Erynn Gordon

Integrated DNA, cDNA, and protein studies in Becker muscular dystrophy show high exception to the reading frame rule

Becker muscular dystrophy (BMD) is a milder form of X‐linked Duchenne muscular dystrophy (DMD). Here, we report a study of 75 patients with immunoblot and/or immunostaining findings of muscle biopsy consistent with BMD (abnormal dystrophin). We utilized multiplex ligation dependent probe amplification (MLPA) on genomic DNA (gDNA) to screen all 79 exons for both deletions and duplications. A total of 19 patients testing negative for MLPA mutations were tested for mRNA splicing abnormalities using cDNA‐MLPA on muscle biopsy. Complete cDNA sequencing was done on MLPA‐negative patients. We identified disease‐causing mutations in 66 (88%) of the patients. Of the mutation‐positive patients, 42 (64%) showed deletions of one or more exons, 14 (21%) showed duplications, and 10 (15%) showed various mutations detected by cDNA‐MLPA and sequencing studies. We found a high rate of “exceptions” to the reading frame rule in this BMD series (out‐of‐frame BMD; 17/56 deletions/duplications; 30%). This was partly explained by the high incidence of 5′ gene deletions in BMD patients (a region known to be a hotspot for exceptions), and due to complex splicing patterns in which a subset of transcripts showed deletions larger than gDNA (exon‐skipping). Comparing our findings in BMD to previously published DMD data, BMD patients have higher proportions of duplications, a different distribution of mutations, and higher exception to the reading frame rule. Hum Mutat 29(5), 728–737, 2008. 2008 Wiley‐Liss, Inc.

Genetic and Clinical Heterogeneity in eIF2B-Related Disorder

Eukaryotic initiation factor 2B (eIF2B)-related disorders are heritable white matter disorders with a variable clinical phenotype (including vanishing white matter disease and ovarioleukodystrophy) and an equally heterogeneous genotype. We report 9 novel mutations in the EIF2B genes in our subject population, increasing the number of known mutations to more than 120. Using homology modeling, we have analyzed the impact of novel mutations on the 5 subunits of the eIF2B protein. Although recurrent mutations have been found at CpG dinucleotides in the EIF2B genes, the high incidence of private or low frequency mutations increases the challenge of providing rapid genetic confirmation of this disorder, and limits the application of EIF2B screening in cases of undiagnosed leukodystrophy.

A C-terminal skeletal muscle sodium channel mutation associated with myotonia disrupts fast inactivation

Missense mutations in the skeletal muscle sodium channel α‐subunit gene (SCN4A) are associated with a group of clinically overlapping diseases caused by alterations in the excitability of the sarcolemma. Sodium channel defects may increase excitability and cause myotonic stiffness or may render fibres transiently inexcitable to produce periodic paralysis. A patient with cold‐aggravated myotonia did not harbour any of the common SCN4A mutations. We therefore screened all 24 exons by denaturing high‐performance liquid chromatography, followed by direct sequencing. Two novel missense changes were found with predicted amino acid substitutions: T323M in the DIS5‐S6 loop and F1705I in the intracellular C‐terminus. The functional impact of these substitutions was assessed by recording whole‐cell Na+ currents from transiently transfected HEK293 cells. T323M currents were indistinguishable from wild‐type (WT). Fast inactivation was impaired for F1705I channels, as demonstrated by an 8.6‐mV rightwards shift in voltage dependence and a two‐fold slowing in the rate of inactivation. Recovery from fast inactivation was not altered, nor was there an increase in the persistent current after a 50‐ ms depolarization. Activation and slow inactivation were not appreciably affected. These data suggest that T323M is a benign polymorphism, whereas F1705I results in fast inactivation defects, which are often observed for myotonia. This is the first example of a C‐terminal mutation in SCN4A associated with human disease. Like the cardiac disorders (long QT syndrome type 3 or Brugada syndrome) and generalized epilepsy with febrile seizures plus (GEFS+) associated with C‐terminal mutations in other NaV channels, the primary effect of F1705I was a partial disruption of fast inactivation.

The ABC's of limb-girdle muscular dystrophy: alpha-sarcoglycanopathy, Bethlem myopathy, calpainopathy and more.

Limb-girdle muscular dystrophy is a class of disorders encompassing many forms of this disease. Variation exists between the inheritance patterns, genes responsible, course of disease and symptoms, with the cohesive factor among these disorders being the predominance of proximal muscle weakness. Here we review each form of limb-girdle muscular dystrophy with attention to molecular genetics, clinical features, inheritance, and diagnostic issues pertaining to each primary genetic cause.

Nondisease genetic testing: reporting of muscle SNPs shows effects on self-concept and health orientation scales

The purpose of this study was to assess the impact of genetic self-knowledge (nondisease genotype information) on individual self-concept and Health Orientation Scale (HOS). Adult volunteers (n=257) were recruited from an ongoing genetic association study identifying muscle quantitative trait loci (QTLs). Participants completed psychosocial assessments before and after 12 weeks of resistance training of the nondominant arm. At study exit, a genetic counselor informed participants of genetic test results on three to four genes that have an association with muscle-related traits, and counseled subjects on the potential significance of these findings. The second psychosocial assessment was performed immediately following this counseling session. The Tennessee Self-Concept Scale v.2 (TSCS:2) and the HOS showed female subjects to have a significantly greater positive change between first and second assessments, relative to male subjects. Most self-concept subscales improved significantly, when ‘neutral’ genotypes (no anticipated beneficial or deleterious impact) were reported, compared to positive genotypes. TSCS:2 subscales showing improvement included: total (P=0.013); physical (P=0.004); satisfaction (P=0.019); and behavioral (P=0.047). HOS subscales showing improvement included health image concern (P=0.006); and health expectations (P=0.047). In conclusion, these results suggest that genetic self-knowledge affects self-concept, consistent with the ‘attribution’ theory. Individuals who received neutral genetic information attributed positive changes from the exercise program to their own abilities, while those who received positive information were more likely to attribute positive changes to their genetics. This study is limited by the ability to determine the direction of the impact of nondisease genetic information presented to participants.