Expanding the disease phenotype of ADSSL1-associated myopathy in non-Korean patients

Two Brothers With ADSS1 Myopathy: A Report of Clinical, Radiological, and Autopsy Findings

ADSS1 myopathy, previously known as adenylosuccinate synthetase-like 1 (ADSSL1) myopathy, is an autosomal recessive muscle disease caused by variants in ADSS1 (adenylosuccinate synthase 1). ADSS1 myopathy is complicated by respiratory muscle weakness or cardiomyopathy as well as limb muscle weakness. We analyzed two siblings with ADSS1 myopathy, both harboring compound heterozygous pathogenic variants (c.781G>A/c.919delA) in ADSS1 and provided details of their phenotypes together with muscle imaging and autopsy findings. Although it was reported that ADSS1 myopathy usually began with lower limb muscle weakness, our cases showed early involvement of the cervical paraspinal muscle, triceps brachii muscle, flexor digitorum superficialis and profundus muscles, rectus abdominis muscle, gluteus maximus and medius muscles, and cardiomyopathy. While a previous study reported that the trunk and hip muscles were relatively spared, atrophy of paraspinal muscles, gluteus medius and maximus muscles, and adductor muscles were observed. Our two siblings allowed for long-term follow-up and will be useful reference cases. We evaluated the frequency of fibers with nemaline bodies in various autopsied muscles and found that the ratio of fibers with nemaline bodies was lower compared to other nemaline myopathies. Postmortem examination revealed, for the first time, nemaline bodies in the diaphragm and myocardium, associated with respiratory failure and cardiomyopathy.

The Impact of Heritable Myopathies on Gastrointestinal Skeletal Muscle Function

Among other contributions to gastrointestinal (GI) function, skeletal muscles regulate transit at both ends of the GI tract by providing propulsive forces for ingested nutrients and controlling the excretion of waste products. At the oropharynx, skeletal muscles provide necessary forces for effective mastication and the transfer of food boluses from the mouth into the proximal esophagus, where skeletal muscle-mediated peristalsis initiates propulsion of food boluses towards the stomach, a function supplanted by the upper esophagus smooth muscle. Consequently, the most prominent manifestation of proximal GI tract skeletal muscle dysfunction is transfer and oropharyngeal dysphagia that may result in repeated episodes of life-threatening choking and pulmonary aspiration. At the anal canal, the external anal sphincter controls the release of gas, liquids, and solids. Skeletal muscles within the pelvic floor play a synergistic role in regulating defection. Hence, distal GI tract skeletal muscle dysfunction may result in the leakage of flatus and fecal matter, whereas, in contrast, pelvic floor dysfunction may contribute to constipation. The balance between such defects may severely impact nutritional status and quality of life. Herein, we provide a comprehensive review of the genetics, molecular biology, and mechanisms underlying heritable disorders of skeletal muscle and how these may impact GI tract function and overall well-being. For organizational purposes, we separate discussions of congenital, mitochondrial, and myofibrillar myopathies and muscular dystrophies. For the sake of completeness, we also briefly consider acquired myopathies that affect GI tract function. As treatment options are currently limited, disorders of skeletal muscle function provide exciting therapeutic opportunities, including innovative approaches to target specific gene modifications.

Vulnerable Nucleotide Pools and Genomic Instability in Yeast Strains with Deletion of the ADE12 Gene Encoding for Adenylosuccinate Synthetase

Adenylosuccinate synthetase (AdSS), encoded by the ADE12 gene in yeast Saccharomyces cerevisiae, plays a critical role in purine biosynthesis, catalyzing the conversion of inosine 5'-monophosphate (IMP) and aspartic acid to adenylosuccinate, a substrate for the following adenosine 5'-monophosphate (AMP) synthesis step. Mutants lacking AdSS activity exhibit a range of pleiotropic phenotypes: slow growth, poor spore germination, accumulation, and secretion of inosine and hypoxanthine. We report new phenotypes of ade12 mutants and explain their molecular mechanisms. A GC-MS analysis showed that ade12 mutants have highly altered metabolite profiles: the accumulation of IMP leads to an impaired cellular energy metabolism, resulting in a dysregulation of key processes-the metabolism of nucleotides, carbohydrates, and amino acids. These metabolic perturbations explain the cell division arrest observed in ade12 yeast strains. A slowed replication in ade12 mutants, because of the insufficient availability of energy, nucleotides, and proteins, leads to the error-prone DNA polymerase ζ-dependent elevation of spontaneous mutagenesis, connecting multiple roles of AdSS in metabolism with genome stability control.

Current insights in ultra-rare adenylosuccinate synthetase 1 myopathy - meeting report on the First Clinical and Scientific Conference. 3 June 2024, National Centre for Advancing Translational Science, Rockville, Maryland, the United States of America

The inaugural Clinical and Scientific Conference on Adenylosuccinate Synthetase 1 (ADSS1) myopathy was held on June 3, 2024, at the National Institutes of Health (NIH) National Center for Advancing Translational Sciences (NCATS) in Rockville, Maryland, USA.

ADSS1 myopathy is an ultra-rare, inherited neuromuscular disease.

Features of geographical patient clusters in South Korea, Japan, India and the United States of America were characterised and discussed.

Pre-clinical animal and cell-based models were discussed, providing unique insight into disease pathogenesis.

The biochemical pathogenesis was discussed, and potential therapeutic targets identified.

Potential clinical and pre-clinical biomarkers were discussed.

An ADSS1 myopathy consortium was established and a roadmap for therapeutic development created.

Variability in Disease Severity in Siblings With Homozygous Missense Variant of ADSSL1: Clinical Genetic Study and Review of Literatures

BACKGROUND

Distal myopathies are genetic muscle disorders caused by mutations in various genes. A study found that mutations in adenylosuccinate synthetase-like 1 (ADSSL1) are associated with distal myopathy in nine patients from six unrelated families in South Korea. Previous research showed that affected individuals experienced distal muscle weakness starting in adolescence, along with mild facial muscle weakness, slightly elevated or normal serum creatine kinase (CK) levels, and the presence of a few rimmed vacuoles in muscle fibers or minimal chronic myopathic damage. Previously reported patients in this category exhibited an early age of symptom onset and severe muscle weakness. In this study, we present a case of two sisters who share the same mutation locus but display distinct disease phenotypes.

METHODS

A literature review was conducted on distal myopathies in patients with ADSSL1 mutations, alongside a retrospective analysis of disease severity variability among siblings with a homozygous missense variant of ADSSL1.

RESULTS

The study focuses on two sisters with differing disease manifestations despite carrying the same genetic mutation. The older sister showed lower ability in running and jumping compared to her peers at age 7 and experienced notable muscle weakness and atrophy by age 27, whereas the younger sister remained free of symptoms at age 30.

CONCLUSION

These findings suggest that mutations at the same locus can result in varying disease outcomes, emphasizing the complexity of predicting disease progression based solely on genetic mutations.

Childhood-Onset Myopathy With Preserved Ambulation Caused by a Recurrent ADSSL1 Missense Variant

Background and Objectives

Distal myopathies are a heterogeneous group of primary muscle disorders with recessive or dominant inheritance. ADSSL1 is a muscle-specific adenylosuccinate synthase isoform involved in adenine nucleotide synthesis. Recessive pathogenic variants in the ADSSL1 gene located in chromosome 14q32.33 cause a distal myopathy phenotype. In this study, we present the clinical and genetic attributes of 6 Indian patients with this myopathy.

Methods

This was a retrospective study describing on Indian patients with genetically confirmed ADSSL1 myopathy. Details were obtained from the medical records.

Results

All patients presented in their first or early second decade. All had onset in the first decade with a mean age at presentation being 17.7 ± 8.4 years (range: 3-27 years) and M:F ratio being 1:2. The mean disease duration was 9.3 ± 5.2 years ranging from 2 to 15 years. All patients were ambulant with wheelchair bound state in 1 patient due to respiratory involvement. The median serum creatine kinase (CK) level was 185.5 IU/L (range: 123-1564 IU/L). In addition to salient features of ptosis, cardiac involvement, bulbar weakness, and proximo-distal limb weakness with fatigue, there were significant seasonal fluctuations and decremental response to repetitive nerve stimulation, which have not been previously reported. Muscle histopathology was heterogenous with the presence of rimmed vacuoles, nemaline rods, intracellular lipid droplets along with chronic myopathic changes. Subtle response to pyridostigmine treatment was reported. While 5 of 6 patients had homozygous c.781G>A (p.Asp261Asn) variation, 1 had homozygous c.794G>A (p.Gly265Glu) in ADSSL1 gene.

Discussion

This study expands the phenotypic spectrum and variability of ADSSL1 myopathy with unusual manifestations in this rare disorder. Because the variant c.781G>A (p.Asp261Asn) is the most common mutation among Indian patients similar to other Asian cohorts, this finding could be useful for genetic screening of suspected patients.

A pilot investigation of muscle integrity in patients with ADSSL1 myopathy using electrical impedance myography

INTRODUCTION/AIMS

ADSSL1 myopathy (OMIM 617030) is a recently discovered, congenital myopathic disease caused by a pathogenic variant in ADSSL1. ADSSL1 is an enzyme involved in the purine nucleotide process and facilitates the conversion of inosine monophosphate to adenosine monophosphate within myocytes. Electrical impedance myography (EIM) is a portable, non-invasive, and cost-effective method for characterizing muscle integrity. Three ADSSL1 patients are presented in whom characterization of muscle integrity using EIM was performed.

