What is Congenital Disorder of Glycosylation? (CDG)
Congenital Disorders of Glycosylation (CDG), formerly called carbohydrate-deficient glycoprotein syndrome, are a group of inherited disorders that affect a process called glycosylation.
Glycosylation is a process by which all human cells build long sugar chains that are attached to proteins. Together the proteins and their attached sugars are called glycoproteins. Glycoproteins, have many very important functions in the human body and are required for the normal growth and function of all tissues and organs. The process or pathway which makes this glycosylation takes at least 100 steps, and specialized proteins called enzymes trigger each step. Hundreds of enzymes are used in making the sugar chains and adding them to thousands of proteins. Sometimes coordinated groups of enzymes work in a specific order to add some sugars, or cleave others from the maturing chain. In individuals born with CDG, one of the many glycosylation enzymes in the pathway malfunctions. However, the impact on the body structures and functions differs depending upon the altered enzyme. CDG is caused by a genetically inherited change or malfunction of one of these enzymes. If one of these enzymes malfunction then the cells in the body of a child or adult cannot glycosylate correctly. This incorrect glycosylation is the underlying basis of the important medical issues in individuals with CDG.
Types of CDG
The CDG’s are divided into groups I and II based on the location in the biochemical pathway which the defect occurs. Defective genes are lettered in chronological order of their discovery. CDG research is quickly evolving, types are now recognized to include Ia through IL and IIa through IIf. Many are so new that only 1 or 2 patients are known. Scientists are working now to identify other types of CDG.
(My ladybug is one of 3 little ones that have CDG SL35A2 and is thus far the only girl in the world with this type of CDG)
Signs and Symptoms
The symptoms and severity of CDG vary from child to child. Some of the symptoms become more prominent at different ages. Most types of CDG are associated with minor differences in facial and body features, neurological problems, slow growth, clotting problems, liver and/or intestinal problems and the most heart breaking of all is that many children with CDG succumb to the disorder as there is no treatment or cure.
Some of the children have significant medical problems during infancy. Physicians should suspect CDG in children who present with the following signs and symptoms:
• hypotonia (low muscle tone)
• failure to thrive (slow growth)
• developmental delay
• hepatopathy (liver disease)
• coagulopathy (bleeding tendancies)
• esotropia (crossed eyes)
• cerebellar hypoplasia (changes in the brain that can be seen on developmental delay)
At a later age, adolescence or adulthood, affected individuals may have these additional clinical features:
• ataxia (poor balance)
• dysarthria (slurred speech)
• absent puberty in females
• retinitis pigmentosa (pigment in the retina of the eye)
• progressive scoliosis (curvature of the spine)
• joint contractures
There is no specific medicine to treat CDG, except for CDG-Ib and some CDG-IIc patients. Current treatment for CDG patients is supportive therapy and treatment of symptoms and sequelae. The effective therapy for CDG-Ib is oral mannose. CDG-Ib presents with protein-losing enteropathy, coagulopathy and liver disease without neurological involvement. These patients have significant gastrointestinal problems, but are neurologically and intellectually normal. Fucose supplements have been used to treat patients with CDG-IIc who have a defective GDP- Fucose transporter. Infections cease and health improves. Unfortunately, fucose does not improve or reverse the developmental delay.