Tau pathology in the olfactory bulb correlates with Braak stage, Lewy body pathology and apolipoprotein ɛ4

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Abstract
Olfactory dysfunction increases with disease severity in Alzheimer's disease (AD), is early and independent of disease severity in Parkinson's disease (PD), but is absent in progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). Previous histopathologic studies of olfactory bulbs in AD have shown neurofibrillary tangles (NFTs) and senile plaques while Lewy bodies (LBs) have been described in PD. Little is known about olfactory bulb pathology in PSP and CBD. Tau and α‐synuclein pathology was assessed with immunohistochemistry in olfactory bulbs of AD (N = 15), Lewy body disease (LBD; N = 10), LBD with concurrent AD (AD/LBD; N = 19), PSP (N = 27), CBD (N = 3) and cases with no significant neurodegenerative pathology (NSP; N = 15). The Braak NFT stage, counts of senile plaques and NFT in cortical and hippocampal sections, and counts of LBs in amygdala and cortical sections were recorded for each case. Apolipoprotein E (APOE) genotypes were determined on DNA prepared from frozen brain tissue. All AD and AD/LBD cases and nine of 10 LBD cases had tau pathology in the anterior olfactory nucleus (AON), but it was uncommon in PSP (9/27), CBD (0/3) and NSP (5/15). Multiple linear regression analysis demonstrated that tau pathology in the AON correlated with Braak stage (P < 0.001), cortical LB counts (P < 0.001), as well as APOE ɛ4. Tau pathology is common in the olfactory bulb of AD and LBD but is minimal or absent in PSP and CBD. It correlates with APOE ɛ4, severity of tau pathology in the brain and surprisingly with cortical and amygdala LBs, suggesting a possible synergistic effect between tau and synuclein in the AON in cases with both pathologic processes.