Selenoprotein N and SEPN1-Related Myopathies: Mechanisms, Models, and Therapeutic Perspectives

: Selenoprotein N (SelN or SELENON) is a selenium-containing protein of the endoplasmic/sarcoplasmic reticulum (ER/SR), encoded by the SEPN1 gene. In skeletal muscle, SelN is particularly important for regulating SR calcium homeostasis. It acts as a calcium sensor, modulating the activity of the sarcoplasmic reticulum calcium pump (SERCA) through a redox-dependent mechanism. Loss-of-function mutations in the SEPN1 gene give rise to a spectrum of skeletal muscle disorders collectively referred to as SEPN1-related myopathies (SEPN1-RM). Histopathologically, SEPN1-RM is characterized by the presence of minicores, which are localized regions within muscle fibers exhibiting mitochondrial depletion (i.e., cores) and sarcomeric disarray. As no effective therapy is currently available for SEPN1-RM, understanding SelN biology through loss-of-function models remains essential for elucidating disease mechanisms and identifying potential therapeutic targets. This review examines the current knowledge on SelN function and the pathological mechanisms underlying SEPN1 loss-of-function, with a particular focus on the connection between calcium handling, oxidative/ER stress, and muscle dysfunction. It also highlights emerging strategies aimed at restoring SelN activity or mitigating downstream defects, outlining potential therapeutic avenues for SEPN1-RM.