Charcot-Marie-Tooth disease type 2D (CMT2D) and distal spinal muscular atrophy type V (dSMA-V) are axonal peripheral neuropathies inherited in an autosomal dominant fashion. Our previous genetic and physical mapping efforts localized the responsible gene(s) to a well-defined region on human chromosome 7p. Here, we report the identification of four disease-associated missense mutations in the glycyl tRNA synthetase gene in families with CMT2D and dSMA-V. This is the first example of an aminoacyl tRNA synthetase being implicated in a human genetic disease, which makes genes that encode these enzymes relevant candidates for other inherited neuropathies and motor neuron diseases. Charcot-Marie-Tooth (CMT) disease constitutes a heterogeneous group of peripheral neuropathies estimated to affect 1 in 2,500 individuals (Skre 1974). The clinical features of CMT include muscular weakness and atrophy in the distal extremities, steppage gait, pes cavus, absent or diminished deep-tendon reflexes, and impaired sensation (Murakami et al. 1996). Through the measurement of motor nerve conductance velocities (MNCVs), CMT can be subdivided into two classes (Dyck and Lambert 1968). In CMT1, patients exhibit decreased MNCVs with demyelinating axons. In CMT2, patients exhibit normal MNCVs and no demyelination but have decreased amplitudes of evoked motor and sensory nerve responses. To date, six subtypes of CMT2 have been reported (CMT2A-F), with the genes responsible for

Fatal familial insomnia (FFI) and a subtype of familial Creutzfeldt-Jakob disease (CJD), two clinically and pathologically distinct diseases, are linked to the same mutation at codon 178 (Asn178) of the prion protein gene. The possibility that a second genetic component modified the phenotypic expression of the Asn178 mutation was investigated. FFI and the familial CJD subtype segregated with different genotypes determined by the Asn178 mutation and the methionine-valine polymorphism at codon 129. The Met129, Asn178 allele segregated with FFI in all 15 affected members of five kindreds whereas the Val129, Asn178 allele segregated with the familial CJD subtype in all 15 affected members of six kindreds. Thus, two distinct disease phenotypes linked to a single pathogenic mutation can be determined by a common polymorphism.

Mutations in the desmin gene affecting intermediate filaments cause a distinct myopathy that is often associated with cardiomyopathy and is termed "desmin myopathy." The mutant desmin interferes with the normal assembly of intermediate filaments, resulting in fragility of the myofibrils and severe dysfunction of skeletal and cardiac muscles.

Desmin-related myopathy (OMIM 601419) is a familial disorder characterized by skeletal muscle weakness associated with cardiac conduction blocks, arrhythmias and restrictive heart failure, and by intracytoplasmic accumulation of desmin-reactive deposits in cardiac and skeletal muscle cells. The underlying molecular mechanisms are unknown. Involvement of the desmin gene (DES) has been excluded in three families diagnosed with desmin-related myopathy. We report two new families with desmin-related cardioskeletal myopathy associated with mutations in the highly conserved carboxy-terminal end of the desmin rod domain. A heterozygous A337P mutation was identified in a family with an adult-onset skeletal myopathy and mild cardiac involvement. Compound heterozygosity for two other mutations, A360P and N393I, was detected in a second family characterized by childhood-onset aggressive course of cardiac and skeletal myopathy.

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.

The PRNP gene, encoding the amyloid precursor protein that is centrally involved in Creutzfeldt-Jakob disease (CJD), has an unstable region of five variant tandem octapeptide coding repeats between codons 51 and 91. We screened a total of 535 individuals for the presence of extra repeats in this region, including patients with sporadic and

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Muscle fiber deterioration resulting in progressive skeletal muscle weakness, heart failure, and respiratory distress occurs in more than 20 inherited myopathies. As discussed in this Review, one of the newly identified myopathies is desminopathy, a disease caused by dysfunctional mutations in desmin, a type III intermediate filament protein, or αB-crystallin, a chaperone for desmin. The range of clinical manifestations in patients with desminopathy is wide and may overlap with those observed in individuals with other myopathies. Awareness of this disease needs to be heightened, diagnostic criteria reliably outlined, and molecular testing readily available; this would ensure prevention of sudden death from cardiac arrhythmias and other complications.