What is Facioscapulohumeral Muscular Dystrophy (FSHD)?

Facioscapulohumeral Muscular Dystrophy (FSHD) is an autosomal dominant genetic disease which weakens and destroys skeletal muscle tissue, affecting muscles in the face, upper body, but in some people could also spread to the lower body over time.¹ FSHD symptoms usually appear in teenage years or early adulthood (around ages of 20 to 30), though childhood onset may occur. 

FSHD is among the most prevalent forms of muscle dystrophy and affects 4 to 10 per 100,000 people worldwide,¹ with most studies showing no identifiable racial group that is more or less affected by FSHD, which has a prevalence of between 3.2 and 4.6 per 100,000. While both sexes can develop FSHD, women have milder symptoms and may be diagnosed at a later age than men, demonstrating that FSHD has a lower disease rate in women.² 

What Causes FSHD?

FSHD can occur without a family history, or be inherited from either parent in an autosomal dominant pattern. The disease is driven by abnormal activation of double homeobox protein 4 gene (DUX4) on chromosome 4, in the 4q35 region, which is expressed incorrectly due to a genetic defect (mutation), which is the most likely cause of FSHD.³ Normally, the DUX4 gene is inactivated in adult muscle cells, but in FSHD, DUX4 exhibits excessive activity, resulting in the production of a toxic protein that disrupts normal gene control and destroys the muscle fibres.⁴

Moreover, a key factor is the complex genetic interaction between the chromosomal location and number of copies of the D4Z4 macrosatellite, located near the DUX4 gene.² In healthy people, this region consists of a lot of repetition units that maintain DUX4 tightly silenced. In FSHD, a reduction in these repeats disrupts normal gene regulation, resulting in abnormal activation of DUX4 protein, which causes muscle damage.²

FSHD occurs in two genetic subtypes, FSHD 1 and FSHD 2, sharing the same symptoms but different causes:

• FSHD1 (~ 95% of cases): Caused by a decrease in the amount of D4Z4 macrosatellite repeat units on chromosome 4q35, which leads to the opening in structure of chromatin and reduction in DNA methylation, allowing DUX4 activation⁵
• FSHD2 (~ 5% of cases): Caused by a mutation in a different gene, such as SMCHD1 or DNMT3B. These mutations lead to loss in DNA methylation (epigenetic changes) that activates the DUX4, even when D4Z4 repeat length is normal⁵

Signs and Symptoms of FSHD 

FSHD signs and symptoms vary from one person to another. Some people may face more serious difficulties during their lives, while others may have fewer symptoms. Facial and upper body muscles are typically where symptoms start, and may progressively spread to other parts of the body.

Early characteristics include:

• Facial weakness: Difficulty in smiling, puckering of the lips, or fully closing the eyes as the muscles surrounding the lips and eyes are the most affected^6,7 
• Shoulder or upper arm weakness: Leads to scapular winging, a condition where the shoulder blades rise and expand further, making it difficult to lift the arms above shoulder height⁷
• Abdominal weakness: Causing a protruding abdomen, making it difficult to maintain core strength¹
• Lower leg weakness: May result in foot drop, causing difficulty climbing stairs and walking on uneven surfaces⁷

Other common characteristics include:

• Asymmetrical weakness: Muscles weakness is uneven, affecting one side of the body more than the other^6,7 
• Muscles pain and inflammation
• Hearing loss
• Eye problems such as retinal blood vessel changes, affecting vision.
• Respiratory difficulties: Rarely severe
• Spinal abnormalities: The spine may curve inward (lordosis) or sideways (scoliosis)^6,7

Symptoms of FSHD usually get worse over time and the condition progresses slowly.Even within the same family, there are significant differences in the severity and rate of progression amongst patients.

How is FSHD Traditionally Diagnosed?

FSHD diagnosis is traditionally performed by a medical professional using a combination of physical examination, laboratory tests and imaging.

Key approaches include:

• Blood tests: Elevated levels of  muscle-related enzymes such as creatine kinase and aldolase may indicate muscular problems¹
• Family history and clinical assessment: Doctors ask about patients' family history and check for features of muscle weakness as the condition is often inherited¹ 
• Magnetic Resonance Imaging (MRI): MRI scans can indicate patterns of fat replacement and muscle loss to help in the diagnosis process⁸
• Muscle biopsy: A muscle tissue sample is examined under the microscope to indicate any structural changes or inflammation^1,8
• Electromyography (EMG): EMG measures electrical activity of muscle weakness to determine muscles relaxing or contracting¹
• Neurological examinations: Tests on coordination, reflexes and nerve function are performed to detect any additional possible problems¹ 
• Genetic testing: Even though it has traditionally been used after clinical assessment, DNA testing is essential to validate the presence of D4Z4 repeat abnormalities or DUX4-related modifications⁸

