Biological CSF Markers of Alzheimer's Disease

Handbook of Clinical Neurology, Vol. 89 (3rd series) Dementias C. Duyckaerts, I. Litvan, Editors # 2008 Elsevier B.V. All rights reserved

Alzheimer’s disease Chapter 24

Biological CSF markers of Alzheimer’s disease HENRIK ZETTERBERG AND KAJ BLENNOW * Institute of Clinical Neuroscience, Department of Experimental Neuroscience, Sahlgrenska University Hospital, Go¨teborg University, Go¨teborg, Sweden

24.1. Introduction The prevalence of Alzheimer’s disease (AD) is rapidly increasing with advancing age and the disease is expected to reach epidemic proportions between 2010 and 2050, when the number of AD patients is projected to more than double. The major neuropathological hallmarks of the disease are loss of neurons and synapses, senile plaques (extracellular aggregates primarily composed of b-amyloid; Ab) and neurofibrillary tangles (aggregates of hyperphosphorylated forms of the microtubule-associated tau protein) throughout cortical and limbic regions of the brain (Selkoe, 2002). During the preclinical phase of AD the neuronal degeneration proceeds and the amount of plaques and tangles increases. At a certain threshold the first symptoms, most often impairment of episodic memory, appear. This preclinical period probably starts 20–30 years before the first recognizable symptoms appear (Davies et al., 1988). According to current diagnostic criteria (McKhann et al., 1984), AD cannot be diagnosed clinically before the disease has progressed so far that dementia is present. This means that the symptoms must be severe enough to significantly interfere with work and social activities or relations. In recent years, however, mild memory impairment without overt dementia has gained increased attention in the medical community, and a new diagnostic entity designated mild cognitive impairment (MCI) has been defined. To make a diagnosis of MCI, memory disturbances must be verified by objective measures adjusted for age and education (Petersen et al., 1999). Like dementia, however, MCI may be caused by several different disorders. Many MCI patients have incipient AD, i.e., early AD pathology,

and will progress to AD with dementia, whereas other MCI patients have a benign form of MCI as part of the normal aging process (DeCarli, 2003). Furthermore, in some MCI cases, cerebrovascular pathology may contribute to the symptoms. This review focuses on the most established cerebrospinal fluid (CSF) biomarkers for MCI and AD (total tau [T-tau], hyperphosphorylated tau [P-tau] and the 42-amino-acid fragment of b-amyloid [Ab42]). We summarize what is known about the clinical usefulness of these biomarkers and how to interpret their results, with emphasis on early and differential diagnosis. We also briefly discuss the possible role of CSF biomarkers in the preclinical diagnosis of AD, when novel diseasemodifying drugs against AD are likely to be the most effective. Finally, we present an updated list of novel possible biological markers that may prove useful for early diagnosis of AD and monitoring of drug effects.

24.2. The importance of biomarkers 24.2.1. The importance of biomarkers in the clinic The introduction of acetylcholine esterase (AChE) inhibitors as symptomatic treatment has highlighted the importance of diagnostic markers for AD. The awareness in the population of the availability of drug treatment has also made patients seek medical advice at an earlier stage of the disease, making the percentage of MCI cases at dementia clinics increase. There is to date no clinical method to determine whether MCI in a certain patient will progress to AD with dementia or remain stable. Hence, new diagnostic tools to aid in the diagnosis of early AD and to identify incipient AD among

*Correspondence to: Kaj Blennow, MD, PhD, Institute of Clinical Neuroscience, Department of Experimental Neuroscience, Sahlgrenska University Hospital/Mo¨lndal, S-431 80 Go¨teborg, Sweden. E-mail: [email protected], Tel: þ46-31-3431791, Fax: þ46-31-343-2426.

262

H. ZETTERBERG AND K. BLENNOW

MCI cases would be of fundamental importance. Such diagnostic markers would be of even greater significance if new drugs such as b-sheet breakers, b-secretase and g-secretase inhibitors, and anti-amyloid vaccination regimes with promise of disease-arresting effects, prove to be clinically useful. These types of drugs may turn out to have the best effects in the early or even preclinical phase of the disease, when the synaptic and neuronal loss has not become too widespread. 24.2.2. The importance of biomarkers in drug development

more than 1 day and/or affecting daily life) was less than 1%. CSF biomarkers for AD should reflect the central pathogenic processes in the brain, i.e., the neuronal loss and synaptic and axonal degeneration, the aggregation of b-amyloid (Ab) with subsequent deposition in plaques, and the hyperphosphorylation and ubiquitination of tau with subsequent formation of tangles.

24.4. Tau proteins and b-amyloid as biomarkers for AD 24.4.1. Tau proteins

Biomarkers may be applied to development of drugs against AD in a number of ways. Firstly, they may be applied as additional diagnostic measures in a population clinically identified as having AD and, hence, provide additional inclusion or exclusion criteria. This would hypothetically ensure the recruitment of “pure” AD cases to studies of anti-AD drugs. Secondly, biomarkers may offer an indirect measure of disease severity and progression. A number of points should be established for such use: the marker must have a scientific rationale, the biomarker should change with disease progression in longitudinal observational studies, and the marker must be measurable and reproducible. Unlike typical diagnostic measures, when biomarkers are used for this purpose, high specificity is not required. Particularly in mid-phase trials, biomarkers can be used to identify appropriate dosage, improve safety assessments, demonstrate pharmacological activity, and identify preliminary evidence of efficacy.

