[Explained by a Doctor] What is Angelman Syndrome?

Q: Is there a treatment for Angelman Syndrome?

Current treatment is primarily symptomatic and supportive. Specifically, it includes (1) seizure management using anti-seizure medications, (2) managing sleep disorders with sleep aids, and (3) physical, occupational, and speech therapy focusing on non-verbal communication training using picture cards or sign language. As a cutting-edge treatment, GTX-102 (apaznosente), which aims to restore paternal UBE3A expression, received Breakthrough Therapy Designation from the FDA in June 2025, and a global Phase 3 clinical trial is currently underway.

Table of Contents

• Introduction
• Basic Knowledge
• Symptoms and Characteristics
• Prognosis and Quality of Life
• Treatment and Care
• Summary

Introduction

For those considering prenatal testing, obtaining accurate information about genetic disorders is essential for making informed decisions.
Here, we explain Angelman syndrome from a medical perspective, covering basic knowledge, symptoms, treatment, and care in an easy-to-understand manner.

Please use this as reference information when considering prenatal testing.

Basic Knowledge

◆ What is Angelman Syndrome?

Angelman syndrome is a genetic disorder caused by a functional abnormality of chromosome 15, and is designated as an intractable disease in Japan.^*1
It is characterized by severe developmental delay, intellectual disability, severe speech impairment, gait ataxia and/or tremulous movement of limbs, and a unique behavioral profile with frequent laughter, smiling, and an easily excited state.

◆ Prevalence and Distribution

Angelman syndrome occurs in approximately 1 in 15,000 live births, with no gender difference.^*3
In Japan, about 500 to 1,000 patients have been identified.^*1

Most cases are not inherited, but it can rarely occur within families, in which case the mother may be a carrier of the genetic abnormality.^*3
Furthermore, it is closely related to Prader-Willi syndrome, which is caused by abnormalities in the same region of chromosome 15, but their clinical symptoms differ significantly.

◆ Genetic Mechanisms

It is caused by a loss of function of the UBE3A gene located on chromosome 15q11-q13.^*1
The loss of function of the UBE3A gene can occur through four known mechanisms:^*3

1
Maternal microdeletion of 15q11-q13 (70%): Deletion of approximately 4 Mb
2
Paternal uniparental disomy (UPD) of chromosome 15 (5%): A phenomenon where both copies of chromosome 15 are inherited from the father
3
UBE3A gene mutation (10%): Presence of a loss-of-function variant in the maternally inherited UBE3A gene
4
Imprinting defect (5%): An abnormal imprinting status in the 15q11-q13 region despite chromosome 15 being inherited from both parents

Note that in the remaining 10%, no genetic abnormality can be identified.^*1

Symptoms and Characteristics

◆ Neurological Symptoms

1
Epileptic Seizures:Epilepsy co-occurs in approximately 80% to 90% of cases.
It most commonly begins in infancy as febrile seizures.^*1
Seizures are frequently generalized (such as tonic-clonic or myoclonic seizures), but absence and focal seizures are also observed.^*3
2
Motor Developmental Delay:It is often first suspected in infancy due to delays in gross motor development or hypotonia (floppiness).
The age at which independent walking starts is generally between 2.5 and 6 years.^*2
In severe cases, a wide-based, stiff, "robot-like" gait with arms raised and elbows flexed is typical.^*2
In fact, about 10% of children do not achieve independent walking.^*2

◆ Behavioral Features

As a behavioral characteristic, easily provoked laughter is well known.^*1
Other traits include hyperactive behavior, a strong curiosity, and a deep interest in water or shiny objects like plastic sheets.^*1

◆ Speech and Cognitive Function

It is rare for them to speak meaningful words, but receptive language (understanding) is relatively well-developed.^*1
Even if they acquire one or two words, consistent and appropriate usage remains rare.^*2

◆ Sleep Disorders

Sleep disorders are commonly associated from infancy to early childhood, with many families struggling with frequent night waking, delayed sleep onset, and early morning awakening.^*1

◆ Physical Features

Distinctive facial features, such as a flat back of the head (occipital flattening), occipital groove, a wide mouth, widely spaced teeth, tongue protrusion, and a prominent jaw (prognathism) have been reported, although their individual occurrence rate is less than 80%.^*2

Prognosis and Quality of Life

Comorbid intractable epilepsy is considered the primary factor affecting life expectancy.^*1 Although there are no precise statistics on lifespan, life expectancy is believed to be almost normal compared to the general population.^*2 However, achieving completely independent living is difficult. As they enter adulthood, physical activity levels tend to decline, which may lead to obesity and loss of walking ability.^*2

Treatment and Care

◆ Current Treatments

Treatment is supportive and symptomatic.^*1 Specifically, the following therapies are administered:

