Sickle Cell Anemia

• Disease Background

  • Sickle cell anemia was first diagnosed in 1904 by Chicago-based cardiologist Dr. James Herrick. He was treating a 20-year old dental student named Walter Clement Noel, who was an exchange student from Grenada. The examination of Noel's blood samples revealed "peculiear elongated and sickle-shaped blood cells," which is the hallmark trait of this disease. Vernon Manson coined the disease's name in 1922.

  • While sickle cell anemia was "discovered" in 1904, the disease was quite possibly described as early as 1846. An autopsy conducted by two students at what is today the Medical University of South Carolina on the corspe of a runaway slave described several key traits of sickle cell disease. The slave, known only as "Jim," was described by his owner to have suffered bouts of chest pain and skin discoloration while alive. The students' inablility to locate Jim's spleen is what keyed off further investigation and led to the description of several more symptoms of sickle cell in the autopsy report. The autopsy was described in the Southern Journal of Medical Pharmacology in the same year. A similar report was made by a physician in Georgia in the 1870's. (MUSC Medical History Club Fall 2008 Lecture Series)

  • Today, the disease is most prevalent in sub-Saharan Africa, although it is also found in India and the Arabian peninsula. Areas where many descendants of former African slaves live, such as the United States and certain islands in the Caribbean Sea, also have carriers and patients of sickle cell present. The disease can be present in any race or gender, but it is overwhelmingly found in people of African descent.

