Brothers Matthew and Michael Clark have developed a rare age-reversing genetic disease which leaves them behaving like young children.
Both had jobs and normal relationships before the disease took hold, but they now live at home with their parents in Lincoln.
Ex-RAF regiment Michael, 42, is now estimated to have a mental age of 10 and giggles constantly, and former factory worker Matthew, 39, whose 19-year-old daughter Lydia is expecting a baby, behaves like a toddler.
After reaching middle age, their behaviour started to change, and both brothers were diagnosed with leukodystrophy last year after their lives had changed dramatically.
The neurological disease affects the brain, nervous system and the spinal cord, and normally only affects newborns. It is so rare there are only 100 people affected in the whole of the UK. Their condition is terminal. Although both Matthew and Michael were perfectly healthy into their 30s, their devastated parents know their children will die before them.
The brothers' father Anthony Clark, 63, drew parallels between the disease and the 2008 fantasy drama, The Curious Case of Benjamin Button, based on the F. Scott Fitzgerald novel, in which an old man played by Brad Pitt gradually gets younger. "It's a devastating disease. Both of them are very childlike now," he said.
"Matthew went out the other day and bought himself a train set and a Mr Potato Head.
He and their mother Christine, 61, have abandoned plans to retire in Benidorm and have moved back to Lincoln to look after their boys. Anthony said:
He also has these awful episodes where he screams and shouts and says 'I don't know what I'm doing'. It is like an adult having a toddler's tantrum. It's obviously worse for him but it is terrible for us too.
"There's nothing we can do to help and we feel absolutely powerless. Michael was evicted from his flat because he stopped looking after himself. If he dropped something it would stay there and he stopped doing the dishes.
"Matthew was fired from his job as a factory worker, and although we don't know the full story we can only assume it is because he was behaving like a child."
Michael moved into Matthew's flat after the eviction but left shortly after a row with a neighbour, believing he was being told to leave. After sleeping in a park for three weeks he found a soup kitchen and was sent to the Salvation Army.
"They knew something was wrong at once and they sent him to a doctor," explains Anthony.
The doctors carried out an MRI scan and they discovered the condition. When they asked him if he had any siblings he said he had a brother and they ran tests on Matthew and discovered he had the same thing.
The 1992 film Lorenzo's Oil, starring Susan Sarandon and Nike Nolte, explored fate of 14-year-old American Lorenzo Odone, who had Adrenoleukodystrophy, and his parents' quest to find a cure.
With the help of a British scientist, they invented an edible mixture of olive and rapeseed oil extract which appeared to halt the destruction caused by the disease, but it is only effective for that particular strain of Leukodystrophy, known as ALD, and must be applied before the symptoms emerge.
Odone died aged 30 in 2008 and his parents later founded the Myelin Project to promote research into diseases which destroy myelin such as multiple sclerosis and the leukodystrophies.
A spokesperson for The Myelin Project, Lynda Carthy, said: "It is so complex because no two patients are the same. I know of around 100 sufferers in this country but in certain circles it can be seen as a curse or an embarrassment and the stigma stops people coming forward.
"There is an estimated 1 in 3 billion chance of two people who carry the gene deficiency meeting and becoming partners.
What a sad story. We wish the family all the best.
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