What Are FKTN-Related Disorders?
FKTN-related disorders, caused by mutations in the FKTN gene, include a spectrum of conditions that cause muscle weakness because of a deficiency in the fukutin (FKTN) protein. Some conditions in the spectrum have a milder course, while others may have severe symptoms and a significantly shortened lifespan. In addition, symptoms may vary greatly among individuals, even within the same family. The various conditions are described below in order of severity.
Walker-Warburg Syndrome (WWS)
The most severe presentation is WWS, which is characterized by significant muscle weakness, vision problems, changes in brain structure, and severe intellectual and developmental disabilities. Impaired vision may result from a wide rage of eye problems, such as unusually small eyes, glaucoma, cataracts, and changes in the retina. Changes in the brain are significant and often include a lack of the normal folding structure resulting in a bumpy “cobblestone” appearance (cobblestone lissencephaly) or a build-up of fluid around the brain (hydrocephalus). Most children with WWS are unable to sit or walk independently, and seizures are common.
Fukuyama Congenital Muscular Dystrophy (FCMD)/Muscle-Eye-Brain Disease (MEB)
FCMD and MEB are two FKTN-related disorders with overlapping symptoms. Individuals with these conditions experience significant muscle weakness, changes in the structure of the brain, and abnormalities of the eyes that are similar to those of WWS but tend to be less severe. Rarely, individuals with these conditions learn to walk at later stages in life and may speak a few words.
Limb Girdle Muscular Dystrophy Type 2M (LGMD2M)
LGMD2M is the mildest presentation of FKTN-related disorders. Age of onset and severity of symptoms vary greatly among individuals. Typically, the only symptom is weakness in the muscles closest to the center of the body, specifically the muscles of the shoulders, upper arms, pelvic area, and thighs. Rarely, patients have a heart condition called dilated cardiomyopathy. The brain and eyes are not affected.
How Common Are FKTN-Related Disorders?
The overall prevalence of FKTN-related disorders has not been established. WWS is considered rare, but may be more common in the Ashkenazi Jewish population, with an estimated prevalence of 1 in 90,000. FCMD is rarely seen outside of the Japanese population, where the prevalence is approximately 1 in 140,000. The precise prevalence of MEB and LGMD2M is currently unknown.
How Are FKTN-Related Disorders Treated?
There is no treatment for the underlying cause of FKTN-related disorders. Available treatments address only the symptoms of the condition. For the more severe conditions, treatment may include medication to control seizures, surgery to reduce fluid build-up around the brain, placement of a feeding tube, and physical and occupational therapy to aid in movement. Management for LGMD2M mainly involves physical and occupational therapy, the use of assistive devices, and monitoring for heart and breathing complications.
What Is the Prognosis for an Individual with an FKTN-Related Disorder?
The prognosis depends on the severity of the condition. Individuals with WWS typically do not survive past three years of age. For those with FCMD or MEB, death typically occurs in childhood or adolescence. Those with LGMD2M generally have a normal lifespan if they do not have heart complications.