A human case of GIMAP6 deficiency: a novel primary immune deficiency

Abstract

The GTPase of immunity-associated proteins (GIMAPs) are a family of genes believed to contribute to lymphocyte development, signaling, and apoptosis, thus playing an important role in immune system homeostasis. While models of gene derangement have been described in both mice and immortalized cell lines, human examples of these diseases remain exceptionally rare. In this manuscript we describe the first documented human cases of a homozygous deleterious GIMAP6 variant in the GIMAP6 gene and their subsequent clinical and immunological phenotype. In order to interrogate the patients' immune defect, we performed whole-exome sequencing, western blot, flow cytometry analysis, lymphocyte activation and proliferation studies, cytokine release assays, and apoptosis studies. We found two siblings with a predicted deleterious homozygous variant in the GIMAP6 gene with no expression of GIMAP6 protein on western blot. Patients demonstrated accelerated apoptosis, but largely normal lymphocyte subpopulations, activation and proliferation and cytokine release. There appears to be a spectrum of clinical features associated with deficiency of GIMAP6 protein, with one patient suffering lymphopenia and recurrent sinopulmonary infections, and the other clinically asymptomatic. Biallelic variants in the GIMAP6 gene have now been shown to demonstrate disease in humans. The absence of GIMAP6 protein is associated with a spectrum of clinical manifestations and much remains to be learnt about the pathogenic mechanisms underlying this disease. We suggest that biallelic variants in the gene for GIMAP6 should be considered in children with lymphopenia and recurrent sinopulmonary infections.

Fig. 1. Immunoblot for GIMAP6 protein expression in PBMCs of the patients and control subjects.

Cell lysates were prepared from P1 and P2 or healthy donor (HD) PBMCs. Cell lysates were separated through 4–20% gradient SDS-PAGE, and immunoblotted for GIMAP6 protein expression. α-Tubulin was used as a loading control.

Fig. 2. Stimulation-induced apoptosis.

PBMC apoptosis in response to anti-CD3/CD28 and PHA stimulation. PBMCs from healthydonor (HD, continuous line), P1 (dashed line), and P2 (dotted line) were analyzed for intracellular activated Caspase-3 expression following exposure to anti-CD3/CD28 or PHA for 0, 3, 16, and 25 h. PBMCs (left panels) gated on CD4+ (middle panels) and CD8+ (right panels) were analyzed for cleaved Caspase-3 expression. Significant values were indicated as follows: *p < 0.05; **p < 0.01; ***p < 0.005.