Asparagine, found in foods such as meat, eggs, and dairy products, was until now considered non-essential because it is produced naturally by the body. Researchers at the University of Montreal and its affiliated CHU Sainte-Justine Hospital found that the amino acid is essential for normal brain development. This is not the case for other organs. “The cells of the body can do without it because they use asparagine provided through diet. Asparagine, however, is not well transported to the brain via the blood-brain barrier,” said senior co-author of the study Dr. Jacques Michaud, who found that brain cells depend on the local synthesis of asparagine to function properly. First co-author José-Mario Capo-Chichi and colleague Grant Mitchell also made major contributions to the study.
In April 2009, a Quebec family experienced the worst tragedy for parents: before the age of one, one of their sons died of a rare genetic disease causing congenital microcephaly, intellectual disability, cerebral atrophy, and refractory seizures. The event was even more tragic because it was the third infant to die in this family from the same disease.
This tragedy led Dr. Michaud to discover the genetic abnormality responsible for this developmental disorder. “We are not at the verge of a miracle drug,” Michaud said, “but we at least know where to look.”
The team identified the gene affected by the mutation code for asparagine synthetase, the enzyme responsible for synthesizing the amino acid asparagine. The study is the first to associate a specific genetic variant with a deficiency of this enzyme. “In healthy subjects, it seems that the level of asparagine synthetase in the brain is sufficient to supply neurons,” Michaud said. “In individuals with the disability, the enzyme is not produced in sufficient quantity, and the resulting asparagine depletion affects the proliferation and survival of cells during brain development.”
Children who are carriers of this mutation suffer, to varying degrees, from a variety of symptoms, including intellectual disability and cerebral atrophy, which can lead to death. The Quebec family lost three infant sons to this disorder. Two of their other children are alive and healthy.
Knowledge about gene mutations can be used to develop treatments. “Our results not only open the door to a better understanding of the disease,” Michaud said, “but they also give us valuable information about the molecular mechanisms involved in brain development, which is important for the development of new treatments.”
For example, asparagine supplement could be given to infants to ensure an adequate level of asparagine in the brain and the latter's normal development. “The amount of supplementation remains to be determined, as well as its effectiveness,” said the geneticist. “Since these children are already born with neurological abnormalities, it is uncertain whether this supplementation would correct the neurological deficits.”
Creating a pediatric clinical genomics centre
To date, nine children from four different families have been identified as carriers of the mutation: three infants from Quebec, three from a Bengali family living in Toronto, and three Israelis, whose symptoms are less severe.
Dr. Michaud's team discovered the genetic mutation by comparing the complete DNA of the Quebec family's children with symptoms of the disease. The researchers then identified children, among other families, who carried the single candidate gene. The gene was revealed only in the affected children, but not in the unaffected children of the families studied.
The discovery comes at a time when CHU Sainte-Justine Mother and Child University Hospital has reached an agreement with Génome Québec to create the first pediatric clinical genomic centre in Canada. “This initiative will transform the quality of care for younger patients to ensure better prevention from childhood,” says Dr. Michaud. “More than 80% of genetic diseases occur in childhood or adolescence. “This new technology will allow us to sequence all the genes in the genome and obtain a genetic portrait of the children more quickly to know which disease they suffer from and to provide treatment, if available, or when it becomes available.”
About this study
Jacques Michaud is a professor at the Université de Montréal Faculty of Medicine and physician at the Division of Medical Genetics at the CHU Sainte-Justine Mother and Child University Hospital. José-Mario Capo-Chichi and colleague Grant Mitchell are both affiliated with CHU Sainte-Justine Hospital Research Centre. The University of Montreal is officially known as Université de Montréal. “Deficiency of Asparagine Synthetase Causes Congenital Microcephaly and a Progressive Form of Encephalopathy” was published in Neuron on October 16, 2013. This study received funding from the Fonds de recherche du Québec – Santé, the Epilepsy Foundation, the Jo Rae Wright Fellowship for outstanding women in science, the Réseau de Médecine Génétique Appliquée du Québec, the Center for HIV/AIDS Vaccine Immunology (‘‘CHAVI'') under a grant from the National Institute of Allergy and Infectious Diseases, National Institutes of Health, Grant Number UO1AIO67854, and by the March of Dimes (grant no. 12-FY10-236) and Canadian Institutes of Health Research (MOP 106499) Additional funding was provided by ARRA 1RC2NS070342-01, NIMH Grant RC2MH089915, NINDS Award RC2NS070344, and the Crown Human Genome Center at the Weizmann Institute of Science.