We identified a member of the BTB/Kelch protein family that is mutated in nemaline myopathy type 6 (NEM6), an autosomal-dominant neuromuscular disorder characterized by the presence of nemaline rods and core lesions in the skeletal myofibers. Analysis of affected families allowed narrowing of the candidate region on chromosome 15q22.31, and mutation screening led to the identification of a previously uncharacterized gene, KBTBD13, coding for a hypothetical protein and containing missense mutations that perfectly cosegregate with nemaline myopathy in the studied families. KBTBD13 contains a BTB/POZ domain and five Kelch repeats and is expressed primarily in skeletal and cardiac muscle. The identified disease-associated mutations, C.742C>A (p.Arg248Ser), c.1170G>C (p.Lys390Asn), and c.1222C>T (p.Arg408Cys), located in conserved domains of Kelch repeats, are predicted to disrupt the molecule's beta-propeller blades. Previously identified BTB/POZ/Kelch-domain-containing proteins have been implicated in a broad variety of biological processes, including cytoskeleton modulation, regulation of gene transcription, ubiquitination, and myofibril assembly. The functional role of KBTBD13 in skeletal muscle and the pathogenesis of NEM6 are subjects for further studies.

Centronuclear myopathies are congenital muscle disorders characterized by type I myofibre predominance and an increased number of muscle fibres with nuclear centralization. The severe neonatal X-linked form is due to mutations in MTM1, autosomal recessive centronuclear myopathy with neonatal or childhood onset results from mutations in BIN1 (amphiphysin 2), and dominant cases were previously associated to mutations in DNM2 (dynamin 2). Our aim was to determine the genetic basis and physiopathology of patients with mild dominant centronuclear myopathy without mutations in DNM2. We hence established and characterized a homogeneous cohort of nine patients from five families with a progressive adult-onset centronuclear myopathy without facial weakness, including three sporadic cases and two families with dominant disease inheritance. All patients had similar histological and ultrastructural features involving type I fibre predominance and hypotrophy, as well as prominent nuclear centralization and clustering. We identified heterozygous BIN1 mutations in all patients and the molecular diagnosis was complemented by functional analyses. Two mutations in the N-terminal amphipathic helix strongly decreased the membrane-deforming properties of amphiphysin 2 and three stop-loss mutations resulted in a stable protein containing 52 supernumerary amino acids. Immunolabelling experiments revealed abnormal central accumulation of dynamin 2, caveolin-3, and the autophagic marker p62, and general membrane alterations of the triad, the sarcolemma, and the basal lamina as potential pathological mechanisms. In conclusion, we identified BIN1 as the second gene for dominant centronuclear myopathy. Our data provide the evidence that specific BIN1 mutations can cause either recessive or dominant centronuclear myopathy and that both disorders involve different pathomechanisms.