Muscular dystrophies (MDs) are a diverse group of genetic diseases that affect various muscle groups depending on the type of dystrophy. So far, the genetic causes of over 30 genetically distinct types of MD have been identified. In these disorders, the progressive degeneration of dystrophic muscles is characterised by: an abundance of small-diameter fibres with central nuclei (indicating regeneration), infiltration of mononucleate cells, accumulating fibrosis, and a greater variation in the size of myofibre cross-sectional areas compared to muscle in healthy individuals. Affected individuals in severe MDs, such as Duchenne MD (DMD), exhibit muscle weakness from early childhood onwards, become wheelchair-dependent in their early teens, and usually die in their early thirties from lung or heart failure. DMD is caused by dystrophin mutations that cause defects in the dystrophin-associated glycoprotein complex (DGC), which connects the cytoskeleton of the muscle fibre to the extracellular matrix (ECM). Deficiencies in other DGC components, such as laminin-2, can cause MD with similar devastating symptoms. On the other end of the clinical spectrum are MDs with very mild symptoms, such as those suffering from Becker MD (BMD), who can be ambulant even late in life in rare cases. Interestingly, BMD is caused by mutations that result in only partial dystrophin function loss. Deficits in the second major sarcolemmal complex, which mediates adhesion via integrins, cause other MDs . Sarcolemma-spanning integrins bind to laminin on the outside of myofibres and interact with actin on the cytosolic side, forming a link between the cytoskeleton of myofibres and the ECM.

For several reasons, zebrafish embryos are particularly well suited to the study of muscle development. For starters, they develop externally, are transparent, somitic muscle comprises a large proportion of the body and is accessible, and they begin to move very soon after gastrulation. Both embryological and genetic studies have successfully used these characteristics to investigate early stages of muscle development in the zebrafish, such as the specification of slow-twitch muscle fibres . However, beyond the initial identification of mutations that affect muscle fibre differentiation, function, and integrity, the later stages of development have received little attention. Somitic muscle in fish develops from segmented paraxial mesoderm, with the posterior compartment of each somite expressing myogenic basic–helix–loop–helix transcription factors from gastrulation’s end. Muscle fibres in the anterior–posterior axis differentiate to span an entire somite. Slowtwitch fibres are the first somitic muscle cells to differentiate. They are specified medially by midline-derived hedgehog family signalling proteins, differentiate by 16 h post-fertilisation (hpf), and migrate to the lateral periphery of the somites. These myosepta, along with the notochord (a primitive stiffening rod shared by all chordates), serve as attachment points for somitic muscle fibres. These are essentially laminar tendons that transmit force to the notochord and, later, the vertebral column. These muscle attachment sites have recently received attention due to the discovery that mechanical failure is the pathological mechanism in a zebrafish mutation that provides the first zebrafish model of an inherited skeletal muscle disease.

Researchers searched current sequence databases for zebrafish orthologs of known human dystrophy genes in order to identify more zebrafish dystrophy mutants. The positioning of these genes allows for rapid candidate identification during genetic mapping of dystrophic zebrafish mutants and may allow for the prioritisation of novel mutants – those with linkage to a genomic region with no known dystrophy-associated ortholog. We have also identified the BAC clone location of these genes due to the evolving nature of the Sanger Centre Zebrafish Genome assembly. When future genome alignments are released, BAC sequences should allow for more consistent local sequence information and easy updating. The researchers discovered orthologous zebrafish transcripts for 24 of the 25 known human dystrophy-associated genes, as well as four additional myopathy-related genes. All 29 of these genes have genomic positions identified, as well as BAC locations for 24 of them. According to the genomic data, at least two dystrophy genes are duplicated in the zebrafish genome. Syntenic relationships are conserved for 19 dystrophy and myopathy-causing genes, according to the localization of the closest mammalian gene neighbours. The zebrafish is rapidly being developed as a system for studying muscle disease, with promising results, and it is hoped that this trend will continue. More laboratories will begin to devote a portion of their time to this approach in the future; both existing muscular dystrophy and zebrafish groups may be converted, hopefully to the benefit of the field and patients. The zebrafish, as a model system, will allow for both genetic and embryological manipulations, and thus may provide insights into both the protein interactions and cellular functions that underpin muscular dystrophy and other muscle diseases, as it has already begun to do with great success in the field of cardiovascular