Singleton-Merten Syndrome-like Skeletal Abnormalities in Mice with Constitutively Activated MDA5

Singleton-Merten syndrome (SMS) is a type I interferonopathy characterized by dental dysplasia, aortic calcification, skeletal abnormalities, glaucoma, and psoriasis. A missense mutation in IFIH1 encoding a cytoplasmic viral RNA sensor MDA5 has recently been identified in the SMS patients as well as in patients with a monogenic form of lupus. We previously reported that Ifih1 mice express a constitutively active MDA5 and spontaneously develop lupus-like nephritis. In this study, we demonstrate that the Ifih1 mice also exhibit SMS-like bone abnormalities, including decreased bone mineral density and thin cortical bone. Histological analysis revealed a low number of osteoclasts, low bone formation rate, and abnormal development of growth plate cartilages in Ifih1 mice. These abnormalities were not observed in Ifih1 ・Mavs and Ifih1 ・Ifnar1 mice, indicating the critical role of type I IFNs induced by MDA5/MAVS-dependent signaling in the bone pathogenesis of Ifih1 mice, affecting bone turnover. Taken together, our findings suggest the inhibition of type I IFN signaling as a possible effective therapeutic strategy for bone disorders in SMS patients.