Rana Almutairi et al, 2020;3(1):45–51.
Correspondence to: Fuad Al Mutairi
*Division of Genetics, Department of Pediatrics, King Abdulaziz Medical City, Ministry of National Guard-Health Affairs (MNGHA), Riyadh, Saudi Arabia.
Email: almutairifu [at] ngha.med.sa
Full list of author information is available at the end of the article.
Received: 02 April 2020 | Accepted: 15 May 2020
Congenital myopathies are a diverse group of diseases that share features from the early onset of symptoms in the first year of life, such as hypotonia, muscle weakness, and developmental delays, and are often associated with respiratory insufficiency and feeding difficulties.
Here, we report an 8-year-old boy having hypotonia and signs of respiratory insufficiency that ended with tracheostomy and ventilator-dependent status. Muscle biopsy showed histological findings of congenital fiber-type disproportion myopathy. The whole exome sequencing revealed a novel hemizygous missense variant (c.530A > C p.Gln177Pro) that confirms the diagnosis of FHL1-associated congenital myopathy.
The findings in this study help to expand the genetic and mutational spectrum of the FHL1 gene associated with respiratory insufficiency and help in formulating a precise strategy for prognosis and future management of patients.
Congenital myopathy, FHL1, hypotonia, congenital fiber-type disproportion myopathy, and X-linked myopathy.
Congenital myopathy (CM) refers to a heterogeneous group of inherited neuromuscular disorders that are exhibited at birth or within the first few months of life (1). CMs are characterized by a delay in gross motor milestones, nonprogressive muscular hypotonia, and immunohistochemical findings, which ranges from myopathic to overtly dystrophic changes on muscle biopsy. These features impair the ability of muscles to contract, ultimately resulting in the loss of muscle fibers (2). CMs are associated with structural changes in some rare disorders with variable degrees of severity, including central core disease, nemaline myopathy, and congenital fiber-type disproportion myopathy (2).
FHL1 is a member of four-and-a-half LIM domains protein 1 located on the Xq26.3 chromosome. The LIM domain proteins play an important role in sarcomeres synthesis and muscle mass regulation, and act as docking sites in a protein complex assembly based on a highly conserved cysteine-rich zinc-binding motif having a double zinc finger domain (3). Furthermore, FHL1 has three isoforms that are highly expressed in skeletal and cardiac muscles known as FHL1A, FHL1B, and FHL1C (4). Recently, FHL1 was identified as a causative gene in several muscle myopathies, including X-linked myopathy with postural muscle atrophy [Online Mendelian Inheritance in Man (OMIM) 300696], X-linked dominant scapuloperoneal myopathy (OMIM 300695), reducing body myopathy (OMIM 300717), rigid spine syndrome, and Emery–Dreifuss muscular dystrophy (OMIM 300696) (5). Up until now, the exact pathomechanism associated with FHL1 mutations is unknown; however, proper genotype–phenotype correlations help in understanding the underlying FHL1 gene pathogenesis.
CMs are not regarded as progressive disorders; however, additional factors such as respiratory muscle weakness, scoliosis, and kyphoscoliosis may coexist, which are associated with extrapulmonary restriction of the lungs, resulting in the impairment of the pulmonary function. Additionally, they compromise the ability of the airways to clear the secretion and to predispose pneumonia and aspiration. As CMs usually involve the muscles of respiration, many patients require ventilatory assistance for a few months or years after the onset of the symptoms.
Herein, using whole exome sequencing, we report a novel hemizygous missense mutation in the FHL1 gene in a patient with congenital fiber-type disproportion myopathy with recurrent aspiration. We reviewed all the previously reported cases to identify the different FHL1 gene mutations that may lead to respiratory impairment in patients with CM and carried out genotype–phenotype correlation for FHL1-associated CM.
The patient is an 8-year-old boy, who is the first and only child of healthy, non-consanguineous parents (Figure 1A). After an uneventful full-term pregnancy, the baby was born by an uncomplicated cesarean section due to prolonged rupture of the membrane; subsequently, he was discharged with his mother in a good condition. At the age of 5 months, the proband had hypotonia with failure to thrive and a relatively weak cry. The proband had developmental delays in the form of gross motor, speech, and language delays. In the following months, he required frequent admissions to the hospital and Pediatrics Intensive Care Unit (PICU) due to respiratory failure, recurrent chocking attacks, and aspiration pneumonia. His medical history included chronic lung disease and bronchial asthma. The family history was unremarkable, except for recurrent miscarriages for the mother where routine investigations were conducted, and all were normal including chromosomal analysis and placental histological findings.
