Article In Brief
Antisense oligonucleotides targeted and lowered humanized APOE4 mutation tau levels in a mouse model of late-onset Alzheimer's disease by about 50 percent. The experiments resulted in decreased aggregation of tau protein, a protein associated with neural degeneration.
Researchers have for the first time used antisense oligonucleotides (ASOs) to target and reduce levels of human apolipoprotein E4 (APOE4) levels by about one-half in a mouse model of late-onset Alzheimer disease (AD).
ASOs—small sequences of DNA that can be used to target RNA transcripts and reduce or modify protein expression—can target and correct the activity caused by the genetic deletions, restoring normal APOE4 activity.
Results of the research by a team of investigators from different institutions—including Washington University School of Medicine, Harvard School of Medicine, Massachusetts General Hospital, Duke University Medical Center, and the Veterans Health Administration—were published online ahead of print on February 11 in Annals of Neurology.
Led by David Holtzman, MD, FAAN, the Andrew B. & Gretchen P. Jones professor and chair of neurology at Washington University, the research team tested the approach in a mouse model of late-onset AD characterized by defects in a key chromosome location that normally controls tau protein in healthy human brains.
Patients with the mutations develop late-onset AD characterized by runaway tau proteins forming neurofibrillary tangles and plaques. The APOE4 genotype is the strongest genetic risk factor for late-onset AD, with the E4 allele increasing risk in a dose-dependent fashion. Moreover, APOE4 plays a crucial role in amyloid-beta deposition. Recent evidence suggests that it also plays an important role in tau pathology and tau-mediated neurodegeneration in late-onset Alzheimer disease, Dr. Holtzman told Neurology Today.
Increased levels of tau and phosphorylated-tau have been detected in the cerebrospinal fluid (CSF) of individuals with AD and at least one copy of APOE4. Moreover, several reports have found that APOE4 carriers diagnosed with frontotemporal dementia (FTD) display significantly higher brain atrophy and exacerbated behavioral deficits as well as lower age of disease onset.
The research team recently documented more severe regional brain degeneration in APOE4 individuals with Pick's disease, corticobasal degeneration
(CBD) and progressive supranuclear palsy (PSP). The APOE4 genotype was also associated with significantly faster rates of clinical disease progression in amyloid-positive individuals with mild dementia in a dose-dependent manner.
“It is not known, however, whether or not therapeutic reduction of APOE4 would exert protective effects on tau-mediated neurodegeneration,” Dr. Holtzman said. “What we discovered in these mice is that using ASOs to reduce levels of APOE4 corresponded with a slowing of neurodegeneration in the animals as well as reduced levels of circulating tau inflammatory markers.”
It is believed that ASOs may make ideal candidates treating neurodegenerative disease caused by genetic mutations, Dr. Holtzman noted.
“ASOs targeting APOE4 have not yet moved into human testing; they have only been tested in animals to date,” Dr. Holtzman noted. “I personally think this would be a very good ASO target to assess in humans in the appropriate diseases. I do not know how close we are to implementing this approach in humans.”
The mouse model expresses only human APOE4. It does not express mouse APOE4. However, expression of APOE4 in the mouse brain using this model is very similar in pattern and level to what is seen in the human brain, where it is mostly expressed by astrocytes and by activated microglia.
The researchers treated the transgenic P301S/APOE4 mice with APOE4 or control ASOs via intracerebroventricular injection at 6 months and 7.5 months of age, and assessed brain pathology at 9 months of age, which is similar in brain age to a middle-aged human.
“Our results indicate that treatment with APOE4 ASOs reduced APOE4 protein levels by approximately 50 percent, significantly protected against tau pathology and associated neurodegeneration, decreased neuroinflammation, and preserved synaptic density,” Dr. Holtzman said.
“These data were also corroborated by a significant reduction in levels of neurofilament light chain (NfL) protein in plasma of ASO-treated mice,” he added. “We feel that reducing APOE4 levels should be further explored as a therapeutic approach for APOE4 carriers with tauopathy, including Alzheimer's disease.”
“If this approach was used in humans with similar results to what we have seen in mouse models—starting treatment in individuals with very mild dementia and/or mild cognitive impairment due to Alzheimer's—one might anticipate that it could decrease amyloid-induced local damage as well as tau-induced damage and lead to significant slowing of both brain atrophy and cognitive decline.”
The cost of screening for APOE4 genotype is very low and easy to get with a simple blood draw, he noted, adding that some insurance carriers will cover this now if indicated, and if there was a treatment based on knowing one's APOE4 genotype, insurance would cover that, Dr. Holtzman said.
“This is a clever manifestation of a therapeutic idea that has been making the rounds for some years, but had a boost when Biogen's antibody trials for Alzheimer disease, designed to remove the amyloid-beta protein, fell on hard times,” said Jeanne Loring, PhD, professor emeritus and director of the Center for Regenerative Medicine in the department of molecular medicine at Scripps Research Institute in La Jolla, CA.
There has been a lot of discussion about whether aggregations of proteins like amyloid-beta in AD, abnormal tau in frontotemporal dementia, or alpha-synuclein in Parkinson's disease, cause disease or are a result of disease, she noted.
“I lean toward the results side because there are so many situations in which a person has a disease, but has no aggregated protein at death, and the opposite—people who do not have the disease in spite of having a lot of aggregated protein.”
“Allan Rose's discovery that having the E4 isoform of APOE, increased the risk of getting AD was quite a breakthrough back in the 1990s and it lit a fire for Dr. Holtzman and others who pursued the APOE issue, even as the big pharma companies were developing their yet-to-fail antibody treatments for amyloid-beta.”
This paper shows that in mice a gene therapy that blocks production of APOE4 —the human form was put into a mouse as a transgene—resulted in decreased aggregation of tau protein, another protein associated with neural degeneration, Dr. Loring said.
“The important result, in my mind, is that the inflammation in the mouse brain was decreased. Neuroinflammation is a common feature of AD, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and multiple sclerosis. Anything that targets and reduces inflammation is worth investigating,” she added.
Gene therapies often fail because the genes don't get into the right cells, Dr. Loring said, or there is an immune reaction to the virus used to deliver the gene.
“It's definitely not a slam dunk, although there have been some moderate successes in rare diseases. I like to keep in mind that enthusiasm from a mouse study needs to be tempered by differences between mice and humans.”
Giulio Maria Pasinetti, MD, PhD, the Saunders Family Chair and professor of neurology, and director of the Center for Molecular Integrative Neuroresilience at the Friedman Brain Institute of the Icahn School of Medicine at Mount Sinai, agreed.
“The strength of this study is that it is based on a robust methodology for investigating the cellular and molecular impacts of knocking down APOE4 levels in a P301S/APOE4 transgenic mouse line,” he commented. The results “are well characterized, showing significant improvements in AD type neuropathologies in vivo, including attenuation of neurodegeneration, preservation of synaptic density, and reduction of NfL in plasma.”
However, the paper did not address any behavioral aspects of treatment, Dr. Pasinetti observed. “While the paper characterizes the impact of ASO treatment of APOE4 at a cellular and molecular level, no behavioral data are presented,” Dr. Pasinetti said. “The behavioral impacts of the attenuation of neurodegeneration, brain inflammation, and preservation of synaptic density would strengthen the author's interpretation of utilizing an APOE4 ASO as a treatment for tauopathies.”