METHODS

A 15-y-old male, 20-y-old female, and 63-y-old male each with a pathogenic variant in ADSSL1 [c.901G > A] as well as three, age-gender matched healthy controls (HCs) were enrolled. Study participants were phenotyped using a virtual EIM procedure.

RESULTS

ADSSL1 myopathy patients presented with variable onset of physical disability, disease progression, and severity of muscle weakness. Across multiple proximal and distal muscles groups and relative to HCs, ADSSL1 myopathy patients demonstrated lower phase and reactance values, while resistance was higher, which together indicated diseased muscle.

DISCUSSION

EIM can provide a novel, non-invasive and objective biomarker to evaluate muscle integrity in patients with ADSSL1 myopathy. Combining EIM with musculoskeletal imaging and histologic assessments in follow-up studies may further inform on the pathophysiology of ADSSL1 myopathy.

Adenylosuccinic Acid: An Orphan Drug with Untapped Potential

Adenylosuccinic acid (ASA) is an orphan drug that was once investigated for clinical application in Duchenne muscular dystrophy (DMD). Endogenous ASA participates in purine recycling and energy homeostasis but might also be crucial for averting inflammation and other forms of cellular stress during intense energy demand and maintaining tissue biomass and glucose disposal. This article documents the known biological functions of ASA and explores its potential application for the treatment of neuromuscular and other chronic diseases.

A review of major causative genes in congenital myopathies

In this review, we focus on congenital myopathies, which are a genetically heterogeneous group of hereditary muscle diseases with slow or minimal progression. They are mainly defined and classified according to pathological features, with the major subtypes being core myopathy (central core disease), nemaline myopathy, myotubular/centronuclear myopathy, and congenital fiber-type disproportion myopathy. Recent advances in molecular genetics, especially next-generation sequencing technology, have rapidly increased the number of known causative genes for congenital myopathies; however, most of the diseases related to the novel causative genes are extremely rare. There remains no cure for congenital myopathies. However, there have been recent promising findings that could inform the development of therapy for several types of congenital myopathies, including myotubular myopathy, which indicates the importance of prompt and correct diagnosis. This review discusses the major causative genes (NEB, ACTA1, ADSSL1, RYR1, SELENON, MTM1, DNM2, and TPM3) for each subtype of congenital myopathies and the relevant latest findings.

Distal myopathy

Distal myopathies are a group of genetic, primary muscle diseases. Patients develop progressive weakness and atrophy of the muscles of forearm, hands, lower leg, or feet. Currently, over 20 different forms, presenting a variable age of onset, clinical presentation, disease progression, muscle involvement, and histological findings, are known. Some of them are dominant and some recessive. Different variants in the same gene are often associated with either dominant or recessive forms, although there is a lack of a comprehensive understanding of the genotype-phenotype correlations. This chapter provides a description of the clinicopathologic and genetic aspects of distal myopathies emphasizing known etiologic and pathophysiologic mechanisms.

Disorders of purine biosynthesis metabolism

Purines are essential molecules that are components of vital biomolecules, such as nucleic acids, coenzymes, signaling molecules, as well as energy transfer molecules. The de novo biosynthesis pathway starts from phosphoribosylpyrophosphate (PRPP) and eventually leads to the synthesis of inosine monophosphate (IMP) by means of 10 sequential steps catalyzed by six different enzymes, three of which are bi-or tri-functional in nature. IMP is then converted into guanosine monophosphate (GMP) or adenosine monophosphate (AMP), which are further phosphorylated into nucleoside di- or tri-phosphates, such as GDP, GTP, ADP and ATP. This review provides an overview of inborn errors of metabolism pertaining to purine synthesis in humans, including either phosphoribosylpyrophosphate synthetase (PRS) overactivity or deficiency, as well as adenylosuccinate lyase (ADSL), 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase (ATIC), phosphoribosylaminoimidazole succinocarboxamide synthetase (PAICS), and adenylosuccinate synthetase (ADSS) deficiencies. ITPase deficiency is being described as well. The clinical spectrum of these disorders is broad, including neurological impairment, such as psychomotor retardation, epilepsy, hypotonia, or microcephaly; sensory involvement, such as deafness and visual disturbances; multiple malformations, as well as muscle presentations or consequences of hyperuricemia, such as gouty arthritis or kidney stones. Clinical signs are often nonspecific and, thus, overlooked. It is to be hoped that this is likely to be gradually overcome by using sensitive biochemical investigations and next-generation sequencing technologies.

