What are congenital disorders of glycosylation (CDG)?

Congenital disorders of glycosylation (CDG) are a large group of rare genetic disorders that affect the addition of sugar building blocks, called glycans, to  proteins in cells throughout the body. The addition of glycans to proteins is critical to the healthy function of cells. People with CDG have a wide range of health problems because of this chemical malfunction.
While glycosylation involves sugar, as glycans are compounds of sugar molecules, CDG are not related to diabetes. Instead, CDG cause problems in the way sugar building blocks are attached to proteins within and on the surfaces of cells, affecting how cells in every part of the body function.

Causes of CDG

CDG are genetic disorders, which means that, in most cases, they are inherited from a child’s parents. In most forms of CDG, that inheritance occurs only when both parents carry the genetic mutation, generally with no symptoms themselves. (This is called an autosomal recessive pattern of inheritance.)

We inherit pairs of each of our genes, one from each parent. In autosomal recessive forms of CDG, if only one copy of a gene’s pair has the mutation, a person will not have CDG, but that person will be a carrier of the disorder. When two carriers have children together, the odds are one in four that any child they produce will have CDG. (The odds are also one in four that a particular child will not inherit the gene mutation at all, and one in two, a 50 percent risk, that the child will be a carrier.)

Forms of CDG

Several forms of CDG, such as EXT1/EXT2-CDG, are autosomal dominant conditions, which means that they can be inherited from either parent. Only one copy of the gene is needed for the condition to be expressed. If a parent has this form of CDG, the risk of passing it on is 50 percent for each child produced, regardless of the sex of the parent or child.

In some cases, the gene mutations that cause CDG are not inherited from the parents. They are simply random mutations and are new in the people who develop the disorders.

Over 400 genes play roles in the healthy expression of glycosylation, and mutations in any of roughly 130 of these have been found to cause different forms of CDG. Because the study of CDG is relatively new, more genes may yet be found to be involved. The first forms of CDG were identified in the 1980s, and knowledge of the conditions continues to expand as new patients are identified and additional research is done.

Signs and symptoms of CDG

CDG affects cell function in many parts of the body, so a combination of unexplained health problems can be an indication of the disorder. Symptoms of CDG in infancy and childhood may include:

• low muscle tone or floppiness (hypotonia)
• poor growth, failure to thrive
• developmental delays
• liver disease (hepatopathy) with elevated liver enzymes
• abnormal bleeding or blood clotting
• misaligned or crossed eyes (strabismus)
• seizures
• stroke-like episodes
• heart problems, including fluid accumulation around the heart or lungs (pericardial or pleural effusion) or thickening and stiffening of the heart muscle (cardiomyopathy)

As children enter adolescence and grow to adulthood, additional symptoms may include:

• balance and coordination problems (ataxia)
• slurred speech (dysarthria)
• no puberty in girls
• progressive curvature of the spine (neuromuscular scoliosis)
• joint contractures
• poor night vision and loss of peripheral vision (signs of retinitis pigmentosa)

Brain imaging may show an undersized cerebellum (cerebellar hypoplasia), another sign of CDG.

Symptoms vary by form of CDG, and can range from mild to severe, even among people with the same form of CDG.

Testing and diagnosis for CDG

Many of the symptoms of CDG are similar to those of other conditions, and patients with CDG are often misdiagnosed at first with different genetic disorders or with unrelated conditions such as cerebral palsy.

Doctors with experience in diagnosing the various forms of CDG suggest that CDG be considered as a possible diagnosis whenever a person has unexplained symptoms affecting multiple body systems or when a single health problem cannot be otherwise explained. Because many forms of CDG have only recently been identified, and because so many are quite rare, it is thought that many people with CDG may remain undiagnosed or misdiagnosed.

When a diagnosis of CDG is suspected — based on symptoms, a detailed patient history and a thorough examination — clinical testing is needed to confirm the diagnosis and identify the specific form of CDG.

• Certain forms of CDG, , may be broadly identified with a blood test to detect abnormal glycans. Once a glycosylation defect is found, additional tests must be done to identify the specific CDG subtype.
• Molecular genetic testing is required to confirm a diagnosis of CDG and to identify the specific form.

Treatment for CDG

There is no known cure for CDG, but treatment is available to manage symptoms and to improve the quality of life for people with the condition. Because there are so many forms of CDG, and because each case presents with different symptoms and different levels of severity, the treatment plan for each child is unique.

Treatments for patients with CDG may include:

• feeding therapy for growth or eating problems, including special formula for a baby, thickening liquid foods or the use of a nasogastric (NG) tube or G-tube
• early intervention with occupational, speech and physical therapy for developmental delays, and ongoing therapy through childhood and adolescence as needed
• antiepileptic medication or epilepsy surgery for seizures
• plasma infusions or blood thinners to treat blood clotting problems
• patching, glasses or corrective surgery to treat misaligned or crossed eyes
• regular monitoring of heart problems, such as fluid accumulation around the heart or thickening and stiffness of the heart muscle, and treatment with medication, drainage of fluid or surgery if warranted
• hormone treatment for thyroid problems
• albumin infusions, vitamin K supplementation and regular monitoring of liver function when there is a risk of developing liver failure
• rare sugar therapy (with mannose or galactose) for the treatment of particular sugar-responsive types of CDG such as MPI-CDG, PGM1-CDG, and SLC35A2-CDG

As children with CDG age into adolescence and adulthood, additional treatment and support may be needed, including:

• therapy, medication, assistive equipment or surgery for orthopedic problems
• low-vision aids, training or therapy for vision loss from retinitis pigmentosa
• life skills and vocational training to enable independent living

Clinical trials may also be an option. The Congenital Disorders of Glycosylation (CDG) Clinic at Children’s Hospital of Philadelphia (CHOP) can tell you about relevant research studies and serve as an access point for enrollment. You can also find a list of ongoing studies at www.clinicaltrials.gov.

Outlook for CDG

Children with CDG are typically happy and engaging. Each has a unique personality.

The outlook for children with CDG depends on the nature and severity of their neurological and health problems. Most will need a team of medical specialists to monitor their health over time and adjust needed treatments. Some will require little medical intervention, while others will deal with life-threatening medical issues and may require frequent or lengthy hospitalizations.

Many children with CDG will deal with cognitive or physical disabilities throughout their lives. The impact of these disabilities on their lives can be minimized and the quality of their lives improved with physical, occupational and speech therapy. That therapy may be needed into adulthood.

Follow-up care for CDG

Children with CDG need regular monitoring by appropriate medical specialists, and most will need ongoing therapy and treatment.

Why choose CHOP?

CHOP’s CDG Clinic offers comprehensive clinical care and diagnostic testing and coordinates with specialists throughout the hospital to provide the most advanced treatments available to infants, children and adolescents living with these genetic conditions. The Clinic also coordinates with local doctors to ensure appropriate monitoring and care.

The medical team at CHOP’s CDG Clinic includes some of the world’s leading experts on diagnosing and treating these conditions. The Clinic’s specialists are engaged in research that may lead to clinical trials, new treatments and potential cures.

For parents of children with CDG, the Clinic provides training on special care needs, as well as genetic counseling to explain the risk of having another child with the condition.