Three Patients With Disseminated Mycobacterial Infections Due to Severe Defects in Interferon Gamma Receptor Signaling: A Challenging Diagnosis

IFN–gamma receptor (IFNGR) signaling via STAT1 is crucial in the defense against intracellular pathogens. Defects in this pathway enhance the susceptibility to infection by otherwise weak pathogenic mycobacteria, resulting in a primary immunodeficiency called mendelian susceptibility to mycobacterial disease (MSMD). Here we describe three patients with MSMD caused by variants in the IFNGR1 or STAT1 genes. All three patients presented with disseminated non-tuberculous mycobacterial infections caused by M. avium , M. persicum or M. bovis BCG respectively. Whole-exome sequencing (WES) was used as the first line diagnostic approach, however in all patients additional analysis was crucial to make the definite diagnosis. In Patient 1, only one heterozygous autosomal recessive variant p.(Val63Gly) in the IFNGR1 gene was identified. Patient 2 was compound heterozygous for the pathogenic null p.(Val68Lysfs*6) variant and the hypomorphic p.(Ile37Thr) variant in IFNGR1. In Patient 3 a novel variant in the STAT1 gene c.1379A>T, p.(Asn460Ile) was identified. Additional genetic analysis identified a second novel complex Alu-insertion in the IFNGR1 gene in Patient 1. Functional analysis showed that Patients 1 and 2 had reduced expression of IFNGR1. All patients had reduced phosphorylation of STAT1 and absent induction of SOCS1 mRNA after IFN-γ stimulation. While STAT1 phosphorylation was normal after IFN–α stimulation in Patient 1 and 2, it was mildly reduced in Patient 3. We conclude that functional assays are crucial to assess the extent of IFNGR signaling defects when new combinations of bi-allelic or non-conclusive genetic variants are found, which is important in the determination of clinical treatment.