24.3. The scientific rationale for AD biomarkers in CSF CSF is in direct contact with the extracellular space of the brain and biochemical changes in the brain are likely to be reflected in this easily accessible biological fluid. Since AD pathology is restricted to the brain, CSF may be considered a superior source of diagnostic biomarkers for AD as compared with peripheral blood. It should also be noted that lumbar punctures and CSF analyses have been used routinely in the practice of neurology for decades. Two large studies performed as part of an evaluation of possible AD biomarkers have shown that the procedure can be applied broadly and that it is well tolerated in the elderly population (Blennow et al., 1993; Andreasen et al., 2001). The only recorded complication was post-lumbar puncture headache. With the use of a small-diameter needle (0.7 mm), the rate of mild headache (duration less than 1 day, not affecting daily life) was less than 4%, and the rate of moderate or severe headache (duration

Tau is a microtubule-associated protein, primarily located in the axons and undetectable in peripheral blood. Alternative splicing of tau mRNA produces six different isoforms of the protein ranging in size between 352 and 441 amino acids and with molecular weights between 50 and 65 kDa (Goedert et al., 1989). By binding to the tubulin of the axonal microtubules, tau promotes their assembly and stability (Buee et al., 2000), which is important for axonal function and transport. Tau is a phosphoprotein, with more than 30 potential phosphorylation sites (Buee et al., 2000; Iqbal et al., 2002). The tangles in AD are made up of an abnormally hyperphosphorylated form of tau (Grundke-Iqbal et al., 1986). When hyperphosphorylated, tau loses its ability to bind to the microtubules and support their assembly (Iqbal et al., 2000). Four different ELISA methods for quantification of T-tau have been published (Vandermeeren et al., 1993; Blennow et al., 1995; Mori et al., 1995; Vigo-Pelfrey et al., 1995). The 20 largest studies including more than 2000 AD patients and 1000 controls, and evaluating the most commonly used ELISA method for T-tau in CSF, have recently been reviewed (Blennow and Hampel, 2003; Blennow, 2004). Taken together, these report an overall sensitivity to discriminate sporadic AD from non-demented age-matched controls of 81% at a specificity level of 91%. Most of the studies are cross-sectional. Furthermore, few studies have examined CSF T-tau in pathologically confirmed AD cases. However, in a study of 131 AD cases the diagnosis was autopsy-verified in 31 cases and the performance of CSF T-tau and Ab42 was similar in both groups (Sunderland et al., 2003). Since CSF T-tau reflects neuronal and axonal degeneration, it is not specific for AD. High levels can be found in all CNS disorders with significant neuronal degeneration or damage. The highest levels are detected in acute stroke (Hesse et al., 2000) and in CJD (Otto et al., 1997). Importantly, several studies have consistently found a very marked increase in CSF T-tau in

BIOLOGICAL CSF MARKERS OF ALZHEIMER’S DISEASE CJD (Otto et al., 1997, 2002; Riemenschneider et al., 2003; Van Everbroeck et al., 2003). The mean level of CSF T-tau in CJD is 10–50-times higher than in controls, resulting in a sensitivity close to 100% and a specificity above 90% against other dementias such as AD. This diagnostic performance is similar to that of CSF 14–3-3 protein (Otto et al., 2002; Van Everbroeck et al., 2003). However, since most of the available 14–3-3 tests are based on qualitative immunoblot, the quantitative ELISA methods for T-tau may be preferable in the clinical laboratory. In vascular dementia, high CSF T-tau has been found in some (Blennow et al., 1995; Andreasen et al., 1999a; Sjogren et al., 2000), but not all (Vigo-Pelfrey et al., 1995; Shoji et al., 1998; Sjogren et al., 2001b), studies. The reason for this discrepancy is unknown but may involve differences in patient characteristics and diagnostic criteria used. High T-tau in clinically diagnosed vascular dementia cases may be caused by concomitant AD pathology, which is a frequent finding at autopsy (Jellinger, 1996; Kosunen et al., 1996) but difficult to identify clinically. A study of longitudinal magnetic resonance tomography scans in vascular dementia cases also found that cases with progressive white-matter changes have normal CSF T-tau (Andreasen et al., 1999a). Furthermore, in patients with nonacute cerebrovascular disease without dementia, CSF T-tau is normal (Arai et al., 1997; Nishimura et al., 1998). These data are compatible with the interpretation that a high CSF-tau level in a patient with clinical and brain imaging findings indicative of vascular dementia suggests mixed (AD/vascular) dementia. Most studies have found normal to mildly increased CSF T-tau levels in other dementias, such as frontotemporal dementia (FTD) and dementia with Lewy bodies (DLB) (Blennow, 2004). Other than in aged non-demented individuals, normal CSF T-tau is found in depression, alcoholic dementia, and in chronic neurological disorders such as Parkinson’s disease (PD) and progressive supranuclear palsy (PSP) (Blennow, 2004). Thus, CSF T-tau has a clear diagnostic value in the differentiation between AD and these important and often difficult alternative diagnoses. Several phosphorylation sites have been identified on tau (Buee et al., 2000). Most of these are Ser-Pro or Thr-Pro motives and are localized outside the microtubule-binding domains. Hyperphosphorylation of tau is found during neuronal development and in several neurodegenerative disorders (Buee et al., 2000; Iqbal et al., 2002). There is no consensus whether or not there are phosphorylation sites that are specific for AD versus other neurodegenerative diseases involving tau pathology, such as FTD. Six different ELISA

