Noonan syndrome

Author: Jonathan Chan, RMO, Waikato Hospital, Hamilton, New Zealand. Editor-in-Chief: A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. Copy editor: Gus Mitchell. September 2017.

What is Noonan syndrome?

Noonan syndrome is a common autosomal dominant disorder associated with mutations in the RAS/MAPK pathway, and is one of a group of conditions collectively known as RASopathies.

Noonan syndrome is characterised by:

Noonan syndrome is also known as:

*Image sourced from Wikipedia

What causes Noonan syndrome?

Noonan syndrome is caused by mutations in genes associated with the RAS/MAPK cell-signalling pathway, which is required for normal cell division, proliferation, differentiation and migration. These mutations lead to loss of regulation of cell growth and division.

Mutations in the PTPN11 gene are implicated in > 50% of cases. Gene mutations in SOS1 are implicated in 10–15% and mutations in RAF1 and RIT1 account for approximately 5% each. A number of other genes account for the rest.

The genetic cause behind Noonan syndrome is unknown in up to 20% of cases.

What are the clinical features of Noonan syndrome

Noonan syndrome affects males and females in all races. 50–70% of people with Noonan syndrome are of short stature. Weight and length at birth are usual normal, but growth slows over time. This is thought to be associated with abnormal levels of growth hormone.

Noonan syndrome presents with distinctive facial features, such as:

Other clinical manifestations of Noonan syndrome include:

Congenital heart disease is common in most cases of Noonan syndrome, with the most common defect being pulmonary valve stenosis.

Cutaneous symptoms of Noonan Syndrome are varied and literature on the topic is limited. Documented cutaneous effects of Noonan syndrome include:

Noonan syndrome with multiple lentigines, formerly known as LEOPARD syndrome, is similar to Noonan syndrome. It has characteristic cutaneous features, including:

What are the complications of Noonan syndrome?

Hypertrophic cardiomyopathy is common in those with critical congenital pulmonary valve stenosis.

Individuals with Noonan syndrome have an eight-fold increased risk of developing haematological malignancies, the most common being leukaemia.

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