Adenosine deaminase deficiency

Adenosine deaminase (ADA) deficiency causes severe combined immunodeficiency disease (SCID). Profound lymphopenia in this disorder has been attributed to toxic levels of ADA substrates, particularly deoxyadenosine, generated from nucleic acid breakdown associated with cell turnover in marrow, thymus, and lymph nodes. Most patients have SCID, which is usually diagnosed in infancy and is often fatal, but some patients are diagnosed later in childhood or as adults. More than 50 ADA mutations are known. Most patients are heteroallelic, and most alleles are rare.