263

methods have been developed for quantification of tau phosphorylated at different epitopes (Blennow, 2004), including threonine 181 þ 231, threonine 181, threonine 231 þ serine 235, serine 199, threonine 231, and serine 396 þ 404. A moderate to marked increase in CSF P-tau has been found using all of these different methods and the mean sensitivity to discriminate AD from nondemented age-matched individuals is 81%, at a specificity level of 91% (Blennow, 2004). The CSF level of P-tau probably reflects the phosphorylation state of tau. This view is based on indirect evidence such as the finding that there is no change in CSF P-tau after acute stroke (Hesse et al., 2000), although there is a marked increase in T-tau. Furthermore, CSF P-tau levels are normal or only mildly increased in CJD despite a very marked increase in T-tau (Riemenschneider et al., 2003). These data suggest that P-tau in CSF is not a marker for neuronal degeneration or damage but that it specifically reflects the phosphorylation state of tau and thus possibly the formation of tangles in AD brains. Accordingly, the specificity of CSF P-tau to differentiate AD from other dementias seems to be higher than for T-tau and Ab42. Normal CSF levels of P-tau are found in psychiatric disorders such as depression, chronic neurological disorders such as amyotrophic lateral sclerosis, PD and PSP, and also in most cases with other dementia disorders such as vascular dementia, FTD and DLB (Blennow, 2004). Furthermore, although there is a very distinct increase in CSF T-tau in CJD, most patients with CJD have normal or only mildly elevated CSF P-tau. In a large set of patients with CJD and other dementias, the ratio of P-tau to T-tau in CSF was found to discriminate CJD from other neurodegenerative disorders without any overlap (Riemenschneider et al., 2003). 24.4.2. b-Amyloid The main component of senile plaques is b-amyloid (Ab) (Masters et al., 1985), which is generated by proteolytic cleavage of its precursor, the amyloid precursor protein (APP) (Selkoe, 2001). After it was found that Ab is generated as a more or less soluble protein during normal cellular metabolism and is secreted in the CSF (Seubert et al., 1992), studies examining CSF Ab as a candidate biomarker for AD were published. However, these initial reports used ELISA methods for “total Ab” that did not discriminate between different Ab isoforms. Although some studies found a slight decrease in the CSF level of total Ab in AD, there was a large overlap between AD patients and controls, and in some studies no change in CSF total Ab was found. It was soon discovered that there

264

H. ZETTERBERG AND K. BLENNOW

are several N- and C-terminally truncated forms of Ab. The two major C-terminal variants of Ab consist of a shorter form ending at Val-40 (Ab40), and a longer form ending at Ala-42 (Ab42). Ab42 was found to aggregate more rapidly than Ab40 (Jarrett et al., 1993), and to be the initial and predominating form of Ab deposited in diffuse plaques (Miller et al., 1993; Iwatsubo et al., 1994; Tamaoka et al., 1994). These data made it logical to develop immunoassays specific for Ab42. To date, 11 different methods have been developed for quantification of Ab42 in CSF (Blennow, 2004). A moderate to marked decrease in CSF Ab42 to about 50% of control levels in AD patients has been found using most of these methods. These studies include more than 650 AD patients and 500 controls. The mean sensitivity to discriminate between AD and normal aging is 86%, at a specificity level of 89% (Blennow, 2004). Other than in non-demented, aged individuals, normal CSF Ab42 is found in psychiatric disorders such as depression, and in chronic neurological disorders such as PD and PSP (Blennow, 2004). Thus, CSF Ab42 helps in the clinical differentiation between AD and these important and often difficult differential diagnoses. However, data on the performance of CSF Ab42 in the discrimination between AD and other dementias and neurological disorders are relatively limited. A mild to moderate decrease in CSF Ab42 is found in a percentage of patients with FTD and vascular dementia (Hulstaert et al., 1999; Sjogren et al., 2000; Riemenschneider et al., 2002b; Sjogren et al., 2002). The reduced CSF level of Ab42 in AD is often hypothesized to be caused by deposition of Ab42 in senile plaques, with lower levels diffusing to CSF. However, some studies have also found a marked reduction in CSF Ab42 in disorders without b-amyloid plaques, such as CJD (Otto et al., 2000), amyotrophic lateral sclerosis (Sjogren et al., 2002), and multiple system atrophy (Holmberg et al., 2003). These findings make a direct correlation questionable between low CSF Ab42 and deposition of Ab in plaques. In contrast, a recent autopsy study found strong correlations between low Ab42 in ventricular CSF and high numbers of plaques in the neocortex and hippocampus (Strozyk et al., 2003), suggesting that the reduction in CSF Ab42 in AD may at least partly be due to sequestration of Ab in plaques. There is no decrease in CSF Ab40 in AD (Kanai et al., 1998; Shoji et al., 1998; Fukuyama et al., 2000; Mehta et al., 2000). As a consequence, a decrease in the ratio of Ab42/Ab40 (or increase in the ratio of Ab40/Ab42) in CSF has been found in

