TORONTO - Researchers in Montreal have discovered a gene mutation responsible for a common form of mental deficiency in children - a finding that could provide some comfort for parents struggling to understand a previously unexplained phenomenon.

Their findings support the hypothesis that what are called "de nuvo" gene mutations - ones that aren't passed from either parent but first arise in the offspring - may be frequent causes of neurodevelopmental disorders. There is a school of thought, for instance, that a significant percentage of autism cases is caused by these new mutations.

The findings could point future research towards drugs that might alleviate the problems the mutations cause, though they are unlikely to lead to a cure, said senior author Dr. Jacques Michaud, a geneticist at the Sainte-Justine University Hospital Research Centre and the Centre of Excellence in Neuromics of the University of Montreal.

But they should lead to a test that could be used to explain to grieving parents why their child has been born with cognitive impairment.

"It's a bit like a black hole when you're telling a family or a parent that their kid has a mental deficiency," said Michaud, who noted many press for answers medical science cannot always provide.

"Not for everybody, but for a lot of parents, especially for mothers, knowing the cause has really allowed them to close the loop or to relieve a lot of anxiety."

The findings were published Wednesday in the New England Journal of Medicine.

Michaud and his colleagues were not looking for the cause of known conditions associated with mental handicaps, things like autism or Fragile X syndrome. Instead they were looking for causes of what science calls "nonsyndromic mental retardation" - cases that occur independent of a known condition and with no apparent genetic explanation.

Mental retardation, as science journals still call it, is the most prevalent severe handicap in children, affecting between one and three per cent of the population. And most cases are of the nonsyndromic kind. Unlike children with some conditions linked to cognitive problems, these children can't be spotted because of small size of their ears or wide spacing of their eyes.

"They don't have any physical distinguishing features," Michaud said. "And we really know very little about (these) kids...."

He and his colleagues had hypothesized that in a lot of cases, the condition is likely caused by new mutations. So they studied the genes of 94 children with nonsyndromic mental retardation, looking particularly at the genes responsible for synaptic processes.

Michaud said there is a fair amount of evidence that in people with mental deficiencies, the synapses or connections between brain cells don't function in the same way as they do in other people.

In three of the 94 children they found what they believe to be causative mutations, on a gene called Syngap1. When they looked for the same mutations in 190 healthy people, 142 people with autism spectrum disorders and 143 with schizophrenia, they did not see the same mutations.

Michaud said in addition to providing some answers and pointing the way towards potential treatments, the findings will also help geneticists counsel parents who have had one child with nonsyndromic retardation who are considering having more children.

Parents often want to know if subsequent children would be at risk of having the same condition, he said, saying these findings suggest the chances would be low.