Improving the efficacy of exome sequencing at a quaternary care referral centre: novel mutations, clinical presentations and diagnostic challenges in rare neurogenetic diseases

BACKGROUND

We used a multimodal approach including detailed phenotyping, whole exome sequencing (WES) and candidate gene filters to diagnose rare neurological diseases in individuals referred by tertiary neurology centres.

METHODS

WES was performed on 66 individuals with neurogenetic diseases using candidate gene filters and stringent algorithms for assessing sequence variants. Pathogenic or likely pathogenic missense variants were interpreted using in silico prediction tools, family segregation analysis, previous publications of disease association and relevant biological assays.

RESULTS

Molecular diagnosis was achieved in 39% (n=26) including 59% of childhood-onset cases and 27% of late-onset cases. Overall, 37% (10/27) of myopathy, 41% (9/22) of neuropathy, 22% (2/9) of MND and 63% (5/8) of complex phenotypes were given genetic diagnosis. Twenty-seven disease-associated variants were identified including ten novel variants in FBXO38, LAMA2, MFN2, MYH7, PNPLA6, SH3TC2 and SPTLC1. Single-nucleotide variants (n=10) affected conserved residues within functional domains and previously identified mutation hot-spots. Established pathogenic variants (n=16) presented with atypical features, such as optic neuropathy in adult polyglucosan body disease, facial dysmorphism and skeletal anomalies in cerebrotendinous xanthomatosis, steroid-responsive weakness in congenital myasthenia syndrome 10. Potentially treatable rare diseases were diagnosed, improving the quality of life in some patients.

CONCLUSIONS

Integrating deep phenotyping, gene filter algorithms and biological assays increased diagnostic yield of exome sequencing, identified novel pathogenic variants and extended phenotypes of difficult to diagnose rare neurogenetic disorders in an outpatient clinic setting.

An autopsied case of ADSSL1 myopathy

ADSSL1 myopathy is an inherited myopathy with limb weakness, respiratory muscle paralysis, dysphagia, and myocardial symptoms. We present an autopsy case of a 66-year-old male carrying compound heterozygous variants c.781G>A (p.D261N) and c.919delA (p.I307fs) in ADSSL1. He had not run fast since school with no family history. He showed a gradual progression of limb weakness and developed dyspnoea, dysphagia, and Brugada syndrome at the age of 56. The magnetic resonance imaging (MRI) revealed bright tongue sign. Muscle biopsy showed only chronic myopathic changes. He died of respiratory muscle weakness at the age of 66. Autopsy revealed that there were many fibres with vacuoles and nemaline rods in the biceps brachii, tongue, diaphragm, and iliopsoas. Many lipopigments and nuclear clumps were also detected. The myocardium and central nervous system had only nonspecific age-related changes. This is the first autopsied case to clarify the terminal state of ADSSL1 myopathy.

Panorama of the distal myopathies

Distal myopathies are genetic primary muscle disorders with a prominent weakness at onset in hands and/or feet. The age of onset (from early childhood to adulthood), the distribution of muscle weakness (upper versus lower limbs) and the histological findings (ranging from nonspecific myopathic changes to myofibrillar disarrays and rimmed vacuoles) are extremely variable. However, despite being characterized by a wide clinical and genetic heterogeneity, the distal myopathies are a category of muscular dystrophies: genetic diseases with progressive loss of muscle fibers. Myopathic congenital arthrogryposis is also a form of distal myopathy usually caused by focal amyoplasia. Massive parallel sequencing has further expanded the long list of genes associated with a distal myopathy, and contributed identifying as distal myopathy-causative rare variants in genes more often related with other skeletal or cardiac muscle diseases. Currently, almost 20 genes (ACTN2, CAV3, CRYAB, DNAJB6, DNM2, FLNC, HNRNPA1, HSPB8, KHLH9, LDB3, MATR3, MB, MYOT, PLIN4, TIA1, VCP, NOTCH2NLC, LRP12, GIPS1) have been associated with an autosomal dominant form of distal myopathy. Pathogenic changes in four genes (ADSSL, ANO5, DYSF, GNE) cause an autosomal recessive form; and disease-causing variants in five genes (DES, MYH7, NEB, RYR1 and TTN) result either in a dominant or in a recessive distal myopathy. Finally, a digenic mechanism, underlying a Welander-like form of distal myopathy, has been recently elucidated. Rare pathogenic mutations in SQSTM1, previously identified with a bone disease (Paget disease), unexpectedly cause a distal myopathy when combined with a common polymorphism in TIA1. The present review aims at describing the genetic basis of distal myopathy and at summarizing the clinical features of the different forms described so far.