Reducing Body Myopathy in Female Patients With FHL1 Variants Showing Rapid and Severe Evolution Mimicking Inflammatory Myopathy

Abstract

Background and Objectives

Reducing body myopathy (RBM) is a rare inherited disorder, due to pathogenic variant in the FHL1 gene. The disease is characterized by protein aggregates in myocytes strongly stained with menadione-nitro blue tetrazolium with α-glycerophosphate and immunoreactive for FHL1 as myopathologic hallmarks. RBM is an X-linked dominant disorder, with the most severe cases occurring in hemizygous sporadic male patients, who usually present with early onset and rapid progression. This study aimed to comprehensively analyse a cohort of FHL1 female patients presenting with a severe and rapidly progressive phenotype clinically mimicking an inflammatory idiopathic myopathy.

Methods

This is retrospective study of a cohort of female patients with RBM harboring pathogenic variants in FHL1 gene from France and Italy. Data regarding clinical onset and progression, myopathologic features, muscle imaging, and genetic testing have been collected retrospectively and analyzed from medical records.

Result

We present 5 young girls and women with FHL1 pathogenic variants presenting a mean age at onset of 13 years. All patients showed an asymmetric pattern of muscle weakness evolving in bilateral proximo-distal involvement and restrictive respiratory syndrome, rapidly leading to severe tetraparesis and loss of ambulation after a mean period of 6.2 years in 4 of them. Serum CK level at onset was slightly elevated with a mean value of 539 UI/L. Muscle MRI revealed severe asymmetric proximo-distal involvement with STIR positive sequences. Muscle biopsies showed reducing bodies and prominent autophagic material accumulation.

Discussion

FHL1-related reducing body myopathy can present with a rapidly progressive muscle weakness mimicking an inflammatory myopathy also in female patient. Muscle biopsy is an useful tool in the contest of rapidly progressive myopathy to distinguish between genetic myopathies and treatable inflammatory myopathy.

Introduction

Congenital myopathies (CMYO) are a heterogeneous group of disorders that may present with a variable phenotypic spectrum ranging from no or slowly progressive muscle weakness to cases with severe progression and loss of ambulation.^1-3 The management of rapidly progressive myopathies still represents a significant clinical challenge especially in acute setting where distinguishing between idiopathic inflammatory myopathies (IIM) and hereditary myopathies is essential to confirm the diagnosis and initiating a targeted therapy, when indicated. Reducing body myopathy (RBM) is a X-linked dominant muscle disorder⁴ due to pathogenic variant on the FHL1 gene located on the Xq26.3 region of the X chromosome and encoding the Four and a Half LIM Domain Protein 1 (FHL1).^5,6 FHL1 is highly expressed in cardiac and skeletal muscle, and it is localized in the I-band and in the M-line of the sarcomere. FHL1 is also involved in protein interactions, nuclear translocation of transcription factors, and muscle hypertrophy.⁷

RBM is defined by the presence in muscle fibers of intracytoplasmic aggregates that reduce nitro-blue tetrazolium (NBT) and stain intensely with menadione-NBT, corresponding to reducing bodies.^4,8 RBM typically presents with congenital or childhood onset, although rare late-onset cases have been reported.^9,10 The clinical spectrum is broad, ranging from mild, often asymmetric proximal or periscapular weakness, to severe, progressive muscle weakness with loss of ambulation, respiratory failure, and cardiac involvement.⁵ In contrast to male patients, where RBM often follows a severe and rapidly progressive course,¹¹ the clinical presentation in women depends on the inheritance. Women with inherited FHL1 variants are usually asymptomatic or presenting a late onset,¹² whereas sporadic de novo RBM in women can presents with early onset in childhood and a rapidly progressive muscle weakness, leading to respiratory failure, similar to that seen in men.^4,7 However, owing to the lack of a clear genotype-phenotype correlation and the mechanism of X-chromosome inactivation, this distinction can be misleading.

In line with this, we report 5 female patients with FHL1-related reducing body myopathy, 4 de novo variants, and 1 inherited variant, presenting with disease severity and progression that may clinically resemble to cases of idiopathic inflammatory myopathies.

