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Genetic counseling: Alpha Thalassemia

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Alpha Thalassemia

Overview of HemoglobinEdit

  • Hemoglobin is the protein in blood that carries oxygen and gives blood its red color
  • A hemoglobin molecule contains four globin chains
  • There are different types of globin chains
  • One of the chains is designated alpha and the others can be grouped together and called non-alpha
  • The combination of two alpha chains and two non-alpha chains produces a complete hemoglobin molecule
  • The combination of two alpha chains and two gamma chains form "fetal" hemoglobin, termed "hemoglobin F"
  • Fetal hemoglobin is the primary hemoglobin in the developing fetus (With the exception of the first 10 to 12 weeks after conception.
  • The combination of two alpha chains and two beta chains form "adult" hemoglobin, also called "hemoglobin A". Although hemoglobin A is called "adult", it becomes the predominate hemoglobin within about 18 to 24 weeks of birth.
  • Must have the right amount of each type of globin being made if there is an imbalance this will result in thalassemia
  • There are different types of thalassemia
  • The severity and type of thalassemia depends on which genes are afffected
  • The type that we will be talking about is alpha thalassemia because it results when there is not enough alpha globin being made by the cells

Alpha GlobinEdit

  • The genes that tell the cell how to make alpha globin are found on chromosome 16
  • Each chromosome 16 has two alpha globin genes that are next to each other on the chromosome
  • Since each cell has two copies of chromosomes 16, a total of four alpha globin genes are usually present in each cell
  • There are several different possible outcomes depending on how many of these copies are not functioning (usually due to being deleted)

Silent CarrierEdit

  • If one of the four copies of the alpha globin gene is not present in an individual they will not have any symptoms because they still make enough alpha globin.
  • This individual is often referred to as a silent carrier

Alpha Thalassemia TraitEdit

  • If two of the genes are not present than the person is said to have the alpha thalassemia trait.
  • Does not cause health problems or symptoms
  • The red blood cells are often small and this is referred to as microcytosis
  • 2 ways of having alpha thalassemia trait which can become important when you want to determine risks to potential children
  • 2 genes on same chromosome not functioning (cis) figure 3
  • 1 gene on each chromosome (trans) figure 4

Hemoglobin H DiseaseEdit

  • When there are three missing copies of the gene then there is even less alpha globin
  • This causes more serious anemia which often requires transfusions
  • Other symptoms include: jaundice, enlarged spleen, and increased risk of infection
  • Called hemoglobin H disease because of an abnormal hemoglobin that forms from excess beta globin(excess beta globin is present and 4 of these come together instead of having 2 alpha globin chains)
  • Hemoglobin H does not carry oxygen and damages the membrane that surrounds the red cell, accelerating cell destruction.
  • The combination of the very low production of alpha chains and destruction of red cells in hemoglobin H disease produces a severe, life-threatening anemia

HB Bart's Hydrops FetalisEdit

  • If there are no genes present than the effects are even more serious
  • Often the fetus is stillborn or dies shortly after birth
  • Rarely, problem is detected in utero, if the disorder occured in an earlier child. In utero blood transfusions have saved some of these children. These patients require life-long transfusions and other medical support.

How does the screening test determine which category a baby fits?Edit

  • Newborn screening test done in Utah since Sep 2001 measures the percentage of different types of hemoglobins
  • Barts hemoglobin consists of 4 gamma globin chains (results from not enough alpha globin to pair up with to form the fetal hemoglobin consisting of 2 alpha and 2 gamma) (gamma globin is no longer made about 18-24 wks after birth)
  • Depending on how much Barts hemoglobin an infant has will help categorize them
  • Less than 5% -- most likely missing one gene and will be silent carrier
  • 5-10% usually indicates loss of 2 genes and will have alpha thalassemia trait
  • >10% (usually 15-20%) will have more severe form

