May 29, 2012
Genes at fault for cystic fibrosis–related intestinal obstruction
Researchers at Johns Hopkins have identified a gene that modifies the risk of newborns with cystic fibrosis developing neonatal intestinal obstruction, a potentially lethal complication of the disease. Their findings, which appeared online March 15 in PlosGenetics, along with the findings of their Toronto-based colleagues, published April 1 in Nature Genetics, may lead to a better understanding of how the intestines work and pave the way for identifying genes involved in secondary complications of other disorders.
Soon after birth, most babies excrete their first stool, a tarlike substance called meconium, but not babies with neonatal intestinal obstruction, or meconium ileus, which affects 15 percent of newborns with cystic fibrosis and, rarely, newborns without cystic fibrosis. Their stool is different.
“It is abnormally viscous due to high protein content and low levels of hydration, and the child can’t move it through the intestine,” said Garry Cutting, a professor of pediatrics at Johns Hopkins’ McKusick-Nathans Institute of Genetic Medicine.
The condition results in death if not treated by surgery or enema. But why some newborns with cystic fibrosis, or CF, get it and others don’t is not well understood. To better understand why this is so and to develop models for finding so-called modifier genes—genes that modify the effects of other genes—Cutting and his colleagues aimed to figure out which modifier genes contribute to the development of meconium ileus, or MI. (From their previous work, they already knew that modifier genes contribute to its development.)
Working with Toronto-based collaborators, members of Cutting’s team looked for gene variants that occur in CF patients with MI. They knew that CF is caused by disruption of the CFTR gene, which encodes for a cell membrane protein, so they thought that the genes that alter CFTR’s activity and cause MI might also encode for cell membrane proteins; after all, a cell membrane protein is more likely to interact with a nearby cell membrane protein than with a protein that’s deep inside the cell.
The researchers tested DNA samples from 3,763 CF patients—611 who had had MI and 3,152 who hadn’t—to compare genes that encode for cell membrane proteins to those that encode for unrelated proteins. Three of the 155 genes tested that encode for cell membrane proteins correlated with risk for MI, compared with none of the 231 genes tested that encode for unrelated proteins.
“These genes have common variants that all of us happen to be carrying around,” Cutting said, “and just by chance, if you have a child with CF, these common variants play a role in modifying risk for meconium ileus.”
The researchers wanted to look for additional gene variants associated with an increased risk for MI, using a different approach.
In an earlier study, Cutting’s team had found a region of human chromosome 8 to be linked to MI. To pinpoint which gene within that region leads to the condition, the researchers analyzed the DNA of 133 families with two or more CF children, at least one of whom previously had MI. The DNA was tested to determine which parts of chromosome 8 parents had passed down to their children who had MI. Using this approach, the researchers found variants of the methionine sulfoxide reductase, or MSRA, gene—in this case, a particular combination of DNA alterations close to and within the gene—that appeared significantly more often in children who had MI. In an unrelated CF patient population from Canada, they found evidence of the same link between MSRA and MI, which helped confirm their results.
While the researchers now knew that CF patients with a certain MSRA gene variant tended to have had MI as newborns, they didn’t yet know whether MSRA actually plays a role in MI and therefore whether it is truly a modifier gene. To address this question, they turned to mice engineered to have CF that tend to die from intestinal obstruction and developed three genetically modified versions of these mice: one with both MSRA genes intact, one with only one intact and one with none intact. The fewer the copies of intact MSRA genes, the more likely the mice were to survive. In other words, the loss of MSRA protected the mice from fatal intestinal obstruction.
Cutting and his colleagues don’t know exactly how the loss of MSRA reduces risk for fatal intestinal obstruction, but they suspect that MSRA’s ability to alter the activity of specific intestinal enzymes may be the key. They surmise that with reduced levels of MSRA, the enzymes are free to do their job breaking down proteins that make up meconium so that it can pass through the intestines and be evacuated normally at birth.
The researchers’ work on MSRA could shed light on how meconium normally gets broken down in the intestines. Moreover, use of the techniques pioneered by Cutting and his colleagues may lead to identification of modifier genes that play roles in other complications of CF, such as lung function, and in other diseases caused by a single gene, such as Huntington’s disease.
The studies were funded by the National Heart, Lung and Blood Institute, the National Institute of Diabetes and Digestive and Kidney Diseases, and the Cystic Fibrosis Foundation.
Authors on both the Plos Genetics and Nature Genetics papers from Johns Hopkins are Lindsay B. Henderson, Vishal K. Doshi, Scott M. Blackman, Kathleen M. Naughton and Cutting.
McKusick-Nathans Institute of Genetic Medicine: