Would carrying a muted version of APOE4, the strongest genetic risk factor of sporadic Alzheimer’s disease, protect against the disorder? Perhaps. Scientists led by Michael Greicius at Stanford University and Chang-En Yu at the VA Puget Sound Health Care System, Seattle, described two such cases in a medRxiv preprint uploaded July 24.
- Scientists found two men with APOE4 loss-of-function variants.
- At 70+ and 90+, both were cognitively sharp, with no signs of amyloid plaques.
- This may bode well for future therapeutics aiming to knock down APOE4.
Both men, who were APOE3/4, had an aberrant stop codon in their E4 allele. One stayed sharp into his 90s, the other until his most recent doctor’s visit at 79. The former had no amyloid plaques in his brain postmortem and the latter had no amyloid markers in his cerebrospinal fluid. “The results provide the strongest human genetics evidence yet available suggesting that ε4 drives AD risk through a gain of abnormal function and support knockdown of APOE ε4 or its protein product as a viable therapeutic option,” the authors concluded. In contrast, a woman who was ApoE3/4 carried a deletion in her E3 allele, rendering it inactive. She developed AD at 75, about average for people with one copy of E4.
Loss-of-function (LoF) variants in APOE are rare. Eight have been reported, but only one, a frameshift in exon 4, was studied for its effects on cognition. A man in his early 40s with both copies of APOE silenced by the frameshift had blood choked with lipids, but his cerebrospinal fluid (CSF) was devoid of AD markers and he seemed cognitively normal, according to his global Mini-Mental State Exam score. Even so, he scored slightly below average for his age on verbal memory and language tasks, leaving it unclear whether the lack of APOE was helpful or harmful to his brain (Aug 2014 news).
To find others like him, co-first authors Augustine Chemparathy and Yann Le Guen, both at Stanford, analyzed whole-genome and whole-exome sequencing data from 20,850 people with AD and 26,600 controls ages 64 to 90 in the Alzheimer’s Disease Sequencing Project (ADSP), a compilation of genetic data from more than 40 cohorts worldwide. They looked for single nucleotide polymorphisms (SNPs) or structural variants in APOE predicted to quash protein levels. Four mutations fit the bill: two SNPs in exon 2, p.W5* and p.L8*, one in exon 3, p.Q39*, and a deletion. The SNPs create a premature stop codon leading to a truncated protein predicted to have no function. The 1,798bp deletion cuts out part of the promoter through to exon 2, preventing transcription (see image below).
Disrupting ApoE. The point mutations p.W5*, p.L8*, and pQ39* create premature stop codons, truncating ApoE. A deletion spanning the promoter to exon 2 silences transcription completely. The mutation p.W228* was more common, but left ApoE protein levels in human cells unchanged. [Courtesy of Chemparathy et al., medRxiv, 2023.]
In the whole ADSP cohort, five controls and one person with AD carried one of these LoF variants. One was the man who was cognitively healthy when he died in his 90s. He was APOE3/4 and had the p.W5* SNP in his E4 allele, rendering him effectively APOE3/-. Postmortem analysis revealed no amyloid plaques despite moderate neurofibrillary tangles at Braak stage IV. Still, he was an outlier for his age among other APOE3/4 carriers (see image below). This pattern of no plaques yet some tangles suggests primary age-related tauopathy, or PART (Nov 2014 news).
Healthy Outlier. Compared to age-related scores for amyloid plaques (top row), cerebral amyloid angiopathy (bottom left), and neurofibrillary tangles (bottom right) among APOE4 heterozygotes, a 90-year-old man with a loss-of-function variant in his APOE4 allele stood out for lack of amyloid and few tangles (red dot). [Courtesy of Chemparathy et al., medRxiv, 2023.]
Another man who was APOE3/4 with the same pW5* SNP, stayed sharp until age 79 and had normal levels of CSF Aβ42/40 and total tau. About two-thirds of APOE3/4 carriers test positive for amyloid by age 75 (Feb 2022 news).
The remaining controls were three cognitively normal women in their 70s and 80s. Two were effectively APOE3/- as the pW5* or pQ39* SNP knocked out their other E3 allele. The third woman was APOE3/4 and had the p.L8* SNP, but the scientists were unable to pinpoint which allele was silenced.
The sixth person was a woman diagnosed with AD at 75 who died in her late 80s. She was APOE3/4 and had the 1,1798 bp deletion in her E3 allele, leaving her with just an intact E4 allele. In ADSP, the mean age of AD onset for APOE4 heterozygotes is 73.5 years old, meaning she developed the disease at around the average age.
All told, these cases suggest that APOE3/4 carriers can lose an APOE4, dodge the risk for AD associated with that allele, and still live healthy lives.
Indeed, researchers are exploring therapeutically silencing the E4 allele. At the Alzheimer’s Association International Conference, held last month in Amsterdam, scientists led by Ornit Chiba-Falek of Duke University School of Medicine, Durham, North Carolina, reported a CRISPR-based epigenome targeting system that represses ApoE expression in human brain organoids and in mice carrying the human gene. The virally delivered CRISPR machinery reduced expression by 50 percent in the organoids and 70 percent in mice. It also slashed phospho-tau production in the organoids by 70 percent. For preclinical studies, others have developed a “switch” mouse to study the effects of knocking down E4 and adding E2 at any point during the AD continuum (see Part 13 of this series).—Chelsea Weidman Burke
- ApoE: One Man’s Brain Can Do Without It
- Scientists Propose a New Definition for Tau-Only Pathology
- Amyloid Lurks in a Third of Cognitively Normal People Over 70
- Meet the Switching Mice: They Flip Their Glia APOE4 to APOE2
No Available Further Reading
- Chemparathy A, LeGuen Y, Chen S, Lee E-, Leong L, Gorzynski J, Xu G, Belloy M, Kasireddy N, Pena-Tauber A, Williams K, Stewart I, Wingo T, Lah J, Jayadev S, Hales C, Peskind E, Child DD, Keene CD, Cong L, Ashley E, Yu C-, Greicus M. APOE loss-of-function variants: Compatible with longevity and associated with resistance to Alzheimer's Disease pathology. 2023 Jul 24 10.1101/2023.07.20.23292771 (version 1) medRxiv.