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Genetic counseling: Hemoglobin C

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Hemoglobin C

Overview of Hemoglobin and genesEdit

  • Hemoglobin is the protein in blood that carries oxygen and gives blood its red color
  • There are hundreds of different types or variants of hemoglobin
    • Hemoglobin A is the most common type of hemoglobin that is made in children and adults
    • Hemoglobin C is one of the many hemoglobin variants
  • The kind of hemoglobin our body makes depends on the genes we inherit
  • Genes are the units of inheritance that tell our bodies how to grow and develop
  • Some genes tell our bodies how to make protein chains that are essential components of hemoglobin
  • One pair of genes, called beta globin, tells the body how to make one of the components of hemoglobin
  • Changes in these beta globin genes can alter the instructions and can result in different types of changes in the structure of hemoglobin
  • We usually get one copy of the beta globin gene from our mother and one from our father
    • We know that you inherited one gene that was altered and results in the variant hemoglobin C being made
    • The other gene produces the usual type of hemoglobin Hb A
    • The presence of only one altered copy of the beta globin gene almost never causes symptoms or health problems for the individual (There have been rare instances where Hb C carriers have had some eye problems)
    • When an individual has one altered and one unaltered copy of a gene, we say that they are carriers

Potential Risks for future offspringEdit

  • A carrier for Hemoglobin C is at an increased risk for having children with a genetic condition (Hb SC disease) similar to, but less severe than sickle cell anemia.
  • Are you familiar with sickle cell anemia?
  • Sickle cell anemia occurs only if a person has inherited two hemoglobin S genes, one from each parent (Hb S is another variant of the beta globin gene and is even more common than hemoglobin C)
  • Symptoms of sickle cell anemia include:
  • Increased susceptibility to frequent infections
  • Anemia (that is not corrected with iron supplementation)
  • Painful episodes caused by the blood cells blocking blood flow to body tissues
  • The risks to children of a carrier are dependent on whether or not the other partner carries a hemoglobin variant
  • The fact that you are a carrier for Hb C means that there may be potential risks to your children, but this is only possible if your partner has sickle cell trait or Hb C trait
  • If the father of the fetus tests positive for sickle cell trait or hemoglobin C trait there is a 25% chance that the fetus will have either hemoglobin SC disease or a mild form of anemia, a 1 in 50% chance that the fetus will be a carrier like one of the parents, and a 25% chance that the fetus will make only the usual hemoglobin (type A).

Symptoms of Hemoglobin SC diseaseEdit

(also called Hemoglobin C disease)

  • Hemoglobin SC disease is milder than sickle cell disease
  • Sometimes the symptoms don't arise until middle or late in life
  • Only physical sign is an enlarged spleen in 65% of individuals with Hb SC disease
  • Mild (hemolytic) anemia may result, accompanied by a mild-to-moderate reduction in the red cell lifespan
  • Patients may have sporadic episodes of joint pain.
  • Continued hemolysis may produce pigmented gallstones, an unusual type of gallstone composed of the dark-colored contents of red blood cells. (The cause of pigment stones is uncertain.)
  • Often goes unrecognized until a serious complication which may include
  • blood in the urine (hematuria)
  • pain in the hip where part of the leg bone tissue has died (aseptic necrosis of the femoral heads)
  • eye problems (proliferative sickle retinopathy (PSR)) which can result in bleeding in the eye and retinal detachment
  • In large study in Jamaica, 1/3 with Hb SC disease had PSR at varying stages

Hb C (homozygous)Edit

  • mild hemolytic anemia with splenomegaly
  • red blood cells contain less water
  • less deformable and are not in circulation as long
  • also have a decreased oxygen affinity
  • may develop gallstones
  • no special treatment necessary