Material and Methods

Patients

We retrospectively studied 5 unrelated women presenting RBM with pathogenic FHL1 variants from one pediatric center and 2 adult centers in France (n = 3) and from one pediatric center and one adult center in Italy (n = 2). The mean age at onset was of 13 years (ranging from 3 to 21 years). P3 and P4 has been previously reported by Malfatti et al.⁸ and Sabatelli et al.¹³

Muscle Biopsy

All patients received an open muscle biopsy. The site of biopsy was biceps for P5, peroneal muscle for P4, while the other patients received a deltoid biopsy. P1 and P3 underwent a second biopsy respectively in deltoid and tibialis anterior due to inconclusive results from the first myopathologic analysis. All samples were processed according to standard histochemical and histoenzymatic procedures described by Malfatti et al.⁸ The presence of RBs was assessed in muscle sections processed for menadione-nitro blue tetrazolium with α-glycerophosphate.

The characterization of the inclusions was performed by immunofluorescence using antidesmin and anti-FHL1 antibodies. Muscle sections were cut at 7 μm and were dried at room temperature for 30 minutes under a fan. They were stored at −80°C until use or stained directly after cutting. Samples were fixed in cold methanol for 6 minutes at −20°C and then were washed in 1X PBS. A working area was delimited using a DakoPen (Cat # S200230-2, Agilent, Dako). They were permeabilized with 0.5% Triton for 5 minutes and rinsed with 1X PBS. Samples were incubated with 10% BSA (Sigma-Aldrich) for 30 minutes at room temperature. Overnight incubation at 4°C was conducted with the primary antibodies: anti-FHL1 (1:50) (Cat # AB255828, Abcam) and antidesmin (1/500) (M0760, Dako). The next day, sections were rinsed in 1X PBS and incubated with appropriate conjugated secondary antibodies for 1 hour and half at 37°C. Samples were rinsed again, mounted, and stored protected from light at 4°C until visualization.

Electron microscopy was performed in P1, P4, and P5, and it was processed according to procedures described by Cowling et al.⁶

Genetics

FHL1 gene was analyzed on genomic DNA from peripheral blood using Sanger sequencing approaches or next-generation sequencing (NGS)–based gene panels according to the French and Italian genetic diagnosis laboratories. Variants were interpreted and classified according to the 2015 American College of Medical Genetics Standards and Guidelines, and only patients with pathogenic or likely pathogenic variants were considered as genetically confirmed.¹⁴

Muscle Imaging

Three patients received a whole-body muscle MRI and expert radiologists in neuromuscular disorders analyzed all the images. STIR sequences were used to assess the presence of inflammation and muscle oedema. The Mercuri score was used to assess and quantify the fibro-fatty replacement as a biomarker of muscle destruction in T1-weighted sequences.¹⁵ P4 received a muscle CT, and no muscle imaging studies were available for P5.

Statistical Methods

Descriptive statistics were calculated, including percentages, medians, and ranges.

Standard Protocol Approvals, Registrations, and Patient Consents

Patients or their guardian gave informed consent following the French or Italian legislation. We obtained the informed consent from genetics and muscle biopsy for the 3 adult patients and that of parents for the 2 minor patients, in agreement with local ethical committees (DC-2012-1535 and AC-2012-1536) and with the 1964 Helsinki declaration and its later amendments. Data regarding clinical onset and progression, myopathologic features, genetic testing, creatine kinase (CK) levels, EMG, respiratory and cardiac function were retrieved.

Data Availability

All clinical and laboratory data supporting the findings of this study are reported directly in the article. Additional methodologic details not included in the article (e.g., muscle biopsy procedures and staining protocols) may be shared upon reasonable request to the corresponding author by qualified researchers for the purpose of replicating the study procedures and results.

Results

Clinical Presentation and Phenotype

The main clinical and genetic features of the cohort are summarized in Table 1.

Table 1.

