Utility of Newborn Screening for Severe Combined Immunodeficiency and X-Linked Agammaglobulinemia Using TREC and KREC Assays

Background Severe combined immune deficiency (SCID) is a potentially fatal primary immunodeficiency due to the absence of T and B lymphocyte function. Early intervention for patients with SCID results in a higher survival rate. From 2017, we launched the first optional newborn screening (NBS) for SCID in Japan based on the detection of T-cell receptor excision circles (TREC). However, NBS for severe B-cell lymphopenia, such as X-linked agammaglobulinemia (XLA), has not been a standard screening test because of a high false-positive rate of Kappa-deleting recombination excision circles (KREC), which reflects the replication of B cells. XLA is characterized by severe B-cell lymphopenia and marked reduction of all classes of serum immunoglobulins. Patients with XLA require early diagnosis and immunoglobulin replacement therapy to prevent the development of bronchiectasis caused by recurrent infections. This study aimed to analyze the results of NBS for SCID and elucidate the utility of NBS for SCID and XLA using the TREC/KREC assay. Patients and Methods We enrolled infants who received NBS for SCID (n = 29,447) between April 2017 and June 2018. Using the EnLiteTM TREC kit, we measured TRECA and β-actin, which are used as controls for monitoring sample amplification. Samples with less than 30 copies/µL and adequate β-actin were defined as positive TRECA. All infants with positive TRECA were followed up for at least 12 months. We measured TRECB and KREC using the EnLiteTM TREC/KREC kit in these infants. As positive controls, we used TRECB and KREC in patients with SCID and XLA, respectively. Furthermore, all infants with positive TRECA were evaluated using flow cytometric analysis and target capture-based next-generation sequencing (NGS) analysis covering 349 primary immunodeficiency- and bone marrow failure-related genes to evaluate CD4+CD45RA+ T-cell counts and identify diagnostic variants. This study was approved by the institutional review board of Nagoya University Graduate School of Medicine. Results Of the infants who underwent NBS for SCID, 43 (0.15%) infants showed positive TRECA. All 43 infants were followed up in Nagoya University for at least 12 months. Of these, we identified one case with DiGeorge syndrome showing severe lymphopenia but did not identify typical SCID. TRECB and KREC were measured in 43 infants with positive TRECA. To determine which kit is more useful to detect T-cell lymphopenia, we compared TRECA with TRECB in 1454 infants with n