A 12-year-old boy who suffers from vanishing bones sydrome - one of the rarest diseases in the world - has started to re-grow bone tissue.

New tissue has been discovered in the spine of Joshua Leighton, who is undergoing a course of experimental treatment to try to cure the condition that effects only 200 people in the world.

Today, his parents Richard Leighton, 46, and Sharon, 45, said they were optimistic that it could be start of the reverse of the disease which struck Joshua three years ago.

And they hope one day he will be able to throw away the body plaster cast he has to wear 24 hours a day.

The condition, officially called Gorham's disease, causes abnormal tissue to produce chemicals that disintegrate bone.

And Joshua is one of only 20 people worldwide with the disease in the spine.

But his medical team at St James's Hospital, Leeds, led by consultant spinal and orthopaedic surgeon Peter Millner, have seen signs of the disease reversing.

Joshua has had to put up with sickness and discomfort from the drugs he takes every day.

And the substance he needs to swallow to undergo a scan makes him sick.

But Mr Millner said he is a model patient and cautiously optimistic about his improvement.

"We hope in time - we don't know when - he will be strong enough to do without his plaster cast jacket.

"It looks like he is growing bone - we have to do another scan to show the bone density but last time it showed improvement."

He praised Joshua and his family for the understanding and co-operation in the experimental treatment.

Joshua takes a cocktail of interferon and calcitonin administered by his mum by injection.

Mr Millner said the success was also down to the wider medical team involved.

Joshua said: "I have been taking drugs to make my spine stronger. I'm coping with it OK. My back doesn't ache as much as it used to. The treatment hurts at times and it makes me feel sick but it is making me better.

"I'm hoping that in a few years time I won't need the plaster cast."

Mrs Leighton, a non-teaching assistant, of Coles Way, Riddlesden, said the news was brilliant.

"We are much more optimistic and we know Mr Millner will do his best.

"We are very proud of him. He is also doing well at school. Nothing phases him."

Joshua has been awarded student of his year at Greenhead School, Keighley, where his 16-year-old brother Jonathan also attends.

Judith Smith, Greenhead's assistant head teacher and special needs co-ordinator, said: "Joshua has a stalwart approach - he gets on with it despite everything. He is a great boy to have in school."