31 May 2001
Missing Enzyme Possibly Responsible For SIDS
by Kate Melville
A missing enzyme may be one possible explanation for sudden infant death syndrome, according to a report by a Wake Forest University School of Medicine gastroenterologist in the June issue of The Journal of Clinical Investigation.
The missing enzyme is called mitochondrial trifunctional protein and it is a key step in providing energy for skeletal muscle and the heart and for metabolism in the liver, according to Jamal A. Ibdah, M.D., Ph.D, assistant professor of internal medicine (gastroenterology) at Wake Forest. The energy ordinarily comes from metabolism of fatty acids.
Ibdah set out to demonstrate effects in mice by "knocking out" the gene that produces mitochondrial trifunctional protein. The mice pups developed low blood sugar (neonatal hypoglycemia) and died suddenly 6 to 36 hours after birth.
This knockout, developed at Wake Forest University, "is the first knockout that models a genetic disease of the protein that breaks down fatty acids," said Ibdah.
In a knockout mouse, a specific gene is removed from the rodent, modeling the disease process in people who are born without that particular gene.
"Our results demonstrate that the mitochondrial-trifunctional-protein-deficient knockout mouse is a valid model for human mitochondrial trifunctional protein deficiency and that it has important implications for human disease," Ibdah said. "Many of the associations that were questioned in humans are confirmed clearly in this knockout."
He said the study demonstrates that the enzyme is essential for fetal development and survival of the newborn. "Deficiency of mitochondrial trifunctional protein causes fetal growth retardation, neonatal hypoglycemia and sudden death," Ibdah said. "All are serious and common human disorders."
He said the mouse model provides mechanisms and understanding for the human disease. "It confirms that there is an association between impairment in breaking down fat and sudden death. It makes it clear cut that human beings after birth require these enzymes to survive."
Ibdah said that because the enzyme is missing, fatty acid products accumulate, producing a toxic effect that probably leads to heart arrythmias as well as respiratory arrest.
The mouse model also demonstrates that these enzymes are important for fetal growth and development. "When they are not present, there is fetal growth retardation," Ibdah said. Previously, he said, it had been thought that these enzymes weren't needed by the fetus because the mother was supplying all the needed nutrients through the placenta.
Making the knockout mouse requires manipulation of the mouse stem cells. "You create a mutation in a specific location on the gene," said Ibdah. "It is much more labor intensive and more tedious than creation of a transgenic mouse, in which you inject DNA into a fertilized egg." He said the characterization of the mouse model was done in conjunction with Mark Cline, D.V.M., Ph.D., associate professor of pathology (comparative medicine.)