Clinical and Genetic Features of the Cohort

	P1	P2	P3	P4	P5
Age at last evaluation (yrs)	32	9	39	46	8
Age at onset (yrs)	18	3	21	20	4
Variant	Pathogenic c.310T>C (p.Cys104Arg) Heterozygous de novo	Pathogenic c386G>A (p.Cys129Tyr) Heterozygous de novo	Pathogenic c.458G>C (p.Cys153Ser) Heterozygous de novo	Pathogenic c.448T>C (p.Cys150Arg) Heterozygous (maternal inheritance)	Likely pathogenic c.369C>A (p.His123Gln) Heterozygous de novo
Clinical presentation at onset	LL Proximo-distal involvement	UL proximal involvement	LL proximal involvement	UL proximal involvement	UL proximal involvement
Asymmetric pattern	Yes	Yes	Yes	Yes	Yes
Clinical progression	Severe Tetra-paresis	Bilateral Proximo-distal involvement	Bilateral Proximo-distal involvement	Severe Tetra-paresis	Bilateral Proximo-distal involvement
Age at loss of ambulation (yrs)	30	5	26	26	Waddling gait
Axial involvement	Scapula alata, neck flexors/extensors Weakness	Scapula alata, rigid spine, Scoliosis	Scapula alata neck flexors/extensors Weakness, rigid spine	Fixed hyperextended neck, rigid spine	Scapula alata, scoliosis
Joint contractures	Knee, elbow	Hip, knee, ankle	Neck, ankle, elbow, fingers	Knee, elbow, wrist, fingers	Elbow, ankle, wrist
Facial involvement	Masseter retraction, high arched palate	Masseter weakness	No	No	No
Bulbar involvement	No	No	Dysphagia for liquids	No	No

Abbreviations: LL = lower limb; UL = upper limb; yrs = years.

At onset, all patients showed an asymmetric muscle involvement, affecting either lower (P1, P3) or upper (P2, P4 and P5) limbs. Disease progression was rapid with a severe bilateral proximo-distal muscular weakness in all patients, leading to loss of ambulation in 4 of them after a main period of 6.2 years. The progression in motor performance status for each patient is depicted in Figure 1A, showing how only P5 is still ambulant with severe waddling gait. The rapid clinical evolution led physicians to consider the possibility of an acquired immune-mediated myopathy.

Figure 1. Motor and Respiratory Function Evolution.

(A) Motor function evolution. Each patient is identified with a different color. The slope of the curve is proportional to the rate of clinical progression. The horizontal red line represents the threshold corresponding to loss of ambulation. (B) Evolution of respiratory function. Green and blue circles mark age at loss of ambulation in patients P3 and P4, respectively. Red squares mark the age at which noninvasive ventilation (NIV) was started.

An axial involvement was found in all patients in form of rigid spine, scapula alata, scoliosis, or neck flexors/extensors muscular weakness. Joint contractures were variable present at elbow, hip, ankle, and neck (Table 1).

Facial involvement was present in form of masseter retraction, high arched palate, and elongated facies in P1, while P2 showed only a limitation in mouth closing. Bulbar involvement was present only in P3 as a dysphagia for liquids.

Laboratory Investigation

The main laboratory investigations are presented in Table 2.

Table 2.

Main Laboratory Features

	P1	P2	P3	P4	P5
CK level at onset (IU/L reference range 30–200 U/L)	267	700	650	Moderate elevation	384
Myositis-associated autoantibodies and myositis-specific autoantibodies (*)	Negative	Negative	Not done	Not done	Not done
EMG	Low amplitude and short duration motor units, with early recruitment	Low amplitude and short duration motor units, with early recruitment	Low amplitude and short duration motor units with fibrillations potentials and pseudo-myotonic discharges	Low amplitude and short duration motor units, with early recruitment	Low amplitude motor units, with early recruitment
Cardiac assessment (Echocardiography)	Normal	Normal	Normal	Normal	Normal
Respiratory involvement (FVC%)	Restrictive respiratory syndrome FVC 51%	Restrictive respiratory syndrome FRC 61%**	Restrictive respiratory syndrome FVC 13%	Restrictive respiratory syndrome FVC 13%	Restrictive respiratory syndrome FVC 67%
NIV (yes/no)	No	Yes	Yes	Yes	No
Age at NIV onset (yrs)	NA	6	36	34	NA
NIV regimen	NA	10/24 H	Nocturnal NIV	24/24 H	NA

(*) Mi2, MDA5, SAE, NXP2, TIF1-gamma, SRP, HMGCR, PL-7, PL12, Jo1, EJ. FVC: forced vital capacity. FRC**: Functional Residual Capacity. NIV: noninvasive ventilation. NA: nonapplicable.

