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

• ACTA1 missense variant in mosaic form causes milder asymmetric myopathy. • Myopathy caused by mosaic missense ACTA1 variant may become milder over time. • Nucleotide c.739 G is prone to recurrent mutations causing a glycine to arginine change. • Asymmetric phenotypes may call for reconsideration in MPS pipeline construction. 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. [ABSTRACT FROM AUTHOR]