A genetic mutation has been linked to the developmental disorder spina bifida, researchers in Montreal say.

Spina bifida is the second most common birth defect in humans after cardiac abnormalities.

"This discovery won't have a major impact on the search for a cure yet, but it can have an immediate impact on diagnosis or risk assessment," said Philippe Gros, a professor of biochemistry at McGill University.

"A pre-natal diagnosis would allow physicians to decide whether to follow a pregnancy more closely."

Scientists have suspected that neural tube defects such as spina bifida have a genetic component.

In Thursday's issue of the New England Journal of Medicine, Gros's team said they have identified three mutations that implicate a gene as a risk factor for neural tube defects in humans.

Neural tube defects, in which the central nervous system fails to develop fully, affect as many as two births in 1,000.

Incomplete closure

In the neural tube defect spina bifida, the spine does not form completely. Effects vary from severe physical deformities and developmental problems to minor bladder problems that may be corrected by surgery, according to the Spina Bifida and Hydrocephalus Association of Canada.

The gene, called VANGL1, codes for a protein that cells need to line up properly during development, the researchers said. The mutation leads cells to lose their orientation, which is needed for neural tubes to close without gaps exposing nerve tissue.

VANGL1 is the first gene shown to cause the disorder in humans, said Gros. The neural defect occurred when the team removed the gene in mice. The same mutations were found when the researchers looked at several children treated at birth defect centres in Italy and France.

Role of diet

Other researchers have looked for mutations linked to neural tube defects without success, which suggests a combination of different genes and environmental factors affect the incidence and severity of the defect, the study's authors wrote.

Neural tube defects occur during the first four weeks of pregnancy when the brain and spinal cord develop. The early onset makes it difficult to detect and treat, since the first signs generally do not appear until the first ultrasound at 18 weeks. At that point, the damage has already occurred.

Given the early onset, women who want to conceive are often advised to take 0.4 milligrams a day of folic acid supplements.

Folic acid is the synthetic form of folate, also called vitamin B-9, which is found in citrus fruits, nuts, liver and dark green, leafy vegetables.

Since the Canadian government introduced mandatory fortification of whole-grain breads, cereals, flour, cornmeal and pasta with folic acid in 1998, the rate of birth defects such as spina bifida has declined.