Four patients showed mildly elevated creatine kinase (CK) levels at onset, with a mean value of 539 IU/L. Myositis-associated autoantibodies and myositis-specific autoantibodies screening performed in 2 patients was negative. EMG revealed a clear myopathic pattern in P1, P2, P4, and P5, while P3 demonstrated a myopathic pattern associated to signs suggestive of irritable myopathy. Cardiac ultrasound findings were unremarkable in all patients. All patients exhibited moderate to severe restrictive respiratory syndrome. The evolution of pulmonary function is depicted in Figure 1B, as percentage of predicted forced vital capacity (FVC) overtime, focusing on the patients with the major respiratory involvement.

P3 and P4 exhibited the most severe restrictive respiratory syndrome in our cohort, with percentages of predicted FVC at the last evaluation of 13%. P3 showed the most rapid decline in respiratory function, with a ΔFVC of −12% over a single year (Figure 1B) between the age of 34 and 35 years. Noninvasive ventilation (NIV) was started in P2, P3, and P4 (Table 2). At the age of 6 years, P2 presented an acute respiratory failure due to bilateral pneumonia requiring hospitalization and admission in intensive care unit. In this context, NIV was started with positive clinical course and it is currently used with a 10/24 hours nocturnal regimen. P3 and P4 started the NIV at the age of 36 and 34 years with a preferential nocturnal regimen and an 24/24 hours regimen, respectively.

Muscle Biopsy

All patients showed the presence of reducing bodies on muscle biopsy, confirming the diagnosis of RBM. The main myopathologic features of P1 and P2 are depicted in Figure 2.

Figure 2. Main Myopathologic Features.

The main myopathologic features of P1 (A, B, E, F, and G) and P2 (C, D, H, I, and J). HE: eosinophilic cytoplasmic aggregates (black arrowheads) (A, C). Menadione α-glycerophosphate: reactive protein material in a cluster of fibers (B, D). Immunofluorescence studies showing desmin aggregates (E, H) and perinuclear FHL1 aggregates (F, I). Merged channels (G, J)

P1 and P2 underwent deltoid muscle biopsy at the age of 27 and 4, respectively. Hematoxylin and eosin (HE) staining highlights a marked increase in fiber size variability in both patients, with some fibers showing a rounded atrophic appearance, more evident in P2. In P1, groups of angular atrophic fibers are observed, compatible with rare foci of neurogenic atrophy, likely related to the previous deltoid biopsy. In addition, some fibers exhibit intracellular eosinophilic aggregates (Figure 2, A and C). The menadione-nitro blue tetrazolium with α-glycerophosphate technique showed the presence of clusters of highly stained fibers, with diffuse or perinuclear intracellular accumulation of reactive protein material corresponding to reducing bodies (Figure 2, B and D). Immunofluorescence confirmed the presence of FHL1 accumulation in the perinuclear aggregates in both patients (Figure 2, F and I).

P3 underwent an initial deltoid muscle biopsy at the age of 24 years that showed only unspecific myopathologic findings. At the age of 25 years, P3 underwent the second muscle biopsy at tibialis anterior. As described by Malfatti et al.,⁸ the main myopathologic lesions in this patient were the presence of pink cytoplasmic inclusions on HE, strongly reactive with menadione-NBT and corresponding to reducing bodies. The immunofluorescence confirms the FHL1 accumulation in cytoplasm and perinuclear region. Myopathologic analysis of P4 is described by Sabatelli et al.¹³ The patient underwent a peroneal muscle biopsy at the age of 34 years, showing severe fiber size variability, nuclear internalization associated with cytoplasmic, and perinuclear menadione-NBT positive aggregates. The same aggregates mostly contain FHL1, p62, and amyloid deposits.

P5 underwent biceps muscle biopsy at the age of 5 years, showing the presence of marked fiber size variability with various hypotrophic fibers. The main myopathologic features in light microscopy were the presence of rimmed vacuoles in multiples fibers and protein aggregates in cytoplasm showing a pattern compatible with a myofibrillar myopathy. Cytoplasmic and perinuclear menadione-NBT positive aggregates were observed in a minority of fibers.

Ultrastructural studies are depicted in Figure 3.

Figure 3. Ultrastructural Myopathologic Features.

EM: electron microscopy. P1 (A, B and C) and P5 (D, E, and F). Reducing bodies surrounding nuclei (A) and lipid droplets (B). (C) Marked sarcomere disorganization. (D, E) Reducing bodies inside protein aggregates. (F) Reducing bodies influencing sarcomere organization. Yellow arrows indicate reducing bodies in all images.

Electron microscopy analysis performed for P1 showed the presence of electron dense protein aggregates with irregular contours and a granular filamentous ultrastructure that resemble truffles, primarily located around the nuclei (Figure 3A) or surrounding lipid droplets (Figure 3B) corresponding to reducing bodies. Marked sarcomere disorganization was also observed (Figure 3C). The ultrastructural analysis of P5 revealed the same electron-dense aggregates inside protein debris (Figure 3, D and E) or next to the sarcomeres (Figure 3F).

Electron microscopy for P4, described by Sabatelli et al.,¹³ revealed the presence of cytoplasmic bodies, tubulofilamentous aggregates and the typical perinuclear protein aggregates.

Muscle Imaging

The main muscle imaging features of P1 and P3 are depicted in Figure 4.

Figure 4. Asymmetric Involvement in Muscle MRI.

Whole-body muscle MRI, T1-weigthed and STIR sequences, axial plane. The green dotted line indicates the spared muscles, compared with the opposite side. Muscle presenting a severe or complete fibro-fatty replacement (Mercuri 3 and 4) are indicate with the red star. Yellow arrow head points muscle showing STIR hypersignal.

P1 underwent a whole-body muscle MRI at 26 years. At the scapular girdle, severe bilateral involvement of the subscapularis and paravertebral muscles was observed, while the supraspinatus muscle showed asymmetric fibro-fatty replacement. In the pelvic girdle sections, marked and global involvement was observed on the right side, affecting the iliopsoas, gluteus maximus, gluteus medius, and gluteus minimus muscles, with sparing of the iliopsoas and gluteus maximus on the left side. At the thigh level, bilateral involvement of the medial and posterior compartments was noted. In the anterior compartment, only the vastus lateralis muscle showed bilateral sparing. At the leg level, relative sparing of the tibialis anterior, peroneus longus, and gastrocnemius lateralis muscles was observed on the left side compared with the opposite, which showed severe global involvement. STIR sequences revealed bilateral muscle hypersignal at vastus lateralis, tibialis anterior, right soleus, and left gastrocnemius lateralis. A whole-body muscle MRI performed for P3 at the age of 22 years showed axial involvement with significant and asymmetric fibro-fatty replacement of the paravertebral muscles at the lumbar level. In the pelvic girdle sections, severe bilateral involvement of the gluteus minimus and gluteus medius muscles was observed. At the thigh level, the semimembranosus and biceps femoris muscles showed bilateral and marked fibro-fatty replacement, with global sparing of the anterior compartment. At the leg level, involvement of the soleus and gastrocnemius medialis muscles was noted, compared with relative sparing of the anterior compartment. STIR hypersignal was noted at tibialis anterior, soleus, and gastrocnemius medialis. P4's muscle CT, described by Sabatelli et al.,¹³ showed global muscle atrophy associated with fat and connective tissue substitution, including the axial muscles, with a minimal sparing of the rectus femoris, the vastus lateralis on the left side, and the peroneus on the right. P2 underwent a whole-body muscle MRI at the age of 4 years, revealing no major fibro-fatty replacement, but only moderate hypotrophy in the medial compartment of the thigh at the distal level. No muscle imaging data of P5 were available.

Genetics

The localization of each variant is depicted in Figure 5.

Figure 5. Pathogenic Variants.

