The first human trial of a gene therapy to counter damage wrought by the ApoE4 protein had results presented at the Clinical Trials in Alzheimer’s Disease meeting, held November 29-December 2 in San Francisco. Five homozygous ApoE4 carriers in different stages of AD received a single dose of LX1001, a virus equipped with the gene for the protective ApoE2 allele. Three months later, its payload was found within the cerebrospinal fluid, where it held steady out to a year, according to Michael Kaplitt of Weill Cornell Medical College in New York. In three women who had 12-month follow-up data, CSF concentrations of Aβ42, phospho-tau, and total tau shrank over the course of the trial.

  • In five women given a low dose, ApoE2 gene therapy LX1001 seems safe.
  • LX1001 induced expression of ApoE2 in CSF of ApoE4 carriers.
  • CSF concentration of Aβ42, phospho-tau, and total tau trended downward.

“Targeting ApoE is a long-sought treatment for AD,” commented Stephen Salloway of Brown University in Providence, Rhode Island. “It is encouraging that there is some evidence of activity of ApoE2 in the CSF in a very small AD sample.”

“There is significant debate on whether APOE should be upregulated or downregulated, with reports showing both protective and pathological roles of ApoE4,” commented Pierre Tariot of Banner Alzheimer’s Institute in Phoenix. “Clinical trials may help answer this critical question,” Tariot added (full comment below).

ApoE4 was pegged as an AD risk factor 30 years ago. Shortly thereafter, ApoE2 emerged as a protective factor, and other rare variants were subsequently reported (Research timeline, ApoE Mutations database). Three decades on, researchers are still exploring which potential mechanisms explain the apolipoprotein’s sway over AD (Li et al., 2020). A recent analysis found that carriers of two copies of ApoE2 enjoy exceptional protection from AD, while other studies have shown that a single copy of ApoE2 can counter the AD risk imparted by a co-inherited copy of ApoE4 (Aug 2019 conference news; Genin et al., 2011).

Viral delivery of ApoE2 reportedly quells amyloid accumulation in transgenic mouse models (Nov 2013 news; Hu et al., 2015; Zhao et al., 2016). Collectively, these findings serve as the rationale behind the strategy of delivering ApoE2 to the brains of ApoE4 carriers. 

At CTAD, Kaplitt reported early findings from an ongoing dose-ranging Phase 1/2 trial of LX1001, an adeno-associated virus (AAV) that carries a copy of the human ApoE2 gene. Developed by Ronald Crystal and colleagues at Weill Cornell, the virus provoked widespread expression of ApoE2 in the cortices and hippocampi of primates following injection into the cisterna magna, a space near the cerebellum to which CSF from the ventricles drains (Rosenberg et al., 2018). Crystal then founded Lexeo Therapeutics to develop LX1001.

The open-label study is evaluating three sequentially higher doses of LX1001 among 15 enrolled homozygous ApoE4 carriers with mild cognitive impairment or mild to moderate AD. So far, only the lowest-dose group has completed the one-year trial. They were five women, including one with MCI and four with moderate AD. They received a single intrathecal injection of 1012 vector genomes of LX1001 via lateral C1/C2 puncture. In this procedure, a needle is inserted just below the ear, and, with guidance from a CT scan, advanced into the subarachnoid space nestled between the top two vertebrae in the neck. Safety was the primary endpoint; secondary endpoints included CSF measurement of ApoE2 and AD biomarkers.

Special Delivery. LX1001 was delivered into the central nervous system of participants via C1/C2 puncture, in which a needle is inserted below the ear and into the space between the top two cervical vertebrae. [Weill Cornell Medical College.]

Kaplitt reported that so far, no serious adverse events have occurred in the low-dose group, or in the mid-dose group, who recently received 3x1012 vector genomes of LX1001. Two women among the former group had a transient headache following injection of LX1001, a common side effect of intrathecal puncture.

Lo and behold, when four of the women returned for CSF sampling three months later, ApoE2 was detected in the CSF of all of them. Only three participants were able to return for CSF sampling at the one-year timepoint, Kaplitt said, as logistical hurdles posed by the pandemic prevented the other two from traveling. In the three who returned, the CSF ApoE2 concentration held steady out to 12 months. “This gives us direct evidence that the virus has been taken up and is working to produce ApoE2,” Kaplitt told the audience. “Whether or not it’s enough to be effective obviously remains to be seen.”

