Successful newborn screening for SCID in the Navajo Nation

Newborn screening (NBS) for severe combined immunodeficiency (SCID) identifies affected infants before the onset of life-threatening infections, permitting optimal treatment. Navajo Native Americans have a founder mutation in the DNA repair enzyme Artemis, resulting in frequent Artemis SCID (SCID-A). A pilot study at 2 Navajo hospitals assessed the feasibility of SCID NBS in this population. Dried blood spots from 1800 infants were assayed by PCR for T-cell receptor excision circles (TRECs), a biomarker for naïve T cells. Starting in February 2012, TREC testing transitioned to standard care throughout the Navajo Area Indian Health Service, and a total of 7900 infants were screened through July 2014. One infant had low TRECs and was diagnosed with non-SCID T lymphopenia, while 4 had undetectable TRECs due to SCID-A, all of whom were referred for hematopoietic cell transplantation. This report establishes the incidence of SCID-A and demonstrates effectiveness of TREC NBS in the Navajo.

Newborn screening for SCID identifies patients with ataxia telangiectasia

Purpose

Severe combined immunodeficiency (SCID) is characterized by failure of T lymphocyte development and absent or very low T cell receptor excision circles (TRECs), DNA byproducts of T cell maturation. Newborn screening for TRECs to identify SCID is now performed in several states using PCR of DNA from universally collected dried blood spots (DBS). In addition to infants with typical SCID, TREC screening identifies infants with T lymphocytopenia who appear healthy and in whom a SCID diagnosis cannot be confirmed. Deep sequencing was employed to find causes of T lymphocytopenia in such infants.

Methods

Whole exome sequencing and analysis were performed in infants and their parents. Upon finding deleterious mutations in the ataxia telangiectasia mutated (ATM) gene, we confirmed the diagnosis of ataxia telangiectasia (AT) in two infants and then tested archival newborn DBS of additional AT patients for TREC copy number.

Results

Exome sequencing and analysis led to 2 unsuspected gene diagnoses of AT. Of 13 older AT patients for whom newborn DBS had been stored, 7 samples tested positive for SCID under the criteria of California’s newborn screening program. AT children with low neonatal TRECs had low CD4 T cell counts subsequently detected (R = 0.64).

Conclusions

T lymphocytopenia in newborns can be a feature of AT, as revealed by TREC screening and exome sequencing. Although there is no current cure for the progressive neurological impairment of AT, early detection permits avoidance of infectious complications, while providing information for families regarding reproductive recurrence risks and increased cancer risks in patients and carriers.

Cellular calibrators to quantitate T-cell receptor excision circles (TRECs) in clinical samples

T-cell receptor excision circles (TRECs) are circular DNA molecules formed during rearrangement of the T-cell receptor (TCR) genes during lymphocyte development. Copy number of the junctional portion of the δRec-ψJα TREC, assessed by quantitative PCR (qPCR) using DNA from dried blood spots (DBS), is a biomarker for newly formed T cells and absent or low numbers of TRECs indicate SCID (severe combined immunodeficiency) or T lymphocytopenia. No quantitation standard for TRECs exists. To permit comparison of TREC qPCR results with a reliable method for counting TRECs across different laboratories, we sought to construct a stable cell line containing a normal human chromosomal constitution and a single copy of the TREC junction sequence. A human EBV (Epstein Barr virus)-transformed B-cell line was transduced with a lentivirus encoding mCherry fluorescence, puromycin resistance and the δRec-ψJα TREC sequence. A TREC-EBV cell line, with each cell carrying a single lentiviral insertion was established, expanded and shown to have one TREC copy per diploid genome. Graded numbers of TREC-EBV cells added to aliquots of T lymphocyte depleted blood showed TREC copy number proportional to TREC-EBV cell number. TREC-EBV cells, therefore, constitute a reproducible cellular calibrator for TREC assays, useful for both population-based screening for severe combined immunodeficiency and evaluation of naïve T-cell production in clinical settings.

Transcription factor zinc finger and BTB domain 1 is essential for lymphocyte development

Absent T lymphocytes were unexpectedly found in homozygotes of a transgenic mouse from an unrelated project. T cell development did not progress beyond double-negative stage 1 thymocytes, resulting in a hypocellular, vestigial thymus. B cells were present, but NK cell number and B cell isotype switching were reduced. Transplantation of wild-type hematopoietic cells corrected the defect, which was traced to a deletion involving five contiguous genes at the transgene insertion site on chromosome 12C3. Complementation using bacterial artificial chromosome transgenesis implicated zinc finger BTB-POZ domain protein 1 (Zbtb1) in the immunodeficiency, confirming its role in T cell development and suggesting involvement in B and NK cell differentiation. Targeted disruption of Zbtb1 recapitulated the T(-)B(+)NK(-) SCID phenotype of the original transgenic animal. Knockouts for Zbtb1 had expanded populations of bone marrow hematopoietic stem cells and also multipotent and early lymphoid lineages, suggesting a differentiation bottleneck for common lymphoid progenitors. Expression of mRNA encoding Zbtb1, a predicted transcription repressor, was greatest in hematopoietic stem cells, thymocytes, and pre-B cells, highlighting its essential role in lymphoid development.