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Noonan syndrome
Noonan syndrome is a relatively common genetic disorder that can impact growth, heart development, and physical appearance. It is caused by changes in genes involved in cell signalling and can affect individuals from birth.
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Overview
Noonan syndrome is a genetic condition that affects many areas of the body, including facial features, heart structure, and growth [PMID:32655959]. It is classified as a RASopathy, a group of disorders caused by pathogenic variants in genes involved in the RAS/MAPK signalling pathway. This pathway is crucial for cell growth, division, and development.
The condition affects approximately 1 in 1,000 to 2,500 people, making it one of the more common genetic syndromes. Noonan syndrome can present with a wide range of signs and symptoms, varying significantly even within the same family. While some individuals experience mild effects, others may have more significant health challenges requiring ongoing medical care.
Symptoms & clinical features
The signs and symptoms of Noonan syndrome can vary widely in their presentation and severity. Characteristic facial features are often present, including a broad forehead, widely spaced eyes (hypertelorism), down-slanting eyes, droopy eyelids (ptosis), a short neck, and low-set ears with a thickened helix [PMID:32655959]. These features can become more subtle with age.
Heart defects are common, affecting up to 80% of individuals with Noonan syndrome. The most frequent heart condition is pulmonary valve stenosis, a narrowing of the valve that controls blood flow from the heart to the lungs. Other potential heart problems include hypertrophic cardiomyopathy, where the heart muscle thickens, and various septal defects (holes in the heart).
Other common features can include short stature, often noticeable by late childhood, and feeding difficulties in infancy. Blood clotting disorders, undescended testicles in males, learning difficulties, and skeletal abnormalities such as a sunken or protruding chest (pectus excavatum or carinatum) may also occur. Skin and hair differences, such as dry skin or sparse hair, can also be part of the syndrome.
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Affected organs
Noonan syndrome can affect multiple organ systems. The heart is frequently involved, with a high percentage of individuals experiencing congenital heart defects. The skeletal system can also be affected, leading to short stature and chest wall deformities. The reproductive system in males may be impacted, commonly presenting as undescended testicles (cryptorchidism).
Other affected areas can include the lymphatic system, potentially causing swelling (lymphoedema), and the blood system, which may result in an increased tendency for bleeding or bruising due to clotting factor deficiencies. The brain and nervous system can also be affected, leading to developmental delays or learning difficulties in some individuals.
Risks & severity
The severity of Noonan syndrome varies considerably between individuals, even those with the same genetic cause. Some people may have very mild signs that go unnoticed for years, while others may experience significant health issues from infancy. The most serious complications are often related to congenital heart defects, which can require surgical intervention.
Individuals with Noonan syndrome may have a slightly increased risk of developing certain cancers, particularly childhood leukaemias (such as juvenile myelomonocytic leukaemia) and solid tumours. However, the overall risk remains relatively low. Developmental delays and learning difficulties are observed in some individuals, ranging from mild to more significant challenges, often requiring educational support. Close monitoring and early intervention are important to manage potential complications and support development.
Genetic causes
Noonan syndrome is caused by pathogenic variants in several different genes, all of which play a role in the RAS/MAPK signalling pathway. This pathway is a critical network within cells that controls many fundamental processes, including cell growth, division, differentiation, and migration. When a gene in this pathway has a pathogenic variant, it can disrupt these normal cellular functions, leading to the diverse features of Noonan syndrome.
The most commonly implicated gene is PTPN11, which accounts for about 50% of cases [PMID:32655959]. PTPN11 produces the SHP-2 protein, which acts as a molecular switch in the RAS/MAPK pathway. Other genes associated with Noonan syndrome include SOS1, RAF1, and RIT1. Variants in SOS1 affect a protein that helps activate other proteins in the pathway. RAF1 encodes a protein involved in transmitting signals for cell proliferation and development. RIT1 produces a protein that helps cells respond to stress by transmitting growth signals. Pathogenic variants in any of these genes lead to an overactive signalling pathway, resulting in the characteristic features of Noonan syndrome.
- PTPN11 protein tyrosine phosphatase non-receptor type 11The PTPN11 gene provides instructions for making the SHP-2 protein, which plays a crucial role in cellular growth, differentiation, migration, and programmed cell death.
- RAF1 Raf-1 proto-oncogene, serine/threonine kinaseThe RAF1 gene provides instructions for a protein that is a key component of the RAS/MAPK signalling pathway, regulating fundamental cellular processes such as growth, division, and differentiation.
