Model Suggests Sporadic Alzheimer is Driven by Neural Gene Network Dysregulation

Bruce A. Yankner, MD, PhD, a professor of genetics in the Blavatnik Institute at Harvard Medical School

Bruce A. Yankner, MD, PhD

A new, model-in-a-dish of sporadic Alzheimer disease has suggested that the dysregulation of neural gene networks may set in motion the pathologic downstream that leads to Alzheimer, potentially removing a major roadblock in the development of therapies for the disease.¹

Specifically, the findings showed identical molecular abnormalities across multiple lines of sporadic Alzheimer for the first time. The model showed that cells differentiated in an accelerated fashion in early development, as well as irregularities in the RE1-Silencing Transcription factor (REST) protein, which regulates this early-phase neuronal differentiation.

“[This] is an intriguing in vitro system that has been missing from the field,” said senior study author Bruce A. Yankner, MD, PhD, a professor of genetics in the Blavatnik Institute at Harvard Medical School, in a statement.² “It’s exciting to uncover a shared phenotype. That was unexpected.”

Yankner did note his skepticism that there could be such a profound defect in sporadic Alzheimer despite the lack of a dominant genetic mutation, though he and his team were able to replicate the findings in cells obtained from 3 additional laboratories. There are some limitations, which he acknowledged.

“These [induced pluripotent stem] cell culture models are valuable because they were derived from human cells and replay the developmental tape. However, they have not aged 80 years like a brain with Alzheimer,” Yankner said.

Yankner, notably, led the investigation in the 1990s that first demonstrated the toxic effects of amyloid beta in Alzheimer. In this assessment, he and colleagues obtained skin cells from 5 patients with sporadic Alzheimer disease and 6 healthy controls. These cells were then reverted to an undifferentiated state as induced pluripotent stem cells (iPSCs), via the retroviral transduction of OCT4, SOX2, KLF4, and cMYC. The iPSCs were then matured into neural progenitors (NPs), at which point drastic differences were observed in the sporadic Alzheimer group.

Enrichment analysis via gene ontology showed gene enrichment in processes related to neurogenesis and neuronal differentiation. The most strongly upregulated genes involved in the induction of neurogenesis were ASCL1/MASH1 (8.9-fold), DLX2 (5.8-fold), and MEIS1 (5.7-fold), while those involved in early neuronal differentiation which were similarly upregulated were DCX (11.4-fold; false discovery rate, 0.054), CD24 (8.2-fold), and STMN2 (13.5-fold). As well, EPHB1, which is involved in axonal targeting, was upregulated 3.7-fold.

“The cells differentiate better, or at least faster. You get more neurons, not fewer,” Yankner explained. Using the appearance of action potentials (APs) to measure the maturation of neurons in each group, Yankner and co-investigators observed APs in the sporadic Alzheimer cells by 4.5 to 7.5 weeks of differentiation, compared to 10 to 12 weeks in the control cells. They noted that even after those 10 to 12 weeks, the Alzheimer cells generated a significantly higher number of AP spikes.

“This difference in the appearance of APs in [sporadic Alzheimer] versus [control] neurons was also observed when neurons were co-cultured with astrocytes,” they wrote.

The investigators additionally observed nuclear differences between the groups, with the sporadic Alzheimer cells appearing more misshapen with a higher occurrence of structural changes in their membranes than the control cells. Previous research has shown such defects in the brains of those who died with Alzheimer, but how and whether they relate to neurodegeneration had not been determined.

Using fluorescence-activated cell sorting (FACS) analysis, Yankner and colleagues showed significantly reduced nuclear REST in the NP cells of those with sporadic Alzheimer, most pronounced in the subset of DCX-positive cells in multiple sporadic Alzheimer NP cell lines.

“In addition, downregulation of REST in normal control NP cells by 2 different short hairpin RNAs (shRNAs) increased DCX expression,” they wrote. “These results suggest that REST function is reduced in SAD NP cells as a result of reduced nuclear translocation and chromatin binding.”

The sporadic Alzheimer risk factor gene APOE, as well as cell cycle progression and neurogenesis-regulator CCND2, were significantly downregulated in the Alzheimer group NP cells. “To determine whether the APOE genotype influences the group effect on gene expression, we excluded APOE4 carriers and re-analyzed the data on APOE3/E3 lines. Importantly, the APOE3/E3 SAD lines consistently showed increased expression of neuronal differentiation genes relative to APOE3/E3 NL lines,” Yankner and colleagues wrote.

Study coauthor George Church, PhD, the Robert Winthrop Professor of Genetics in the Blavatnik Institute at Harvard Medical School, noted in a statement that he excited by the potential of this model to uncover contributing factors for other diseases without clear genetic drivers.

“We can not only create appropriate human cell types, but even skip over the many decades normally needed to develop a phenotype and find assays that detect predisposition to late-onset diseases even when the known genetics is inadequate,” he said.

REFERENCES

1. Meyer K, Feldman HM, Lu T, et al. REST and neural gene network dysregulation in iPSC models of Alzheimer’s disease. Cell Reports. Published online January 29, 2019. doi.org/10.1016/j.celrep.2019.01.023. Accessed January 29, 2019.

2. Dutchen S. Sporadic Alzheimer’s in a dish [press release]. Boston, MA: Harvard Medical School; Published January 29, 2019. hms.harvard.edu/news/sporadic-alzheimers-dish. Accessed January 29, 2019.