Dementia is a syndrome that can be caused by a number of progressive disorders that affect memory, thinking, behavior and the ability to perform everyday activities. About 36 million people worldwide are suffering from dementia today. The number of people with dementia is projected to double by 2030, and more than triple to 115.4 million people by 2050. Alzheimer's disease (AD) is the most common type of dementia. Currently, one in nine people age 65 and older (11 percent) and nearly half of those over age 85 have Alzheimer's disease. According to Alzheimer's Disease International, current global costs of caring for these patients exceeds $600 billion annually. These costs are likely to rise even faster than the prevalence of disease, especially in the developing world, as more formal social care systems emerge, and rising incomes lead to higher opportunity costs (B. Winblad and L. Jonsson, World Alzheimer Report 2010).
The brains of AD patients have an abundance of two abnormal structures, amyloid plaques and neurofibrillary tangles. This is especially true in certain regions of the brain that are important in memory. There is also a substantial loss of neurons and synapses in the cerebral cortex and certain subcortical regions. Both neurofibrillary tangles and neuronal loss increase in parallel with the duration and severity of illness (T. Gomez-Isla et al., Ann. Neurol. 1997; 41:17-24) and neurofibrillary load has been shown to correlate with cognitive decline (H. Braak and E. Braak, Neurobiol. Aging 1997 July-August; 18(4):351-7).
Neurofibrillary tangles are intraneuronal lesions that are composed of hyperphosphorylated and insoluble accumulations of the microtubule-associated protein, tau. These accumulations are a histopathological feature of many neurodegenerative diseases, which are collectively known as tauopathies. Tauopathies include, e.g., Alzheimer's disease (AD), Pick's disease (PiD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and frontotemporal lobar degeneration (FTLD). In human tauopathies, pathology progresses from one brain region to another in disease-specific patterns (H. Braak and E. Braak, Neurobiol. Aging 1997 July-August; 18(4):351-7; Raj et al., Neuron. 2012; 73:1204-1215; Seeley et al., Neuron. 2009; 62:42-52; and Zhou et al. Neuron. 2012; 73:1216-1227), the underlying mechanism of which is not yet clear.
Tau pathology is involved in and may be a cause of many tauopathies. In its normal form, tau is a highly soluble microtubule-associated protein (Jeganathan et al., Biochemistry 2008, 47:10526-10539) that binds and promotes the assembly of microtubules (Drechsel et al., Mol. Biol. Cell. 1992, 3:1141-1154). However, in tauopathies, tau becomes hyperphosphorylated, causing detachment from microtubules, and ultimately accumulation as neurofibrillary tangles that are visualized within dystrophic neurites and cell bodies (Mandelkow and Mandelkow, Cold Spring Harbor, Perspect. Med. 2, 2012: a006247). The amount of tau pathology correlates with progressive neuronal dysfunction, synaptic loss, and functional decline in humans and transgenic mouse models (Arriagada et al., Neurology 1992 March; 42(3 Pt 1):631-9; Bancher et al., Neurosci. Lett. 1993, 162:179-182; Polydoro et al., J. Neuroscience 2009, 29:10741-10749; and Small and Duff, Neuron. 2008 Nov. 26; 60(4):534-42). While there have been no tau mutations observed in Alzheimer's disease, mutations in the tau gene appear to cause some forms of frontotemporal dementia (Cairns et al., Am. J. Pathol. 2007, 171:227-40), presenting with tau-positive inclusions and signifying that tau dysfunction is sufficient to cause neurodegeneration. Moreover, pathological tau appears to be an integral part of Aβ-induced neurotoxicity in cell culture and transgenic animal models (M. Rapoport, PNAS 2002, 99:9, 6364-6369; E. D. Roberson et al., Science 2007, 316:750-754; A. M. Nicholson and A. Ferreira, J. Neurosci. 2009, 29:4640-4651; H. Oakley, J. Neurosci. 2006, 26(40):10129-10140).
Passive and active immunizations against tau have been analyzed in mice using several different mouse models, including different phospho-tau peptides for active immunizations and anti-tau antibodies for passive immunotherapy (A. A. Asuni et al., J. Neurosci. 2007, 27(34):9115-9129; E. M. Sigurdsson, Curr. Alzheimer Res. 2009, 6(5):446-450; A. Boutajangout et al., J. Neurosci. 2010, 30(49):16559-16566; H. Rosenmann et al., Arch. Neurol. 2006, 63(10):1459-1467; M. Boimel et al., Exp. Neurol. 2010, 224(2):472-485). In the first report describing immunizations with a 30-amino acid phosphorylated tau peptide, an effect on the ratios of soluble and insoluble tau, reduction of tangle formation in the immunized mice, and functional benefits observed in behavior testing for these mice were shown (A. Boutajangout et al., J. Neuroscience 2010, 30:16559-16566). Passive immunization with well-characterized anti-tau antibodies, which react with phosphorylated Ser396 and Ser404 of the hyperphosphorylated tau protein at an early pathologic conformational epitope on tau, confirmed the results seen in active immunization studies. Mice treated with these antibodies showed marked reductions in tau pathology, which was measured by biochemical methods and histology, as well as a significant delay in loss of motor-function decline that was assessed in behavioral testing (A. Boutajangout et al., J. Neurochem. 2011, 118(4):658-667; X. Chai et al., J. Biol. Chem. 2011, 286(39):34457-34467).
Tau-based therapies have only been analyzed in mouse models to date. But in view of the severity of tauopathies in general, and to the cost to society of Alzheimer's disease specifically, there is an ongoing need for effective means to diagnose, monitor, prevent and treat tauopathies.