- RIT1 Ras like without CAAX 1RIT1 encodes a small GTPase that regulates cellular stress responses and RAS/MAPK pathway signalling, with variants causing Noonan syndrome and linked to certain blood cancers.
- SOS1 SOS Ras/Rac guanine nucleotide exchange factor 1The SOS1 gene provides instructions for a protein that plays a key role in regulating cell growth, division, and differentiation by controlling the RAS/MAPK signalling pathway.
Inheritance pattern
Noonan syndrome is typically inherited in an autosomal dominant pattern. This means that only one copy of the altered gene is needed to cause the condition. In most cases (about 50%), the pathogenic variant occurs spontaneously as a new change (de novo) in the affected individual and is not inherited from either parent. In these situations, neither parent carries the variant, and the chance of it recurring in future children is very low, though not zero due to germline mosaicism.
When Noonan syndrome is inherited from a parent, that parent also has the condition, though they may have milder symptoms. In such cases, there is a 50% chance for each child of an affected parent to inherit the pathogenic variant and therefore develop Noonan syndrome. Genetic counselling can help families understand the specific inheritance pattern and implications for future children.
Each child has a 50% chance of inheriting the pathogenic variant, regardless of sex.
Diagnosis & testing
A diagnosis of Noonan syndrome is often suspected based on the presence of characteristic physical features, such as the facial appearance, short stature, and heart defects. A healthcare professional, typically a paediatrician or clinical geneticist, will conduct a thorough physical examination and review the individual's medical history.
Confirmation of Noonan syndrome usually involves genetic testing. This can be performed using a blood sample to look for pathogenic variants in the genes known to cause the condition, such as PTPN11, SOS1, RAF1, and RIT1. The NHS Genomic Medicine Service (GMS) offers genomic testing for Noonan syndrome under specific National Genomic Test Directory R-codes. Referral for genetic testing is typically made by a clinical geneticist or another specialist following an initial clinical assessment.
Management & lifestyle
Management of Noonan syndrome is multidisciplinary and focuses on addressing the specific symptoms and complications an individual experiences. Regular medical follow-up is important, often involving specialists such as cardiologists for heart conditions, endocrinologists for growth issues, and developmental paediatricians for developmental progress.
Treatment for heart defects may include medication or surgery, depending on the type and severity. Growth hormone therapy may be considered for children with significant short stature. Early intervention programmes, including physical, occupational, and speech therapy, can be beneficial for developmental delays. Regular monitoring for potential complications like blood clotting issues and hearing or vision problems is also part of routine care. Individuals and families are encouraged to work closely with their healthcare team to develop a personalised care plan.
UK care pathway
In the UK, individuals suspected of having Noonan syndrome are typically referred to a clinical genetics service. Clinical geneticists and genetic counsellors can provide diagnosis, information about the condition, and discuss inheritance patterns and risks. Genetic testing for Noonan syndrome is available through the NHS Genomic Medicine Service (GMS) as part of the National Genomic Test Directory (e.g., under relevant R-codes for RASopathies).
Following a diagnosis, care is usually managed by a multidisciplinary team of specialists tailored to the individual's specific needs, often coordinated through paediatric services. Genetic counsellors offer support and information throughout the diagnostic process and beyond, helping families understand the implications of a genetic diagnosis.
Frequently asked questions
Is Noonan syndrome always inherited from a parent?
No, about half of all cases of Noonan syndrome occur due to a new genetic change (de novo) in the affected individual, meaning it is not inherited from either parent. In the other half of cases, it is inherited from a parent who also has the condition.
What are the most common health problems associated with Noonan syndrome?
Heart defects, particularly pulmonary valve stenosis, are very common. Short stature, feeding difficulties in infancy, and developmental delays can also occur. The severity of these issues can vary widely.
Will my child with Noonan syndrome have learning difficulties?
Learning difficulties can occur in some individuals with Noonan syndrome, ranging from mild to more significant challenges. However, many individuals have typical intelligence. Early developmental assessments can help identify needs and provide appropriate support.
Is there a cure for Noonan syndrome?
There is no cure for Noonan syndrome, as it is a lifelong genetic condition. However, symptoms can be managed effectively through multidisciplinary medical care, including therapies, medications, and sometimes surgery, to improve quality of life and address specific health concerns.
How often should someone with Noonan syndrome be monitored?
The frequency of monitoring depends on the individual's specific symptoms and age. Regular check-ups with a cardiologist, endocrinologist, and developmental paediatrician are often recommended. Your healthcare team will establish a personalised follow-up schedule.
References
- Zenker M. Clinical overview on RASopathies. American journal of medical genetics. Part C, Seminars in medical genetics. 2022. PMID: 36428239