A Mercer Island girl with a rare and fatal genetic condition that slowly damages the brain was enrolled in July in a University of Minnesota study, one that may be the first step in learning how to treat the as of yet incurable disease.

Shannon Tootill, 16, was diagnosed with Sanfilippo syndrome when she was 9 years old after years of testing to try to understand why she seemed to do worse every year at school. About one in 70,000 children is born with Sanfilippo, and those with the disease slowly lose their ability to learn, their memories and eventually the capacity to walk and talk. Most die in their teens or 20s.

“As the first really bad result came trickling in seven years ago, it was sort of like losing a child,” said Shannon’s mother, An Tootill. “She was this bright child. We were thinking about putting money aside for college and looking at schools. She had the world on a string.”

When Shannon was young, her mother said, the family had no idea that anything was wrong. It was not until she started school that teachers began suggesting that she might have attention deficit disorder or autism. The most frustrating part of the whole experience, An said, was seeing Shannon’s performance deteriorate and not knowing what was wrong.

“Even worse than the diagnosis was having a child who seems to be doing less well every year — worse was every year, expecting things to get better and then hearing from teachers that Shannon is moderately retarded, she’s never going to go to college, she’s never going to do these things,” said An.

For a long time, she said, nobody believed her when she said that Shannon was losing her mental abilities each year at a time when most children are learning rapidly.

“I’d say she’s talking less, she’s doing less, and they’d say I was wrong,” An said. “There were years of dismissiveness.”

But ever since An and her husband, David Tootill, received Shannon’s diagnosis, An said that Shannon has been doing better, which is surprising considering that Sanfilippo victims ordinarily lose most of their mental abilities by their teenage years.

“She’s an unusual case at her age still to be walking and talking lucidly,” said An. “I thought in high school that we would have to be shopping for a wheelchair.”

Shannon’s continuing ability to function mentally along with her lack of other medical problems made her a good candidate for the University of Minnesota study, which will track 20 children with Sanfilippo over the course of a year to see what the syndrome does to their brains. This information, in turn, can be used to measure the effectiveness of an experimental treatment for the disease that the FDA may approve after this study is complete, said Dr. Chester Whitley, a professor at the University of Minnesota who started work on the Sanfilippo study in 2000.

Whitley said Sanfilippo was first discovered in 1960 at the University of Minnesota, and he has been studying it since working on his Ph.D. in the 1970s.

He said he could not talk about specific patients, but in general, Sanfilippo syndrome is a mutation in a particular gene that would ordinarily allow the body to produce enzymes that break down something called mucopolysaccarides, which naturally occur in humans. For some reason that is not entirely understood, he said, a buildup of these leads to brain damage in a person.

“It’s frankly a slow demise,” he said. “Many describe it as worse than cancer.”

Whitley said scientists are hoping to manufacture the missing enzyme and give it to children in a process that is underway in Europe. So far the FDA has not approved this treatment in the United States, but with the completion of this study, called a natural history study, Whitley said he hoped it would.

An said that Shannon leaves for her first trip to Minnesota on Sunday, where she is to undergo IQ tests for several days. She is to return twice more for the same tests over the course of a year.

“It’s a big deal for us,” said An. “Shannon’s making a contribution to science now. She may not grow up to be a doctor, but she’s still doing something.”

Overall, though, An said Shannon’s case it tragic, and it is a source of constant hardship for the family to know that her brain damage is getting worse all the time.

“She’s going to lose the skills she has, her memories; she’s not going to recognize me,” said An. “The one other girl I know who has this just sort of drifts in and out.”

For now, An and David Tootill, and their 10-year-old son, Eric, can only wait and hope that Shannon’s impressive resistance to Sanfilippo so far means that she truly is an exceptional case.

“We have our fingers crossed,” An said. “We’ve had a miracle so far, so we’re hoping for another miracle.”