Myhre syndrome

Myhre syndrome is a condition with features affecting many systems and functions of the body.

People with Myhre syndrome usually have delayed development of language and motor skills such as crawling and walking. Most have intellectual disability that ranges from mild to moderate. Some have behavioral issues such as features of autism or related developmental disorders affecting communication and social interaction.

People with Myhre syndrome often have hearing loss, which can be caused by changes in the inner ear (sensorineural deafness), changes in the middle ear (conductive hearing loss), or both (mixed hearing loss).

Growth is reduced in people with this disorder, beginning before birth and continuing through adolescence. Affected individuals have a low birth weight and are generally shorter than about 97 percent of their peers throughout life.

People with Myhre syndrome typically have stiffness of the skin and are usually described as having a muscular appearance. Skeletal abnormalities associated with this disorder include thickening of the skull bones, flattened bones of the spine (platyspondyly), broad ribs, underdevelopment of the winglike structures of the pelvis (hypoplastic iliac wings), and unusually short fingers and toes (brachydactyly). Affected individuals often have joint problems (arthropathy), including stiffness and limited mobility.

Typical facial features in people with Myhre syndrome include narrow openings of the eyelids (short palpebral fissures), a shortened distance between the nose and upper lip (a short philtrum), a sunken appearance of the middle of the face (midface hypoplasia), a small mouth with a thin upper lip, and a protruding jaw (prognathism). Some affected individuals also have an opening in the roof of the mouth (a cleft palate), a split in the lip (a cleft lip), or both.

Other features that occur in some people with this disorder include constriction of the throat (laryngotracheal stenosis), high blood pressure (hypertension), heart or eye abnormalities, and in males, undescended testes (cryptorchidism).

A disorder sometimes called laryngotracheal stenosis, arthropathy, prognathism, and short stature (LAPS) syndrome is now generally considered to be the same condition as Myhre syndrome because it has similar symptoms and the same genetic cause.

Myhre syndrome is a rare disorder. Only about 30 cases have been documented in the medical literature. For reasons that are unknown, most affected individuals have been males.

Mutations in the SMAD4 gene cause Myhre syndrome. The SMAD4 gene provides instructions for making a protein involved in transmitting chemical signals from the cell surface to the nucleus. This signaling pathway, called the transforming growth factor beta (TGF-β) pathway, allows the environment outside the cell to affect how the cell produces other proteins. As part of this pathway, the SMAD4 protein interacts with other proteins to control the activity of particular genes. These genes influence many areas of development.

Some researchers believe that the SMAD4 gene mutations that cause Myhre syndrome impair the ability of the SMAD4 protein to attach (bind) properly with the other proteins involved in the signaling pathway. Other studies have suggested that these mutations result in an abnormally stable SMAD4 protein that remains active in the cell longer. Changes in SMAD4 binding or availability may result in abnormal signaling in many cell types, which affects development of several body systems and leads to the signs and symptoms of Myhre syndrome.

Read more about the SMAD4 gene.

This condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder.

Most cases result from new mutations in the gene and occur in people with no history of the disorder in their family.