A recent study conducted over seven years has streamlined the testing and diagnosis of newborns with severe combined immunodeficiency (SCID), facilitating early intervention and treatment of infants before the development of symptoms.
The U.S. National Institute of Health (NIH) defines SCID as a rare group of genetic disorders that impair the functioning of infection-fighting immune cells. Individuals with SCID show little to no defense against pathogenic agents, making them highly susceptible to infections on even the slightest exposure to antigens. While newborn babies do not exhibit any obvious symptoms of the disorder, the condition is fatal if not treated within the first two or three years of life.
There are over 461 genes linked with SCID, including those coding for the protein phytohemagglutinin (PHA), and the enzyme adenosine deaminase (ADA). Another rare variant called X-linked SCID arises due to a mutation on the X-chromosome, resulting in an absence of B-cell proliferation. The defective X-chromosome is inherited from the female parent, thus inordinately affecting male progeny.
SCID gained public attention with the case of David Vetter, popularly known as the 'Bubble Boy'. Born in the 1970s, Vetter was diagnosed with X-linked SCID before birth, and lived virtually his whole life in a sterile, plastic-lined environment. He passed away at 16-years-old following a failed bone marrow transplant, raising significant global awareness for SCID and fueling interest in further research.
In 1990 the first experimental gene therapy was administered for the treatment of ADA deficiency in a 4-year-old girl and subsequently showed promising results. Till date, most treatments involve the transfer of hematopoietic stem cells from healthy donors, or enzyme replacement therapies, but early diagnosis is a key factor in determining the success of these.
Newborn screening for SCID began in the U.S. as an optional test within 24-72 hours of birth, and has since been adopted in Taiwan, Israel, and parts of Europe. Diagnostic tests involve blood counts, immunoglobulin tests, urine analyses, and lymphoproliferative assays of PHA or ADA. T-receptor expansion circles (TRECs) act as biomarkers also used in diagnosis, following analysis of dried blood spots in the heel-prick test.
Ana Ramirez et al utilized such methods during their seven-year study in Catalonia, Spain, the results of which were published last month in the journal Frontiers in Immunology. Initiated in 2008, the investigation screened approximately 420,263 newborns in a Catalonian hospital, 104 of which tested positive for some form of SCID.
Of the babies that tested positive, the most frequent gene variant observed was the RAG2 mutant, common in progeny arising from consanguineous parents. Only one patient tested positive for X-linked SCID. Thirty-three cases were subsequently found to be false positives.
Patients were further referred to medical professionals, including pediatric immunologists and psychologists. A thorough medical history was extracted from the patients' families, and confirmed cases were subject to immediate isolation to prevent pathogenic exposure. Their samples underwent next-generation sequencing to determine the specific genetic variant of SCID, human leukocyte antigen (HLA) typing, and a search for healthy donors of bone marrow was initiated. During the course of the study, one patient with X-linked SCID received a successful gene therapy in London, and six underwent hematopoietic stem cell transplants, resulting in one death due to allograft rejection. Gene therapies and thymus transplantation resulted in overall successful treatment, with a majority of patients showing good recovery in the seven-year follow-up period.
Additionally, 13 of the cases were discovered to have non-SCID T-cell lymphopenia, another serious immune disease. The next-generation sequencing allowed for the diagnosis, which would otherwise been classified incorrectly as SCID.
The results of the study were collated over six years between 2017 and 2023. At the time of publication, the survival rate of patients was 88%, indicating a positive outcome.
The Catalonian trial thus serves as a promising model for SCID newborn screening, facilitating early detection and more feasible results through treatment.
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