Supplementary file 1_A novel stop-loss variant in ISG15 causes C-terminal dysfunction and impairs USP18-mediated negative feedback.docx

The patient was treated with the JAK inhibitor baricitinib.</p>Results<p>Two novel compound heterozygous ISG15 variants were identified: c.234C>A, which produced a truncated ISG15 protein, and c.498A>T, a stop-loss variant predicted to extend the ISG15 C-terminus and associated with markedly impaired ISGylation. Patient PBMCs and skin tissue showed increased p-STAT1 signaling and IFN-I pathway activation, with decreased USP18 expression and altered ISG15-USP18 regulation in PBMCs.

In 293T cells, c.498A>T alone did not reduce USP18 abundance, suggesting that this variant contributes to disease through impaired C-terminal functional presentation rather than USP18 loss alone. Structural analysis suggested that the C-terminal extension may alter LRLRGG motif presentation, weakening ISG15-USP18-mediated negative feedback. Baricitinib reduced IFN-I-related markers and was associated with improvement of ulcerative skin lesions and liver enzyme abnormalities.</p>Discussion<p>This study expands the genetic and mechanistic spectrum of ISG15-associated type I interferonopathy, and supports that JAK-STAT inhibition with baricitinib can effectively control IFN-I-driven inflammation.</p>