Kaplitt also reported AD biomarker findings for three of the participants. CSF Aβ42 trended down in two of them, by 24 and 33 percent between baseline and 12 months. Phospho-tau and total tau nudged downward in all three participants, dropping by 9 percent to 16 percent for phospho-tau, and by 4 percent to 22 percent for total tau. With so few people in the trial, the findings are not statistically significant.

Researchers were cautiously optimistic about these first findings. They noted a litany of questions that remain unanswered. “Trans-cisternal delivery of ApoE2 is an approach that is very early in development, with many challenges, such as how to ensure sufficient delivery and uptake into cells that matter to produce a clinical benefit, what stage of disease makes the most sense, and how to scale this treatment if it proves to be effective,” Salloway commented.

“It was gratifying to see the first data on this gene therapy approach,” commented Lawrence Honig of Columbia University in New York. Honig also pointed to unknowns. For one, it is unclear whether the brain itself expressed APOE2, let alone which cell types there (full comment below).

When Warren Hirst of Biogen posed a similar question at CTAD, Kaplitt said that he does not know the answer in humans. In primates, however, delivery of LX1001 via a similar route was shown to promote widespread expression in neurons and glial cells in the cortex and hippocampus.—Jessica Shugart


  1. ApoE plays an important role in increased Aβ accumulation, Aβ-induced neurotoxicity, reduced inflammation, reduced energy metabolism, mitochondrial/metabolic processes relevant to AD risk, lipid transport to neurons, synaptogenesis, cerebrovascular integrity and cerebral blood flow, hippocampal neurogenesis, neuroimmune modulation, and reduced Aβ clearance. The mechanisms by which APOE and its variants account for AD are not known, and it remains unclear whether APOE variants differ in the extent to which they represent a pathological gain of function (e.g., APOE4> 3> 2 >Christchurch >no APOE) or a pathological loss of function (APOE4<3<2).

    There is significant debate on whether APOE should be upregulated or downregulated, with reports showing both protective and pathological roles of ApoE4. Clinical trials may help answer this critical question. Novel approaches targeting APOE as a therapeutic strategy for AD include use of adeno-associated virus gene delivery of APOE2 to reduce Aβ pathology such as presented by Dr. Kaplitt; an ABCA1 peptide agonist derived from the carboxy terminus of APOE to attempt to decrease lipidation of astrocytic APOE4, or drugs such as bexarotene and 9-cis retinoic acid to achieve the same ends.

    They also include APOE4 structure correctors to convert neuronal APOE4 to APOE3 and/or mitigate neuronal toxicity caused by APOE4 fragments; APOE4 antibodies to target unlipidated APOE associated with plaques and increase Aβ clearance; and APOE antisense oligonucleotides (ASO’s) and small-interfering RNAs (RNAi’s) to decrease expression of APOE4 in the brain. The early results from the ApoE2 gene therapy trial are promising in terms of safety and achieving the desired protein expression and at least a signal suggesting possible normalization of certain AD biomarkers; hopefully it will continue to yield crucial information as the trial progresses.

  2. It was gratifying to see the first data on this gene therapy approach. It is a reasonable AD experimental strategy, although by no means fully supported preclinically, to try to convert APOE4 individuals, through AAV-carried viral gene therapy, into a mixed APOE2/APOE4 constitution. While understanding of APOE4 in AD is still quite incomplete, it is indisputable that APOE4 genotype is associated with greater cerebral Aβ42 and AD.

    The investigators have shown that they successfully intracisternally dosed five persons, reasonably safely (so far), that there was at least some APOE2 expression for at least three of four persons with viral measurements at 12 months.

    The preliminary biomarker data on three subjects showing Aβ42, t-tau, and p-tau showing possible small decreases over 12 months, are not convincingly consistent, large, or statistically or clinically significant. Furthermore, arguably one might expect to see an increase, not a decrease, in CSF Aβ42, if the therapy was directionally beneficial towards cerebral amyloid deposition.

    There are also unpresented data, and a number of unknowns:

    (a) it would be helpful to see data on any CSF inflammatory response, as can be seen with viral vectors, any MRI effects, any amyloid PET effects, and the clinical effects;

    (b) is not clear whether the magnitude of the APOE2 gene expression is potentially meaningful;

    (c) it is not clear whether there is any APOE2 expressed in the brain itself (rather than “CSF,” which could be white-cell or ependymal-cell expression) and whether there is systemic measurable expression;

    (d) it is not clear that if there is intracerebral expression, in what cell types it might be, at what magnitude (e.g., ratio to APOE4), and how widespread regionally.

