Unusual cutaneous features associated with a heterozygous gain‐of‐function mutation in IFIH1: overlap between Aicardi‐Goutières and Singleton‐Merten syndromes

Cutaneous lesions described as chilblain lupus occur in the context of familial chilblain lupus or Aicardi-Goutieres syndrome. To date, seven genes related to Aicardi-Goutieres syndrome have been described. The most recently described encodes the cytosolic double-stranded RNA receptor IFIH1 (also known as MDA5), a key component of the antiviral type I interferon-mediated innate immune response. Enhanced type I interferon signalling secondary to gain-offunction mutations in IFIH1 can result in a range of neuroinflammatory phenotypes including classical Aicardi-Goutieres syndrome. It is of note that none of the patients with a neurological phenotype so far described with mutations in this gene was reported to demonstrate cutaneous involvement. We present a family segregating a heterozygous pathogenic mutation in IFIH1 showing dermatological involvement as a prominent feature, variably associated with neurological disturbance and premature tooth loss. All three affected individuals exhibited increased expression of interferon-stimulated genes in whole blood, and the mutant protein resulted in enhanced interferon signalling in vitro, both in the basal state and following ligand stimulation. Our results further extend the phenotypic spectrum associated with mutations in IFIH1, indicating that the disease can be confined predominantly to the skin, while also highlighting phenotypic overlap with both Aicardi-Goutieres syndrome and Singleton-Merten syndrome.