What Is It And Where Did It Come From · What is Sickle Cell Disease? Sickle cell disease is a...
Transcript of What Is It And Where Did It Come From · What is Sickle Cell Disease? Sickle cell disease is a...
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What Is It And
Where Did It Come From
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What is Sickle Cell Disease?
Sickle cell disease is a disease of the blood It is due to an abnormal hemoglobin (the red
pigment of blood)
It is inherited (it comes from the parents and is passed to children)
It causes many problems, including: Pain
Disability
Loss of organs (strokes, kidney failure, bone damage, etc.
Premature death
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What is Hemoglobin?
Hemoglobin is the red pigment of blood
It carries oxygen from the lungs to every
part of the body
It consists of 4 units called chains
Each chain has about 145 building
blocks (amino acids)
These chains a wound around in a very
complicated way
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How Is the Hemoglobin of Sickle Cell
Disease Different?
Two of the building blocks (amino acids)
in sickle cell hemoglobin are different
from normal
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How Is the Hemoglobin of Sickle Cell
Disease Different?
Two of the building blocks (amino acids)
in sickle cell hemoglobin are different
from normal
This is due to a change in the gene for
hemoglobin
One piece out of about 2,000 is changed
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Fundamental Defect in Sickle Cell Disease
Hb A Hb S
-PRO- -GLU- -PRO- -GLU- VAL
-GGA-C C-CTC- A -GGA-C C-CTC- GENE T
GLU Hemoglobin
Chain
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How Is the Hemoglobin of Sickle Cell
Disease Different?
Sickle cell hemoglobin can be detected
by examining the blood in the laboratory
An individual may have: normal hemoglobin,
Both normal and sickle hemoglobin (sickle
cell trait), or
Only sickle hemoglobin (sickle cell disease)
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GLOBIN GENES ELECTROPHORESIS
HEMOGLOBIN
Sickle Cell Trait (Hb
AS)
Identification of Hb S Status by
Electrophoresis
Sickle Cell Disease
(HbSS)
Normal (Hb
AA) +
+
+
A S
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How is Sickle Cell Disease Inherited
(Passed in the Family)
Most inherited characteristics are
determined by two genes
One gene comes from the father
One gene comes from the mother
When one parent has the trait, half of
the children are likely to have the trait
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Genetics of Sickle Cell Disease
Normal Trait
Trait Normal
Normal Trait
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How is Sickle Cell Disease Inherited
(Passed in the Family)
When one parent has the trait, half of
the children are likely to have the trait
When both parents have the trait
One fourth of the children will likely be
normal
One half will likely have the trait
One quarter will likely have the disease
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Genetics of Sickle Cell Disease
Trait Trait
Trait Trait
Normal Disease
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Why Does the Hemoglobin of Sickle
Cell Disease Cause Problems?
Two of the building blocks in the
hemoglobin are different from normal
This difference allows the molecules of
hemoglobin to stick together in a
particular pattern
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Formation of "Crystals" of Sickle
Hemoglobin
Unpoly- merized
Nucleation Random Rods
Parallel Rods
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THE SHAPE CHANGE OF SICKLING
NORMAL
SICKLED
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ARTERIOLE CAPILLARY VENULE
O 2
SICKLING IN THE PERIPHERAL
CIRCULATION
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Sickling in a Small Vessel
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Sickling in a Small Vessel
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Area of
Low Oxygen
Sickling in a Small Vessel
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A Tale of Five Individuals
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How Did Sickle Cell Arise?
The change in the sickle hemoglobin
molecule (Hb S) is caused by a mistake
in the gene responsible for that chain of
hemoglobin
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Where Did This Happen?
In India in one individual, ~3,500 years
ago
It spread to Eastern Arabia
Sickle cell persists in the “Hill Tribes” of India
○ Sickle cell disease is more plentiful in one
state of India than in the entire United States
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The Harappa-Dilmun Axis
Elam
Akkad
Dilmun
Indus Valley
Civilization
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Where Did This Happen?
In India in one individual, ~3,500 years ago
It spread to Eastern Arabia
Sickle cell persists in the “Hill Tribes” of India
○ The gene is more plentiful in one state of India
than in the entire United States
In Africa in four individuals
Near the Congo, ~2,000 years ago
Near Nigeria, just a little later
In West Africa, sometime later
In Cameroons, fairly recently
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Cameroons
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Where Did It Spread?
Locally at each site of origin The area covered indicates how long the gene
has been there
By trade routes
Wherever people from the area migrated or were taken – especially by the African slave trade To Sicily, Greece, and Turkey by the Arab trade
in African slaves
To the New World by the transatlantic slave trade
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Why Did The Abnormal Gene Become
So Common?
By Darwinian (natural) selection
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Population Before Disease Strikes
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Disease Strikes
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Population After Disease Strikes
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Some People (in Green) Are Resistant
to the Disease
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Some People (in Green) Are Resistant
to the Disease When It Hits
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The Percentage of Resistent People Is
Increased
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What Is Malaria?
Human malaria started many, many years ago, probably from birds
Organism develops within red blood cells
Transmitted by tropical mosquitos
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Malaria
Became a public health problem after settled life began in tropics
People lived close enough together that the mosquitoes could spread the organism
Malaria occurs throughout the tropics
1-2 million people affected each year
216 million cases in 2101
>300,000 deaths in 2010
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Sickle Cell Disease and Malaria
Sickle cell trait protects against malaria The reason is not known
Based on population studies
Malaria is fatal in sickle cell disease
This is called “trait advantage” One dose of the gene is helpful
Two doses of the gene are harmful
Why everybody in Africa does not have sickle cell trait: If everybody had the trait, too many people
would have the disease
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Other Changes In Red Cells That Confer
Protection Against Malaria
Hemoglobin C Started in one person in Burkina Faso about 1000
years ago
Hemoglobin E Started in people in Southeast Asia
Thalassemia Many kinds, mostly with “trait advantage”
One kind (a thalassemia) has so little downside in people with two doses of the gene as to become universal in the population
G6PD deficiency
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Other Changes In Red Cells That
Confer Protection Against Malaria
Absence of Duffy blood group
Almost all Africans lack this factor; all other
people have it
One person started the gene many, many years
ago
Two doses of the gene have no downside
The absence of the factor means that one
type of malaria (called “vivax malaria”)
cannot invade the red cell
Vivax malaria does not now exist in Africa
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What Has Been The Effect of These
Protections or Their Lack on History
People lacking them could not live in tropical, malarious Africa
Natives of the tropical parts of the New World were decimated when malaria was introduced
Descendants of African slaves, who were resistant, replaced them
Local patterns of living in the southern States were altered
“Upcountry” homes in South Carolina
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What Has Been The Effect of These
Protections or Their Lack on History
People lacking them could not live in tropical, malarious Africa
Natives of the tropical parts of the New World were decimated when malaria was introduced
Descendants of African slaves, who were resistant, replaced them
Local patterns of living in the southern States were altered
“Upcountry” homes in South Carolina
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What Has Been The Effect of These
Protections or Their Lack on History
People lacking them could not live in tropical, malarious Africa
Natives of the tropical parts of the New World were decimated when malaria was introduced
Descendants of African slaves, who were resistant, replaced them
Local patterns of living in the southern States were altered
“Upcountry” homes in South Carolina
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Summary
Sickle cell arose as a protection against
malaria
It started from no more than 5
individuals
It increased in the population by natural
selection of individuals with the trait
Thus, it is one of several
“improvements” that allowed people to
live where malaria is common