Markesbery disease: autosomal dominant late-onset distal myopathy: from phenotype to ZASP gene identification

Myopathy associated LDB3 mutation causes Z-disc disassembly and protein aggregation through PKCα and TSC2-mTOR downregulation

Mechanical stress induced by contractions constantly threatens the integrity of muscle Z-disc, a crucial force-bearing structure in striated muscle. The PDZ-LIM proteins have been proposed to function as adaptors in transducing mechanical signals to preserve the Z-disc structure, however the underlying mechanisms remain poorly understood. Here, we show that LDB3, a well-characterized striated muscle PDZ-LIM protein, modulates mechanical stress signaling through interactions with the mechanosensing domain in filamin C, its chaperone HSPA8, and PKCα in the Z-disc of skeletal muscle. Studies of Ldb3Ala165Val/+ mice indicate that the myopathy-associated LDB3 p.Ala165Val mutation triggers early aggregation of filamin C and its chaperones at muscle Z-disc before aggregation of the mutant protein. The mutation causes protein aggregation and eventually Z-disc myofibrillar disruption by impairing PKCα and TSC2-mTOR, two important signaling pathways regulating protein stability and disposal of damaged cytoskeletal components at a major mechanosensor hub in the Z-disc of skeletal muscle.

A novel mutation in the PDZ-like motif of ZASP causes distal ZASP-related myofibrillar myopathy

Mutations in the LDB3 gene have been identified in patients with Z-disc-associated, alternatively spliced, PDZ motif-containing protein (ZASP)-related myofibrillar myopathy (ZASP-MFM) characterized by late-onset distal myopathy with signs of cardiomyopathy and neuropathy. We describe an autosomal dominant inherited pedigree with ZASP-MFM that is in line with the typical phenotype of distal myopathy without cardiomyopathy and neuropathy, while mild asymmetrical muscle atrophy can be observed in some affected members. Muscle MRI revealed considerable fatty degeneration involved in the posterior compartment of thigh and lower leg, but relatively preserved in rectus femoris, sartorius, gracilis, adductor longus and biceps femoris breve muscles in the later stage. In addition, fatty infiltration of medial gastrocnemius muscle can be initiated as early as in the third decade in asymptomatic individuals. Myopathological features showed sarcoplasmic accumulation of multiple protein deposits and electron dense filamentous bundle aggregates. A novel heterozygous missense mutation (p.N155H) in a highly conserved PDZ-like motif of ZASP was identified. The results indicate that typical ZASP-MFM presenting with late-onset distal myopathy is commonly associated with mutations in PDZ-like motif of ZASP. The development of fatty degeneration is consistent with the typical pattern of ZASP-MFM, and the initial fatty infiltration might be started from medial gastrocnemius muscle. Our study expands the clinical and mutational spectrum of ZASP-MFM.

Z-disc-associated, alternatively spliced, PDZ motif-containing protein (ZASP) mutations in the actin-binding domain cause disruption of skeletal muscle actin filaments in myofibrillar myopathy

The core of skeletal muscle Z-discs consists of actin filaments from adjacent sarcomeres that are cross-linked by α-actinin homodimers. Z-disc-associated, alternatively spliced, PDZ motif-containing protein (ZASP)/Cypher interacts with α-actinin, myotilin, and other Z-disc proteins via the PDZ domain. However, these interactions are not sufficient to maintain the Z-disc structure. We show that ZASP directly interacts with skeletal actin filaments. The actin-binding domain is between the modular PDZ and LIM domains. This ZASP region is alternatively spliced so that each isoform has unique actin-binding domains. All ZASP isoforms contain the exon 6-encoded ZASP-like motif that is mutated in zaspopathy, a myofibrillar myopathy (MFM), whereas the exon 8-11 junction-encoded peptide is exclusive to the postnatal long ZASP isoform (ZASP-LΔex10). MFM is characterized by disruption of skeletal muscle Z-discs and accumulation of myofibrillar degradation products. Wild-type and mutant ZASP interact with α-actin, α-actinin, and myotilin. Expression of mutant, but not wild-type, ZASP leads to Z-disc disruption and F-actin accumulation in mouse skeletal muscle, as in MFM. Mutations in the actin-binding domain of ZASP-LΔex10, but not other isoforms, cause disruption of the actin cytoskeleton in muscle cells. These isoform-specific mutation effects highlight the essential role of the ZASP-LΔex10 isoform in F-actin organization. Our results show that MFM-associated ZASP mutations in the actin-binding domain have deleterious effects on the core structure of the Z-discs in skeletal muscle.