The womans genetic profile showed she would develop Alzheimers by the time she turned 50.

She, like thousands of her relatives, going back generations, was born with a gene mutation that causes people to begin having memory and thinking problems in their 40s and deteriorate rapidly towards death around age 60.

But remarkably, she experienced no cognitive decline at all until her 70s, nearly three decades later than expected.

How did that happen?

New research provides an answer, one that experts say could change the scientific understanding of Alzheimers disease and inspire new ideas about how to prevent and treat it.

In a study published in the journal Nature Medicine, researchers say the woman, whose name they withheld to protect her privacy, has another mutation that has protected her from dementia, even though her brain has developed a major neurological feature of Alzheimers disease. This ultra-rare mutation appears to help stave off the disease by minimising the binding of a particular sugar compound to an important gene. That finding suggests that treatments could be developed to give other people that same protective mechanism.

A drug or gene therapy would not be available any time soon because scientists first need to replicate the protective mechanism found in this one patient by testing it in laboratory animals and human brain cells. Still, this case comes at a time when the Alzheimers field is craving new approaches after billions of dollars have been spent on developing and testing treatments and some 200 drug trials have failed. It has been more than 15 years since the last treatment for dementia was approved, and the few drugs available do not work very well for very long.

The woman is entering her late 70s now and lives in Medelln, the epicentre for the worlds largest family to experience Alzheimers. It is an extended Colombian family of about 6,000 people whose members have been plagued with dementia for centuries, a condition they called La Bobera the foolishness and attributed to superstitious causes.

Decades ago Colombian neurologist Dr Francisco Lopera began painstakingly collecting the familys birth and death records in Medelln and remote Andes mountain villages. He documented the sprawling family tree and took dangerous risks in guerrilla and drug-trafficking territory to cajole relatives of people who died with dementia into giving him their brains for analysis.

Through this work, Dr Lopera, whose brain bank at the University of Antioquia now contains 300 brains, helped discover that their Alzheimers was caused by a mutation on a gene called Presenilin 1. Although this type of hereditary early-onset dementia accounts for only a small proportion of the roughly 30 million people worldwide with Alzheimers, it is important because, unlike most forms of the disease, the Colombian version has been traced to a specific cause and a consistent pattern. So Dr Lopera and a team of American scientists have spent years studying the family, searching for answers both to help the Colombians and to address the mounting epidemic of the more typical old-age Alzheimers disease.

When they found that the woman had the Presenilin 1 mutation, but had not yet even developed a pre-Alzheimers condition called mild cognitive impairment, the scientists were mystified. We have a single person who is resilient to Alzheimers disease when she should be at high risk, said Dr Eric Reiman, executive director of the Banner Alzheimers Institute in Phoenix and a leader of the research team.

The woman was flown to Boston, where some of the researchers are based, for brain scans and other tests. Those results were puzzling, said Yakeel Quiroz, a Colombian neuropsychologist who directs the familial dementia neuroimaging lab at Massachusetts General Hospital.

The womans brain was laden with the foremost hallmark of Alzheimers: plaques of amyloid protein. She had the highest levels of amyloid that we have seen so far, said Quiroz, adding that the excessive amyloid probably accumulated because the woman had lived much longer than other family members with the Alzheimers-causing mutation.

But the woman had few other neurological signs of the disease not much of a protein called tau, which forms tangles in Alzheimers brains, and little neurodegeneration or brain atrophy. Her brain was functioning really well, said Quiroz, who, like Dr Reiman, is a senior author of the study. Compared to people who are 45 or 50, shes actually better.

She said the woman, who raised four children, had only one year of formal education and could barely read or write, so it was unlikely her cognitive protection came from educational stimulation. She has a secret in her biology, Dr Lopera said. This case is a big window to discover new approaches.

Quiroz consulted Dr Joseph Arboleda-Velasquez, who, like her, is an assistant professor at Harvard Medical School (he is also Quirozs husband). Dr Arboleda-Velasquez, a cell biologist at Massachusetts eye and ear, conducted extensive genetic testing and sequencing, determining that the woman had an extremely rare mutation on a gene called APOE.

APOE is important in general-population Alzheimers. It has three variants. One, APOE4, greatly increases risk and is present in 40 per cent of people with Alzheimers. The Colombian woman has two copies of APOE3, the variant that most people are born with but both copies have a mutation called Christchurch (for the New Zealand city where it was discovered). The Christchurch mutation is extremely rare, but several years ago, Reimans daughter Rebecca, a technologist, helped determine that a handful of Colombian family members have that mutation on one of their APOE genes. They developed Alzheimers as early as their family members typically did. The fact that she had two copies, not just one, really kind of sealed the deal, Dr Arboleda-Velasquez said.

Dr Guojun Bu, chairman of the neuroscience department at the Mayo Clinic in Jacksonville, Florida, who studies APOE, said that while the findings involved a single case and more research was needed, the implications could be profound.

When you have delayed onset of Alzheimers by three decades, you say wow. New York Times

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The woman who should have got Alzheimers by 50, but didnt - The Irish Times

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