Pathologic Variability Associated With Phenotypic Differences in Siblings With <scp>ACTA1</scp> Myopathy

Pathogenic variants in the ACTA1 gene cause a diverse range of myopathies. These variants are often associated with autosomal dominant nemaline myopathy, responsible for 15%–25% of typical cases and about 50% of lethal forms. The histopathological spectrum of ACTA1 myopathies includes cores, cap-like structures, fiber-type disproportion, type I fiber predominance, dystrophic features, and zebra bodies [1, 2]. A rare pathological finding of cytoplasmic bodies has been reported in three severe cases [1]. Here we describe two maternal half-brothers (Table 1) with the same ACTA1 genotype but differing phenotypes, with the more severely affected sibling exhibiting cytoplasmic bodies in muscle pathology. Both siblings were born at term after uneventful pregnancies. The older sibling (sibling 1) was hospitalized at 2 months old for feeding difficulties and was noted to have hypotonia and pectus excavatum. Family history was notable for leg weakness in mother. Mother exhibited proximal lower > upper extremity weakness and waddling gait at age 22 years. Genetic testing for spinal muscular atrophy and myotonic dystrophy was unremarkable. He required frequent hospitalizations between 2 and 10 months old for respiratory distress associated with infection and feeding difficulties. Whole exome sequencing at 5 months old revealed a maternally inherited heterozygous likely pathogenic variant (p.Gly44Cys c.130G>T) in ACTA1. Muscle biopsy at 10 months of age showed myopathic changes with intranuclear inclusions and nemaline rods. Myopathic changes were nonspecific and included increased endomysial and perimysial connective tissue, fiber size variation due to the presence of variably rounded atrophic myofibers, a few regenerating fibers, and increased numbers of myofibers with internal nuclei (Figure 1). He continued to exhibit hypotonia, mild weakness, and gross motor delay but had normal speech, language, and cognitive development. He has not had any further hospitalizations. Sibling 2 presented at 3 weeks old after an apneic episode and experienced hypoxic brain injury due to cardiac arrest from pulseless electrical activity, requiring a 2-month hospitalization. Brain magnetic resonance imaging revealed bilateral symmetric areas of diffusion restriction in the perirolandic cortex, centrum semiovale, posterior lentiform nucleus, and thalamus. Targeted ACTA1 testing confirmed the same change found in his half-brother. Whole exome sequencing did not identify additional variants. He was readmitted immediately for respiratory failure, leading to an 8-month hospital stay that included gastrostomy tube placement and tracheostomy with continuous ventilatory support. He was profoundly hypotonic and weak and was eventually discharged to a long-term care facility. Since then, he has had multiple hospitalizations for acute on chronic respiratory failure. His muscle biopsy at 2 years of age showed findings similar to his half-brother and, additionally, numerous cytoplasmic bodies (Figure 1). These cytoplasmic bodies were not seen in sibling 1's biopsy. He continued to experience severe motor delay and profound hypotonia and was non-ambulatory at his last follow-up at 4.5 years old. The likely pathogenic G44C variant identified in the half-brothers has not been previously reported and was absent in large population cohorts. This variant affects a highly conserved amino acid position across species. In silico analysis predicted that G44C may disrupt a natural splice acceptor site in intron 2, potentially leading to abnormal splicing. RNA sequencing could not be performed in our patients to confirm the abnormal splicing. Similar missense variants in the same or nearby residues (P40L, Q43R, G44V, V45F, M46T) have been linked to nemaline myopathy, highlighting the importance of this region in the protein's function [3]. Additionally, missense variants close by (Q41R, G42V) are associated with a milder phenotype, with patients remaining ambulant and living up to their 40s [4-6]. The family described here exhibits notable phenotype variability, with the younger sibling's severe case possibly linked to the presence of cytoplasmic bodies, though a contribution from hypoxic injury cannot be fully excluded. These cytoplasmic bodies, originating from Z-disk material, suggest rapid turnover or disruption of myofibrillar components and are rare in ACTA1 myopathy. Previously, three severe ACTA1 myopathy cases with the c.282C>A; p.Asn94Lys variant were reported, all showing cytoplasmic bodies but no nemaline rods upon histopathological examination of muscle biopsies. Two of these patients died at 6 months of age [1]. In summary, these cases suggest a relationship between cytoplasmic bodies and severe ACTA1 myopathy, while also underscoring the phenotypic and histopathological variability within a family. Robert Kiss: data collection and drafting manuscript. Steven A. Moore: data collection, concept, and revising manuscript. Vikki Stefans: revising manuscript. Murat Gokden: data collection, revising manuscript. Aravindhan Veerapandiyan: study concept, drafting and revising manuscript. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this report is consistent with those guidelines. A.V. has received compensation for ad-hoc advisory boards/consulting activity with Novartis, Sarepta therapeutics, PTC therapeutics, Pfizer, Catalyst, Lupin, Entrada therapeutics, Percheron, Edgewise, Regenxbio, solid biosciences, Precisionbio, Keros, Gruenenthal, Biogen, Avidity, Entrada, and Italfarmaco outside of the submitted work. Other authors declare no conflicts of interest. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.