Written By: Kathleen Cunningham
Original Article: Pant et al. Journal of Clinical Investigation 2019
The Gist of It:
In your brain and spinal cord, neurons have a fatty covering that protects them, similar to the plastic coating around your cell phone charge cord. This covering is made of myelin. Myelin is made by cells in your brain called oligodendrocytes and it has many roles, including helping to maintain the proper electrical signals in the neurons in your brain. However, in patients with leukodystrophies (LD), a group of rare disorders that affect the nervous system, myelin is not made or broken down abnormally Patients with LD develop the disorder in infancy and can have severe symptoms, such as poor motor function, involuntary muscle contractions, seizures, and death. Using genetic sequencing, Pant and colleagues found a change in the gene DEGS1 that caused LD in 19 patients from 13 different families. The DEGS1 gene makes a protein that can change specific types of fats in your cells from a saturated form to a desaturated form, an important step in making myelin. The authors measured the amount of the saturated lipid and found that it is increased in patient cells. To test whether changes in DEGS1 may really cause LD, the authors created a zebrafish model by decreasing the amount of DEGS1. Zebrafish larvae with less DEGS1 had unusual shape during development, as well as abnormal movement and fewer oligodendrocytes making myelin. These changes are consistent with what is observed in patients. Excitingly, the authors tested a drug (FTY720) that is known to affect fat saturation levels and block the accumulation of this saturated fat that was previously approved by the Food and Drug Administration (FDA) as a treatment for multiple sclerosis. This drug was able to reduce the fat accumulation and improve the number of oligodendrocytes and the movements of the drug-treated zebrafish. The success of this drug in zebrafish opens the door to the potential of finding a treatment for this rare but deadly childhood disease.
The Nitty Gritty:
Pant et al. first conducted whole exome sequencing in a series of patients with undiagnosed leukodystrophies. They found a frameshift mutation in the delta-4 desaturase, sphingolipid 1 (DEGS1) gene in the proband patient which was not present in multiple genomic databases in healthy individuals. They next used GeneMatcher and communications with the Reference Center for Leukodystrophies to find an additional 18 patients with candidate causative mutations in the DEGS1 gene. All of the patients were homozygous or compound heterozygotes for DEGS1 mutations. The researchers also categorized the specific common features of these LD patients and found that 79% of patients shared poor psychomotor development, dystonia, and spasticity. The patients also presented with eye movement defects and seizures. All patients were confirmed by MRI to have white matter lesions and hypomyelinating LD, with a median patient age of 3.6 years. Interestingly, patients with the mutations predicted in silico to confer the greatest loss of function had the most severe disease, all dying before the age of 7. Using fibroblasts collected from patients, the researchers next observed possible consequences of the loss of DEGS1 function; they showed a dramatic accumulation of the DEGS1 lipid substrate Dihydroceramide (DhCer). As DhCer accumulation can cause a buildup of excess reactive oxygen species (ROS), Pant et. al next measured ROS levels and showed that patients’ fibroblasts have increased levels. Further, when treated with exogenous DhCer, control fibroblasts showed an increase in ROS levels, while patient fibroblast ROS levels were saturated. To further confirm that DEGS1 was important in key myelinating cell types in vivo, the researchers turned to zebrafish. Zebrafish are one of the smallest and easiest laboratory models that preserve key components of myelinated axons. Indeed, zebrafish DEGS1 was expressed in myelinating oligodendrocytes in zebrafish larvae. The researchers designed a morpholino to knock down DEGS1 in zebrafish. This resulted in accumulation of DhCer, loss of myelination and myelinating cells, and locomotor disability. Since the enzymatic pathway for DEGS1 is known, the authors examined an FDA-approved drug known to target the ceramide synthesis pathway, FTY720. FTY720 inhibits the enzyme that synthesizes DhCer prior to the DEGS1 step in the pathway. Excitingly, the drug rescued DhCer levels as well as myelination and motor function in zebrafish; it also rescued DhCer and ROS levels in patient fibroblasts. Therefore, the authors have identified an exciting potential treatment for a rare but devastating childhood disease.