It claimed the lives of four family members and has left his brother bedbound. Now, the cruel disease that has gripped several generations of Chris Graham's family has struck again. Mr Graham himself was diagnosed with early-onset Alzheimer's disease at just 34. The ex-serviceman, now 39, discovered in 2010 that he carries the same faulty gene that claimed the lives of his father, aunt, cousin and granddad, all in their 40s. This rare, inherited, or 'familial' form of Alzheimer's disease has also left his 43-year-old brother, Tony, bedbound in a care home and being fed through a tube. Scroll down for video . Chris Graham was diagnosed with early-onset Alzheimer's disease at just 34. Several members of his family have been killed by the cruel, genetic form of the condition . Like his brother, Mr Graham carries a faulty version of PSEN-1 gene, which affects around 400 families worldwide. The father-of-three, from Carterton in Oxfordshire, is experiencing mild memory problems, but knows it is a case of when, not if, his dementia progresses. Although he is young, the disease process will affect him in the same way as the estimated 500,000 people with Alzheimer's in the UK. Mr Graham, who was medically discharged from the Army in January after 23 years' service, said: 'My friends didn't believe me when I told them I had Alzheimer's disease. 'At the age of 39, it just doesn't make sense to people: I'm fighting fit – how can I have something like that? We still think of Alzheimer's as a bit of forgetfulness as we get older. The truth is far worse.' He continued: 'My family has been hit hard by Alzheimer's. Dad died aged 42. He was in hospital so long I don't really remember him, I was only six or seven at the time. 'His sister, my aunt Thelma, died at 41 as she inherited the faulty gene too. I also lost a cousin, Thelma's daughter, at 41, and a granddad, Dad's father, at 46 to the disease.' This rare, inherited, or 'familial' form of Alzheimer's disease has also left his 43-year-old brother, Tony (pictured with his son), bedbound in a care home and being fed through a tube . The disease also claimed the life of Mr Graham's father. He said: 'My family has been hit hard by Alzheimer's. Dad died aged 42. He was in hospital so long I don't really remember him, I was only six or seven at the time' As one of four children, each of Mr Graham and his siblings had a 50/50 chance of inheriting the gene from their father. While his two sisters avoided the genetic curse, tragically, both he and his brother Tony have been struck by it. Speaking about his brother, who was diagnosed in 2006, said: 'He's in an old folk's home, he can't move and has to be fed through a tube. He can't speak but can raise a smile sometimes.' Alzheimer's disease is the most common form of dementia and is responsible for killing brain cells and shrinking the brain 400 per cent faster than the normal rate of ageing. Mr Graham is now taking part in research studies at University College London's Institute of Neurology to help scientists understand the earliest signs of Alzheimer's to boost the search for new treatments. He said: 'Although I know what will happen to me in the coming years, I now have direction in life. 'I wanted to do something to fight back against the disease – to do as much as I can while I can. It's simple for me, you have to hit the enemy directly, so I've taken on a challenge to help support research and I'm taking part in studies.' Now, to raise awareness of dementia and vital funds for Alzheimer's Research UK, Mr Graham is heading off on a 16,000 mile cycle ride around Canada and America in April. His year-long challenge has earned the support of Prime Minister David Cameron, who praised Mr Graham's 'extraordinary grit and determination' to raise money for the charity. Mr Graham said of his brother (pictured right with his own children): 'He's in an old folk's home, he can't move and has to be fed through a tube. He can't speak but can raise a smile sometimes'. Tony is pictured with his partner Jane . Now, to raise awareness of dementia and vital funds for Alzheimer's Research UK, Mr Graham is heading off on a 16,000 mile cycle ride around Canada and America in April . Mr Graham, who is also supporting ABF The Soldiers' Charity through his cycle ride, said: 'It's going to take me a year to do it, but I hope giving one of my good years to this challenge will help give many more good years to people with dementia. 'A cure may come too late for me, but it will help my kids. We've got man to the moon, so we'll get to the answers if we put enough into it. I hope what I'm doing will help inspire people to support Alzheimer's Research UK.' He is sharing his story in the same week a YouGov survey commissioned by Alzheimer's Research UK. Early onset Alzheimer's tends to cluster within families, sometimes with several generations affected, in which case it is called familial disease. In some of these cases, it is caused by mutations in one of three genes. People with any of these extremely rare mutations tend to develop Alzheimer's disease in their 30s or 40s. These genes, and the prevalence of their defective versions  is as follows: . These mutations are extremely rare and account for fewer than one in 1,000 cases of Alzheimer's disease. It is likely that all of those who inherit faulty versions of any of these three genes will develop Alzheimer's disease at a comparatively early age. On average, half of the children of a person with one of these rare genetic mutations will inherit the disease. People who do not inherit the mutation cannot pass it on. Someone who has two or more close relatives (a parent, brother or sister) who developed Alzheimer's disease before the age of 60, is advised to ask their GP about genetic testing and counselling for these rare mutations, who can also refer patients to a geneticist, if appropriate. Source: Alzheimer's Society . This revealed that, when asked what they think dementia is and who it affects, just 23 per cent of British adults specifically mentioned brain disease or degeneration, despite a previous survey from the charity showing more than one in three people know a close friend or family member with the condition. Hilary Evans, Director at Alzheimer's Research UK, said: 'Chris embodies everything that goes against the dementia stereotype – he's young and fit and his zest for life in the face of adversity is truly inspiring. 'Although Chris' form of Alzheimer's is rare, the tragic consequences it has had on his relatives will resonate with thousands of families across the UK who have experienced Alzheimer's. 'Chris' situation shows how profound the impact of Alzheimer's can be – this is not forgetfulness in old age, the disease is a destructive process that takes the ultimate toll. 'Our new survey data shows we have a long way to go in dementia education. We need to recognise the physical process of Alzheimer's – in doing so we'll see that there's something real we can attack and overcome with research. 'Until we do, our own misunderstanding or prejudice risks becoming the biggest barrier to research. Prime Minister David Cameron said: 'Alzheimer's is one of the key health challenges of this generation. 'Chris has my absolute backing as he undertakes this mammoth challenge in aid of Alzheimer's Research UK and ABF The Soldiers' Charity. 'Travelling unsupported on an expedition which will see him cycle 16,000 miles around the coastline of North America, Chris is a great example of someone who, when faced with a difficult diagnosis, has shown extraordinary grit and determination to overcome his circumstances and ensure that future generations can benefit from potentially life-saving research. 'These are important causes, and I urge everyone to get involved and support Chris as he sets off on this daunting, once-in-a-lifetime journey.' To find out more about Mr Graham, his challenge, his family's story and about inherited Alzheimer's disease, visit www.alzheimersresearchuk.org/chris . To sponsor Chris, visit www.justgiving.com/Christopher-Graham8 or text CHRIS to 70800 to donate £5 to Alzheimer's Research UK.
Chris Graham was diagnosed with early onset Alzheimer's five years ago . Disease has claimed the lives of four other close family members . Killed his father at the age of 42, and Chris' brother is bedbound at just 43 . He is now fundraising for research into Alzheimer's disease . Plans to cycle 16,000 miles around Canada and America next month .