(A) schematic representation of FHL1 protein. The blue box indicates the region of the protein that contains all the variants. (B) 3D representation of variant localization on FHL1 protein (C) Variant localization according to domain. Predicted 3-dimensional structure of protein FHL1 obtained from the AlphaFold database (alphafold.ebi.ac.uk/).

All patients carried pathogenic variants (Class 5) in the FHL1 gene, except for P5, who harbored a likely pathogenic variant (Class 4). P4 showed a pathogenic variant with maternal inheritance,¹³ while all the other patients showed heterozygous de novo variants. P1 carries the c.310T>C (p.Cys104Ar) variant, resulting in a cysteine to arginine substitution at position 104 of the FHL1 protein. This variant located in a critical domain of the protein and is known to disrupt its structure, impairing its function. P2 harbors the c.386G>A (p.Cys129Tyr) variant, causing a cysteine to tyrosine substitution at position 129, which similarly affects the protein's stability and activity. P3 carries the c.458G>C (p.Cys153Ser) variant, leading to the substitution of cysteine with serine at position 153. This variant already reported by Cowling et al.,⁶ is predicted to disrupt the protein structure, resulting in functional impairment. P4 showed a pathogenic missense variant c.448T>C in exon 5 resulting in the replacement at codon 150 of a cysteine residue with an arginine residue (p.Cys150Arg) in a region of FHL1 gene that is highly conserved among species and evolution. P4 harbored the pathogenic variant from her mother who only present mild proximal muscle weakness, in the absence of axial involvement or muscle atrophy; no muscle biopsy was available for P4's mother.¹³ P5 harbors the c.369C>A (p.His123Gln) missense variant resulting in the substitution of histidine with glutamine at position 123 already described by Schessl et al.⁴ This variant, classified as likely pathogenic, is already reported in patients suffering from RBM.

Discussion

RBM is an X-linked dominant disorder in which female patients may present with 2 main phenotypes: (1) benign forms revealing as slowly progressive congenital myopathy, often familial cases with asymmetric muscle weakness and late onset,^12,16 and (2) severe and rapidly progressive forms, usually presenting an early childhood onset and clinical evolution comparable to male patients.^4,7,17 These cases are generally associated to de novo variants with a skewed chromosome X-inactivation. In this work, we present a case series of 5 female patients with FHL1-related RBM, showing an asymmetric muscle weakness at onset, followed by rapid bilateral and severe proximo-distal progression, and loss of ambulation in young age. Rapidly progressive muscle weakness may suggest an idiopathic inflammatory myopathy, which is a potentially treatable condition.

A comprehensive clinical neuromuscular examination is the first step to evaluate a rapidly progressive myopathy. Among the clinical red flags that may help the clinician to differentiate a CMYO with rapid and severe progression from an IIM there are: (1) the presence of facial or extraocular muscle involvement; (2) dysmorphic features such as high-arched palate, myopathic facies, or prominent bulbar symptoms; and (3) asymmetrical muscle involvement, which is rare in the context of IIM. Our patients presented at onset a clear asymmetrical muscle involvement affecting both proximal upper or lower limbs while IIM, except for inclusion body myositis, usually show symmetrical muscle weakness variably associated with myalgias.¹⁸ In the context of inherited myopathy, the clinical phenotype described for our patients may share some features with other genetic disorders. The presence of early onset muscle weakness and joint contractures may raise the clinical suspicion of Emery-Dreifuss muscular dystrophy (EDMD).¹⁹ However, the preferential scapulo-humeral distribution of muscle weakness, the early onset of joint contractures, and, in particular, a prominent cardiac involvement can help the clinician focus on EDMD.¹⁹ All our patients present a moderate to severe restrictive respiratory defect, associated with variable axial involvement. The prominent restrictive syndrome is strongly related to skeletal deformities, especially severe scoliosis and rigid spine, as seen in some congenital myopathies such as SELENON-related CMYO, in which the major axial involvement lead to early respiratory disfunction also in patient remained ambulant.^1,20 Another inherited myopathy presenting with early onset severe respiratory involvement is Early-onset Myopathy with Areflexia, Respiratory Distress and Dysphagia (EMARDD), caused by pathogenic variants in the MEGF10 gene.²¹ Clinically, these patients show neonatal or early onset, associated with areflexia and early-onset dysphagia, compared with patients with RBM.²² In IIM respiratory involvement can be usually seen in form of Interstitial lung disease especially in antisynthetase syndrome and anti–MDA5-positive dermatomyositis.¹⁸ In our cohort, the most significant respiratory involvement was observed in patients P3 and P4, who received a close respiratory follow-up. Both patients presented with clinical onset at the ages of 20 and 21 years, respectively, followed by a rapid progression leading to loss of ambulation at the age of 26. As shown in Figure 1B, P3 and P4 exhibited a marked decline in FVC% after the loss of ambulation, suggesting that exclusive wheelchair use may have an effect on respiratory mechanic in patients with severe axial involvement. However, the lack of serial respiratory assessments before loss of ambulation makes it difficult to estimate the rate of respiratory decline when patients were still ambulant. P3 experienced the most severe deterioration in respiratory function, with a 12% decrease in FVC within a single year. These data emphasize the importance of close respiratory monitoring in such patients and prompt initiation of noninvasive mechanical ventilation if necessary also in young still ambulant patient as P5.

