Cofilin-2, encoded by CFL2, is an actin-binding protein essential for regulating actin filament dynamics in skeletal muscle. Biallelic variants in CFL2 are associated with an ultra-rare, early-onset myopathy typically presenting as nemaline myopathy. Only 10 patients have been described to date from five unrelated families. Here, we describe two new cases from two unrelated families. The first proband presented clinically with rigid spine syndrome and a biopsy keeping with nemaline myopathy. The second proband presented with a relatively mild congenital myopathy which became rapidly progressive in the fourth decade, the muscle biopsy showed cytoplasmic bodies, internal nuclei and ringbinden. Exome and genome sequencing revealed three novel biallelic missense variants in CFL2, a homozygous c.115 T > G; p.(Cys39Gly) in the proband of Family 1, and bi-allelic heterozygous c.256G > A:(p.Asp86Asn), and c.283A > G (p.Lys95Glu) variants in the proband of Family 2. We characterised the effects of these substitutions using an in vitro F-actin depolymerisation assay and showed all three were associated with significantly reduced filamentous actin depolymerisation rates compared to the wildtype. Taken together, our findings are highly suggestive of a CFL2-related disease in these patients. Since CFL2-related myopathies are ultrarare, the application of ACMG/AMP guidelines and the diagnostic reportability of CFL2 variants identified in patients remains a challenge. The actin depolymerisation assay may be useful to elucidate the impact and pathogenicity of additional CFL2 variants and has the potential to guide variant classification in future.

There is a range of phenotypes associated with pathogenic variants in the FHL1 gene, including X-linked dominant scapuloperoneal myopathy, X-linked myopathy with postural muscle atrophy, reducing body myopathy, Emery-Dreifuss muscular dystrophy, rigid-spine syndrome, and hypertrophic cardiomyopathy. This gene encodes the four-and-a-half LIM domain protein 1 which is highly expressed in skeletal and cardiac muscle. The function of this protein includes influencing cellular architecture, myoblast differentiation, mechanotransduction, and skeletal muscle fiber size. We report a case of a 6-year-old boy with a novel FHL1 gene mutation who presented to the neuromuscular clinic for evaluation of stiffness, joint contractures, and mild proximal weakness. Symptoms first noted in the newborn period have been slowly progressive. The child's presentation has not been described before and represents a new clinical phenotype within the spectrum of FHL1-related disorders.

Rigid spine syndrome is a rare childhood-onset myopathy characterized by slowly progressive or non-progressive scoliosis, neck and spine contractures, hypotonia and respiratory insufficiency. Biallelic variants in SELENON account for most cases of rigid spine syndrome, however, the underlying genetic cause in some patients remains unexplained. We used exome and genome sequencing to investigate the genetic basis of rigid spine syndrome in patients without a genetic diagnosis. In five patients from four unrelated families, we identified biallelic variants in HMGCS1 (3-hydroxy-3-methylglutaryl-coenzyme A synthase). These included six missense variants and one frameshift variant distributed throughout HMGCS1. All patients presented with spinal rigidity primarily affecting the cervical and dorso-lumbar regions, scoliosis and respiratory insufficiency. Creatine kinase levels were variably elevated. The clinical course worsened with intercurrent disease or certain drugs in some patients; one patient died from respiratory failure following infection. Muscle biopsies revealed irregularities in oxidative enzyme staining with occasional internal nuclei and rimmed vacuoles. HMGCS1 encodes a critical enzyme of the mevalonate pathway and has not yet been associated with disease. Notably, biallelic hypomorphic variants in downstream enzymes including HMGCR and GGPS1 are associated with muscular dystrophy resembling our cohort's presentation. Analyses of recombinant human HMGCS1 protein and four variants (p.S447P, p.Q29L, p.M70T, p.C268S) showed that all mutants maintained their dimerization state. Three of the four mutants exhibited reduced thermal stability, and two mutants showed subtle changes in enzymatic activity compared to the wildtype. Hmgcs1 mutant zebrafish displayed severe early defects, including immobility at 2 days and death by Day 3 post-fertilisation and were rescued by HMGCS1 mRNA. We demonstrate that the four variants tested (S447P, Q29L, M70T and C268S) have reduced function compared to wild-type HMGCS1 in zebrafish rescue assays. Additionally, we demonstrate the potential for mevalonic acid supplementation to reduce phenotypic severity in mutant zebrafish. Overall, our analyses suggest that these missense variants in HMGCS1 act through a hypomorphic mechanism. Here, we report an additional component of the mevalonate pathway associated with disease and suggest biallelic variants in HMGCS1 should be considered in patients presenting with an unresolved rigid spine myopathy phenotype. Additionally, we highlight mevalonoic acid supplementation as a potential treatment for patients with HMGCS1-related disease.

Mutations in the FHL1 gene have been associated with a diverse spectrum of X-linked diseases affecting skeletal and cardiac muscle. Six clinically distinct human myopathies can be recognized, including reducing body myopathy (RBM), X-linked dominant scapuloperoneal myopathy (SPM), X-linked myopathy with postural muscle atrophy (XMPMA), rigid spine syndrome (RSS), hypertrophic cardiomyopathy (HCM) and type 6 Emery- Dreifuss muscular dystrophy (EDMD). The core features of all described FHL1opathies are mostly scapuloperoneal muscle weakness, rigid spine, cardiac involvement, and cytoplasmic bodies in the muscle biopsy. We systematically reviewed the medical literature between the years 2000 and 2024 regarding the phenotype and genotype description of FHL1-associated myopathies. Here, we report two novel patients presenting with an X-linked myopathy with postural muscle atrophy (XMPMA) caused by the c.672 C > G FHL1 gene mutation. When encountering these features in a patient, one may consider screening for an FHL1 mutation. The course ranges from a severe fatal course with early onset to very mild features with late onset. Once a dystrophinopathy has been excluded, increased CK values in male subjects with possible X-linked inheritance should always trigger FHL1 gene screening.