Figure 1. Pedigree of the index family. (B) Thoracic-spine radiograph showing moderate thoracolumbar dextroscoliosis estimated by Cobb’s angle measuring 37° taken from the upper end plate of T12-2 lower end plate of the vertebral bodies, and there is a significant downward right-sided pelvic tilt. (C) DNA chromatogram, the index, and two healthy family members.
On physical examination, the boy weighted 17.8 kg (< 3rd percentile), was 108 cm long (< 3rd percentile), and his head circumference was 50.5 cm (10th–25th percentile). He had dysmorphic features, including the myopathic face, low hairline, bilateral epicanthal folds, and gingival hyperplasia.
Neurological examination revealed a generalized weakness that mainly involved both upper and lower limbs with poor head control and hyporeflexia. Joint hyperlaxity was observed through musculoskeletal examination without any signs of contracture. On auscultation, the air entry was reduced and crackles were heard over all the lung fields. A thoracic spine X-ray revealed bilateral perihilar air space opacity and dextroscoliosis with a Cobb angle of 37° taken from the upper end plate of T12-2 lower end plate of the vertebral bodies (Figure 1B). His electromyogram (EMG), nerve conduction study, and brain Magnetic resonance imaging (MRI) were all normal. During his disease course, his condition did not improve and he was frequently admitted and also needed to be intubated and ventilated; he was admitted to the PICU several times due to hypercarbia. By the age of 18 months, the patient was tracheotomized and became ventilator-dependent. Now, at the age of 8, the patient is stable and saturating well on a home ventilator. The proband is having a global developmental delay, wherein he is unable to sit or stand independently; however, according to his mother, he can write alphabets, numbers, and talk fluently for his age.
Muscle biopsy, from an unspecified site, was carried out at the age of 9 months. Histological analyses of the muscle showed a marked variation in muscle fibers size, due to the presence of evenly distributed fibers around the atrophic fibers, alternating with the normal-sized fibers. Additionally, ATPase reactions revealed type 1 fiber atrophy, up to 50% smaller in size than type 2 fibers, with a tendency of type 1 fibers clustering. The presence of atrophic fibers with sarcolemmal folds was confirmed using ultrastructural examination. The fiber illustrated disorganization of myofibrils; however, no ring fibers were observed. There were occasional collections of enlarged mitochondria with cristae. Therefore, congenital fiber-type disproportion myopathy was compatible with the histomorphology of the biopsied muscle.
Chromosomal analysis, array Array based comparative genomic hybridization (CGH), and Sanger sequencing of RYR1 and TMP3 genes were unremarkable. Additionally, molecular testing for SMN gene and SNRPN gene was carried out using standard methods and the result were unremarkable. Subsequently, trio-Whole Exome Sequencing (WES) was carried out for the proband and parents using standard methods. The WES revealed a novel hemizygous missense variant (c.530A>C; p.Gln177Pro) in exon 6 of the FHL1 gene (NM_001159702.3) located on chromosome Xq26.3 (Figure 1). Using Sanger sequencing, the identified variant segregated perfectly from the disease phenotype and was found in the mother in a heterozygous status, while the father and two maternal uncles’ results revealed normal wild type. This variant classified as likely pathogenic based on the American College of Medical Genetics (ACMG) guidelines and has not been previously observed in large-scale sequencing databases, such as Exome Aggregation Consortium, dbSNP/1,000 genome, Exome Sequencing Projects or Genome Aggregation Database, and local database. This substitution (c.530A>C; p.Gln177Pro) was predicted to be deleterious by several online computational prediction tools [PolyPhen2, MutationTaster, and Sorting Intolerant From Tolerant (SIFT)]. Complete attention to the TPM3, ACTA1, and RYR1 genes did not reveal any possible diseases-causing variants in any of them. Furthermore, manual analysis of the raw data generated from WES, including Binary Alignment Map (BAM) file, failed to identify deletion or duplication in the above-mentioned gene.
CMs are diagnosed based on clinical features associated with respiratory insufficiency, feeding difficulties, and histological changes that are seen in the patients’ biopsied muscles. However, of late, genetic testing is considered as one of the preferred methods since it can detect a breadth of phenotypic variability associated with each gene (2). Recently, most of the studies have identified the mutations in the FHL1 (which plays a critical role in the development and function of the skeletal muscles) as a causative gene in different human myopathies, considering its high level of expression in the skeletal as well as cardiac muscles (6).