1
Management of Epilepsy:Seizures can often be controlled, or at least reduced to a level that does not interfere with daily life, with the appropriate use of anti-seizure medications.^*1
2
Addressing Sleep Disorders:In cases of severe sleep disturbance, the use of sleep-inducing medications may be considered.^*1
3
Therapy and Rehabilitation:It is recommended to implement physical therapy, occupational therapy, and speech therapy with a strong emphasis on non-verbal communication methods, such as augmentative and alternative communication (AAC) devices (e.g., picture cards, communication boards) or sign language.^*2

◆ Development of New Therapies

Antisense oligonucleotide (ASO) therapy and gene therapy targeting the restoration of UBE3A gene expression are currently under development and hold significant promise.^*3

Summary

Angelman syndrome is a congenital disorder caused by abnormalities of chromosome 15, characterized primarily by intellectual disability, epilepsy, speech delay, and a characteristic smiling appearance.
Most cases are not inherited.

Although life expectancy is believed to be almost normal, it can be influenced by severe epilepsy or obesity-related complications.
Moreover, because developmental and motor functions are affected long-term, ongoing medical management, rehabilitation, and educational support are crucial.
While current treatments focus on managing symptoms, research into novel gene therapies is underway.

As a cutting-edge treatment, GTX-102 (apaznosente), an antisense oligonucleotide (ASO) therapy aimed at restoring paternal UBE3A expression, has gained significant attention.
With this therapeutic candidate receiving Breakthrough Therapy Designation from the U.S. Food and Drug Administration (FDA) in June 2025, expedited development in the United States is anticipated.

This drug is developed by Ultragenyx Pharmaceutical Inc., and a global Phase 3 clinical trial (the Aspire study) is currently ongoing.^*4

For those considering prenatal testing, it is crucial to make decisions based on accurate and reliable information.

We hope this article helps deepen your understanding of Angelman syndrome and leads to the best choice for you and your family.

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References

Author

Tasuku Hiroshige, M.D., Ph.D.

M.D., Ph.D., Board Certified Specialist and Instructor of the Japanese Urological Association, Certified Physician of the Japan Society of Clinical Oncology, Board Certified Specialist of the Japanese Society of Anti-Aging Medicine, Certified Occupational Physician of the Japan Medical Association, Board Certified Physician of the Japanese Society of Chemotherapy, Board Certified Specialist of the Japanese Society for Sexually Transmitted Infections, Certificate of da Vinci System Training As a Console Surgeon, etc.
After graduating from Kagoshima University Faculty of Medicine in 2010, he has built extensive clinical experience as a urologist. In addition to clinical practice, he actively engages in academic activities, including presentations at conferences, writing papers, and obtaining research grants. He holds specialist certifications across a broad spectrum of medical fields, including urology, cancer therapy, anti-aging medicine, and infectious disease management. Utilizing his rich medical knowledge and skills, he provides compassionate, patient-centered care.

Frequently Asked Questions

Q.What is Angelman Syndrome?

Angelman syndrome is a designated intractable disease caused by the loss of function of the UBE3A gene located on chromosome 15q11-q13 (1). It presents with severe intellectual disability, epilepsy, speech impairment, and characteristic behaviors including frequent laughter and smiling (2). It occurs in approximately 1 in 15,000 live births, with no gender difference (3).

Q.What causes Angelman Syndrome?

The cause is the loss of function of the UBE3A gene, which can occur through four confirmed mechanisms (3): (1) maternal microdeletion (70%), (2) paternal uniparental disomy (5%), (3) UBE3A gene mutation (10%), and (4) imprinting defects (5%). In the remaining 10% of cases, the genetic abnormality cannot be identified (1). Most cases are sporadic and not inherited, but family occurrence can rarely occur where the mother is a carrier.

Q.What are the main symptoms of Angelman Syndrome?

There are five main symptoms: (1) epileptic seizures (co-occurring in 80–90%) (1); (2) motor developmental delays (walking starts between 2.5–6 years of age, and about 10% cannot walk independently) (2); (3) severe language impairment (meaningful speech is rare) (2); (4) behavioral characteristics such as easily provoked laughter and excitement (1); and (5) sleep disorders starting from infancy (1).

Q.What is the lifespan and prognosis for Angelman Syndrome?

Life expectancy is considered almost equivalent to that of the general population (2). However, comorbid intractable epilepsy can affect the prognosis (1). Complete independent living is difficult. In adulthood, risks of obesity and loss of walking ability due to decreased activity levels exist, making long-term medical management, rehabilitation, and educational support crucial (2).

Q.Is there a treatment for Angelman Syndrome?

Current treatment is primarily symptomatic and supportive (1). Specifically, it includes (1) seizure management using anti-seizure medications, (2) managing sleep disorders with sleep aids, and (3) physical, occupational, and speech therapy focusing on non-verbal communication training using picture cards or sign language (2). As a cutting-edge treatment, GTX-102 (apaznosente), which aims to restore paternal UBE3A expression, received Breakthrough Therapy Designation from the FDA in June 2025, and a global Phase 3 clinical trial is currently underway (4).