Gonadal mosaicism for ACTA1 gene masquerading as autosomal recessive nemaline myopathy

A recurrent ACTA1 amino acid change in mosaic form causes milder asymmetric myopathy

We describe three patients with asymmetric congenital myopathy without definite nemaline bodies and one patient with severe nemaline myopathy. In all four patients, the phenotype had been caused by pathogenic missense variants in ACTA1 leading to the same amino acid change, p.(Gly247Arg). The three patients with milder myopathy were mosaic for their variants. In contrast, in the severely affected patient, the missense variant was present in a de novo, constitutional form. The grade of mosaicism in the three mosaic patients ranged between 20 % and 40 %. We speculate that the milder clinical and histological manifestations of the same ACTA1 variant in the patients with mosaicism reflect the lower abundance of mutant actin in their muscle tissue. Similarly, the asymmetry of body growth and muscle weakness may be a consequence of the affected cells being unevenly distributed. The partial improvement in muscle strength with age in patients with mosaicism might be due to an increased proportion over time of nuclei carrying and expressing two normal alleles.

Pediatric Nemaline Myopathy: A Systematic Review Using Individual Patient Data

Nemaline myopathy is a skeletal muscle disease that affects 1 in 50 000 live births. The objective of this study was to develop a narrative synthesis of the findings of a systematic review of the latest case descriptions of patients with NM. A systematic search of MEDLINE, Embase, CINAHL, Web of Science, and Scopus was performed using Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) guidelines using the keywords pediatric, child, NM, nemaline rod, and rod myopathy. Case studies focused on pediatric NM and published in English between January 1, 2010, and December 31, 2020, in order to represent the most recent findings. Information was collected about the age of first signs, earliest presenting neuromuscular signs and symptoms, systems affected, progression, death, pathologic description, and genetic changes. Of a total of 385 records, 55 case reports or series were reviewed, covering 101 pediatric patients from 23 countries. We review varying presentations in children ranging in severity despite being caused by the same mutation, in addition to current and future clinical considerations relevant to the care of patients with NM. This review synthesizes genetic, histopathologic, and disease presentation findings from pediatric NM case reports. These data strengthen our understanding of the wide spectrum of disease seen in NM. Future studies are needed to identify the underlying molecular mechanism of pathology, to improve diagnostics, and to develop better methods to improve the quality of life for these patients.

Lessons Learned from Large-Scale, First-Tier Clinical Exome Sequencing in a Highly Consanguineous Population

We report the results of clinical exome sequencing (CES) on >2,200 previously unpublished Saudi families as a first-tier test. The predominance of autosomal-recessive causes allowed us to make several key observations. We highlight 155 genes that we propose to be recessive, disease-related candidates. We report additional mutational events in 64 previously reported candidates (40 recessive), and these events support their candidacy. We report recessive forms of genes that were previously associated only with dominant disorders and that have phenotypes ranging from consistent with to conspicuously distinct from the known dominant phenotypes. We also report homozygous loss-of-function events that can inform the genetics of complex diseases. We were also able to deduce the likely causal variant in most couples who presented after the loss of one or more children, but we lack samples from those children. Although a similar pattern of mostly recessive causes was observed in the prenatal setting, the higher proportion of loss-of-function events in these cases was notable. The allelic series presented by the wealth of recessive variants greatly expanded the phenotypic expression of the respective genes. We also make important observations about dominant disorders; these observations include the pattern of de novo variants, the identification of 74 candidate dominant, disease-related genes, and the potential confirmation of 21 previously reported candidates. Finally, we describe the influence of a predominantly autosomal-recessive landscape on the clinical utility of rapid sequencing (Flash Exome). Our cohort's genotypic and phenotypic data represent a unique resource that can contribute to improved variant interpretation through data sharing.

Update on the Genetics of Congenital Myopathies

The congenital myopathies form a large clinically and genetically heterogeneous group of disorders. Currently mutations in at least 27 different genes have been reported to cause a congenital myopathy, but the number is expected to increase due to the accelerated use of next-generation sequencing methods. There is substantial overlap between the causative genes and the clinical and histopathologic features of the congenital myopathies. The mode of inheritance can be autosomal recessive, autosomal dominant or X-linked. Both dominant and recessive mutations in the same gene can cause a similar disease phenotype, and the same clinical phenotype can also be caused by mutations in different genes. Clear genotype-phenotype correlations are few and far between.

Parental mosaicism in RYR1-related Central Core Disease

Myopathies due to mutations in the skeletal muscle ryanodine receptor (RYR1) gene are amongst the most common non-dystrophic neuromuscular disorders and have been associated with both dominant and recessive inheritance. Several cases with apparently de novo dominant inheritance have been reported. Here we report two siblings with features of Central Core Disease (CCD) born to unaffected parents. Genetic testing revealed a heterozygous dominant RYR1 c.14582G>A (p. Arg4861His) mutation previously identified in other CCD pedigrees. The variant was absent in blood from the asymptomatic mother but detected at low but variable levels in blood- and saliva-derived DNA from the unaffected father, suggesting that this mutation has arisen as a paternal post-zygotic de novo event. These findings suggest that parental mosaicism should be considered in RYR1-related myopathies, and may provide one possible explanation for the marked intergenerational variability seen in some RYR1 pedigrees.