    However, in addition to the now-proven efficacious strategy of Aβ clearance via monoclonal antibodies, it is certainly worthwhile to consider other approaches such as this one, that might also be useful.

  3. This is encouraging work that can help to move the gene therapy field forward. However, ICM one-time delivery is not going to provide optimal global delivery of vector to brain structures affected by AD, and ApoE2 is unlikely to protect from neurodegeneration on its own.

  4. Our lab (Dodart et al, 2005) first showed that intra-parenchymal lentiviral delivery of apoE2 reduces brain amyloid burden in APP transgenic mice, and over a relatively short period of time. We, together with the Crystal laboratory (Zhao et al. 2016), subsequently showed that intra-parenchymal AAV delivery of apoE2 markedly reduced brain amyloid burden, this time in an APP transgenic mouse where apoE4 was essential for the development of amyloid pathology.

    However, in both studies relatively high levels of apoE2 were necessary to reduce amyloid burden, and this was particularly true of APP mice expressing apoE4. Whether sufficient and widespread brain delivery of apoE2 can be safely achieved following intra-cisternal or intrathecal administration of a viral vector in humans has yet to be determined but will hopefully be learned from the pioneering work of Kaplitt and colleagues.

    Success may require systemic administration of an AAV capsid that readily crosses the blood brain barrier to deliver apoE2 more broadly throughout the brain. As we also pointed out, intra-parenchymal administration of current viral vectors results in apoE2 expression primarily in neurons (expression in glia, the cell type where apoE2 is physiologically expressed, is much less prominent). Thus, where, how much, and in what cell type apoE2 expression occurs following intrathecal administration of a viral vector maybe important from both an efficacy and safety perspective. Successful gene delivery of apoE2 for preventing/treating AD may require the use of novel capsids and cell-specific promoters.


    . Gene delivery of human apolipoprotein E alters brain Abeta burden in a mouse model of Alzheimer's disease. Proc Natl Acad Sci U S A. 2005 Jan 25;102(4):1211-6. PubMed.

    . Intracerebral adeno-associated virus gene delivery of apolipoprotein E2 markedly reduces brain amyloid pathology in Alzheimer's disease mouse models. Neurobiol Aging. 2016 Aug;44:159-72. Epub 2016 Apr 30 PubMed.

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Mutation Interactive Images Citations

  1. APOE

News Citations

  1. Rare Luck: Two Copies of ApoE2 Shield Against Alzheimer’s
  2. Averting a Late-life Crisis: Midlife ApoE2 Clears Plaques in Mice

Therapeutics Citations

  1. LX1001

Paper Citations

  1. . APOE2: protective mechanism and therapeutic implications for Alzheimer's disease. Mol Neurodegener. 2020 Nov 4;15(1):63. PubMed.
  2. . APOE and Alzheimer disease: a major gene with semi-dominant inheritance. Mol Psychiatry. 2011 Sep;16(9):903-7. Epub 2011 May 10 PubMed.
  3. . Opposing effects of viral mediated brain expression of apolipoprotein E2 (apoE2) and apoE4 on apoE lipidation and Aβ metabolism in apoE4-targeted replacement mice. Mol Neurodegener. 2015 Mar 5;10:6. PubMed.
  4. . Intracerebral adeno-associated virus gene delivery of apolipoprotein E2 markedly reduces brain amyloid pathology in Alzheimer's disease mouse models. Neurobiol Aging. 2016 Aug;44:159-72. Epub 2016 Apr 30 PubMed.
  5. . AAVrh.10-Mediated APOE2 Central Nervous System Gene Therapy for APOE4-Associated Alzheimer's Disease. Hum Gene Ther Clin Dev. 2018 Mar;29(1):24-47. Epub 2018 Mar 13 PubMed.

Other Citations

  1. Research timeline

Further Reading


  1. . Gene delivery of human apolipoprotein E alters brain Abeta burden in a mouse model of Alzheimer's disease. Proc Natl Acad Sci U S A. 2005 Jan 25;102(4):1211-6. PubMed.