Hee-Suk Lee

Missense mutations in desmin associated with familial cardiac and skeletal myopathy.

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.

Increased Susceptibility to Kuru of Carriers of the PRNP 129 Methionine/Methionine Genotype

Kuru reached epidemic proportions by the mid-twentieth century among the Fore people of New Guinea and disappeared after the abolition of cannibalistic rituals. To determine susceptibility to kuru and its role in the spread and elimination of the epidemic, we analyzed the PRNP gene coding sequences in 5 kuru patients; no germline mutations were found. Analysis of the PRNP 129 methionine (M)/valine (V) polymorphism in 80 patients and 95 unaffected controls demonstrated that the kuru epidemic preferentially affected individuals with the M/M genotype. A higher representation of M/M carriers was observed among the affected young Fore males entering the age of risk, whereas a lower frequency of M/M homozygotes was found among the survivors. M/V and V/V genotypes predisposed to a lower risk of disease development and longer incubation times. These findings are relevant to the current outbreak of variant Creutzfeldt-Jakob disease (vCJD) in the United Kingdom, because all vCJD patients tested thus far have been M/M carriers.

Inheritance patterns and phenotypic features of myofibrillar myopathy associated with a BAG3 mutation

Myofibrillar myopathies are a heterogeneous group of neuromuscular disorders characterized by disintegration of myofibrils. The inheritance pattern is commonly autosomal dominant, but there has been a striking absence of secondary cases noted in a BAG3-associated subtype. We studied three families with BAG3 p.Pro209Leu mutation showing a severe phenotype of myofibrillar myopathy and axonal neuropathy with giant axons. In one family, transmission to a pair of siblings has occurred from their asymptomatic father who showed somatic mosaicism. In two other families, neither of the parents was affected or showed detectable level of somatic mosaicism. These observations suggest that the BAG3 variant of myofibrillar myopathy may result from a spontaneous mutation at an early point of embryonic development and that transmission from a mosaic parent may occur more than once. The study underlines the importance of parental evaluation as it may have implications for genetic counseling.

In-frame deletion in the seventh immunoglobulin-like repeat of filamin C in a family with myofibrillar myopathy.

Myofibrillar myopathies (MFMs) are an expanding and increasingly recognized group of neuromuscular disorders caused by mutations in DES, CRYAB, MYOT, and ZASP. The latest gene to be associated with MFM was FLNC; a p.W2710X mutation in the 24th immunoglobulin-like repeat of filamin C was shown to be the cause of a distinct type of MFM in several German families. We studied an International cohort of 46 patients from 39 families with clinically and myopathologically confirmed MFM, in which DES, CRYAB, MYOT, and ZASP mutations have been excluded. In patients from an unrelated family a 12-nucleotide deletion (c.2997_3008del) in FLNC resulting in a predicted in-frame four-residue deletion (p.Val930_Thr933del) in the seventh repeat of filamin C was identified. Both affected family members, mother and daughter, but not unrelated control individuals, carried the p.Val930_Thr933del mutation. The mutation is transcribed and, based on myopathological features and immunoblot analysis, it leads to an accumulation of dysfunctional filamin C in the myocytes. The study results suggest that the novel p.Val930_Thr933del mutation in filamin C is the cause of MFM but also indicate that filamin C mutations are a comparatively rare cause of MFM.

Identical de novo Mutation in the Type 1 Ryanodine Receptor Gene Associated with Fatal, Stress-induced Malignant Hyperthermia in Two Unrelated Families

Background: Mutations in the type 1 ryanodine receptor gene (RYR1) result in malignant hyperthermia, a pharmacogenetic disorder typically triggered by administration of anesthetics. However, cases of sudden death during exertion, heat challenge, and febrile illness in the absence of triggering drugs have been reported. The underlying causes of such drug-free fatal “awake” episodes are unknown. Methods: De novo R3983C variant in RYR1 was identified in two unrelated children who experienced fatal, nonanesthetic awake episodes associated with febrile illness and heat stress. One of the children also had a second novel, maternally inherited D4505H variant located on a separate haplotype. Effects of all possible heterotypic expression conditions on RYR1 sensitivity to caffeine-induced Ca2+ release were determined in expressing RYR1-null myotubes. Results: Compared with wild-type RYR1 alone (EC50 = 2.85 ± 0.49 mM), average (±SEM) caffeine sensitivity of Ca2+ release was modestly increased after coexpression with either R3983C (EC50 = 2.00 ± 0.39 mM) or D4505H (EC50 = 1.64 ± 0.24 mM). Remarkably, coexpression of wild-type RYR1 with the double mutant in cis (R3983C-D4505H) produced a significantly stronger sensitization of caffeine-induced Ca2+ release (EC50 = 0.64 ± 0.17 mM) compared with that observed after coexpression of the two variants on separate subunits (EC50 = 1.53 ± 0.18 mM). Conclusions: The R3983C mutation potentiates D4505H-mediated sensitization of caffeine-induced RYR1 Ca2+ release when the mutations are in cis (on the same subunit) but not when present on separate subunits. Nevertheless, coexpression of the two variants on separate subunits still resulted in a ∼2-fold increase in caffeine sensitivity, consistent with the observed awake episodes and heat sensitivity.

