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Diseases

Genetic and Rare Diseases Information Center (GARD)

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X-linked severe combined immunodeficiency


Other Names for this Disease

  • SCID, X-linked
  • SCIDX
  • SCIDX1
  • Severe combined immunodeficiency T- B+ due to gamma chain deficiency
  • Severe combined immunodeficiency T- B+, X-linked
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Inheritance

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How is X-linked severe combined immunodeficiency inherited?

X-linked severe combined immunodeficiency (X-SCID) is inherited in an X-linked recessive manner.[1] A condition is X-linked if the changed (mutated) gene responsible for the condition is located on the X chromosome. The X chromosome is one of the two sex chromosomes; females have two X chromosomes, and males have one X and one Y chromosome. In males, one mutated copy of the responsible gene causes signs and symptoms of the condition because they don't have another X chromosome with a working copy of the gene. In females, having one mutated copy of the gene would make them an unaffected carrier; a mutation would have to occur in both copies of the gene to cause the condition. This is why X-linked recessive disorders, including X-linked SCID, occur much more frequently in males. In female infants who have symptoms of SCID, non-X-linked forms of SCID (autosomal recessive forms) should be considered.[1]

The father of a boy with X-SCID cannot have the disease and cannot be a carrier (having a mutated copy of the responsible gene would make him affected). If an affected boy is the only affected family member, either his mother is a carrier or the mutation occurred for the first time in the affected boy (a de novo mutation). It is thought that about 2/3 of these cases are inherited from a carrier mother, and about 1/3 are due to new de novo mutations.[1]

If there is more than one affected family member, it is assumed that the mother of the affected boy is a carrier. A carrier mother has a 50% risk to pass on the mutation in each pregnancy. Males who inherit the mutation will be affected, and females who inherit the mutation will be unaffected carriers. A man with X-SCID will pass the mutation to all of his daughters and none of his sons.[1] 

Genetic testing can often determine which family member had the first mutation. Identifying this family member is important for learning which branches of a family are at risk for X-SCID. Carrier testing for at-risk relatives and prenatal testing for pregnancies at increased risk are possible if the disease-causing IL2RG mutation has been identified in the family.[1]

People with a family history of X-SCID who are interested in learning about genetic risks to themselves and family members should speak with a genetics professional.
Last updated: 5/27/2014

References
  1. Eric Allenspach, David J Rawlings, and Andrew M Scharenberg. X-Linked Severe Combined Immunodeficiency. GeneReviews. January 24, 2013; http://www.ncbi.nlm.nih.gov/books/NBK1410/. Accessed 5/28/2014.


Other Names for this Disease
  • SCID, X-linked
  • SCIDX
  • SCIDX1
  • Severe combined immunodeficiency T- B+ due to gamma chain deficiency
  • Severe combined immunodeficiency T- B+, X-linked
See Disclaimer regarding information on this site. Some links on this page may take you to organizations outside of the National Institutes of Health.