These clinical clues may be useful; however, cases of immuno-mediated necrotizing myopathies with early onset mimicking a limb-girdle muscular dystrophy have been reported,^23,24 thus making comprehensive laboratory investigations essential to discriminate between hereditary and acquired disorders. A complete testing for myositis-specific and myositis-associated autoantibodies (including anti-SRP and anti-HMGCR) is mandatory, but it does not solve the seronegative cases. Anti-HMGCR antibodies are associated with immune-mediated necrotizing myopathy, which can mimic a limb-girdle muscular dystrophy in both pediatric and adult patients.^25,26 The HMGCR gene encodes HMGCR, a key enzyme in the cholesterol synthesis pathway.²⁷ In recent years, pathogenic variants in this gene have been associated with rare cases of LGMD with respiratory involvement.²⁸ The functions of FHL1 in myocytes have not been fully elucidated, and some studies suggest that it could play a role in the pathophysiologic mechanisms of inflammatory myopathies. Recently, Galindo-Feria et al. performed a longitudinal study on a large cohort of 449 patients with idiopathic inflammatory myopathy to determine the prevalence of anti-FHL1 antibodies in this population. They showed at baseline the presence of anti-FHL1 antibodies in 27% of patients, namely, female patients presenting with muscle weakness.²⁹ They also confirmed that anti-FHL1+ patients correlate with higher CK level and worst clinical assessment. The effect of anti-FHL1 antibodies on muscle phenotype in the context of inflammatory myopathies has already been proposed by Albrecht et al.³⁰ showing how patients anti-FHL1+ presented higher necrosis and fibro-fatty replacement in muscle biopsy compared with negative patients. To assess the potential pathogenic role of anti-FHL1 immunity in IIM, the authors used a MHC class I-dependent mouse model (genetically prone to inflammatory myopathy) compared with control mice. Mice prone to inflammatory myopathy that were immunized with FHL1 developed more severe muscle weakness, significant weight loss, and higher mortality than control mice immunized with FHL1.³⁰

In the context of laboratory studies, muscle MRI may help distinguish between idiopathic inflammatory myopathies and hereditary myopathies. Muscle hypersignal in STIR sequences, highlighting inflammation and muscle oedema, could be a common feature seen in both inflammatory and genetic myopathies (Figure 4) as marker of disease activity. By contrast, a specific pattern of fibro-fatty replacement in T1 sequences, with massively involved muscles next to spared muscles is typical of hereditary disorders, although anti-SRP necrotizing myopathies could easily present a fibro-fatty substitution and atrophy on muscle imaging.³¹ In our cohort, muscle imaging revealed an asymmetric pattern of fibro-fatty replacement with relative sparing of gluteus maximus, gracilis, and gastrocnemius medialis, compatible with studies reported in literature focusing on FHL1-related RBM.^32,33 However, in the context of a rapidly progressive myopathy, limited accessibility to muscle MRI restricts its use in the acute phase. In cases where the clinical presentation and laboratory investigations do not allow for a prompt distinction between inflammatory and hereditary myopathies, muscle biopsy remains an essential and rapid tool for myopathologic characterization and treatment. The myopathologic analysis of P1 and P2 revealed the presence of intracellular protein aggregates characterized using the menadione-nitro blue tetrazolium with α-glycerophosphate and immunofluorescence, revealing a prominent accumulation of FHL1, particularly at the perinuclear region. Electron microscopy performed for P1 and P5 demonstrated the typical electron-dense “truffle-like” appearance of reducing bodies and areas of marked sarcomere disorganization.