AD in several papers (Kanai et al., 1998; Shoji et al., 1998; Fukuyama et al., 2000). This decrease seems more pronounced than the reduction of CSF Ab42 alone and, hence, further studies are warranted to determine whether the CSF Ab42/Ab40 ratio has a higher diagnostic potential than CSF Ab42 alone. Using mass spectrometry, it has been found that there is a quite heterogeneous set of Ab peptides in CSF (Vigo-Pelfrey et al., 1993). Also using urea-based sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western immunoblot (Wiltfang et al., 2002), it is possible to separate several C-terminally truncated Ab peptides in CSF, including Ab1–37, Ab1–38, Ab1–39, Ab1–40, and Ab1–42. A recent finding is that the second most abundant Ab peptide is Ab1–38, which is more abundant than Ab1– 42 (Wiltfang et al., 2002). In AD, increased CSF levels of both Ab1–40 and Ab1–38 were found, along with a decrease in Ab1–42. Similar data have been obtained using surface-enhanced laser desorption/ionization (SELDI) time-of-flight (TOF) mass spectrometry (Lewczuk et al., 2003). Further studies on large patient and control series are needed to determine the diagnostic potential of these Ab variants. Using two-dimensional electrophoresis combined with Western blotting and mass spectrometry for characterization of formic acid-extracted insoluble Ab from brain tissue, it was found that N-terminally truncated Ab42 variants are present in the earliest stages of AD (Sergeant et al., 2003). We have recently shown that these N-terminally truncated Ab42 species are also detectable in CSF and may be of diagnostic use in early AD (Vanderstichele et al., 2005). 24.4.3. Combination of CSF biomarkers for AD There are several studies in which the diagnostic potential of the combination of CSF T-tau and Ab42 have been evaluated. For the most commonly used ELISA methods for T-tau and Ab42, sensitivity and specificity figures are available from 12 studies (Blennow, 2004). The sensitivity and specificity for the combination of CSF T-tau and Ab42 are slightly higher (89% and 90%, respectively) than for the respective marker alone. Other combinations of CSF markers also result in slightly better diagnostic performance than the use of single markers. In a study on the combination of CSF P-Tau181 and Ab42, the sensitivity for AD was 86% at a specificity of 97% (Maddalena et al., 2003), and in another study the combination of CSF T-tau and P-tau396/404 resulted in a sensitivity of 96% at a specificity of 100% (Hu et al., 2002).

BIOLOGICAL CSF MARKERS OF ALZHEIMER’S DISEASE

24.5. CSF biomarkers in early AD and MCI The performance of CSF T-tau, P-tau, and Ab42 in early AD cases with Mini-Mental State Examination (MMSE) scores above 23–25 has been examined in some studies. Also in this early phase of the disease, the sensitivity figures are similar to those found in more advanced AD cases (Blennow, 2004). Several studies have evaluated the performance of CSF markers in MCI cases that developed AD during a clinical follow-up period of 1–2 years, finding sensitivity figures similar to those found in AD cases with clinical dementia (Andreasen et al., 1999b; Arai et al., 2000; Gottfries et al., 2001; Lautenschlager et al., 2001; Maruyama et al., 2001; Buerger et al., 2002; Riemenschneider et al., 2002a; Andreasen et al., 2003; Zetterberg et al., 2003). A population-based study also showed that reduced CSF Ab42 is present in asymptomatic elderly who developed dementia during a 3-year follow-up period (Skoog et al., 2003). Furthermore, an association between lower CSF Ab42 concentrations and the e4 allele of the apolipoprotein E gene in non-demented controls was recently reported (Prince et al., 2004). One interpretation of this result is that Ab42 concentrations in CSF may decline before the onset of clinical cognitive change and, thus, Ab42 should be further explored as a potential antecedent biomarker that may predict development of dementia due to AD.

