What is CDG?
Congenital disorders of glycosylation (CDG) is a large group of rare, inherited disorders that affect a complex process in the body called glycosylation. Most children who have CDG have neurological issues and symptoms, developmental problems, growth delays and problems with organs not working like they should.
Congenital means that CDG is a condition that happens at or before birth.
Notice that “disorders” is plural. This is because CDG is not just one disorder, but rather, a group of disorders. There are many types. Which type your child has depends on which body system is affected.
Glycans are sometimes called “sugar trees,” “antennas” or “sugar chains” by health care providers. They are built from sugar “building blocks.”
When some people hear sugar, they think of blood sugar, blood glucose or diabetes. This is not the case with glycans, which are not used for burning calories.
Glycosylation is the process of creating, changing and attaching these sugar building blocks to proteins and lipids.
- When the sugar building blocks attach to proteins, they are called “glycoproteins.”
- When the sugar building blocks attach to lipids, they are called “glycolipids.”
When someone has CDG, his or her body cannot properly add or attach the sugar building blocks to proteins or lipids. Every single system in the body needs the process of glycosylation to work right so the body can function normally. This is why people with CDG have many health problems—because many body systems are affected by glycosylation not working correctly.
What causes CDG?
To understand the cause of CDG, it can help to understand some basic concepts about genes and heredity.
What are genes?
Most people have 46 chromosomes in all their cells. Each chromosome is made up of a long chain of a chemical called deoxyribonucleic acid, or as you likely have heard it called, DNA.
A gene is a section of one of these DNA chains.
Everyone is unique because of their genes. Genes are like an instruction book that tells your cells how to work. For example, genes determine everything from your eye color to your blood type to your height.
CDG is caused by mistakes or changes in genes. These used to be called mutations. They are now called pathogenic variants.
Genetic variants are like words in a sentence that are spelled wrong or are in the wrong order. The sentence cannot be read correctly when this happens. Inherited genetic variants pass from a parent to a child. They are present at birth in every cell of the child’s body. You can inherit a genetic variant from either your mother or your father.
Pathogenic genetic variants can keep cells from doing their work correctly, which can lead to genetic diseases like CDG. Genes determine how well the process of glycosylation works in your body. There are over 400 genes in the human genome that have a role in the process of glycosylation.
How is CDG diagnosed?
CDG is usually diagnosed when your child is a baby. To diagnose CDG, your child’s health care provider does a thorough physical exam and talks to you about your and your family’s medical history.
Children with CDG are often misdiagnosed because their symptoms are similar to symptoms of other conditions. Some are mistakenly diagnosed with cerebral palsy or other neurological or genetic disorders. An important part of diagnosis is ruling out other conditions.
Your child’s health care provider will likely order tests. These tests may include:
- Genetic testing, which is the most reliable way to diagnose CDG. Genetic testing can also determine the type.
- Blood tests to check for many things, including missing sugar building blocks on hormone proteins, coagulation factors, transport proteins, and elevated liver enzymes (ALT and AST).
- A transferrin glycosylation test which is used to check for missing or incomplete sugar chains. Transferrin glycosylation results are often normal in many children with CDG. Special laboratory tests might be needed.
Depending on symptoms or problems your child has, you may be referred to specialists for tests of other conditions that are often associated with CDG.
Your child may have imaging exams done, such as liver ultrasound, X-rays and MRI scans. Results of these can be used by your provider to plan treatment.
What is the treatment for CDG?
Currently, there is no cure for CDG, but many treatments and therapies are available to help your child. These treatments are focused on treating the symptoms and the problems caused by the condition. This type of treatment is called supportive therapy.
Supportive therapy may include:
Physical therapy ─ A physical therapist (PT) can work with your child to develop muscle training and exercises that may help your child's strength, flexibility, balance, motor development, and mobility.
Speech therapy ─ Speech-language pathologists can help improve your child's ability to speak clearly or to communicate using sign language.
Occupational therapy ─ Occupational therapists can develop an exercise program for your child that includes stretching, strengthening, and functional use of the upper extremities.
There are two CDG types where a diet that includes a special sugar supplement can help improve most symptoms.
- For the type called MPI-CDG, mannose can help.
- For the type called PGM1-CDG, galactose can help.
Other treatment may include:
- Seizure management
- Nutrition care including tube feeding if needed
- Heart medication
- Hormone supplements to treat growth problems
- Antibiotics to treat or prevent infection
- Organ transplants