In our cohort, 4 patients present a de novo pathogenic variant in FHL1, while only P4 shows a pathogenic variant with maternal inheritance. As described by Sabatelli et al.,¹³ the P4's mother displayed a mild clinical phenotype with only moderate limbs muscular weakness. This marked intrafamilial phenotypic variability in women can be explained by a probable mechanism of skewed X-chromosome inactivation affecting P4, who exhibited a phenotype and clinical course comparable to that typically observed in male patients. All variants are located in the LIM2 domain, a well-known hotspot of recurrent variants, or in the first part of LIM3 domain (Figure 5). Each LIM domain is characterized by 2 zinc fingers that are essential for the protein's function. Each zinc finger composed by 4 highly conserved cysteine residues, which are critical for its structural integrity and activity. In our cohort, 4 patients carry variants affecting these cysteine residues, resulting in a strong pathogenic impact leading to protein misfolding and aggregation. P5, the only patient still ambulant of the cohort, harbors a likely pathogenic variant affecting a highly conserved histidine in a key position within LIM2, located immediately after the first 4 cysteine residues that coordinate the zinc ion. The effect of this variant is therefore similar to that of the cysteine variants, leading to marked domain instability and protein aggregation.

In this work, we present a series of female patients with reducing body myopathy, due to pathogenic FHL1 variants, exhibiting a rapid and severe progression comparable with that observed in male patients. We want to emphasize how rare inherited myopathies can manifest with a clinical course that may mimic idiopathic inflammatory myopathies. In this context, the detection of myositis-associated autoantibodies and myositis-specific autoantibodies is essential to rule out seropositive IIM. However, in such clinical presentations, muscle biopsy remains a cardinal tool to characterize muscle involvement and to identify features of seronegative IIM or inherited muscle disorders. This study has limitations due to the small number of patients. Additional clinical studies with larger cohorts will be necessary to expand current knowledge on this peculiar topic.

Glossary

CK

creatine kinase

CMYO

congenital myopathies

EDMD

X-linked Emery-Dreifuss muscular dystrophy

EMARDD

early-onset myopathy with areflexia, respiratory distress and dysphagia

EMG

electromyography

FVC

forced vital capacity

HCM

hypertrophic cardiomyopathy

HE

haematoxylin and eosin

IIM

idiopathic inflammatory myopathy

NBT

nitro-blue tetrazolium

RBM

reducing body myopathy

Author Contributions

G. Severa: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; analysis or interpretation of data. C. Barnerias: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data. C. Gitiaux: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data. P. Laforet: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data. I. Desguerre: major role in the acquisition of data. S. Souvannanorath: major role in the acquisition of data. B. Periou: analysis or interpretation of data. S. Bastu: analysis or interpretation of data. H. Prigent: major role in the acquisition of data. N.B. Romero: major role in the acquisition of data. P. Bonaldo: major role in the acquisition of data. L. Merlini: major role in the acquisition of data. G. Cenacchi: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data. E. Pegoraro: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data. L. Bello: major role in the acquisition of data. J. Rendu: major role in the acquisition of data. M. Villanova: major role in the acquisition of data. R.Y. Carlier: major role in the acquisition of data. E. Malfatti: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; study concept or design; analysis or interpretation of data. F.-J. Authier: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; study concept or design; analysis or interpretation of data.

Study Funding

This work was supported by funding from the Association Française contre les Myopathies (AFM) via TRANSLAMUSCLE (PROJECT 22946).

Disclosure

The authors report no relevant disclosures. Go to Neurology.org/NG for full disclosures.