Adenosine deaminase deficiency

Description

Adenosine deaminase (ADA) deficiency is a disorder that affects the immune system. Specifically, ADA deficiency impairs the development and function of immune cells called lymphocytes. Lymphocytes are white blood cells that help the body fight infections. As a result, people with ADA deficiency often develop pneumonia, chronic diarrhea, and widespread skin rashes. Additional signs and symptoms of ADA deficiency include slow growth and developmental delays.

About 80 percent of individuals with ADA deficiency also have severe combined immunodeficiency (SCID). People with SCID lack virtually all immune protection from bacteria, viruses, and fungi. They are prone to repeated and persistent infections that can be serious or life-threatening. These infections are often caused by "opportunistic" organisms that ordinarily do not cause illness in people with a normal immune system. People with ADA deficiency with SCID (ADA-SCID) typically develop health problems within the first 6 months of life. Without treatment, these babies usually do not survive past age 2.

About 15 to 20 percent of people with ADA deficiency develop health problems that begin between 1 and 10 years of age (delayed onset) or in adulthood (late onset). In people with this form of ADA deficiency (known as delayed or late-onset combined immunodeficiency or ADA-CID), the immune deficiency tends to be less severe than in people with ADA-SCID. People with ADA-CID typically have recurrent upper respiratory and ear infections. Over time, affected individuals may develop chronic lung damage, malnutrition, and other health problems.

In some individuals, ADA deficiency only impacts red blood cells. Since white blood cells are not affected, these individuals have normal immune systems. This form of the condition is known as partial ADA deficiency. Individuals with this form do not have any health problems related to the condition. They often only find out they have ADA deficiency when they undergo testing because of an affected relative or during a normal health screening.

Frequency

Adenosine deaminase deficiency is very rare. It is estimated to occur in approximately 1 in 500,000  newborns worldwide. Approximately 15 percent of people with SCID have ADA deficiency.

Causes

Adenosine deaminase deficiency is caused by variants (also called mutations) in the ADA gene. This gene provides instructions for producing the enzyme adenosine deaminase. This enzyme is found throughout the body but is most active in lymphocytes. These cells protect the body against foreign invaders, such as bacteria and viruses. Lymphocytes are produced in specialized lymphoid tissues throughout the body, including in a gland located behind the breastbone called the thymus and in the lymph nodes.

The adenosine deaminase enzyme works to get rid of a molecule called deoxyadenosine, which is generated when DNA is broken down. A buildup of deoxyadenosine in cells can lead to early cell death. Adenosine deaminase converts deoxyadenosine to another molecule called deoxyinosine, which is not harmful. Variants in the ADA gene reduce or eliminate the activity of adenosine deaminase and allow deoxyadenosine to build up in cells. The severity of ADA deficiency generally depends on how much working enzyme is available. Individuals with ADA-SCID have no enzyme activity. Those with ADA-CID have greatly reduced enzyme activity, and people with partial ADA deficiency  have somewhat reduced enzyme activity.

Immature lymphocytes in the thymus are particularly vulnerable to a toxic buildup of deoxyadenosine. These cells die before they can mature and help fight infection. The number of lymphocytes in other lymphoid tissues is also greatly reduced. The loss of infection-fighting cells results in the signs and symptoms of ADA deficiency.

Inheritance

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell must have a variant to cause the disorder. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

Autosomal recessive inheritance. Credit: U.S. National Library of Medicine.

Other Names for This Condition

• ADA deficiency
• ADA-Related Immune Deficiency, Adenosine Deaminase 1 Deficiency
• ADA-SCID
• ADA1 Deficiency
• Adenosine deaminase deficient severe combined immunodeficiency
• SCID due to ADA deficiency
• Severe combined immunodeficiency due to ADA deficiency
• Severe combined immunodeficiency, autosomal recessive, T cell-negative, B cell-negative, NK cell-negative, due to adenosine deaminase deficiency

Where this page came from

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