265

24.6. Use of CSF biomarkers in clinical practice The diagnostic performance of AD biomarkers in clinical practice has been evaluated in two studies (Andreasen et al., 1999a, 2001). In these, the CSF markers were evaluated on prospective patient samples from clinical practice, and ELISA assays were routinely run each week in a neurochemistry laboratory. The diagnostic performance of CSF T-tau and the combination of CSF T-tau and Ab42 was similar to that found in other studies, with a high ability to differentiate AD from normal aging, depression and PD, but lower specificity against other dementias such as vascular dementia and LBD. Taken together, the high sensitivity and specificity figures reported in most investigations of CSF T-tau, P-tau and Ab42 as markers of AD suggest that these are ready for use in the clinical routine. Table 24.1 summarizes the typical results of this three-marker test in AD and the most important differential diagnoses. The diagnostic performance of the CSF markers seems to be highest in the differentiation between AD and normal aging, depression, B-vitamin deficiency, alcohol dementia, PD and PSP. Another useful clinical application is the identification of CJD in cases with rapidly progressive dementia, in which the combination of a very marked increase in CSF T-tau concentration and normal or only mildly increased P-tau has

Table 24.1 Typical three-marker (T-tau, P-tau and Ab42) test results in Alzheimer’s disease and important differential diagnoses CSF biomarkers Diagnosis

T-tau

P-tau

Ab42

AD Normal aging Depression B-vitamin deficiency Alcohol dementia PD PSP CJD FTD LBD Vascular dementia Acute stroke

Increase Normal Normal Normal Normal Normal Normal Very marked increase Normal to mild increase Normal to mild increase Normal to mild increase Mild to very marked increase Normal

Increase Normal Normal Normal Normal Normal Normal Normal Normal Normal to mild increase Normal Normal

Decrease Normal Normal Normal Normal Normal Normal Normal to marked decrease Normal to moderate decrease Moderate decrease Mild to moderate decrease Normal

Not examined

Normal

Non-acute CVD without dementia

AD, Alzheimer’s disease; PD, Parkinson’s disease; PSP, progressive supranuclear palsy; CJD, Creutzfeldt-Jakob disease; FTD, frontotemporal dementia; LBD, Lewy body dementia; CVD, cerebrovascular disease.

266

H. ZETTERBERG AND K. BLENNOW

high diagnostic value. Lastly, these CSF markers may be useful in identifying mixed AD/vascular dementia. The lower specificity against other dementias, such as LBD and FTD, may hamper the clinical utility of the currently available CSF markers. However, in the clinic, dementias with differing history, symptoms, and findings on brain imaging (e.g., FTD, vascular dementia) can often be identified by means of the medical history, clinical examination, and auxiliary investigations (e.g., blood tests, SPECT, and computerized tomography or magnetic resonance tomography). A major diagnostic challenge is whether a patient with MCI will progress to AD with dementia or not. Although larger, longitudinal studies are needed, the three-marker test seems very useful for this application.

24.7. Candidate CSF biomarkers for AD There are several promising candidate CSF biomarkers that to date have only been evaluated in a few studies. These include among others ubiquitin (Wang et al., 1991; Blennow et al., 1994), neurofilament proteins (Rosengren et al., 1999; Sjogren et al., 2001a), neuronal thread protein (Monte et al., 1997), GAP43 (neuromodulin) (Sjogren et al., 2000), different markers for oxidative damage and defective lipid peroxidation, such as isoprostanes (Pratico, 2005) and 12/15 lipoxygenase products (Yao et al., 2005), and several cytokines involved in microglia and astrocyte activation (Mrak and Griffin, 2005). Even more biomarkers are being discovered in the explorative studies that are employing proteomics technology on samples from different well-defined dementia disorders (Davidsson and Sjogren, 2005). Further studies are warranted on the diagnostic potential of all of these markers. Furthermore, the time seems ripe for larger longitudinal studies combining CSF biomarkers with recently developed and very promising neuroimaging techniques, such as 2-[18F]fluoro-2-deoxy-d-glucose positron emission tomography (FDG-PET) (Mosconi et al., 2005) and amyloid visualization using molecular tracers in combination with PET or single photon emission computed tomography (SPECT) (Mathis et al., 2004). Possibly, we may end up with a CSF multimarker profile that can be used in conjunction with clinical examination, neuropsychological testing and neuroimaging techniques for early diagnosis of AD, preclinical diagnosis of the disease in certain risk groups of patients, and drug selection and monitoring of drug effects for optimal treatment using upcoming disease-modifying drugs.

