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Myofibrillar myopathy (BAG3)
This condition leads to the gradual weakening of muscles throughout the body, including those used for movement and potentially the heart. It often appears in adolescence or adulthood, impacting daily activities and requiring ongoing medical support.
Overview
Myofibrillar myopathy (MFM) refers to a group of genetic muscle disorders where the muscle fibres, specifically the myofibrils, break down. This leads to progressive muscle weakness and wasting. When caused by changes in the BAG3 gene, it is known as BAG3-related myofibrillar myopathy, and can also impact other organs, most notably the heart [PMID:29665324]. The condition is typically progressive, meaning symptoms tend to worsen over time. While rare, understanding its genetic basis is crucial for diagnosis and management.
Symptoms & clinical features
Individuals with BAG3-related myofibrillar myopathy often experience muscle weakness that slowly progresses, especially in the muscles closer to the body's centre (proximal muscles). This can lead to difficulties with activities such as walking, climbing stairs, or lifting objects [PMID:22080490]. Weakness can also affect muscles further from the body's centre (distal muscles), leading to issues with fine motor skills. In some individuals, respiratory muscles may be affected, potentially leading to breathing difficulties.
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Affected organs
The primary organs affected in BAG3-related myofibrillar myopathy are skeletal muscles, which are responsible for movement. However, the heart is also frequently involved, with individuals potentially developing cardiomyopathy, a condition where the heart muscle becomes enlarged, thickened, or rigid, affecting its ability to pump blood effectively [PMID:22080490]. This cardiac involvement can sometimes be the most serious aspect of the condition. In some cases, the peripheral nerves may also be affected, leading to neuropathy, which can cause numbness, tingling, or further weakness.
Risks & severity
The severity of BAG3-related myofibrillar myopathy can vary significantly between individuals, even within the same family. Symptoms typically begin in adolescence or early adulthood, but onset can range from childhood to later adulthood [PMID:29665324]. The condition is progressive, meaning muscle weakness tends to worsen over time. Cardiac involvement is a major concern, as cardiomyopathy can lead to serious heart complications, including heart failure. Respiratory muscle weakness can also pose significant health risks, sometimes requiring support for breathing.
Genetic causes
Myofibrillar myopathy linked to BAG3 is caused by pathogenic variants in the BAG3 gene. The BAG3 gene provides instructions for making a protein called BAG3, which plays a critical role in cellular processes, including protein quality control and maintaining the structural integrity of muscle fibres [PMID:21179044]. It helps to ensure that damaged or misfolded proteins are cleared from cells, which is particularly important in high-stress tissues like muscle. When there are pathogenic variants in BAG3, the protein's normal function is disrupted, leading to the accumulation of abnormal protein aggregates within muscle cells. This accumulation is thought to contribute to the progressive degeneration of muscle fibres characteristic of myofibrillar myopathy.
- BAG3 BAG cochaperone 3
Inheritance pattern
BAG3-related myofibrillar myopathy has an autosomal dominant inheritance pattern. This means that inheriting just one copy of the altered BAG3 gene is enough to cause the condition. If a person has a pathogenic variant in BAG3, each of their children has a 50% chance of inheriting the variant and therefore developing the condition. This inheritance pattern affects both males and females equally. Family members who do not inherit the variant will not develop the condition and cannot pass it on to their own children.
Each child has a 50% chance of inheriting the pathogenic variant, regardless of sex.
Diagnosis & testing
Diagnosis of BAG3-related myofibrillar myopathy typically begins with a clinical evaluation based on symptoms such as progressive muscle weakness and potential cardiac issues. Muscle biopsies can show characteristic changes in muscle tissue under a microscope, indicating myofibrillar pathology. Genetic testing is then used to confirm the diagnosis by identifying a pathogenic variant in the BAG3 gene [PMID:22080490]. In the UK, genetic testing for neuromuscular disorders, including myofibrillar myopathies, is available through the NHS Genomic Medicine Service (GMS) via the Rare and Inherited Disease eligibility criteria. Referrals for genetic testing are usually made by a neurologist or a clinical genetics specialist, often after initial assessments. The relevant R-code for genetic testing for limb girdle muscle dystrophy and congenital myopathy, which can include BAG3-related conditions, would be part of the R101 panel for limb girdle muscular dystrophies or R102 for congenital myopathies.
Management & lifestyle
Management for BAG3-related myofibrillar myopathy focuses on addressing symptoms and preventing complications, as there is currently no cure. A multidisciplinary team, including neurologists, cardiologists, respiratory specialists, and physiotherapists, typically provides care. Regular cardiac monitoring is crucial due to the risk of cardiomyopathy, and interventions like medications or pacemakers may be considered if heart problems develop. Physiotherapy and occupational therapy can help maintain muscle function and mobility, and provide adaptive strategies for daily living. Respiratory support may be necessary if breathing muscles are significantly affected. Management plans are individualised and developed within the NHS care pathway, focusing on improving quality of life and managing specific symptoms.
UK care pathway
In the UK, individuals suspected of having a genetic muscle disorder like BAG3-related myofibrillar myopathy would typically be referred by their GP or a specialist to a regional Clinical Genetics service or a specialist neuromuscular centre. These services operate within the NHS Genomic Medicine Service (GMS) framework. A clinical geneticist or neurologist can assess the need for genetic testing, which is performed using GMS-approved panels (e.g., R101 or R102). Genetic counsellors are also an integral part of this pathway, providing support, information, and guidance on inheritance patterns and family planning.
Frequently asked questions
What does 'myofibrillar' mean?
Myofibrillar refers to the myofibrils, which are the basic rod-like units that make up muscle fibres. In myofibrillar myopathy, these structures become disorganised and break down, leading to muscle weakness.
Is BAG3-related myofibrillar myopathy always inherited?
Yes, BAG3-related myofibrillar myopathy is an inherited genetic condition. It follows an autosomal dominant pattern, meaning it is passed down through families, and only one copy of the altered gene is needed for the condition to develop.
Can children get BAG3-related myofibrillar myopathy?
While symptoms most often begin in adolescence or adulthood, in some cases, the onset of BAG3-related myofibrillar myopathy can occur in childhood. The specific age of onset can vary significantly among affected individuals.
What kind of doctors treat this condition?
Individuals with BAG3-related myofibrillar myopathy are typically cared for by a team of specialists, including neurologists (for muscle weakness), cardiologists (for heart issues), respiratory specialists, and physiotherapists. Geneticists and genetic counsellors also play a key role in diagnosis and family support.
Is there a cure for BAG3-related myofibrillar myopathy?
Currently, there is no cure for BAG3-related myofibrillar myopathy. Management focuses on treating the symptoms, preventing complications, and improving quality of life through a combination of therapies, medications, and supportive care.