Identification of nonsense variants in the ATM gene mimicking SCID phenotype: a brief report.

Severe combined immunodeficiency (SCID) represents a life-threatening inborn error of immunity, necessitating rapid diagnosis and intervention to prevent fatal outcomes. While SCID is characterized by profound T-cell lymphopenia, it may overlap with other conditions like ataxia-telangiectasia (AT), which also presents with T-cell deficiencies. This study examines two cases of suspected SCID in infants, later identified as AT due to pathogenic variants in the ATM gene. Despite initial negative results from SCID-targeted gene panels, further genetic testing revealed nonsense mutations (p.Y2036X and p.E1996X) in the FAT domain of the ATM gene, confirmed by Sanger sequencing. The patients exhibited significant T-cell lymphopenia and reduced ATM protein activity, indicative of AT. These findings highlight the importance of comprehensive genetic screening beyond common SCID-associated genes, especially in patients with atypical presentations. Early and accurate diagnosis can prevent mismanagement and guide appropriate therapies, improving patient outcomes.