• Description of Disease

  • Sickle Cell Anemia is a disease of the blood where the body makes sickle-shaped red blood cells, as opposed to normal circular “disc-shaped” red blood cells.  It is a type of anemia, which means that a person's red blood cell count is lower than normal. Another sign of anemia is if the red blood cells don't have enough hemoglobin, as is the case with Sickle Cell Anemia. (NHLBI) Some other names for Sickle Cell Anemia are SCD, Sickle cell disorder, Hemoglobin S Disease, HbS Disease, and Sickling disorder due to hemoglobin S. (Genetic Home Reference)
    • Normal Red Blood Cells:
      • Normal red blood cells are similar to donuts (without the hole in the middle) in shape and easily flow through blood vessels. Red blood cells also contain an iron-rich protein called hemoglobin that gives them their red color and allows for the transport of oxygen from the lungs to all other parts of the body.
    • Sickle Cell Anemia:
      • Sickle-shaped red blood cells have abnormal hemoglobin which causes the undesired “crescent” shape.  They are problematic because they don’t flow as easily through blood vessels and they don't last as long as normal red blood cells. Their stiff and sticky nature creates a tendency to stick together to form clumps of cells that blocks blood flow to extremities and vital organs, thus causing serious problems. (NHLBI)
      • 
      • This video shows an animation of how sickle cells appear in the blood. Note the characterstic sickle shape and the smaller size.
• Description of Symptoms
  • The signs and symptoms of Sickle Cell Anemia can run from mild to severe depending on the case. In the more serious cases people can be hospitalized. Although this disease is present at birth, signs usually don’t begin to show until 4 months of age. Most common symptoms are associated with anemia and pain. Other symptoms will present with the complications of the disease.
  • Symptoms linked to anemia:
    • Fatigue is the most common symptom of anemia. Other anemia related symptoms include:
      • Headache
      • Dizziness
      • Loss of Breath
      • Coldness of Hands and Feet
      • Chest Pain
      • Pale Skin
    • These symptoms can occur because in order for the body to receive adequate oxygen, the heart must work harder to pump the abnormally shaped cells through the blood vessels.  
  • Symptoms linked to pain
    • Known as “sickle cell crisis”, many people with sickle cell anemia can have sudden pain throughout their entire body. A sickle cell crisis can occur in the joints, bones, abdomen, and lungs. This is when the abnormal sickle cells clump together to create a blockage in the small vessels of organs and extremities. The blockage can cause acute or chronic pain and even organ damage.
      • Acute pain:
        • This is more common with people who have Sickle Cell Anemia. Acute pain occurs suddenly and can last anywhere from a couple hours to days.
      • Chronic pain:
        • Chronic pain is less common but more severe. It can last for weeks to months. Chronic pain can limit one’s daily activities due to the intensity of the pain.
• Inheritance of Disease
  • Sickle Cell Anemia is has an autosomal recessive inheritance pattern. This means that each parent is a carrier for the mutated gene, even though neither one has the disease, and can pass it on from generation to generation.
    • Homozygous Recessive:
      • There is a 25% chance that their offspring will be homozygous recessive and have the disease along with the accompanying symptoms. The child will have two recessive mutated sickle cell genes. This phenotype would be the presentation of Sickle Cell Anemia. Homozygous recessive offspring would have the possibility of passing this disease on.
    • Heterozygous:
      • There is a 50% chance that their offspring will be heterozygous for the disease (like the parents). The child would have one normal gene (dominant) gene and one mutated sickle cell gene (recessive). This person would not have the disease nor have any symptoms. These offspring would be carriers and therefore be able to pass on the mutated sickle cell gene.
    • Homozygous Dominant:
      • There is also a 25% chance that the offspring will not have to Sickle Cell gene at all and be homozygous dominant. This child would have two normal genes (dominant). This offspring would not be able to pass the mutated sickle cell gene on because they are not a carrier or a homozygous recessive.  (Wrong Diagnosis)
      • 
• Gene that codes for Sickle Cell
  • Normal Function of gene
    • "The genes of the β-globin gene cluster (ε, Gγ, Aγ, δ, and β) are present on chromosome 11 in the same order in which they are expressed during development. The β–locus control region (β–LCR) is a major regulatory element located far upstream of the genes of the cluster that is necessary for the high level of expression of those genes."---http://www.jci.org/articles/view/30920/figure/2
    • The β-globin gene is one of 5 genes at the HBB sequence located at position 15.5 on chromosome 11. This gene codes for the β-globin sheet of hemoglobin. This protein is part of the makeup of the hemoglobin molecule and plays a role in binding with oxygen. Hemoglobin's main function is to transport oxygen to the body's cells for use in cellular respiration.
  • Mutation that causes sickle cell (a point mutation in the β-globin chain of hemoglobin)
    • "patients with disorders of the β-globin genes start manifesting the clinical features of their diseases"
    • There is a point mutation in the gene that codes for β-globin that causes a non-functional β-globin to be formed due to a change in the amino acid chain that is used to construct the protein. The resultant mutant is unable to correctly bind with oxygen and is responsible for the iconic sickle shape that mutated red blood cells exhibit when they have a mutant β-globin.
    • 
    • Comparison of Normal Hemoglobin DNA and Sickle Cell Mutant DNA
      • Normal DNA:                           ATG GTG CAC CTG ACT CCT GAG GAG AAG TCT GCC GTT ACT
      • Sickle Cell Mutant DNA:      ATG GTG CAC CTG ACT CCT GTG GAG AAG TCT GCC GTT ACT
      • Source: http://www.carnegieinstitution.org/first_light_case/horn/lessons/sickle.html
      • The mutant DNA has a point mutation in which an adenine base has undergone a mutation into thymine. This thymine causes the GAG sequence, which codes for glutamic acid, to now code for the amino acid valine, which has the code of GTG.
      • A fundamental aspect of Moleculary Biology is the "Central Dogma" that was first elucidated by Francis Crick. Crick stated that DNA replicates to preserve genetic information, and it is transcribed into RNA. These RNA transcripts are then translated into proteins, which then help perform the necessary life functions of the body as enzymes and basic materials. The sickle cell mutation interferes with the normal operation of this process due to the fact that there is "false" information in the DNA coding sequence on chromosome 11, specifically at the locus where alleles pertaining to hemoglobin is located. By having mutant DNA present, the RNA transcripts are of a different makeup than they should be normally, and thus, the proteins translated from these RNA transcripts are functionally being made from the "wrong" genetic blueprint. This mutation causes the hemoglobin to be less functional in binding with oxygen and to take on the characteristic sickle shape, thus leading to further problems regarding blood vessel blockage.
  • Diagnosing Sickle Cell Mutation
    • Gel electrophoresis is used to test for the presence of sickled red blood cells. Sickle cells have a "characteristic migration...by gel electrophoresis" that "is sufficient to make a diagnosis of a sickling disorder" (http://www.jci.org/articles/view/30920#B7). When compared to red blood cells in gel electrophoresis, sickle cells present a different result and can be identified based off a characterestic position.
    • http://www.iupui.edu/~wellsctr/MMIA/images/gel.jpg
      • KEY: AA = normal hemoglobin     AS = Sickle Cell Heterozygote     SS = Sickle Cell Homozygote
    • PCR and Reverse Dot Blotting
      • Polymerase Chain Reaction can be used to replicate samples taken from fetal genetic testing methods. Upon replication, these samples may be tested using reverse dot analysis to look for sickle cell genetic sequencing. "The polymerase chain reaction (PCR) has provided improved methods for detecting sickle cell genes in fetal DNA obtained by chorionic villus sampling or amniocentesis. One PCR method, reverse dot blotting, can screen in a single reaction for the several genes that cause sickle cell diseasehemoglobin (Hb) S, Hb C, and all the ,-thalassemia alleles of African Americans" ('Molecular Basis of Apolipoprotein (a) Isoform Size Heterogeneity as Revealed by Pulsed-Field Gel Electrophoresis')
• Treatment of Sickle Cell