In the previous studies, FHL1 mutation has been identified in various phenotypes of X-linked myopathy, such as X-linked dominant scapuloperoneal myopathy, distal myopathy with hypertrophic cardiomyopathy, Emery–Dreifuss muscular dystrophy with rigid spine, and many other phenotypes (7). However, the association between FHL1 mutation and respiratory insufficiency is discussed without clear phenotype delineation. Only a few studies have identified the coexistence of respiratory impairment in association with FHL1 mutation in their patients (Table 1).
In this study, we report a patient with a novel hemizygous missense mutation (c.530A>C; p.Gln177Pro) in exon 6 of the FHL1 gene associated with congenital myopathy and early respiratory muscle involvement. The identified mutation changes a highly conserved Gln amino acid at position 177 into a Pro amino acid. Glutamine is a polar amino acid, while proline is a hydrophobic aliphatic amino acid. This mutation (p.Gln177Pro) results in secondary structure disability and improper FHL1 function. There are around 12 different isoforms in the RefSeq and Ensemble database for the FHL1 gene, and 9 out of 12 results in the same protein changes from Gln to Pro at different amino acid positions (177 or 206), and in the three remaining isoforms the variant is considered as a non-coding exon. The FHL1 protein consists of four LIM domains (LIM1–4), a half-LIM domain (Z), and an N-terminal and C-terminal (Figure 2B). The mutation (p.Gln177Pro) identified in our patient is located in the highly conserved LIM3 domain (Figure 2B,C).
Table 1. FHL1 mutations, clinical characteristics and respiratory involvement of the reported cases.
Figure 2. (A-B) Schematic representation of FHL1 exons and protein domain representation. FHL1 consists of four LIM domains (LIM1–4), an N-terminal, and a half-LIM domain [Z]. (C) The mutation identified in the present study is located in the highly conserved LIM 3 domain.
Some of the previously reported patients who had FHL1 mutations were severely affected, as they required ventilatory support either permanently or while sleeping, and had various symptoms from childhood to late adulthood (4,8). About five patients died from respiratory failure and the age of the deceased individuals ranged widely from the age of 6 to 50 (4,8–10). Until now, few pathogenic mutations in the FHL1 gene have been reported and mostly they appear in the second and fourth LIM domains. The mutations in the FHL1 gene were identified at positions c.367C>T, c.369C>G, c.395G>T, and c.672C>G, where c.367C>T, c.369C>G, and c.395G>T were reported mostly in early childhood, while the c.672C>G variant has been associated with the later onset of the symptoms (4,8–10). Other mutations, such as c.381_382insATC, c.827G>A, c.457T>C, c.377G>A, and c.451–459del, have been associated with various phenotypes. However, most of them present at a later stage with respiratory insufficiency (8,9,11–14). Because of the small number of available patients with an unclear description of respiratory status, there is no clear phenotype–genotype correlation neither with the onset nor with severity of the respiratory complications.
The findings in this study increase the mutational spectrum of the FHL1 gene associated with respiratory insufficiency and also ensure that clinicians and respiratory therapists are aware of the respiratory involvement in the patients with FHL1 gene mutations. Further studies are required to dissect the pathophysiology of the FHL1 mutations in terms of respiratory muscle involvement to obtain a precise future management strategy.
The authors would like to thank the members of the family enrolled in this study.
|ACMG||American College of Medical Genetics|
|BAM||Binary Alignment Map|
|CGH||Array based comparative genomic hybridization|
|FHL1||Four-and-a-Half Lim Domains 1|
|LIM||LIM-domain proteins 1|
|MRI||Magnetic resonance imaging|
|OMIM||Online Mendelian Inheritance in Man|
|PICU||Pediatrics Intensive Care Unit|
|SIFT||Sorting Intolerant From Tolerant|
|WES||Whole Exome Sequencing|
The authors of this article have no affiliations or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.
Ethical approval is not required at our institution to publish an anonymous case report.
Informed consent was obtained from the parents.
Rana Almutairi1, Sara Alrashidi1, Muhammed Umair2, Maha Alshalan3, Lamia Alsubaie2,3, Taghrid Aloraini4, Ahmed Al Ahmad4, Ahmed Alfares4,5, Fuad Al Mutairi1,2,3*