JAK Inhibition in PNPT1-Related Mitochondrial Interferonopathy: A Case Report and Review of Mitochondrial-Immune Crosstalk.

Biallelic pathogenic variants in PNPT1 cause combined oxidative phosphorylation deficiency 13 (COXPD13) (MIM #614932), linking mitochondrial dysfunction to type I interferon (IFN) activation through cytosolic leakage of mitochondrial double-stranded RNA (mt-dsRNA). This mechanism connects mitochondrial disease to interferonopathies such as Aicardi-Goutières syndrome (AGS). We describe a 7-month-old female infant with compound heterozygous PNPT1 variants presenting with severe hypotonia, feeding difficulties necessitating gastrostomy, dystonia, and elevated serum lactate. Brain magnetic resonance imaging (MRI) demonstrated marked cerebellar, brainstem, and basal ganglia atrophy, with a lactate peak on MR spectroscopy (consistent with an inverted doublet). Serum immune profiling revealed a mild but elevated type I IFN signature. Given the mechanistic overlap with AGS, off-label tofacitinib, a Janus kinase (JAK) inhibitor that blocks IFN-driven JAK/STAT signaling, was initiated following pediatric interferonopathy dosing protocols. Tofacitinib was associated with normalization of serum type I IFN biomarkers, reduction in lactate and transaminases, improvement in dystonic movements, ventilatory stability, and improved growth/nutrition without treatment-limiting adverse events. To our knowledge, this represents the first reported use of JAK inhibition in COXPD13. The observed clinical and biochemical stabilization supports defining COXPD13 as a "mitochondrial interferonopathy" and suggests that IFN-signature screening may identify mitochondrial disease patients who could benefit from targeted immunomodulation.