Sporadic cardiac and skeletal myopathy caused by a de novo desmin mutation

Desmin myopathy is a familial or sporadic disorder characterized by intracytoplasmic accumulation of desmin in the muscle cells. We and others have previously identified desmin gene mutations in patients with familial myopathy, but close to 45% of the patients do not report previous family history of the disease. The present study was conducted to determine the cause of desmin myopathy in a sporadic patient presenting with symmetrical muscle weakness and atrophy combined with atrioventricular conduction block requiring a permanent pacemaker. A novel heterozygous R406W mutation in the desmin gene was identified by sequencing cDNA and genomic DNA. Expression of a construct containing the patients mutant desmin cDNA in SW13 (vim‐) cells demonstrated a high pathogenic potential of the R406W mutation. This mutation was not found in the patients father, mother or sister by sequencing and restriction analysis. Testing with five microsatellite markers and four intragenic single nucleotide polymorphisms excluded alternative paternity. Haplotype analysis indicates that the patients father was germ‐line mosaic for the desmin mutation. We conclude that de novo mutations in the desmin gene may be the cause of sporadic forms of desmin‐related cardiac and skeletal myopathy.

Novel PRNP sequence variant associated with familial encephalopathy.

Human transmissible spongiform encephalopathies (TSEs) are a group of chronic progressive neurodegenerative disorders that may be hereditary, infectious, or sporadic. Hereditary TSEs are associated with mutations in the PRNP gene on chromosome 20p12-pter. We report on a family in which seven patients developed limb and truncal ataxia, dysarthria, myoclonic jerks, and cognitive decline. The age of onset in the 30s, 40s, or 50s, prolonged disease duration, cerebellar atrophy on imaging, and the presence of synchronic periodic discharges on electroencephalogram suggested a familial encephalopathy resembling Gerstmann-Sträussler-Scheinker disease. A novel H187R mutation has been identified in affected, but not in unaffected, family members or unrelated controls suggesting a pathogenic role for this mutation. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 88:653-656, 1999. Published 1999 Wiley-Liss, Inc.

Nemaline myopathy type 6: clinical and myopathological features.

Nemaline myopathy (NEM) is one of the most common congenital myopathies. A unique subtype, NEM6, maps to chromosome 15q21‐q23 in two pedigrees, but the causative gene has not been determined. We conducted clinical examination and myopathological studies in a new NEM family. Genotyping and gene screening were accomplished by searching known and 18 new candidate genes. The disease started in childhood by affecting proximal and distal muscles and causing slowness of movements. Muscle biopsies showed numerous nemaline rods and core‐like formations. Suggestive linkage to chromosome 15q22‐q23 was established. Genes known to be mutated in NEM or core‐rod myopathy were screened and excluded. No pathogenic mutations were identified in other candidate genes. The disease in this Spanish family was classified as NEM6. It is phenotypically similar and probably allelic to the two previously reported NEM6 pedigrees. Further studies of these families will lead to the identification of the NEM6 gene. Muscle Nerve 42: 901–907, 2010

The Functional Impact of Cis Acting Ryanodine Receptor Type 1 Mutations in a Child with a Fatal Spontaneous MH Event

Mutations in the RYR1 gene result in MH, a pharmacogenetic disorder of skeletal muscle typically triggered by administration of anesthetics. However, cases of sudden death during exertion, heat challenge, and febrile illness in the absence of triggering drugs have been reported in individuals with RYR1 gene variants. We describe the clinical history and genetic analysis of a child that suffered a fatal, non-anesthetic MH episode associated with febrile illness who was heterozygous for two novel RYR1 variants where one variant, R3983C, occurred de-novo and another, D4505H, was inherited. Effects of the two variants on RYR1 sensitivity to activation by caffeine were assessed following expression in RYR1-null myotubes. The single (R3983C and D4505H) and double (R3983C-D4505H) variants were engineered into a full-length rabbit RYR1 cDNA and introduced into RyR1-null myotubes via nuclear microinjection. Effects of the different heterotypic expression conditions (WT+R3983C, WT+D4505H, R3983C+D4505H, and WT+R3983C-D4505H) on RYR1 activation by caffeine were determined in indo-1-loaded myotubes. Compared to WT RYR1 alone (EC50 = 1.5 mM), the caffeine sensitivity of RYR1 Ca2+ release was only modestly increased following co-expression of either R3983C (EC50 = 1.3 mM) or D4505H (EC50 = 0.9 mM). Remarkably, co-expression of WT RYR1 with the double mutant in cis (R3983C-D4505H) produced a much stronger sensitization of caffeine-induced release (EC50 = 0.3) than that observed following co-expression of the two variants on separate subunits (EC50 = 0.9). Thus, the R3893C mutation potentiates D4505H-mediated sensitization of caffeine-induced RYR1 Ca2+ release when these mutations are in cis or present on the same subunit, but not when present on separate subunits. These results indicate that allelic segregation can be a critical, and heretofore unappreciated, pathogenic factor in compound heterozygous MH individuals.