MN1 C-terminal truncation syndrome

MN1 C-terminal truncation (MCTT) syndrome is a condition characterized by intellectual disability, developmental delay, distinctive facial features, and brain abnormalities.

Most people with MCTT syndrome have mild to moderate intellectual disability. Many affected individuals are nonverbal, but some have speech limited to one or two words or communicate using sign language. Most children with this condition have delayed development of motor skills, such as crawling or walking, but are able to walk by age 2 or 3. However, they often need help with fine-motor skills, such as getting dressed or using a fork when eating.

Individuals with MCTT syndrome often have distinctive facial features that include a sunken appearance of the middle of the face (midface hypoplasia); a high arch in the roof of the mouth (high-arched palate); outside corners of the eyes that point downward (downslanting palpebral fissures); widely spaced eyes (hypertelorism ); shallow and bulging eyes (exophthalmos); a short, upturned nose; and small, low-set ears. Some affected individuals have dental abnormalities, such as cone-shaped (conical ), jagged, or crowded teeth. Rarely, people with MCTT syndrome have premature fusion of certain skull bones (craniosynostosis).

People with MCTT syndrome often have characteristic brain abnormalities. The surface of the brain normally has many ridges or folds, called gyri. A common brain abnormality in people with MCTT syndrome is called perisylvian polymicrogyria, in which an area of the brain called the perisylvian region develops too many gyri, and the folds are irregular and unusually small. Individuals with MCTT syndrome can also have a malformation of the part of the brain that coordinates movement (the cerebellum ). This malformation, called atypical rhombencephalosynapsis, is characterized by tissue loss in the central part of the cerebellum (known as the vermis) and fusion of the two sides of the cerebellum. These brain abnormalities likely contribute to the movement problems and intellectual disability that are common in MCTT syndrome. 

Less common features of MCTT syndrome include hearing loss, seizures, abnormal curvature of the spine, and heart abnormalities.