Malaria and Sickle-Cell
Anemia Map
Malaria Pathogen
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Plasmodium is a protozoan parasite that causes
malaria
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It consumes glucose and enzymes of blood cells,
and as a result the cells die.
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The cells are destroyed at 2 to 3 day intervals
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The release of the waste products causes chills
and fever
Plasmodium Life Cycle
There are sexual and asexual parts
of the Plasmodium life cycle
Malaria: Vectors
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Anopheles gambiae breeds an open
sunny pools
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Anopheles funestus breeds along
shaded river edges and vegetative
swamps in undisturbed forest
When agriculture was introduced two
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thousand years ago in sub-Saharan Africa
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The forests were cleared & sunny pools
of water became more common.
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Anopheles gambiae mosquitoes
increased
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And so did people contracting malaria
Malaria: Epidemiological Issues
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The current prevalence of malaria is between
300,000,000 - 500,000,000 cases.
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Mortality rate: 650,000 people die of malaria each
year
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Highest among small children
Malaria also causes miscarriage and premature
birth.
Malaria: Clinical Features
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Malaria can lead to kidney failure, anemia,
pneumonia
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Low birth weight, miscarriage, and stillbirths
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It is a strong selective pressure, especially on
children
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Adults who survive malaria in childhood develop
a protective immunity
Sickle-Cell Anemia
Sickle-Cell Anemia: Genetic Aspects
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Hemoglobin is a protein found in red blood cells
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It binds with, carries, and releases oxygen to the
cells of the body
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A hemoglobin molecule is large with a great
potential for DNA point mutations to occur
Hemoglobin: Genotypes and Phenotypes
• Homozygous Dominant (HbA, HbA)
• Normal hemoglobin
• Homozygous Recessive (HbS, HbS)
• Abnormal hemoglobin, Sickle Cell Anemia
• Heterozygous (HbA, HbS)
• Sickle-Cell Trait
• Normal and abnormal hemoglobin, with normal
hemoglobin more abundant.
Diagnosis of Sickle-Cell Anemia
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Amniocentesis of fetal cells can be used in
the 16th week of pregnancy
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Chorionic villi biopsy can be done at 8 to
10 weeks
The Heterozygous Genotype and Malaria
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Heterozygous individuals produce red blood cells that
live only 2 to 3 weeks instead of 120 days
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These red blood cells have both normal and
abnormal hemoglobin
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They inhibit the reproduction of the malaria parasite
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When heterozygous individuals get malaria it is less
severe than when homozygous dominant individuals
get malaria
The Homozygous Recessive Genotype
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This Genotype causes Sickle Cell Anemia
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Red blood cells with abnormal
hemoglobin become sickle shaped.
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They clump, blocking blood flow.
Clinical Signs and Symptoms
• Pain crises
• Acute chest syndrome (a life-threatening
pneumonia-like illness)
• Cerebrovascular accidents
• Splenic and renal dysfunction
• Chronic anemia
• Susceptibility to infections
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Homozygous recessive individuals who are infected with
the malaria parasite die of an anemia crisis
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Children with sickle cell anemia must be cautious in play
and sports; bruises and scrapes can lead to crises
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before modern medical care, the average lifespan was 20
years
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many individuals suffering with sickle cell disease can
now live into their mid-40s and beyond
Epidemiology
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Malaria is found in only a few people in the U.S.
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Recent immigrants and tourists
As a result, there is only a selection against the sickling
hemoglobin allele (HbS) in the U.S.
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For example, HbS allele has decreased in frequency in
blacks from about 8.5% to about 1%
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70,000 to 100,000 people in the U.S. have sickle
cell anemia
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2 million have sickle cell trait.
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Sickle cell anemia is more common in certain
ethnic groups, including:
• People of African descent, including AfricanAmericans
• Hispanic–Americans from Central and South
America
• People of Middle Eastern, Asian, Indian, and
Mediterranean descent
Cultural and Psychological Adaptation
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Teachers and friends of children with sickle cell
anemia are often silent; they are uncomfortable
talking about serious illness and death
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Parents may feel guilty because sickle cell anemia
is hereditary.
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Adolescents are sometimes pessimistic about
getting a job, continuing school, getting married
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Adolescents may become depressed
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Women with sickle cell anemia:
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have a 50-50 chance of not surviving after giving birth
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there is also a risk of bearing an injured child.
Genetic counseling, sterilization, and abortion are options
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However abortion is not acceptable to all.
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And it’s difficult to predict how severely a child will be
affected by sickle cell anemia.
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All newborns in the United States are now tested for the
disease.