Loss of Hectd1 disrupts muscle sarcomere assembly and causes combined desmin-nemaline myopathy

The sarcomere is the muscle contractile unit, whose assembly and disassembly are intimately linked to changes in myofiber size and function. Mutations in sarcomeric proteins are common causes of myopathies, ranging from severe neonatal to adult-onset forms. Here, we identify HECTD1 as an E3 ubiquitin ligase necessary for sarcomere maintenance. We show that silencingHectd1 in myotubesreduces the levels of proteins critical for sarcomere integrity including a-actin, titin, troponins, tropomyosins, and several myosin heavy chain isoforms. We further establish that Hectd1 ubiquitylates and stabilizes the molecular chaperones KLHL40/41, that when dysfunctional destabilize nebulin and other sarcomere thin filament proteins (causing nemaline myopathy). Consequently, skeletal muscle-specific Hectd1 knockout mice (Hectd1 mKO) show progressive muscle weakness, exercise intolerance, and unresolved tissue remodeling. Molecular and histological analysis of Hectd1 mKO muscle revealed subsarcolemmal storage of the scaffold protein desmin, mitochondrial abnormalities, and severe sarcomere disorganization. This work uncovers Hectd1 as an E3 ligase critical for sarcomere assembly, and a novel diagnostic gene for myopathy with features of nemaline and desmin myopathy.