1. Health
Send to a Friend via Email

Severe Combined Immunodeficiency

Gene therapy may hold key to cure

By

Updated January 25, 2009

The immune system

The immune system is defective in SCID

Photo © A.D.A.M.
In the 1970s, the world learned the story of David Vetter, a boy born with severe combined immunodeficiency (SCID). David lived all of his 12 years of life inside a sealed plastic bubble which protected him from infections. The 1976 TV movie "The Boy in the Plastic Bubble," which starred John Travolta, was based on his story. Scientists are still working on developing a cure for SCID.

What is SCID?

The body's immune system fights against diseases and infections. The SCID syndromes are inherited disorders that result in severe defects in the immune system. White blood cells (which fight infection) are produced in the bone marrow by stem cells. In people with SCID, the bone marrow stem cells are absent or defective. This leaves the affected person open to any and all germs around him because he has no way to fight them off.

Who gets SCID?

There are several different types of SCID. The most common type is called X-linked SCID because its gene is linked to the X chromosome that a mother provides to her child during reproduction. To break down the genetics simply, males are more likely to develop this type of SCID because they only have one X chromosome--the traits tied to it are more likely to be expressed. Females, on the other hand, have two X chromosomes--both must carry the SCID gene in order for a girl to have the disease.

The second most common type comes from the lack of the enzyme adenosine deaminase. The disorder is inherited when both parents contribute an abnormal gene to a child during reproduction. This is referred to as autosomal recessive inheritance.

What are the symptoms?

Since a child is born with SCID, the diagnosis is usually made before age 6 months. The child has a high number of infections that are more serious and less responsive to antibiotics than normal. These infections may be ones that are unusual for a child to have, such as Pneumocystis carinii pneumonia (PCP). The child may have persistent diarrhea with weight loss and chronic skin infections.

How is SCID treated?

Since 1968, the treatment of choice has been replacing the defective bone marrow with normal bone marrow through stem cell (bone marrow) transplant. This is a difficult treatment that does not guarantee a cure, but it does work in many cases.

Children with SCID are usually kept isolated from other people, including relatives, and take Bactrim (trimethoprim/sulfamethoxazole) to prevent PCP infection. Without a bone marrow transplant, the child is always at risk for a severe or fatal infection and will have difficulty surviving beyond his first birthday.

New treatments

In recent years, new treatments for SCID have been developed:
  • In-utero transplantation
    Stem cell transplantation on a fetus before it is born has been successfully done.
  • Pretransplantation therapy
    New treatments before bone marrow transplantation to make it more successful are being developed.
  • Gene therapy
    Some of the child's own marrow stem cells are removed. The "normal" version of the defective gene responsible for SCID is inserted into the cells. The "corrected" cells are then put back into the child's bone marrow.
Gene therapy for SCID has been tried successfully. On April 3, 2002, physicians in England announced that they had successfully restored the immune system of a boy with X-linked SCID. Eighteen-month-old Rhys Evans appeared to have been "cured" of the disorder, although whether one treatment will last a lifetime is not yet known. Other children have also been treated successfully.

A complication developed with gene therapy trials in France, however, beginning in September 2002. Unfortunately, three boys treated as infants later developed a leukemia-like blood disorder. They have been treated with chemotherapy. Researchers are working on discovering why this happened, and how to ensure that other children with SCID who are treated enjoy the success of gene therapy without the negative consequences.

Based on the French cases, the U.S. Food and Drug Administration (FDA) suspended two similar American studies.

Sources:

"Primary Immunodeficiency Diseases - Severe Combined Immunodeficiency." INFO4PI.ORG. Jeffrey Modell Foundation. 8 Oct 2007.

Weinberg, K.I., N. Kapoor, A.J. Shah, G.M. Crooks, D.B. Kohn, & R. Parkman. "Hematopoietic stem cell transplantation for severe combined immune deficiency." Curr Allergy Asthma Rep 1(2001): 416-420.

Weiss, Rick. "Boy's Cancer Prompts FDA to Halt Gene Therapy." The Washington Post 04 Mar 2005: A02.

©2014 About.com. All rights reserved.

We comply with the HONcode standard
for trustworthy health
information: verify here.