Mutation in BAG3 causes severe dominant childhood muscular dystrophy

Abstract

Objective-Myofibrillar myopathies (MFM) are morphologically distinct but genetically heterogeneous muscular dystrophies in which disintegration of Z disks and then of myofibrils is followed by ectopic accumulation of multiple proteins. Cardiomyopathy, neuropathy, and dominant inheritance are frequent associated features. Mutations in αB-crystallin, desmin, myotilin, Zasp, or filamin-C can cause MFM, and were detected in 32/85 patients of the Mayo MFM cohort. Bag3, another Z-disk associated protein, has antiapoptotic properties and its targeted deletion in mice causes fulminant myopathy with early lethality. We therefore searched for mutations in BAG3 in 53 unrelated MFM patients.Methods-We searched for mutations in BAG3 by direct sequencing and excluded polymorphism using allele-specific PCR in relatives and 200 control subjects. We analyzed structural changes in muscle by histochemistry, immunocytochemistry and electron microscopy, examined mobility of the mutant Bag3 by nondenaturing electrophoresis, and searched for abnormal aggregation of the mutant protein in COS-7 cells.Results-We identified a heterozygous p.Pro209Leu mutation in three patients. All presented in childhood, had progressive limb and axial muscle weakness, and developed cardiomyopathy and severe respiratory insufficiency in their teens; two had rigid spines and one a peripheral neuropathy. Electron microscopy showed disintegration of Z disks, extensive accumulation of granular debris and larger inclusions, and apoptosis of 8% of the nuclei. On nondenaturing electrophoresis of muscle extracts, the Bag3 complex migrated faster in patient than control extracts, and expression of FLAG-labeled mutant and wild-type Bag3 in COS cells revealed abnormal aggregation of the mutant protein.Interpretation-We conclude mutation in Bag3 defines a novel severe autosomal dominant childhood muscular dystrophy.Myofibrillar myopathies (MFMs) represent a morphologically distinct but genetically heterogeneous subset of muscular dystrophies. Cardiomyopathy, peripheral neuropathy, and dominant inheritance are frequent associated features.1 -7 The histologic findings are similar in that disintegration of the myofiber Z disk is an early pathologic alteration; this is followed

Dilated cardiomyopathy-associatedBAG3mutations impair Z-disc assembly and enhance sensitivity to apoptosis in cardiomyocytes

“…In addition, Bag3 knockout mice displayed degeneration of muscle fibers with apoptotic nuclei in the striated muscles, resulting in a severe form of skeletal myopathy and cardiomyopathy, which lead to a hypothesis that BAG3 protein might play a role as a Z-disc signaling molecule [Homma et al, 2006]. In accordance with the hypothesis, apart from the association with DCM described above [Norton et al, 2011;Villard et al, 2011], a heterozygous Pro209Leu mutation was found in patients with myofibrillar myopathy (MFM) accompanied by cardiomyopathy (MFM6; MIM# 612954) [Lee et al, 2011;Odgerel et al, 2010;Selcen et al, 2009]. Moreover, it was demonstrated that knockdown of bag3 in a zebrafish model developed heart failure resembling to human DCM [Norton et al, 2011].…”

Chaperone-assisted degradation: multiple paths to destruction

“…In addition to aged neuronal cells, muscle cells apparently highly rely on CASA. This is further emphasized by the fact that a mutation in human BAG-3 (P209L) causes a rapidly progressing myopathy in affected children (Selcen et al, 2009). How the observed pathologies relate to the regulatory role of BAG-3 in chaperone-assisted selective autophagy was recently elucidated (Arndt et al, 2010).…”

“…The chaperone machinery then mediates the processing and ubiquitylation of Z-disk components that were damaged during muscle contraction to initiate their disposal through CASA (Arndt et al, 2010). BAG-3 was found to be upregulated in contracting muscles (Arndt et al, 2010), and impairment of BAG-3 activity in transgenic animals and patients resulted in a contraction-dependent disintegration of the Z-disk (Homma et al, 2006;Selcen et al, 2009). The Zdisk component filamin was identified as a critical substrate of CASA in muscle cells (Arndt et al, 2010).…”

“…Moreover, recent studies demonstrate that an impairment of chaperone-assisted degradation causes diverse diseases and age-related pathologies (Zhang and Cuervo, 2008;Kajiro et al, 2009;Selcen et al, 2009). The 'degrading' activity of certain chaperones apparently fulfills an essential function within the proteostasis network.…”

Molecular chaperones and neuronal proteostasis

“…HSJ1a can bind tau, reduce tau phosphorylation and aggregation (Novoselov and Cheetham unpublished observations). [167,168] …”