  • Currently there is no cure for Sickle Cell. However, given the dangerous nature of the disease, research is ongoing to find a cure. As it is a genetic disease, a cure must address the genetic nature of the disease by either altering the mutant genes or prevent them from coding mutant sickle cells. While there is no cure, many treatments for the symptoms of sickle cell are available. Most of these treatments focus on the demethylation of fetal hemaglobin (Hb F) genes, allowing for increased red blood cell production, thus alleviating the conditions caused by sickle cell anemia. These treatments range from decades old to recent discoveries.
  • The most common vector to treating the symptoms and secondary problems caused by sickle cell condition are pharmacological agents. These drugs work primarily by attempting to increase the patient's supply of healthy red blood cells through stimulating properly functioning hemaglobin production.
  • Several non-profit organizations exist to educate both patients and the public about sickle cell condition and sickle cell anemia. They also provide advice and support to patients and those affected by the disease.
    • National Heart, Lung, and Blood Institute
    • Sickle Cell Disease Association of America
    • American Sickle Cell Anemia Orgnization
  • Treatment of symptoms
    • A primary focus of treating Sickle Cell disease is the mediated reactiavation of fetal hemaglobin genes by using pharmacological agents. Fetal hemaglobin reactivation results in a marked decrease of severity of symptoms and generally improves prognosis for the patient. Fetal hemaglobin (Hb F) production is also desirable "because it inhibits Hb S polymerization" ('Treatment of sickle cell anemia with 5-azacytidine results in increased fetal hemoglobin production')
      • This process of reactivating genes relates to Molecular Biology in that drugs are used to alter DNA structure or conformation to bring deactivated DNA sequences back into an actively transcribing state. As they develop, eukaryotic orgnansisms frequently undergo DNA gene deactivation when genes' transcription products are no longer needed. By reversing whatever changes prevent the DNA from being transcribed, these genes can be transcribed again. One of the most common methods of silencing genes in eukaryotes is to methylate the DNA of a particular sequence. The methylated DNA prevents proteins such as activators and polymerases from binding, thus preventing transcription and gene expression. By removing the methyl groups, DNA can be reactivated to be expressed again. This is the case for the fetal hemaglobin genes.
    • 5-Azacytidine
      • The molecule 5-azacytidine causes hypomethylation in the DNA region that codes for γ-globin. This methylation state of DNA is believed to allow for increased expression of γ-globin, which in turn causes levels of fetal hemaglobin to rise. (http://www.pnas.org/content/80/15/4842.full.pdf+html). Functionally, it is similar to other agents that increase hemaglobin levels, but it is mechanistically different. The inhibation of methylation of the fetal hemaglobin genes is believed to allow them to be expressed, thus raising hemaglobin expression.
      • New research however suggests that the hypomethylation caused by 5-azactyidine is not the mechanism responsible for the increase fetal hemaglobin expression. By using alternative methods so lower the methylation state of the DNA at the γ-globin sequence, researches found no change in fetal hemaglobin expression. The previously accepted mechanism of DNA hypomethylation is thus in doubt. (http://bloodjournal.hematologylibrary.org/cgi/content/abstract/111/1/411)
    • Hydroxyurea
      • Hydroxyurea is a compound that has been connected with increased production of fetal-type hemoglobin molecules. These molecules are first detected in the human fetus, and are present in the body until the child is around 6 months of age. The precise mechanism of this is unclear, although it might be related to the fact that hydroxyurea increases nitric acid levels in the blood.
      • Hyrdroxyurea has also been found to lower transcranial dopplar flow velocities in young children. Diagnosing elevated transcranial dopplar (TCD) flow velocities in children is a primary method of diagnosing sickle cell anemia, and hydroxyurea has been found to lower TCD flow velocities closer to normal levels. This therapy could be very useful in the future, as children with high TCD values need frequent transfusions. (http://bloodjournal.hematologylibrary.org/cgi/content/full/110/3/1043)
        • 
        • Hydroxyurea also has benefits in adults. Hydroxyurea "in adults substantially decreases the severity of anemia and the frequency of pain crisis, acute chest syndrome, hospital admission, and transfusion." ('Long-term hydroxyurea therapy for infants with sickle cell anemia: the HUSOFT extension study.')
        • The following video features Dr. Ludovico Guarini, the Director of Pediatric Hematology of Maimonides Medical Center. In the video, he briefly discusses some of the background and importance of hydroxyurea in the treatment of sickle cell anemia.
        • 
    • Butyrate
      • Butyrate is another Hb F inducer that works by hypomethylating the DNA at the γ-globin sequence.
    • Decitabine
      • Decitabine works to reactivate fetal hemaglobin expression by hypomethylating DNA coding for fetal hemaglobin by inhibiting DNA methyltransferase. (http://bloodjournal.hematologylibrary.org/cgi/content/full/102/12/3865?ijkey=f84017e05a0cf730e740dcacc748a2459ed680f8)
    • Blood Transfusion
      • Blood transfusion is one of the oldest treatment methods for sickle cell. By providing the patient with a supply of matching, fully functional red blood cells, symptoms can be alleviated as the patient has an increased supply of functioning red blood cells. Transfusions are not a cure, and must be done at normal intervals, as when the red blood cells die, the patient has no means with which to produce an adequate supply of replacement red blood cells.
  • Treatment of secondary problems cauesed by sickle cell
    • Sickle cell can cause multiple health issues not directly related to the genetic mutation. An example of this is the finding that "a small but significant" percentage of young children and infants with sickle cell anemia have been found to suffer from silent bran infarcts (strokes) as a result of "low Hb F" and other factors caused by sickle cell anemia. (http://www.ncbi.nlm.nih.gov/pubmed/18478575?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DiscoveryPanel.Pubmed_RVAbstractPlus)
  • Treatment of mutation
    • Currently, there is no cure for sickle cell. While treatment of symptoms has advanced considerably, no breakthrough for a cure has occurred yet. However, many new therapies are currently being researched.
    • Proposed and theoretical treatments of the actual chromosomal mutation that causes sickle cell condition is using the principle of Molecular Biology that if DNA is in the proper sequence, the transcriptional and translational products will be correct, assuming there are no errors in those processes. If the DNA of a person with sickle cell where to be somehow corrected to the normal genotype, then the correct mRNA transcript, and thus the correct peptide chain, would be the resultant products. By fixing the mutation in the DNA, functional hemoglobin and red blood cells would be readily available to the person.
    • Bone marrow transplants
      • As the sickle cells, like all red blood cells, are produced by tissues in the bone marrow, replacing the bone marrow the mutant genetic composition with bone marrow of normal genetic make-up could quite possibly present a cure. As the new, healthy bone marrow would produce normal red blood cells, sickle cell could be alleviated or cured. (http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=18788247&tool=MedlinePlus)./
    • Vial Gene Therapy
      • By utilizing the mechanics of viral replication, in which a virus infects a host cell with its genetic material that is then inserted into the host's genome and replicated along with it, geneticists and physicians can effectively "edit" the human genome.
      • This therapy applies to sickle cell, in that if it is possible to use a virus vector to insert a normal-coding globin gene into the DNA sequence that codes for hemoglobin, then sickle cell could effectively be cured. Promise has been shown in experiments focsing on inserting "a normal globin gene into hematopoietic stem cells...with the use of the adenoassociated virus (AAV) as a vector. This virus does not cause human disease, integrates transgenes into the host genome, accommodates high-level expression, and infects a wide variety of human cells." (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1303549&blobtype=pdf).
      • By using viruses such as AAV, transgenes coding for normal hemaglobin can be added into the human genome.
      • Experiments in mice have shown that viral vector gene therapy can lower the amount of sickle cells being produced. ('Correction of Sickle Cell Disease in Transgenic Mouse Models by Gene Therapy)

