By the looks of their home, Tony and Christine Clark are raising two rambunctious 7-year-old boys. Model train tracks and Monopoly pieces are scattered on tables and cartoons flicker on the TV set.



But the Clark's two sons are grown men who share only the same interests and emotional fluctuations of little boys. Like the character portrayed by Brad Pitt in the 2008 film "The Curious Case of Benjamin Buttons," Matthew, 39, and Michael, 42, are aging backwards.



Diagnosed with a terminal form of leukodystrophy, one of some 40 extremely rare genetic disorders that attack the Myelin, or white matter, in the nervous system, spinal cord and brain. In the Clarks' case, the condition has not only eroded their physical capacities, but their emotional and mental states, as well.



Only six years ago, both brothers were holding down jobs and growing their families. Today, they spend their days in the care of their parents, both in their sixties, playing with Mr. Potato Head, fighting over Monopoly, and in rare lucid moments, struggling to understand why their lives have changed so dramatically.





Before the Clark Brothers were diagnosed, they were living independent lives. Michael served in the Royal Air Force and later became a cabinet maker. Matthew, worked in a factory, and was raising a teenage daughter. Tony and Christine, meanwhile, had retired and moved from their UK-home to Spain. Then in 2007, both their sons fell off the radar. They stopped returning their parents' calls and texts, and as Clark brothers' conditions developed, their lives fell apart.



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Michael surfaced in a soup kitchen, and was referred to medical experts by social workers. After an MRI scan he was diagnosed with the incurable degenerative disorder, and soon after Matthew received the same news. In the U.S. alone, about one in 40,000 children are born with a form of the neuro-degenerative disease, according to Dr. William Kintner, President of the United Leukodystrophy Foundation. While some forms of the disorder are potentially treatable if discovered in the earliest stages, once symptoms start showing it's unlikely to reverse the damage. "It's very difficult to do anything once progression has occurred," Dr. Kitner tells Yahoo! Shine.

As of April, when the Clarks were first written about in the British press, their mental age was 10.



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"We will be out walking and things which might interest a toddler interest them, the other day we were walking home when Michael saw a balloon and pointed it out to us," father Tony Clark, told The Telegraph last spring.

Today, the brothers are even younger mentally.

"Just like small children, they wake up a lot during the night," mom Christine said in an interview published in The Independent this week. "I was up seven times with them last night."

After learning of their diagnoses, Tony and Christine returned to the UK and moved in with their sons. Their daily struggles as a family have been chronicled in a British documentary, "The Curious Case of the Clark Brothers," airing Monday in the UK.



Earlier this year, Matthew became a grandfather, when his daughter had a son. But the news for the family was bittersweet, as the Clark brothers' mental age continued to creep backwards.



"There's no return to them being cute little boys," said Christine, who regularly manages the men's tantrums and fights over Monopoly. "They're big strong men - and that presents a quite different set of problems."

More recently, even their physical strength began deteriorating.

"A few weeks ago, they could still manage with a knife and fork, but now that's getting too difficult for them - they get the food on to their forks, but somehow it all falls off before it reaches their mouths," she said.



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Now walking is the next hurdle; Matthew is already confined to a wheelchair.



"The likeliehood that they're on a terminal course are fairly certain but who knows," says Dr. Kintner, who is familiar with the Clark case but didn't meet the brothers. "If they were citizens of U.S., we'd try to get them to the National Institute of Health for diagnostic work, but in the UK the system is different. There is no comparable organization with genetic diseases, so it's a little more difficult there."



Dr. Kintner estimates there are several million cases of Leukodystrophies in the U.S, but an exact number is hard to pinpoint because the different many forms of the disorder are still being identified. "It's going to take a long time," he says, "I hope in my lifetime I see a cure for some of them."