Two Unique Cases of X-linked SCID: A Diagnostic Challenge in the Era of Newborn Screening.

Pooja Purswani,Luigi D Notarangelo,Jessica Trotter,Taylor Alberdi,Hans D Ochs,Jennifer W Leiding,Anna K Meyer,Troy R Torgersen,Benjamin Oshrine,Sonia Joychan,Carla Duff,Panida Sriaroon,Hye Sun Kuehn,Jaime E Hale,Sergio D Rosenzweig,Maryssa Ellison,Yi‐Lu Chang ,Anne Marie Comeau ,Jack J Bleesing ,Boglárka Ujházi ,Krisztián Csomós ,Aleksandra Petrovič ,Joseph Dasso ,Cristina Meehan ,D A Lindsay ,Attila Kumánovics ,Jolán E Walter

doi:10.3389/fped.2019.00055

Abstract

In the era of newborn screening (NBS) for severe combined immunodeficiency (SCID) and the possibility of gene therapy (GT), it is important to link SCID phenotype to the underlying genetic disease. In western countries, X-linked interleukin 2 receptor gamma chain (IL2RG) and adenosine deaminase (ADA) deficiency SCID are two of the most common types of SCID and can be treated by GT. As a challenge, both IL2RG and ADA genes are highly polymorphic and a gene–based diagnosis may be difficult if the variant is of unknown significance or if it is located in non-coding areas of the genes that are not routinely evaluated with exon-based genetic testing (e.g., introns, promoters, and the 5′and 3′ untranslated regions). Therefore, it is important to extend evaluation to non-coding areas of a SCID gene if the exon-based sequencing is inconclusive and there is strong suspicion that a variant in that gene is the cause for disease. Functional studies are often required in these cases to confirm a pathogenic variant. We present here two unique examples of X-linked SCID with variable immune phenotypes, where IL2R gamma chain expression was detected and no pathogenic variant was identified on initial genetic testing. Pathogenic IL2RG variants were subsequently confirmed by functional assay of gamma chain signaling and maternal X-inactivation studies. We propose that such tests can facilitate confirmation of suspected cases of X-linked SCID in newborns when initial genetic testing is inconclusive. Early identification of pathogenic IL2RG variants is especially important to ensure eligibility for gene therapy.

Highlights

severe combined immunodeficiency (SCID) is caused by a defect in cellular and humoral immunity, primarily stemming from abnormal T cell development and/or function
We report two unique cases of X-linked SCID, in which a diseasecausing variant could not be identified on initial genetic testing, but functional assays and additional DNA sequencing led to the discovery and/or confirmation of a pathogenic variant
Maternal X chromosome inactivation (XCI) studies by two independent laboratories using peripheral blood mononuclear cells (PBMCs) demonstrated mild skewing, while subsequent testing of maternal T cells indicated complete skewing toward inactivation of the X-chromosome with the allele containing the pathogenic variant carried by the patient (Table 1 and Figure 2A lower panel)

Summary

INTRODUCTION

SCID is caused by a defect in cellular and humoral immunity, primarily stemming from abnormal T cell development and/or function. If one allele carries a pathogenic variant in IL2RG, inactivation is typically highly skewed toward the mutant allele [18] This is especially evident in T cells that are dependent on common γc for their development. The impaired signaling in γc dependent cytokines and the skewing of X-inactivation in maternal T cells and carrier sisters’ PBMCs, taken together, strongly support that the p.E59Q variant detected in IL2RG is pathogenic. Maternal XCI studies by two independent laboratories using PBMCs demonstrated mild skewing, while subsequent testing of maternal T cells indicated complete skewing toward inactivation of the X-chromosome with the allele containing the pathogenic variant carried by the patient (Table 1 and Figure 2A lower panel). Patient engrafted neutrophils on day and platelets on day and subsequently demonstrated full T and B cell immune reconstitution

METHODS

Chromosome Inactivation Assay

DISCUSSION

ETHICS STATEMENT