References Andreasen N, Minthon L, Clarberg A, et al. (1999a). Sensitivity, specificity, and stability of CSF-tau in AD in a communitybased patient sample. Neurology 53: 1488–1494. Andreasen N, Minthon L, Vanmechelen E, et al. (1999b). Cerebrospinal fluid tau and Abeta42 as predictors of development of Alzheimer’s disease in patients with mild cognitive impairment. Neurosci Lett 273: 5–8. Andreasen N, Minthon L, Davidsson P, et al. (2001). Evaluation of CSF-tau and CSF-Abeta42 as diagnostic markers for Alzheimer disease in clinical practice. Arch Neurol 58: 373–379. Andreasen N, Vanmechelen E, Vanderstichele H, et al. (2003). Cerebrospinal fluid levels of total-tau, phosphotau and A beta 42 predicts development of Alzheimer’s disease in patients with mild cognitive impairment. Acta Neurol Scand Suppl 179: 47–51. Arai H, Morikawa Y, Higuchi M, et al. (1997). Cerebrospinal fluid tau levels in neurodegenerative diseases with distinct tau-related pathology. Biochem Biophys Res Commun 236: 262–264. Arai H, Ishiguro K, Ohno H, et al. (2000). CSF phosphorylated tau protein and mild cognitive impairment: a prospective study. Exp Neurol 166: 201–203. Blennow K (2004). Cerebrospinal fluid protein biomarkers for Alzheimer’s disease. NeuroRx 1: 213–225. Blennow K, Hampel H (2003). CSF markers for incipient Alzheimer’s disease. Lancet Neurol 2: 605–613. Blennow K, Wallin A, Hager O (1993). Low frequency of post-lumbar puncture headache in demented patients. Acta Neurol Scand 88: 221–223. Blennow K, Davidsson P, Wallin A, et al. (1994). Ubiquitin in cerebrospinal fluid in Alzheimer’s disease and vascular dementia. Int Psychogeriatr 6: 13–22. Blennow K, Wallin A, Agren H, et al. (1995). Tau protein in cerebrospinal fluid: a biochemical marker for axonal degeneration in Alzheimer disease?. Mol Chem Neuropathol 26: 231–245. Buee L, Bussiere T, Buee-Scherrer V, et al. (2000). Tau protein isoforms, phosphorylation and role in neurodegenerative disorders. Brain Res Rev 33: 95–130. Buerger K, Teipel SJ, Zinkowski R, et al. (2002). CSF tau protein phosphorylated at threonine 231 correlates with cognitive decline in MCI subjects. Neurology 59: 627–629. Davidsson P, Sjogren M (2005). The use of proteomics in biomarker discovery in neurodegenerative diseases. Dis Markers 21: 81–92. Davies L, Wolska B, Hilbich C, et al. (1988). A4 amyloid protein deposition and the diagnosis of Alzheimer’s disease: Prevalence in aged brains determined by immunocytochemistry compared with conventional neuropathologic techniques. Neurology 38: 1688–1693. DeCarli C (2003). Mild cognitive impairment: prevalence, prognosis, aetiology, and treatment. Lancet Neurol 2: 15–21.

BIOLOGICAL CSF MARKERS OF ALZHEIMER’S DISEASE Fukuyama R, Mizuno T, Mori S, et al. (2000). Age-dependent change in the levels of Abeta40 and Abeta42 in cerebrospinal fluid from control subjects, and a decrease in the ratio of Abeta42 to Abeta40 level in cerebrospinal fluid from Alzheimer’s disease patients. Eur Neurol 43: 155–160. Goedert M, Spillantini MG, Potier MC, et al. (1989). Cloning and sequencing of the cdna encoding an isoform of microtubule-associated protein tau containing four tandem repeats: Differential expression of tau protein mrnas in human brain. Embo J 8: 393–399. Gottfries J, Blennow K, Lehmann MW, et al. (2001). Onecarbon metabolism and other biochemical correlates of cognitive impairment as visualized by principal component analysis. J Geriatr Psychiatry Neurol 14: 109–114. Grundke-Iqbal I, Iqbal K, Tung YC, et al. (1986). Abnormal phosphorylation of the microtubule-associated protein tau (tau) in Alzheimer cytoskeletal pathology. Proc Natl Acad Sci USA 83: 4913–4917. Hesse C, Rosengren L, Vanmechelen E, et al. (2000). Cerebrospinal fluid markers for Alzheimer’s disease evaluated after acute ischemic stroke. J Alzheimers Dis 2: 199–206. Holmberg B, Johnels B, Blennow K, et al. (2003). Cerebrospinal fluid Abeta42 is reduced in multiple system atrophy but normal in Parkinson’s disease and progressive supranuclear palsy. Mov Disord 18: 186–190. Hu YY, He SS, Wang X, et al. (2002). Levels of nonphosphorylated and phosphorylated tau in cerebrospinal fluid of Alzheimer’s disease patients : An ultrasensitive bienzyme-substrate-recycle enzyme-linked immunosorbent assay. Am J Pathol 160: 1269–1278. Hulstaert F, Blennow K, Ivanoiu A, et al. (1999). Improved discrimination of AD patients using beta-amyloid(1–42) and tau levels in CSF. Neurology 52: 1555–1562. Iqbal K, Alonso AD, Gondal JA, et al. (2000). Mechanism of neurofibrillary degeneration and pharmacologic therapeutic approach. J Neural Transm Suppl 59: 213–222. Iqbal K, Alonso Adel C, El-Akkad E, et al. (2002). Pharmacological targets to inhibit Alzheimer neurofibrillary degeneration. J Neural Transm Suppl 62: 309–319. Iwatsubo T, Odaka A, Suzuki N, et al. (1994). Visualization of A beta 42(43) and A beta 40 in senile plaques with endspecific A beta monoclonals: Evidence that an initially deposited species is A beta 42(43). Neuron 13: 45–53. Jarrett JT, Berger EP, Lansbury PT Jr (1993). The carboxy terminus of the beta amyloid protein is critical for the seeding of amyloid formation: Implications for the pathogenesis of Alzheimer’s disease. Biochemistry 32: 4693–4697. Jellinger KA (1996). Diagnostic accuracy of Alzheimer’s disease: A clinicopathological study. Acta Neuropathol (Berl) 91: 219–220. Kanai M, Matsubara E, Isoe K, et al. (1998). Longitudinal study of cerebrospinal fluid levels of tau, A beta1–40, and A beta1–42(43) in Alzheimer’s disease: A study in Japan. Ann Neurol 44: 17–26. Kosunen O, Soininen H, Paljarvi L, et al. (1996). Diagnostic accuracy of Alzheimer’s disease: A neuropathological study. Acta Neuropathol (Berl) 91: 185–193.

