Cerebrospinal fluid (CSF) tau and phospho-tau are well established biomarkers of Alzheimer’s disease. While these measures are conventionally referred to as ‘total tau’ (T-tau) and ‘phospho-tau’ (P-tau), several truncated and modified tau forms exist that may relay additional diagnostic information. We evaluated the diagnostic performance of an endogenous tau peptide in CSF, tau 175–190, in the phosphorylated and non-phosphorylated state. A liquid chromatography-mass spectrometry (LC-MS) method was established to measure these peptides in CSF and was used to analyze two independent clinical cohorts; the first cohort included patients with Alzheimer’s disease (AD, n = 15), Parkinson’s disease (PD, n = 15), progressive supranuclear palsy (PSP, n = 15), and healthy controls (n = 15), the second cohort included AD patients (n = 16), and healthy controls (n = 24). In both cohorts T-tau and P-tau concentrations were determined by immunoassay. While tau 175–190 and P-tau 175–190 did not differentiate the study groups, the separation of AD and controls by T-tau (area under the ROC Curve (AUC) = 95%) and P-tau (AUC = 92%) was improved when normalizing the ELISA measurements to the concentrations of the endogenous peptides: T-tau/tau 175–190 (AUC = 100%), P-tau/P-tau 175–190 (AUC = 95%). The separation between patients and controls by T-tau (AUC = 88%) and P-tau (AUC = 82%) was similarly improved in the second cohort by taking the ratios of T-tau/tau 175–190 (AUC = 97%) and P-tau/P-tau 175–190 (AUC = 98%). In conclusion, our results suggest that the performance of the AD biomarkers T-tau and P-tau could be improved by normalizing their measurements to the endogenous peptides tau 175–190 and P-tau 175–190, possibly because these endogenous tau peptides serve to normalize for physiological, and disease-independent, secretion of tau from neurons to the extracellular space and the CSF. Finally, the observations made here add to the general applicability of mass spectrometry as a tool for rapid identification and accurate quantification of biomarker candidates.

脑脊液 (CSF) tau 和磷酸化 tau 是公认的阿尔茨海默病生物标志物。虽然这些测量通常被称为“总 tau”（T-tau）和“磷酸化 tau”（P-tau），但存在几种截断和修改的 tau 形式，可以传递额外的诊断信息。我们评估了 CSF 中内源性 tau 肽 tau 175-190 在磷酸化和非磷酸化状态下的诊断性能。建立了液相色谱-质谱（LC-MS）方法来测量脑脊液中的这些肽，并用于分析两个独立的临床队列；第一个队列包括患有阿尔茨海默病（AD，n = 15）、帕金森病（PD，n = 15）、进行性核上性麻痹（PSP，n = 15）和健康对照组（n = 15）的患者，第二个队列包括AD患者（n = 16），和健康对照（n = 24）。在两个队列中，T-tau 和 P-tau 浓度均通过免疫测定法确定。虽然 tau 175-190 和 P-tau 175-190 并没有区分研究组，但 AD 和对照组的分离通过 T-tau（ROC 曲线下面积 (AUC) = 95%）和 P-tau（AUC = 92 %）在将 ELISA 测量值标准化为内源性肽浓度时得到改善：T-tau/tau 175-190（AUC = 100%），P-tau/P-tau 175-190（AUC = 95%）。T-tau (AUC = 88%) 和 P-tau (AUC = 82%) 的患者和对照组之间的分离在第二个队列中通过采用 T-tau/tau 的比率 175-190 (AUC = 97) 同样得到改善%) 和 P-tau/P-tau 175–190 (AUC = 98%)。综上所述，我们的结果表明，AD 生物标志物 T-tau 和 P-tau 的性能可以通过将其测量值标准化为内源性肽 tau 175-190 和 P-tau 175-190 来提高，可能是因为这些内源性 tau 肽用于标准化tau 从神经元到细胞外空间和脑脊液的生理性和与疾病无关的分泌。最后，这里所做的观察增加了质谱作为快速识别和准确量化候选生物标志物的工具的普遍适用性。