Works Cited

 "Bone marrow transplantation (BMT) and gene replacement therapy (GRT) in sickle cell anemia." Nigerian Journal of Medicine. 17 July 2008. US National Library of Medicine. 22 Nov. 2008 <http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=18788247&tool=MedlinePlus>

"Correction of Sickle Cell Disease in Transgenic Mouse Models by Gene Therapy." Science. 14 October 2008. American Academy for the Advancement of Science. 1 Nov. 2008.

"Effects of 5-aza-2'-deoxycytidine on fetal hemoglobin levels, red cell adhesion, and hematopoietic differentiation in patients with sickle cell disease." Blood. 7 August 2003. American Society of Hematology. 1 Nov. 2008 <http://bloodjournal.hematologylibrary.org/cgi/reprint/102/12/3865>

Gel Electrophoresis Picture: http://www.iupui.edu/~wellsctr/MMIA/htm/gelelectro.htm Indiana

University Purdue University at Indianapolsis Science Department 28 Feb. 2008. Accessed 1 Nov. 2008 

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Inheritance Picture - http://content.revolutionhealth.com/contentimages/images-image_popup-r7_autosomalrecessive.jpg 

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MUSC Medical History Club Fall 2008 Lecture Series, "Jim: The First Case of Sickle Cell?," October 2008, Warring Historical Library, Medical University of South Carolina.

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"Long-term hydroxyurea therapy for infants with sickle cell anemia: the HUSOFT extension study." Blood. 14 June 2005. American Society of Hematology. 1 Nov. 2008. 

"

Molecular Basis of Apolipoprotein (a) Isoform Size Heterogeneity as Revealed by Pulsed-Field Gel Electrophoresis." Journal of Clinical Investigation. June 1991. American Society for Clinical Investigation 1 Nov. 2008.

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http://ghr.nlm.nih.gov/condition=sicklecelldisease>. 

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