267

Lautenschlager NT, Riemenschneider M, Drzezga A, et al. (2001). Primary degenerative mild cognitive impairment: study population, clinical, brain imaging and biochemical findings. Dement Geriatr Cogn Disord 12: 379–386. Lewczuk P, Esselmann H, Meyer M, et al. (2003). The amyloid-beta (Abeta) peptide pattern in cerebrospinal fluid in Alzheimer’s disease: evidence of a novel carboxyterminally elongated Abeta peptide. Rapid Commun Mass Spectrom 17: 1291–1296. Maddalena A, Papassotiropoulos A, Muller-Tillmanns B, et al. (2003). Biochemical diagnosis of Alzheimer disease by measuring the cerebrospinal fluid ratio of phosphorylated tau protein to beta-amyloid peptide42. Arch Neurol 60: 1202–1206. Maruyama M, Arai H, Sugita M, et al. (2001). Cerebrospinal fluid amyloid beta(1–42) levels in the mild cognitive impairment stage of Alzheimer’s disease. Exp Neurol 172: 433–436. Masters CL, Simms G, Weinman NA, et al. (1985). Amyloid plaque core protein in Alzheimer disease and Down syndrome. Proc Natl Acad Sci U S A 82: 4245–4249. Mathis CA, Wang Y, Klunk WE (2004). Imaging betaamyloid plaques and neurofibrillary tangles in the aging human brain. Curr Pharm Des 10: 1469–1492. McKhann G, Drachman D, Folstein M, et al. (1984). Clinical diagnosis of Alzheimer’s disease: Report of the NINCDSADRDA Work Group under the auspices of Department of Health and Human Services Task Force on Alzheimer’s Disease. Neurology 34: 939–944. Mehta PD, Pirttila T, Mehta SP, et al. (2000). Plasma and cerebrospinal fluid levels of amyloid beta proteins 1–40 and 1–42 in Alzheimer disease. Arch Neurol 57: 100–105. Miller DL, Papayannopoulos IA, Styles J, et al. (1993). Peptide compositions of the cerebrovascular and senile plaque core amyloid deposits of Alzheimer’s disease. Arch Biochem Biophys 301: 41–52. Monte SM, Ghanbari K, Frey WH, et al. (1997). Characterization of the AD7C-NTP cdna expression in Alzheimer’s disease and measurement of a 41-kd protein in cerebrospinal fluid. J Clin Invest 100: 3093–3104. Mori H, Hosoda K, Matsubara E, et al. (1995). Tau in cerebrospinal fluids: establishment of the sandwich ELISA with antibody specific to the repeat sequence in tau. Neurosci Lett 186: 181–183. Mosconi L, Tsui WH, De Santi S, et al. (2005). Reduced hippocampal metabolism in MCI and AD: automated FDGPET image analysis. Neurology 64: 1860–1867. Mrak RE, Griffin WS (2005). Glia and their cytokines in progression of neurodegeneration. Neurobiol Aging 26: 349–354. Nishimura T, Takeda M, Nakamura Y, et al. (1998). Basic and clinical studies on the measurement of tau protein in cerebrospinal fluid as a biological marker for Alzheimer’s disease and related disorders: Multicenter study in Japan. Methods Find Exp Clin Pharmacol 20: 227–235. Otto M, Wiltfang J, Tumani H, et al. (1997). Elevated levels of tau-protein in cerebrospinal fluid of patients with Creutzfeldt-Jakob disease. Neurosci Lett 225: 210–212.

