Dominantly Inherited Alzheimer Network Trial: An Opportunity to Prevent Dementia. A Study of Potential Disease Modifying Treatments in Individuals With a Type of Early Onset Alzheimer's Disease Caused by a Genetic Mutation (DIAN-TU)

Alzheimer's disease (AD) is characterized pathologically by the presence of accumulation of amyloid plaques and tau-containing neurofibrillary tangles (NFTs) in the brain. Amyloid plaques can be detectable within the brain some years before symptoms manifest whereas tau-mediated toxicity has been hypothesized to appear later during the course of disease. Physiologically, tau is predominantly a neuronal microtubule-associated protein that plays a fundamental role in the stabilization of microtubules. Under pathological conditions however, short motifs in the microtubule binding region (MTBR) domains of tau adopt a beta-sheet conformation, inducing self-assembly with other tau molecules that lead to the formation of insoluble aggregates. Insoluble tau is also a feature of a number of different neurodegenerative diseases collectively termed tauopathies. The accumulation of insoluble deposits has been suggested to result in altered distribution and function of organelles to adversely affect neuronal cell function as well as causing synapse loss, ultimately leading to cell death. In AD, evidence also suggests a direct correlation between the number of NFTs found in the brain at postmortem and the degree of dementia observed in subjects with AD at the time of death.

The microtubule-associated protein tau (MAPT) gene is located on chromosome 17 of the human genome. Through alternative splicing, 6 possible tau protein isoforms are expressed from this gene in the adult brain. A number of studies have suggested that pathological forms of tau protein transmit from neuron to neuron in human brain to cause disease, including AD. It has also been reported that tau can form seeds, which when applied extracellularly, can cause the initiation and propagation of intracellular tau aggregation. To form tau seeds, the MTBR of the protein is required. Furthermore, the tau MTBR is important in initiating the tau aggregation process and forming the core of fibrils pathologically associated with disease. Together, these observations suggest that therapeutic intervention with an antibody that binds to the MTBR region of tau in the brain, thereby disrupting tau aggregation may prevent initiation or slow down the neurodegeneration in AD or other tauopathies.

Upcoming Dominantly Inherited Alzheimer Network Trials Unit (DIAN-TU) trials are designed to investigate therapies targeting tau in combination with amyloid as pre-specified in the approved and NIH-funded NexGen Prevention Trial grants. As with other amyloid lowering drug trials, clinical benefit was not definitively demonstrated in the symptomatic population after tau pathology has been established. Determining the role of tau in disease biology and progression is critically important. Based on beneficial effects on amyloid, tau, and neurodegeneration markers associated with amyloid removal in the gantenerumab trial arm in DIAN-TU, the DIAN-TU will now implement amyloid removal treatment in mutation carriers and add placebo- controlled tau treatment arms. The platform trial design is exceptionally well-suited for the investigation of treatments used in combination because of ongoing multiple arms in a single trial and operational platform.