TestingEdit

  • Testing can be performed to determine the parents' risks for future pregnancies. A primary care physician can order the following blood tests.
    • complete blood count (CBC) - a measurement of size, number, and maturity of different blood cells in a specific volume of blood.
    • hemoglobin electrophoresis with A2 and F quantitation - differentiation of the types of hemoglobin present
    • FEP (free-erythrocyte protoporphyrin) and ferritin - to exclude iron deficiency anemia.
  • The above testing is fairly inexpensive and covered by most types of insurance
  • Gene testing also available, but more expensive and usually not needed
  • FISH can be performed to determine if there is a large deletion on chromosome 16 in the presence of severe developmental delay

Types of mutationsEdit

  • Most are deletions of the gene(s)
  • About 5% of alpha thalassemia is due to point mutations
  • Often cause more severe thalassemia than single gene deletions
  • Hemoglobin Constant Spring -- most common point mutation
  • Stop codon mutated results in large alpha globin protein (RNA unstable and little protein is made)
  • Mutations have also been found in regions of DNA that regulate expression of alpha globin gene

Hydrops FetalisEdit

  • Can be due to a number of different reasons (immune response to blood that is incompatible such as Rh incompatibility or nonimmune hydrops)
  • Nonimmune hydrops includes a number of explanations like cardiovascular, chromosome abnormality, pulmonary infection and homozygous alpha thalassemia (4 copies of alpha globin gene missing has even been seen in some with 3 missing copies)
  • Sometimes the reason can not be determined
  • Treatment for hydrops caused by some is starting to become available
  • Prognosis still is poor

How common is Alpha thalassemiaEdit

  • Occurs in people of all ethnic backgrounds
  • More common in Mediterranean, African, and South-east Asian populations
  • Individuals of African descent are more likely to be silent carriers or have alpha thalassemia trait
  • Individuals in Southeast Asia are more likely to have all four possibilities (estimates of carrier frequency are 1/30)
  • Carrier frequency in other populations is not certain

Rare cases of learning difficulties have been associated with alpha thalassemia (not very likely to be the case in_____) but is something that we wanted to make you aware of so that if you notice delays in her development we can see her again to follow up with her.

  • Some due to large deletions of chromosome 16 (other genes presumably deleted too that cause learning difficulties)
  • If you start to notice
  • Can also be due to mutation of gene XH2 (DNA helicase/repair enzyme) not clear why it causes alpha thalassemia and this is X-linked so very very unlikely

Types of mutationsEdit

  • Most are deletions
  • About 5% of alpha thalassemia is due to point mutations
  • Often cause more severe thalassemia than single gene deletions
  • Hemoglobin Constant Spring -- most common point mutation
  • Stop codon mutated results in large alpha globin protein (RNA unstable and little protein is made)
  • Mutations have also been found in regions of DNA that regulate expression of alpha globin gene

Are you planning more children?Edit

  • May want to consider testing for you and husband in the future so that it can be determined if there are risks for future pregnancies

A simple blood test can be performed that will detect most types of thalassemia trait

  • We will have this information in letter so when and if you want to have the testing your physician can order it
  • May be potential risks to _____ offspring
  • Keep this in mind because she may decide with her partner to have him tested

Psychosocial concernsEdit

  • may be nervous and think there is something serious
  • may be a lot of information to understand at once so talk about letter
  • guilt about passing on something
  • concern that other pregnancy with hydrops is related to this
  • help understand that other reasons for hydrops are more likely due to their ethnic background

WebsitesEdit

ReferencesEdit

  • Giardina P, Hilgartner M. Update on thalassemia. Pediatr Rev 1992;13:55-62.
  • Rund, D Rachmilewitz, E. Thalassemia major 1995: older patients, new therapies. Blood Rev 1995; 9:25-32
  • Piomelli S, Loew T. Management of thalassemia major (Cooley's anemia). Hematol Oncol Clin North Am 1991;5:557-69.
  • 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.
  • Guideline: The laboratory diagnosis of haemoglobinopathies. British Journal of Haematology. 1998. 101 (783-792)

NotesEdit

The information in this outline was last updated in 2002.


Material obtained under GFDL Licence from http://en.wikibooks.org/wiki/Handbook_of_Genetic_Counseling

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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