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Alpha thalassemia X-linked intellectual disability syndrome
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Reviewed August 2009
What is alpha thalassemia X-linked intellectual disability syndrome?
Alpha thalassemia X-linked intellectual disability syndrome is an inherited disorder that affects many parts of the body. This condition occurs almost exclusively in males.
Males with alpha thalassemia X-linked intellectual disability syndrome have intellectual disability and delayed development. Their speech is significantly delayed, and most never speak or sign more than a few words. Most affected children have weak muscle tone (hypotonia), which delays motor skills such as sitting, standing, and walking. Some people with this disorder are never able to walk independently.
Almost everyone with alpha thalassemia X-linked intellectual disability syndrome has distinctive facial features, including widely spaced eyes, a small nose with upturned nostrils, and low-set ears. The upper lip is shaped like an upside-down "V," and the lower lip tends to be prominent. These facial characteristics are most apparent in early childhood. Over time, the facial features become coarser, including a flatter face with a shortened nose.
Most affected individuals have mild signs of a blood disorder called alpha thalassemia. This disorder reduces the production of hemoglobin, which is the protein in red blood cells that carries oxygen to cells throughout the body. A reduction in the amount of hemoglobin prevents enough oxygen from reaching the body's tissues. Rarely, affected individuals also have a shortage of red blood cells (anemia), which can cause pale skin, weakness, and fatigue.
Additional features of alpha thalassemia X-linked intellectual disability syndrome include an unusually small head size (microcephaly), short stature, and skeletal abnormalities. Many affected individuals have problems with the digestive system, such as a backflow of stomach acids into the esophagus (gastroesophageal reflux) and chronic constipation. Genital abnormalities are also common; affected males may have undescended testes and the opening of the urethra on the underside of the penis (hypospadias). In more severe cases, the external genitalia do not look clearly male or female (ambiguous genitalia).
Read more about alpha thalassemia.
How common is alpha thalassemia X-linked intellectual disability syndrome?
Alpha thalassemia X-linked intellectual disability syndrome appears to be a rare condition, although its exact prevalence is unknown. More than 200 affected individuals have been reported.
What genes are related to alpha thalassemia X-linked intellectual disability syndrome?
Alpha thalassemia X-linked intellectual disability syndrome results from mutations in the ATRX gene. This gene provides instructions for making a protein that plays an essential role in normal development. Although the exact function of the ATRX protein is unknown, studies suggest that it helps regulate the activity (expression) of other genes. Among these genes are HBA1 and HBA2, which are necessary for normal hemoglobin production.
Mutations in the ATRX gene change the structure of the ATRX protein, which likely prevents it from effectively regulating gene expression. Reduced activity of the HBA1 and HBA2 genes causes alpha thalassemia. Abnormal expression of other genes, which have not been identified, probably causes developmental delay, distinctive facial features, and the other signs and symptoms of alpha thalassemia X-linked intellectual disability syndrome.
Read more about the ATRX gene.
How do people inherit alpha thalassemia X-linked intellectual disability syndrome?
This condition is inherited in an X-linked recessive pattern. The ATRX gene is located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), one working copy of the ATRX gene can usually compensate for the mutated copy. Therefore, females who carry a single mutated ATRX gene almost never have signs of alpha thalassemia X-linked intellectual disability syndrome.
A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.
Where can I find information about diagnosis or management of alpha thalassemia X-linked intellectual disability syndrome?
These resources address the diagnosis or management of alpha thalassemia X-linked intellectual disability syndrome and may include treatment providers.
General information about the diagnosis and management of genetic conditions is available in the Handbook. Read more about genetic testing, particularly the difference between clinical tests and research tests.
To locate a healthcare provider, see How can I find a genetics professional in my area? in the Handbook.
Where can I find additional information about alpha thalassemia X-linked intellectual disability syndrome?
You may find the following resources about alpha thalassemia X-linked intellectual disability syndrome helpful. These materials are written for the general public.
You may also be interested in these resources, which are designed for healthcare professionals and researchers.
What other names do people use for alpha thalassemia X-linked intellectual disability syndrome?
What if I still have specific questions about alpha thalassemia X-linked intellectual disability syndrome?
Where can I find general information about genetic conditions?
The Handbook provides basic information about genetics in clear language.
These links provide additional genetics resources that may be useful.
What glossary definitions help with understanding alpha thalassemia X-linked intellectual disability syndrome?
acids ; anemia ; cell ; chromosome ; chronic ; constipation ; developmental delay ; digestive ; digestive system ; disability ; esophagus ; gastroesophageal reflux ; gene ; gene expression ; genitalia ; hemoglobin ; hypospadias ; hypotonia ; inheritance ; inherited ; mental retardation ; microcephaly ; motor ; muscle tone ; newborn screening ; oxygen ; prevalence ; protein ; recessive ; screening ; sex chromosomes ; short stature ; sign ; stature ; stomach ; syndrome ; testes ; thalassemia ; X-linked recessive
You may find definitions for these and many other terms in the Genetics Home Reference Glossary.
See also Understanding Medical Terminology.
References (9 links)
The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? in the Handbook.