Article In Brief
Researchers suggest that KLOTHO, a variant of the ‘longevity gene,’ may protect some carriers of the APOE risk gene from Alzheimer's disease. Experts comment on its protective synaptic qualities and its impact on brain function in older age.
A variant of the so-called longevity gene—KLOTHO—appears to protect some apolipoprotein e4 (APOE4) carriers against developing Alzheimer's disease (AD).
Scientists have long wondered why APOE4, a high-risk variant for late-stage AD, leads to dementia in some carriers and not others.
Scientists at Stanford University's Alzheimer's Disease Research Center became interested in the genetics of the KLOTHO gene and its role in protecting synaptic function and hints that it may be associated with APOE4. But the findings of past studies regarding the association were mixed.
Past analyses were conducted on small groups, however. Michael D. Greicius, MD, MPH, associate professor of neurology and director of the Stanford Center for Memory Disorders, and his colleagues believed that they could combine the numbers from more than 20 case-control AD studies representing tens of thousands of people on whom they had genetic information and they would have a more definitive answer. It worked.
In the study, published in the April 13 online issue of JAMA Neurology, they were able to show that people who had one or two copies of the APOE4 allele and also one copy of the KLOTHO-VS allele were less likely to develop AD and also had reduced levels of amyloid-beta (Abeta) on brain scans.
The findings open up the possibility that KLOTHO itself, or proteins in the klotho pathway, could provide new targets to treat AD. Several companies are trying to develop ways to deliver the Klotho protein in order to increase klotho levels.
Since the gene's longevity prowess was identified in 1997, scientists have learned that it is produced in the periphery (primarily, the kidney) and in the brain. It is made in the choroid plexus and cranked out into the cerebrospinal fluid that bathes the brain. Nerve cells in the hippocampus also produce low levels of klotho. In the kidney, it regulates vitamin D and phosphate metabolism and certain signaling pathways.
How the Study Developed
Dr. Greicius and his colleagues have an ongoing study of “protected” APOE4 carriers (people older than 70 who carry two copies of APOE4 and people over age 80 who have one copy). Last year, one of the study subjects, a retired Stanford professor, sent him a paper from a team at the University of Wisconsin that showed that people who had the APOE4 gene and carried one copy of the KLOTHO gene variant, KL-VS, had significantly less amyloid-beta (Abeta) in their brains. The study, which was published in Neurology, suggested that Klotho could block or slow the toxic effects of the APOE4 variant.
A post-doctoral student in his laboratory—Michael Belloy, PhD, asked if he could do a larger analysis. Two weeks after the Wisconsin paper was published, another paper from an Australian group was published in the Neurobiology of Aging journal suggesting that there was no protective link between the two gene variants.
Given the discrepancies, Dr. Greicius thought that he'd forgo the study, but Dr. Belloy had already dug deep into the collected databases, gathering data on tens of thousands of people. Dr. Greicius agreed to move forward with the data.
They had data from more than 20,000 men and women over 60 who had been part of 22 independent case-control longitudinal studies. Most were of Northwestern European ancestry and represented older people who were either cognitively normal or who had been diagnosed with AD. They also had data from 3,008 people enrolled in conversion studies—progressing from mild cognitive impairment (MCI) to AD—and another 556 others who had CSF testing to measure Abeta, and another 251 who had had Abeta PET imaging. In addition, they had genetic data from the single nucleotide protein microarrays.
They split the group into people with AD and normal controls. Then, they further divided the groups into those who were APOE4-positive and those who are APOE4-negative. About 20 to 25 percent of the population have one APOE4 allele and about 20 percent of the population have one copy of the variant, KL-VS HET+.
Their question: Did having one copy of KL-VS HET+ protect against AD? First, they found that among APOE4 carriers, people in the control group were more likely to carry one copy of the KLOTHO variant compared with those who entered the studies with AD.
They found that KL-VS HET+ people carrying an APOE4 allele had a 30 percent reduced risk for AD compared with those without the KLOTHO variant. The reduced risk was only seen in the entire group of people between ages 60 and 100 but was considerably stronger in those who were in the 60-80 age range.
In the longitudinal component of the study, the 3000-plus subjects were also less likely to convert from a normal control to MCI or AD. Lastly, they found that among healthy controls with amyloid biomarkers, in the APOE4 group, KL-VS HET+ subjects were more likely to have higher Abeta levels in the CSF and lower levels on the PET scans. The higher Abeta CSF levels and lower Abeta PET is an indication that the brain is clearing out the toxic protein, the study authors said. Studies have shown that many healthy older APOE4 carriers already have Abeta deposits in their brains, but this effect was significantly less in those APOE4 carriers with the KLOTHO variant.
