Muscle Integrity

Congenital Muscular Dystrophy

Figure 1.

Figure 1.

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Muscular Dystrophies (MDs) are a group of hereditary diseases, characterised by progressive weakness and degeneration of skeletal muscle. To date, identified gene mutations causative to MDs affect a functionally diverse set of genes, ranging from genes encoding structural proteins such as Dystrophin, to sarcomeric proteins such as Telethonin, and membrane regulators such as Dysferlin.

List of glycosyltransferases with associated deseases and functions.

List of glycosyltransferases with associated deseases and functions.

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Congenital Muscular Dystrophy (CMD) is a clinically and genetically heterogeneous group of muscular disorders. CMD patients show a wide spectrum of clinical defects, characterised by apparent muscle weakness at birth that may be associated with brain malformation and mental retardation. The first form of CMD to be identified genetically was found to be a deficiency of Laminin α2, a basement membrane component, which leads to MDC1A type muscular dystrophy. Recent studies have suggested that hypoglycosylation of α-Dystroglycan is the primary cause of some forms of Laminin α2-positive CMD, in which mutations affect known or putative glycosyltransferase genes (See table). In particular, CMD associated with hypoglycosylation of α-Dystroglycan are termed as dystroglycanopathies.

Side view of 72hpf embryos.

Side view of 72hpf embryos.

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Dystroglycan is a central component of the Dystrophin-associated Glycoprotein Complex, linking the actin cytoskeleton of a muscle cell with extracellular ligands, such as Laminin. To date, POMT1/2 and POMGnT1 have been shown to catalyze specific steps of O-linked glycosylation of α-Dystroglycan. The enzymatic activities of Fukutin, FKRP and LARGE have not been demonstrated. In addition, the pathological mechanisms of CMD remain unclear.

Due to the early onset of the disease, it might be challenging to model CMD if embryonic lethality occurs. Nevertheless, zebrafish embryos develop ex uterus; therefore, serve as a useful model to study the roles of these known or putative glycosyltransferases during late embryogenesis. To gain insights into the etiology of CMD, we have designed antisense morpholino oligonucleotides (MOs) for targeted gene knockdown. MO-injected embryos were analysed for expression of sarcomeric and DGC components. To establish stable mutation carriers, we are currently generating targeted knockout fish under the Zebrafish Mutation Project.

Myopathies

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Figure 1.

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Congenital myopathies are a heterogeneous group of rare neuromuscular diseases characterised by early onset muscle weakness and hypotonia. In contrast to muscular dystrophies congenital myopathies generally do not cause muscle tissue degeneration. The most common form of congenital myopathy is Nemaline Myopathy, which leads to mostly non-progressive muscle weakness and hypotonia. Thus far six genes (α-Actin, Nebulin, Troponin-T, Cofilin-2, Tropomyosin-2 and Tropomyosin-3) have been implicated as causative for nemaline myopathies. These genes all encode components of the sarcomeric thin filament in skeletal muscle. Nebulin is a giant protein of 600-900 kD, which spans the length of sarcomeric thin filament. Nebulin is thought to be the most commonly affected gene in nemaline myopathy, but its size and splice variants complicate the identification of mutations. The severity of symptoms varies greatly ranging from a delayed ability to walk or late onset hypotonia to respiratory failure at birth. In some cases affected members of a family carrying the same mutation display very different degrees of muscle function impairment. In muscle biopsies so-called "nemaline rods" are an important diagnostic marker for NM. These rods are electron dense protein aggregates within skeletal muscle tissue that are comprised of sarcomeric proteins such as α-Actinin and filamentous Actin. Often nemaline rods are accompanied by ectopic thin and/or thick filament accumulations (called cores and caps) and in some cases intranuclear rods. It is unknown how some mutations give rise to the full spectrum of sarcomeric protein aggregates combined with a relatively mild muscle weakness, whereas other defects lead to extensive hypotonia with near normal myofibrils on a structural level. The lack of correlation between identified mutations and severity of symptoms as well as our poor understanding of the mechanisms leading to nemaline rods and other sarcomeric abnormalities make it very difficult to diagnose, let alone develop treatments for nemaline myopathy. We have studied several zebrafish mutants in thin and thick filament proteins in single and double loss of function experiments to understand sarcomere assembly. Some of these mutants show all the hallmarks of nemaline myopathy and thus allow us to dissect the process by which impaired function of some, but not every thin filament protein causes nemaline myopathy.

* quick link - http://q.sanger.ac.uk/qxjy3h2b