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What is Noonan syndrome, and what are the signs and symptoms of this condition?

Noonan syndrome is a developmental disorder characterized by unusual facial characteristics, short stature, heart defects, bleeding problems, and skeletal malformations. Eye abnormalities occur in up to 95 percent of people with Noonan syndrome. Problems with language and speech are common. Puberty for both males and females with Noonan syndrome is usually delayed for approximately two years. Most males with this disorder have undescended testicles (cryptorchidism) which can lead to infertility (inability to father a child) later in life. The majority of children diagnosed with Noonan syndrome have normal intelligence, but a small percentage have special educational needs, and some have mental retardation.

How common is Noonan syndrome?

Noonan syndrome occurs in approximately 1 in 1,000 to 2,500 people.

What genes are related to Noonan syndrome?

Mutations in the KRAS, PTPN11, RAF1, and SOS1 genes cause Noonan syndrome.

Approximately 50 percent of individuals with Noonan syndrome have mutations in the PTPN11 gene. Mutations in the SOS1 gene are seen in 20 percent of those with Noonan syndrome. Mutations in the RAF1 gene account for between 10 and 15 percent of Noonan syndrome cases. About 5 percent of people with Noonan syndrome have mutations in the KRAS gene and usually have a more severe or atypical form of the disorder. The cause of Noonan syndrome in the remaining 10 to 15 percent of people with this disorder is unknown.

The PTPN11, SOS1, KRAS, and RAF1 genes all provide instructions for making proteins that are important for the proper formation of several types of tissue during development. These proteins also play roles in cell division, cell movement, and cell differentiation (the process by which cells mature to carry out specific functions).

Mutations in the PTPN11 gene, SOS1 gene, KRAS gene, or RAF1 gene cause the resulting protein to be continuously active, rather than switching on and off in response to signals that control growth and development. This constant activation disrupts the regulation of systems that control cell growth and division, leading to the characteristic features of Noonan syndrome.