Newborn Screening for Severe Combined Immunodeficiency in 11 Screening Programs in the United States

The purpose of newborn screening is early detection of inborn conditions for which prompt treatments mitigate mortality or irreversible damage. The first heritable immune disorders to which newborn screening has been applied are those that together comprise severe combined immunodeficiency (SCID), caused by defects in any of a diverse group of gene products essential for development of adaptive immunity provided by T and B lymphocytes.1,2 A feature of all SCID is defective production of T cells. In most SCID, B cells are also defective, but even normal B cells cannot produce antibodies without T-cell help. Thus, infants with SCID are susceptible to life-threatening infections. Early detection and treatment optimize survival.3-5 Provided that SCID is diagnosed before infections become overwhelming, affected infants can be rescued with hematopoietic stem cell transplantation; gene therapy; or, for adenosine deaminase deficiency, enzyme replacement therapy.2,5-8 Population-based screening is the only means to detect SCID prior to the onset of infections in most cases, as more than 80% lack a positive family history.9,10 T-cell receptor excision circles (TRECs), a biomarker for T lymphopoiesis,11 can be measured by polymerase chain reaction (PCR) using DNA isolated from infant dried blood spots collected for newborn screening.9 Dried blood spots from apparently healthy newborns who were later diagnosed with SCID lacked TRECs.9 Confirmation of the utility of the TREC test,12 adaptation for pilot newborn screening programs in Wisconsin13 and Massachusetts,14 and an evidence-based review led to the recommendation by the US Department of Health and Human Services Secretary in 2010 that SCID be added to the Uniform Screening Panel for all newborns, with related T-cell deficiencies added to the list of secondary targets.15 Currently, 23 states, the District of Columbia, and the Navajo Nation screen approximately two-thirds of all infants born in the United States for SCID. Individual states have confirmed detection of SCID as well as additional disorders with low T-cell numbers, which also may benefit from further assessment of immune dysfunction and from protective treatments.13,16-18 Here we present the first combined analysis of more than 3 million infants screened for SCID in 10 states and the Navajo Nation, providing a population-based overview of SCID and non-SCID T-cell lymphopenia.