09/05/2013 08:31 BST | Updated 22/05/2015 06:12 BST

Why This Loving Dad Says: 'I Wish My Son Had Cancer'.

A dad has defended a shocking advert in which he declares: "I wish my son had cancer."

For behind the frankly gobsmacking statement lies a heartbreaking reality.

Alex Smith's six-year-old son has a condition that can never be cured and could kill him before he reaches 25.

And the dad's logic is that cancer would be preferable – because at least there would be the potential for a cure.

Harrison Smith was born with duchenne muscular dystrophy, which leads to loss of movement and, eventually, paralysis.

There is no treatment and no cure, but his father Alex has given up his job to dedicate himself to finding one. He hopes to achieve this by funding research through his charity Harrison's Fund.

A campaign for the charity features a picture of Alex and Harrison and carries the slogan: "I wish my son had cancer".

Explaining the advert, which appeared in the London Evening Standard, Alex said: "For many people the notion of even wishing your child had cancer would be tough to understand, how could I wish a terrible disease on my own child?

"But the brutal truth is that there is a treatment in most cases. With duchenne there is no treatment, no cure, it's 100 per cent fatal and at the moment one day he will be dead."

Alex, from Cobham, Surrey, who has another son William, four, with his wife Donna, 37, said when Harrison was first diagnosed in 2011, 'the world turned upside down'.

He said: "Coming to terms with a child that has a life-limiting condition, it's not what we had mapped out for ourselves. We were also worried that we could have two sons with duchenne.

"I'm a very determined guy and I can't accept being told, 'Look after your son, give him the best life you can, he is going to die.' I couldn't accept that solution."

As a result, he decided to give up his job in company branding to start a charity named after Harrison in a bid to find a practical solution for him and other sufferers.

Duchenne muscular dystrophy, which is caused by a genetic mutation in the dystrophin gene located on the X chromosome, affects around one in 3,500 boys in the UK.