Noonan syndrome

Noonan syndrome presents with a characteristic phenotype that evolves with age. In infancy, features include a webbed neck, low-set ears, ptosis, and hypertelorism. Short stature is common, with growth hormone deficiency in some cases. Congenital heart defects occur in about 50–80% of individuals, most commonly pulmonary valve stenosis and hypertrophic cardiomyopathy. Other findings include chest deformities (pectus excavatum or carinatum), cryptorchidism in males, and mild intellectual disability in about 25% of cases. Diagnosis is primarily clinical, using scoring systems such as the van der Burgt criteria, and is confirmed by molecular genetic testing.

Noonan syndrome is caused by mutations in genes encoding components of the RAS-mitogen-activated protein kinase (MAPK) pathway, which regulates cell proliferation and differentiation. Approximately 50% of cases are due to mutations in PTPN11, which encodes the protein tyrosine phosphatase SHP-2. Other implicated genes include SOS1, RAF1, KRAS, NRAS, BRAF, and MAP2K1. These mutations lead to dysregulation of the pathway, resulting in the characteristic developmental abnormalities. The condition is inherited in an autosomal dominant manner, but many cases are sporadic. Genetic testing is available and can aid in diagnosis and genetic counseling.

Management of Noonan syndrome is multidisciplinary, addressing cardiac, growth, and developmental issues. Cardiac defects are treated with medication or surgery, such as balloon valvuloplasty for pulmonary stenosis. Growth hormone therapy is approved to improve final adult height in children with short stature. Developmental delays and learning difficulties are managed with educational support and early intervention. Regular monitoring for complications, including bleeding disorders and lymphatic abnormalities, is recommended. With appropriate care, most individuals have a normal lifespan, though outcomes depend on the severity of cardiac and other complications.

Beyond the classic features, Noonan syndrome has several lesser-known associations. It is part of a family of RASopathies, which include Costello syndrome and cardiofaciocutaneous syndrome, with overlapping phenotypes. Hematologic abnormalities, such as thrombocytopenia and platelet dysfunction, can cause bleeding tendencies. Lymphatic dysplasia may lead to peripheral lymphedema or chylothorax. Some individuals have a predisposition to juvenile myelomonocytic leukemia, particularly those with PTPN11 or KRAS mutations. The condition has been identified in historical figures, including a possible diagnosis in King Henry VIII, based on clinical features and reproductive history. Additionally, the first description by Jacqueline Noonan was initially met with skepticism, but the syndrome is now widely recognized.

RASopathy

A group of genetic syndromes caused by mutations in genes of the RAS-MAPK pathway.

Hypertrophic cardiomyopathy

A condition in which the heart muscle becomes abnormally thick.

PTPN11

A gene encoding the protein tyrosine phosphatase SHP-2, commonly mutated in Noonan syndrome.