Desmin myopathy is a recently identified disease associated with mutations in
desmin or alphaB-
crystallin. Typically, the illness presents with lower limb
muscle weakness slowly spreading to involve truncal, neck-flexor, facial, bulbar and respiratory muscles. Skeletal
myopathy is often combined with
cardiomyopathy manifested by conduction blocks and arrhythmias resulting in premature
sudden death. Sections of the affected skeletal and cardiac muscles show abnormal fibre areas containing amorphous eosinophilic deposits seen as granular or granulofilamentous material on electron microscopic examination. Immuno-staining for
desmin is positive in each region containing abnormal structures. The inheritance pattern in familial
desmin myopathy is autosomal dominant or autosomal recessive, but many cases have no family history. At least some, and probably most, non-familial
desmin myopathy cases are associated with de novo
desmin mutations. Age of disease onset and rate of progression may vary depending on the type of inheritance and location of the causative mutation. Multiple mutations have been identified in the
desmin gene: point substitutions, insertion, small in-frame deletions and a larger exon-skipping deletion. The majority of these mutations are located in conserved alpha-helical segments of
desmin. Many of the missense mutations result in changing the original
amino acid into
proline, which is known as a helix breaker. Studies of transfected cell cultures indicate that mutant
desmin is assembly-incompetent and able to disrupt a pre-existing filamentous network in dominant-negative fashion. Disease-associated
desmin mutations in humans or transgenic mice cause accumulation of chimeric intracellular aggregates containing
desmin and other
cytoskeletal proteins. alphaB-
crystallin serves in the muscle as a chaperone preventing
desmin aggregation under various forms of stress. If mutated, alphaB-
crystallin may cause a
myopathy similar to those resulting from
desmin mutations. Routine genetic testing of patients for mutations in
desmin and alphaB-
crystallin genes is now available and necessary for establishing an accurate diagnosis and providing appropriate genetic counselling. Better understanding of disease pathogenesis would stimulate research focused on developing specific treatments for these conditions.