Sickle Cell Anemia


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Definition

Sickle cell anemia is a blood disorder that affects hemoglobin (pronounced: hee-muh-glow-bin), the protein found in red blood cells (RBCs) that helps carry oxygen throughout the body.
Sickle cell anemia occurs when a person inherits two abnormal genes (one from each parent) that cause their RBCs to change shape. Instead of being flexible and disc-shaped, these cells are more stiff and curved in the shape of the old farm tool known as a sickle — that's where the disease gets its name. The shape is similar to a crescent moon.
Red blood cells with normal hemoglobin (hemoglobin A, or HbA) move easily through the bloodstream, delivering oxygen to all of the cells of the body. Normal RBCs are shaped like discs or doughnuts with the centers partially "scooped out" and are soft and flexible. They can easily "squeeze" through even very small blood vessels.


Nature of the Genetic Difficulty

Approximate gene location is based on Chromosome 11 map from NCBI Entrez Map Viewer. Official Gene Symbol: HBB
Name of Gene Product: hemoglobin, beta
Alternate Name of Gene Product: beta globin
Locus: 11p15.5 - The HBB gene is found in region 15.5 on the short (p) arm of human chromosome 11.
Gene Structure: The normal allelic variant for this gene is 1600 base pairs (bp) long and contains three exons.
mRNA: The intron-free mRNA transcript for the HBB gene is 626 base pairs long. See the NCBI sequence record Sickle_cell.gifNM_000518 to access the mRNA sequence data.
Coding Sequence (CDS): 444 base pairs within the mRNA code for the amino acid sequence of the gene's protein product.
Protein Size: The HBB protein is 146 amino acids long and has a molecular weight of 15,867 Da. See the annotated HBB protein sequence record in Swiss-Prot.
Protein Function: Hemoglobin molecules, which reside in red blood cells, are responsible for carrying oxygen from the lungs to various parts of the body for use in respiration. The HBB gene codes for one of the two types of polypeptide chains found in adult hemoglobin. Normal adult hemoglobin is a tetrameric protein consisting of two alpha chains and two beta chains. HBB codes for the beta chain, which is often referred to as beta globin. Mutant beta globin is responsible for the sickling of red blood cells seen in sickle cell anemia.
Is there a point mutation, or any insertion/deletion entirely inside one gene? Is there a (gene or genes) missing? A whole chromosone extra, missing, or both? Or might the gene be extended in length?
Sickle cell anemia is a disease passed down through families in which red blood cells form an abnormal crescent shape. (Red blood cells are normally shaped like a disc.)

Basis Facts of Disease

Sickle cell anemia is a deadly disease that leads to the formation abnormal sickle-shaped red cells. Sickle cells could easily lead to blockage of blood vessels that supply body organs and limbs with nutrients and oxygen. The condition could easily cause infections, pain and organ damage. Mile Davis, Paul Williams, Tionne, and Georgeanna Tillman are famous celebrities that live with sickle cell disease.
Sickle cell anemia is inherited from both parents. Sickle cell disease is much more common in people of African and Mediterranean descent. It is also seen in people from South and Central America, the Caribbean, and the Middle East.Common symptoms include:Attacks of abdominal painBone painBreathlessnessDelayed growth and pubertyFatigueFeverJaundicePalenessRapid heart rateUlcers on the lower legsChest painExcessive thirstFrequent urinationPainful and prolonged erection (priapism - occurs in 10 - 40% of men with the disease)Poor eyesight/blindnessStrokesSkin ulcers

Prognosis

Prognosis of Sickle Cell Anemia: There is no cure for the condition though bone marrow transplants can significantly help a small number of people. People with the condition tend to have a shorter life span but careful management of symptoms and complications can improve the quality of life and increase the life span. Some people can survive past the age of 50 if they manage their condition carefully.
Estimated mortality rate for Sickle Cell Anemia from prevalence and deaths statistics:
These numbers are the averages for men and women in the US.
Men with sickle cell disease live for about 42 years, on average. For women, the average is 48 years
Deaths: 500 (USA annual deaths calculated from this data: 501 deaths Incidence: 8,000 (USA annual incidence calculated from this data: 1 per 500 African American births; 1 per 1,000-1,400 Hispanic-American births 0.092% (ratio of deaths to incidence). See also Deaths and Sickle Cell Anemia and Overview of Sickle Cell Anemia.
In some parts of Africa, where good health care isn't always available, about half of children with sickle cell disease die before their first birthday
In the past, sickle cell patients often died from organ failure between ages 20 and 40. Thanks to a better understanding and management of the disease, today, patients can live into their 50s or beyond.
Causes of death include organ failure and infection. Some people with the disease experience minor, brief, infrequent episodes. Others experience severe, long-term, frequent episodes with many complications.

Sources Cited

Platt OS, Brambilla DJ, Rosse WF, et al. Mortality in sickle cell disease: life expectancy and risk factors for early death. New England Journal of Medicine. 1994; 330: 1639-1643. http://kidshealth.org/teen/diseases_conditions/blood/sickle_cell_anemia.html#