Small sugar chains protruding from the surface of proteins ensure that they fold properly and are sent to the right places in the body. The mechanism for attaching these chains to proteins is called glycosylation. Errors in this mechanism are often fatal, and the few children born with this type of disorder are often misdiagnosed. Using gene matching, researchers identified the novel GALNT2 congenital disorder of glycosylation (GALNT2-CDG), which is caused by an error in the sugar chains. The disorder, which is too rare for medication to be developed, can be screened for in future pregnancies and its discovery provides important clarification for the people affected.
Children with congenital disorders of glycosylation may suffer from epilepsy, developmental delay, autistic features, decreased stature and chronic insomnia. However, children are often misdiagnosed, since these disorders are rare or unknown. For the parents of these children, the uncertainty about what is wrong with their child can be almost unbearable. Researchers have identified a novel congenital disorder of glycosylation, GALNT2-CDG, caused by loss of function of the enzyme GALNT2 (polypeptide N-acetyl-D-galactosamine-transferase 2).
“We monitored two siblings with epilepsy, intellectual disabilities and decreased stature at the Children’s Outpatient Clinic of Filadelfia Epilepsy Hospital. We had a really hard time figuring out what was wrong with these children. We used a genetic test and then made comparisons with other reported gene mutations in the GeneMatcher database. We found five more children worldwide with similar symptoms, all with the same genetic disorder. Although there is no treatment today, we can now explain what is wrong and we can advise and screen for this if the parents wish to have more children,” explains the joint first author, Monica Zilmer, neuropediatrician, Department of Paediatrics, Filadelfia Epilepsy Hospital, Dianalund, Denmark.
Help was right around the corner
Congenital disorders of glycosylation are a growing group of rare genetic disorders caused by errors in sugar chains on proteins and lipids. Glycosylation is one of the most frequent modifications of proteins. It takes place after the protein has been translated and can induce proteins to fold in a special way, be transported to a special place or to communicate with other proteins in the body. Without glycosylation, this information is lost.
“These people have a serious disorder. The disorder is very rare, in part also because we suspect that many fetuses with this disorder do not survive. Since the disorder affects all the cells in the body and produces many types of symptoms, diagnosis can be difficult. Genetic testing is a very effective tool and knowing which gene is affected and the type of mutation allows you to ask around the world to see whether others have found the similar mutations in other patients. We had the genetic profile of our siblings with the disorder and searched in GeneMatcher, the equivalent of a dating app for genetics. And we got hits,” explains another main author, Rikke Steensbjerre Møller, Professor and Head, Department of Epilepsy Genetics and Personalized Medicine, Filadelfia Epilepsy Hospital.
In addition to two previously known cases, the researchers used GeneMatcher to identify five new patients with similar symptoms and similar genetic mutations from Afghanistan, Egypt, Italy and Syria – including siblings with the same symptoms.
“When we discovered that the gene affected encodes an enzyme that attaches sugar chains to proteins, we searched for experts with whom we could collaborate to help us understand the challenges these children face in more detail,” says Rikke Steensbjerre Møller.
Help was just around the corner, since Katrine T. Schjoldager’s group at the Copenhagen Center for Glycomics, University of Copenhagen studies the biological role of GALNT2 and glycosylation in health and disease.
“They all have an error in a specific enzyme that attaches sugar chains to the serine and threonine amino acids. This process is called O-linked glycosylation, and we recently found that this modification is much more frequent than we thought. In fact, we estimate that 80% of all proteins have this O-linked glycosylation, so we were not surprised that disruption of this process would have the dramatic effect we found,” says Katrine T. Schjoldager.
Clarity reassures
The GALNT2 gene encodes the GALNT2 enzyme, and a mutation that disrupts the function of the encoded enzyme causes the congenital disorder. For the seven patients with GALNT2-CDG, the defective GALNT2 gene was inherited from healthy parents, who each passed a recessive copy of the gene to their child, who then got the disorder because of two recessive genes.
“GALNT2 is expressed in most types of cells and catalyses the cell to attach these sugar chains. We found one of the proteins, apolipoprotein C-III, that requires this specific enzyme to attach sugar chains. Using apolipoprotein C-III as the basis for a test, we could then distinguish the patients who had active GALNT2 from those who had an error. In collaboration with the joint first author, Andrew C. Edmondson, attending physician, Division of Human Genetics, Children’s Hospital of Philadelphia and Mayo Clinic, Rochester, MN, USA, we have now developed a clinically available test that can determine whether a specific mutation destroys the function of GALNT2, so clinicians around the world can use this test to identify children with the same disorder in the future,” explains Katrine T. Schjoldager.
More than 130 congenital disorders of glycosylation have been identified, all of which significantly affect the brain and the rest of the nervous system. Most people with these disorders have developmental delay, low muscle tone, decreased stature, well-being problems and epilepsy and may have a host of other symptoms. Although no treatment exists, the new discovery is significant.
“GALNT2-CDG is too rare for an actual drug to be developed, so apart from being able to test for this genetic mutation in the future, clarification is vitally important – at least for the parents of the affected children. Some parents think that they might have done something wrong during pregnancy that has made their child sick. An explanation can help them to focus on caring for their child since they then know how the disease develops. We are very grateful to the participating families, and the current study has only been possible because of wide-ranging international collaboration with many experts in Europe, the United States and Egypt, which is required when mapping a rare disease,” concludes Monica Zilmer.
“Novel congenital disorder of O-linked glycosylation caused by GALNT2 loss of function” has been published in Brain. In 2017, the Novo Nordisk Foundation awarded a grant to Katrine T. Schjoldager for the project Novel Proteoforms of Peptide Hormones Provide Exciting Options for Improving Drug Design. In 2019, the Foundation also awarded a grant to Rikke Steensbjerre Møller for the project "Clinical and Functional Characterization of GABAA-receptor Related Disorders: Translating Genetic Diagnostics into Personalized Treatment".
