Introduction

Nemaline myopathy is a rare genetic condition that affects skeletal muscles, leading to muscle weakness and low tone. It is named after rod-like structures, called nemaline bodies, which appear in the muscle fibres of people with the condition. These structures help doctors confirm a diagnosis but do not cause the symptoms directly. The condition varies widely. Some people are born with serious symptoms and need support to breathe and eat. Others develop mild symptoms later in life, such as difficulty climbing stairs or lifting objects.

This article explains what causes nemaline myopathy, focusing on the genes involved and how these changes are inherited. Understanding the condition can help people make informed decisions about medical care, testing, and family planning. The three key takeaways are: nemaline myopathy is mainly inherited through autosomal recessive or dominant patterns; most cases are caused by mutations in the NEB or ACTA1 genes; and several other genes can also lead to the condition, contributing to the wide range of symptoms people experience.

Answering the main question

Nemaline myopathy is caused by mutations in genes that are responsible for making the proteins that support muscle structure and movement. These proteins form part of the “thin filaments” in muscle fibres. If these filaments are unstable or faulty, the muscle cannot contract properly. This causes weakness, which is often most obvious in the arms, legs, face, and breathing muscles.

The NEB gene is the most common gene involved. It provides the instructions for a protein called nebulin, which helps maintain the structure of the thin filaments. When nebulin is missing or defective, the muscle fibres cannot function properly. NEB mutations usually follow an autosomal recessive inheritance pattern. This means a person needs to inherit two faulty copies, one from each parent, to develop the condition. Carriers, who have only one faulty copy, typically have no symptoms but can pass the gene to their children.¹

The second most common gene is ACTA1, which codes for alpha-actin, another key protein in the thin filaments. Mutations in ACTA1 are often inherited in an autosomal dominant pattern. That means only one faulty copy is needed to cause the disease. In many cases, the mutation arises spontaneously in the affected child (called a de novo mutation), without being inherited from either parent.² ACTA1-related cases often have a more severe presentation, especially when symptoms start at birth.

Other genes can also cause nemaline myopathy. These include TPM2, TPM3, TNNT1, CFL2, KBTBD13, KLHL40, KLHL41, LMOD3, and MYPN. Each of these genes plays a role in either forming or stabilising the thin filaments of skeletal muscle. For example, TPM3 mutations often lead to a milder form of the disease that develops slowly over time, while KLHL40 mutations can cause a very severe neonatal type that presents even before birth.³

Inheritance patterns vary depending on the gene. NEB mutations are typically recessive, ACTA1 mutations are usually dominant, and TNNT1 follows a recessive pattern as seen in the Amish population. In some rare cases, NEB has also been shown to have a dominant mutation causing a milder, later-onset form⁴. This diversity in genetic causes is part of why symptoms vary so much from person to person.

The severity of symptoms even among people with the same mutation can be different. This is known as variable expressivity. For example, siblings with the same mutation may have very different levels of muscle weakness. One might need a wheelchair, while the other walks independently. This makes it hard to predict the future for any one person and means that regular follow-up and monitoring are essential.

Diagnosis is usually based on clinical signs, muscle biopsy, and increasingly on genetic testing. In a muscle biopsy, doctors may find nemaline bodies, but not always. Advances in genetic testing mean that it’s now possible to test for many different gene mutations at once using techniques like exome sequencing. These tests help identify the cause in most cases, around 80%, but some remain undiagnosed, suggesting that there are still genes yet to be discovered.¹

Sometimes, testing also reveals unusual findings. A person might have two different mutations or one that’s not linked to disease. Genetic counselors can help interpret these results and explain what they mean for the individual and their family. This information is especially helpful for parents considering future pregnancies.

For families, understanding inheritance patterns is important. In recessive inheritance, if both parents are carriers, each child has a 25% chance of being affected, a 50% chance of being a carrier, and a 25% chance of inheriting two normal copies. In dominant inheritance, a parent with the condition has a 50% chance of passing it on to each child. De novo mutations usually don’t carry a high recurrence risk, but very rarely, they can reappear due to a condition called germline mosaicism.²

Animal studies have helped us understand how mutations affect muscle function. For example, mice with ACTA1 mutations show similar muscle problems to humans. Researchers have tried potential treatments in these animals, like drugs that increase muscle strength or help muscles contract more efficiently. While promising, these treatments are not yet available to people. At the moment, management focuses on supportive care. This includes physiotherapy, help with feeding or breathing, and other interventions to improve daily life.²

Living with nemaline myopathy can be challenging, not only physically but also emotionally. Families may struggle with uncertainty, complex care needs, and medical decisions. Support groups and specialist clinics can provide information, emotional support, and practical help. Having a clear diagnosis and understanding the genetic cause can help people feel more in control.

Read On

Nemaline myopathy is a complex and evolving condition. Scientists continue to discover new gene mutations, study their effects on muscle function, and test new treatments. Although there’s still no cure, research offers hope for future therapies. For now, understanding the genetics helps patients and families manage care and make informed decisions.

FAQs

What causes nemaline myopathy?

It’s caused by genetic mutations that affect proteins in the muscle, stopping the muscle from working properly.

How is nemaline myopathy inherited?

It can be inherited in two main ways: recessive (both parents are carriers) or dominant (one faulty gene is enough). Sometimes it happens spontaneously.

Can genetic testing help?

Yes. It can confirm the diagnosis, help plan treatment, and guide family planning decisions.

Summary

Nemaline myopathy is a genetic condition that weakens muscles. It is usually caused by mutations in the NEB or ACTA1 genes, though many other genes can be involved. These genes help control how muscle fibres are built and function. Depending on the gene, the condition can be inherited in different ways, most commonly recessive or dominant. Diagnosis often includes a muscle biopsy and genetic testing. There’s no cure yet, but treatments can help manage symptoms. Knowing the genetic cause helps with planning care and understanding risks for other family members.