The KL-VS HET+ allele did not offer any protection against AD in those who were APOE3/E3.
Dr. Greicius said that the findings suggest that genetic testing should include KLOTHO and APOE status to better understand who is at greater risk for AD. Such information could also be used to enter people into clinical treatment trials or be used for personalized genetic counseling. The finding also paves the way to study klotho-based treatments to prevent or slow the pathological signs of AD. Even without the protective KLOTHO allele it may be possible to ramp up klotho and see if it has an effect on brain circuits and at synapses, the Stanford scientists said.
“I hope these compelling findings will motivate more of my basic science colleagues to start investigating klotho, its protein network partners, and its potential interactions with APOE,” said Dr. Greicius. In the meantime, his group is collaborating with research groups across the world to replicate and expand on the amyloid findings and to start looking into the role of KLOTHO levels in the blood and CSF of older controls and patients with AD.
“We know that klotho has protective effects at the synapse,” said Dena Dubal, MD, PhD, associate professor and the David A. Coulter endowed chair in aging and neurodegenerative disease at the Weill Institute for Neurosciences at University of California, San Francisco. “And we also know that the APOE4 allele can be toxic at the synapse.
Dr. Dubal, an associate editor of JAMA Neurology who wrote an accompanying editorial, also studies KLOTHO. Her work had spurred the Stanford scientists' interest in looking at the relationship between klotho and Alzheimer's. “We found that people with this KLOTHO variant leading to higher levels of klotho protein had better brain function in old age,” she said. “Remarkably, when we tested mice genetically engineered to produce more klotho, they were also smarter in old age. Increasing klotho in mouse models of AD also enhanced memory, even though the brain pathology did not improve.”
“The Stanford findings are a big deal for many reasons,” she added. “From a personal genomics standpoint, this should alter our counseling of APOE4 carriers. Not everyone is going to develop AD, and we can now test to see if they also have the protective KLOTHO genotype.”
In addition, she said, knowing a person's KLOTHO genotype before they are enrolled into AD clinical trials would be important. “Having people without the protective genotype may increase the ability to detect a therapeutic effect between those on active medication and those on a placebo,” she added.
“This data really supports the possibility that these two gene variants have a biological link,” she explained. “How might Klotho block the harmful biology of the APOE4 gene? And could klotho be a new target for AD treatment?”
(Dr. Dubal said that UCSF has an exclusive licensing agreement with Unity Biotechnology to develop medicines for healthy aging and to protect against neurodegenerative diseases. Another company called Klogene Therapeutics is working with scientists at Boston University to develop genetic strategies to target klotho.)
“The association between APOE4 and the KLOTHO variant is exciting,” said Sudha Seshadri, MD, FAAN, director of the Glenn Biggs Institute for Alzheimer's & Neurodegenerative Diseases, and the Robert R. Barker distinguished university professor of neurology at UT Health in San Antonio. “It confirms that this KLOTHO variant is protective in people at highest risk for Alzheimer's.”
Dr. Seshadri and colleagues have researched klotho and fibroblast growth factor 23 (FGF23) that appears to be involved with cerebral small vascular disease and poor cognition in patients on dialysis. This growth factor also interacts with klotho, said Dr. Seshadri. “It is important to understand the biology of this pathway,” suggesting that studies moving forward should include assays to measure blood levels of klotho.
Dr. Seshadri explained that Klotho also has an endocrine function, and it may be possible to develop a Klotho-like analog and use it to treat neurodegenerative conditions like AD.
“I think the data are strong that suggest that klotho heterozygosity reduces risk in E4+ people for AD, reduced risk of conversion from MCI/very mild dementia to AD, and somewhat reduced amyloid deposition as measured by CSF Abeta42 and brain imaging,” said David M. Holtzman, MD, FAAN, the Andrew B. and Gretchen P. Jones professor and chair of neurology at Washington University School of Medicine. “The caveats are that this was only studied to date in people of northwestern European descent, so we don't yet know if this applies to everyone yet.”
Dr. Holtzman said that he also thinks that his genetic change should be incorporated into ongoing and future clinical research related to AD.
“It also further supports that it will be important to better understand the details of how klotho partially protects against the effects of APOE4,” he added. “This will require not only human but also cellular and animal studies. For example, there may be several possible ways klotho is protective. It will also be important to sort out as if it protects against something very early versus later in pathogenesis could determine when targeting it therapeutically would have maximal therapeutic benefit.”