Background/Purpose: A novel candidate gene, Mosaic (Multi-Organ System Autoimmunity in Canines), was identified as the culprit causing an early and severe multiorgan autoimmunity in a unique subset of dogs. Affected dogs develop early onset Addison’s disease, arthritis, autoimmune cytopenias, hepatitis and uveitis. Adrenal tissue from affected dogs reveal a T cell infiltrate suggesting a T cell mediated autoimmune process. Little is known about Mosaic function. It is conserved across all known vertebrae species including humans and mice, and it is highly expressed in immune cells, particularly in T regulatory cells based on BioGPS. Thus, we hypothesize that disruption of Mosaic function results in a breach in T cell tolerance and leads to multiorgan autoimmunity.

Methods: To assess the effects of Mosaic deficiency, two mouse models were produced. First, Mosaic knockout mice were generated using a B6 ES cell line by deleting at 2.5kb region containing the start codon and inserting a reporter cassette. This reporter will allow tracing of the endogenous gene expression pattern. The 5’ and 3’ regions of the targeted locus was confirmed using long-range PCR and Mosaic transcript was absent. Second, floxed Mosaic mice were generated using CRISPR-Cas9 by inserting two LoxP sequences flanking exon 3 which contains the start codon. Two gRNAs, two oligonucleotides, and Cas9 protein were injected into B6 fertilized eggs. Mice were screened for insertion of LoxP sequences that were in cis position and had no other indels in the targeted region. 3 founders were generated and bred to the CD4-Cre strain to generate deletion of Mosaic in the T cell compartment. Mosaic transcript was absent in the T cells. These mouse models were phenotyped using flow cytometry and histology.

Results: Using the reporter cassette, Mosaic was identified to have the highest expression level in memory T cells and T regulatory cells. Global deletion of Mosaic resulted in partial embryonic lethality. However, viable knockout pups showed increased memory T cells and decreased naïve T cells. Aged mice showed lymphocytic infiltrates in the adrenal and lacrimal glands compared to littermate controls. Analysis of the Mosaic^fl/flCD4-Cre mice also showed a skewed memory T cell compartment in mice as young as 1 month of age.

Conclusion: Deletion of Mosaic in mice resulted in an autoimmune phenotype with lymphocytic infiltrates in the adrenal and lacrimal glands. These mice also have a skewed memory T cell phenotype which is consistent with other autoimmune mouse models. Further investigation on Mosaic will reveal the molecular mechanism on how Mosaic functions in tolerance.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/mosaic-a-novel-gene-mediating-autoimmunity-in-mice/