Incidence of sickle cell trait and Sickle cell anemiaEdit

  • Sickle cell trait occurs in about 8% (or 1 in 12) African Americans
  • Prevalence among Hispanics whose families originated from the Caribbean, Central America, or South America is approximately 4% (or 1 in 24)
  • Carrier rate among 2 million screened over 4 years in California showed 6,921 nonblack infants had sickle cell trait ( .35% or approximately 1 in 300)
  • 58 out of 2 million non black children had sickle cell disease from Calif. Newborn screen
  • Carrier rate in Caucasians has been reported to be 1 in 600

Frequency of hemoglobin SC disease and Hb C traitEdit

  • In the US: Hemoglobin SC disease has an incidence of 0.017% less than 1 in 5000 n African Americans.
  • Internationally: In northern Africa, the incidence of Hb C disease is approximately 0.03%.
  • Hb SC disease is more common in individuals of African descent, but it also has been reported in people of Hispanic and Sicilian ancestry
  • Gene frequency in African Americans is about 1 in every 100 to 1 in every 50 (1-2%)
  • Gene frequencies are highest in Ghana and Upper Volta approaching about 1 in 7 (15%)

Possible to have Hb C with other beta globin variantsEdit

  • Hb C and Beta Thalassemia if the Beta thal mutation is null (resulting in not functional beta chain production)
  • Causes similar symptoms and problems as Hb SC disease

TestingEdit

  • Discuss ability to do simple blood test for partner to determine if they carry a Beta globin variant or Beta thalassemia so we can clarify risks for future children (hemoglobin electrophoresis with an A2 quanitiation to identify possible beta thalassemia)
  • Covered by most insurance companies and can be ordered by primary care physician
  • Some may require preauthorization so you may want to find out if this is necessary before ordering testing
  • Direct mutation analysis by PCR possible but I will not discuss this because Hb electrophoresis should be sufficient

http://www.hopkinsmedicine.org/dnadiagnostic/betavariants.htm (lab performs complete gene sequencing and testing of known mutations once electrophoresis has been performed)

Psychosocial concernsEdit

  • May be nervous and think it is more serious initially
  • May be a lot of information to understand at once so talk about how it will be in letter
  • Guilt about possibly passing on something that could cause health problems
  • Ethnic issues surrounding the higher prevalence among minority populations that have been oppressed in the past.

Other Facts about Hb CEdit

  • Hemoglobin C is composed of 2 normal alpha-chains and 2 variant beta-chains, where lysine has replaced glutamic acid at position 6
  • This unstable hemoglobin is less soluble and it tends to crystallize
  • Intracellular crystals lead to a decrease in red cell deformability and blood that is more viscous
  • The spleen effectively removes these crystal-containing cells
  • Hemoglobin C trait is not detected by solubility testing or by a sickle cell preparation.
  • Heterozygotes have as much as 35% hemoglobin C

ReferencesEdit

  • The Metabolic and Molecular Bases of Inherited Disease (8th edition). 2001. McGraw Hill. Chapter 181 Hemoglobinopathies . Weatherall, D.J., Clegg, J.B., Higgs, D.R., Wood, W.G.
  • Newborn screening for sickle cell disease: 4 years of experience from California's newborn screening program. Journal of Pediatric Hematology Oncology. 18(1): 36-41. (1996).
  • De Caluwe, J P; Alexander, M; Bondue, H. Study of 19 heterozygote AC carriers and of 5 cases of double hemoglobinopathy SC. ACTA CLINICA BELGICA. vol. 48, no. 5 (1993): 297-306.
  • Rana, S R; Sekhsaria, S; Castro, O L. Hemoglobin S and C traits: contributing causes for decreased mean hematocrit in African-American children. PEDIATRICS. vol. 91, no. 4 (1993 Apr): 800-802.

Web resourcesEdit

NotesEdit

The information in this outline was last updated in 2002.


This material has been imported fom the wikibook "Genetic counseling"[ http://en.wikibooks.org/wiki/Genetic_counseling] under the GNU Free Documentation License.

Heckert GNU white Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License."

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