Traditional Genetic Testing 

Southern blots can be utilised to quantify the D4Z4 repeat array size on chromosome 4q35 and identify the pathogenic contraction of repeat units that are a hallmark of FSHD1.⁹

Despite being the gold standard for many years, it is labour-intensive, requires large high-quality DNA samples, and has limited resolution, making it less effective in identifying complex structural variations or FSHD2.⁹

Advances in Genetic Testing Technologies

Advanced testing tools enable direct visualisation of repeat arrays, detection of minor structural changes, and identification of epigenetic alterations. These techniques have increased the speed, accuracy, and resolution of FSHD diagnosis, avoiding a number of the drawbacks associated with traditional Southern blot analysis.

• Molecular Combing

A high resolution single molecule method used in complicated disease mutations identify difficult structural variants and determining the D4Z4 repeat number as it directly visualises genetic combinations associated with FSHD.¹⁰ 

• Optical Genome Mapping

Uses large DNA molecules, allowing for highly precise and sensitive direct measurement of D4Z4 repeat size and structural variations. This method is quicker and uses less DNA than Southern blotting.⁹

• Next-generation sequencing (NGS)

Allows for the rapid and accurate sequencing of SMCHD1 or DNMT3B, two important epigenetic regulators in FSHD2. In cases where the D4Z4 repeat number is normal but the disease is caused by epigenetic changes, NGS is crucial,providing a deeper analysis in a shorter period of time.¹¹

• Long-read sequencing techniques, such the Oxford Nanopore Technologies (ONT)

Ability to sequence long DNA strands in a single read, including repeating regions such D4Z4. This enables extensive analysis of methylation patterns, repetition length, and sequence variation, which are challenging to identify using short-read techniques.¹²

• Epigenetic Testing (DNA Methylation Analysis) 

Assesses the D4Z4 region's methylation levels, aiding separation between FSHD1 and FSHD2 and confirming conditions in which DUX4 is activated by abnormal hypomethylation.¹³

Why These Advances Matter

Recent advances in genetic testing have transformed the techniques to diagnose FSHD.

• Early and more precise diagnosis: New technologies are able to identify abnormal structural variations, epigenetic modifications, and minor D4Z4 changes that went frequently undetected by previous techniques
• Improved subtype classification: Accurate genetic counselling requires the ability to differentiate between FSHD1 and FSHD2, which epigenetic testing and NGS can effectively detect
• Early diagnosis and knowledgeable family plans: Patient and family decisions on future pregnancies, lifestyle modifications, and medical care are made easier by quicker disease confirmation
• Targeted therapy foundation: Precise mapping of genetic and epigenetic pathways provides essential data for the development of researched gene-based or epigenetic therapeutics currently being researched

Challenges and Limitations

These innovative techniques have drawbacks despite their benefits:

• Cost and accessibility: A lot of these techniques remain expensive and require specialised labs, which limits their availability in some areas
• Technical complexity: Advanced technology and highly skilled staff are required for techniques such as molecular combing, optical genome mapping, and long-read sequencing
• Data interpretation: Diagnosis and counselling may be significantly more difficult for variants with unclear significance, for example, complex methylation patterns
• Lack of understanding of disease progression and severity: Long-term management of FSHD is difficult since current testing cannot accurately predict the age of onset, rate of muscle weakness, or overall severity of the disease, even with accurate genetic and epigenetic information¹⁴
• Lack of treatment strategies: Since there is currently no cure for many genetic diseases, including FSHD, management often focuses on symptom management along with supportive care rather than preventing the disease's progression¹⁴

Summary

FSDH is a common genetic muscle disorder caused by abnormal DUX4 gene activation, resulting in progressive weakness in the muscles of the face, shoulders, and upper body. In previous years, the diagnosis of D4Z4 repeat contractions has been determined by clinical examination, enzyme testing, imaging, and Southern blot genetic testing. However, these techniques are time-consuming, slow, and have a low resolution.

Innovations in genetic testing, such as optical genome mapping, NGS, long-read sequencing, molecular combing, and DNA methylation analysis, have contributed to the ability to discover structural variants, epigenetic modifications, and FSHD subtypes (FSHD1 versus FSHD2) with higher precision. Early family planning decisions are made easier by these technologies, which also help in the development of targeted gene and epigenetic therapies and offer a faster and more accurate diagnosis.

However, challenges and limitations remain such as high costs, limited accessibility, and the inability to accurately predict the duration or severity of the diseases. Despite these drawbacks, current genetic techniques indicate a significant advancement in FSHD future treatment approaches and personalised treatments.