268

H. ZETTERBERG AND K. BLENNOW

Otto M, Esselmann H, Schulz-Shaeffer W, et al. (2000). Decreased beta-amyloid1–42 in cerebrospinal fluid of patients with Creutzfeldt-Jakob disease. Neurology 54: 1099–1102. Otto M, Wiltfang J, Cepek L, et al. (2002). Tau protein and 14-3-3 protein in the differential diagnosis of CreutzfeldtJakob disease. Neurology 58: 192–197. Petersen RC, Smith GE, Waring SC, et al. (1999). Mild cognitive impairment: Clinical characterization and outcome. Arch Neurol 56: 303–308. Pratico D (2005). Peripheral biomarkers of oxidative damage in Alzheimer’s disease: The road ahead. Neurobiol Aging 26: 581–583. Prince JA, Zetterberg H, Andreasen N, et al. (2004). APOE epsilon4 allele is associated with reduced cerebrospinal fluid levels of Abeta42. Neurology 62: 2116–2118. Riemenschneider M, Lautenschlager N, Wagenpfeil S, et al. (2002a). Cerebrospinal fluid tau and beta-amyloid 42 proteins identify Alzheimer disease in subjects with mild cognitive impairment. Arch Neurol 59: 1729–1734. Riemenschneider M, Wagenpfeil S, Diehl J, et al. (2002b). Tau and Abeta42 protein in CSF of patients with frontotemporal degeneration. Neurology 58: 1622–1628. Riemenschneider M, Wagenpfeil S, Vanderstichele H, et al. (2003). Phospho-tau/total tau ratio in cerebrospinal fluid discriminates Creutzfeldt-Jakob disease from other dementias. Mol Psychiatry 8: 343–347. Rosengren LE, Karlsson JE, Sjogren M, et al. (1999). Neurofilament protein levels in CSF are increased in dementia. Neurology 52: 1090–1093. Selkoe DJ (2001). Alzheimer’s disease: Genes, proteins, and therapy. Physiol Rev 81: 741–766. Selkoe DJ (2002). Alzheimer’s disease is a synaptic failure. Science 298: 789–791. Sergeant N, Bombois S, Ghestem A, et al. (2003). Truncated beta-amyloid peptide species in pre-clinical Alzheimer’s disease as new targets for the vaccination approach. J Neurochem 85: 1581–1591. Seubert P, Vigo-Pelfrey C, Esch F, et al. (1992). Isolation and quantification of soluble Alzheimer’s beta-peptide from biological fluids. Nature 359: 325–327. Shoji M, Matsubara E, Kanai M, et al. (1998). Combination assay of CSF tau, A beta 1–40 and A beta 1–42(43) as a biochemical marker of Alzheimer’s disease. J Neurol Sci 158: 134–140. Sjogren M, Minthon L, Davidsson P, et al. (2000). CSF levels of tau, beta-amyloid(1–42) and GAP-43 in frontotemporal dementia, other types of dementia and normal aging. J Neural Transm 107: 563–579. Sjogren M, Blomberg M, Jonsson M, et al. (2001a). Neurofilament protein in cerebrospinal fluid: a marker of white matter changes. J Neurosci Res 66: 510–516. Sjogren M, Davidsson P, Gottfries J, et al. (2001b). The cerebrospinal fluid levels of tau, growth-associated protein-43 and soluble amyloid precursor protein correlate in Alzheimer’s disease, reflecting a common pathophysiological process. Dement Geriatr Cogn Disord 12: 257–264.

Sjogren M, Davidsson P, Wallin A, et al. (2002). Decreased CSF-beta-amyloid 42 in Alzheimer’s disease and amyotrophic lateral sclerosis may reflect mismetabolism of beta-amyloid induced by disparate mechanisms. Dement Geriatr Cogn Disord 13: 112–118. Skoog I, Davidsson P, Aevarsson O, et al. (2003). Cerebrospinal fluid beta-amyloid 42 is reduced before the onset of sporadic dementia: A population-based study in 85-yearolds. Dement Geriatr Cogn Disord 15: 169–176. Strozyk D, Blennow K, White LR, et al. (2003). CSF Abeta 42 levels correlate with amyloid-neuropathology in a population-based autopsy study. Neurology 60: 652–656. Sunderland T, Linker G, Mirza N, et al. (2003). Decreased beta-amyloid1–42 and increased tau levels in cerebrospinal fluid of patients with Alzheimer disease. JAMA 289: 2094–2103. Tamaoka A, Kondo T, Odaka A, et al. (1994). Biochemical evidence for the long-tail form (Abeta 1–42/43) of amyloid beta protein as a seed molecule in cerebral deposits of Alzheimer’s disease. Biochem Biophys Res Commun 205: 834–842. Vandermeeren M, Mercken M, Vanmechelen E, et al. (1993). Detection of tau proteins in normal and Alzheimer’s disease cerebrospinal fluid with a sensitive sandwich enzyme-linked immunosorbent assay. J Neurochem 61: 1828–1834. Vanderstichele H, De Meyer G, Andreasen N, et al. (2005). Amino-truncated {beta}-amyloid42 peptides in cerebrospinal fluid and prediction of progression of mild cognitive impairment. Clin Chem 51: 1650–1660. Van Everbroeck B, Quoilin S, Boons J, et al. (2003). A prospective study of CSF markers in 250 patients with possible Creutzfeldt-Jakob disease. J Neurol Neurosurg Psychiatry 74: 1210–1214. Vigo-Pelfrey C, Lee D, Keim P, et al. (1993). Characterization of beta-amyloid peptide from human cerebrospinal fluid. J Neurochem 61: 1965–1968. Vigo-Pelfrey C, Seubert P, Barbour R, et al. (1995). Elevation of microtubule-associated protein tau in the cerebrospinal fluid of patients with Alzheimer’s disease. Neurology 45: 788–793. Wang GP, Khatoon S, Iqbal K, et al. (1991). Brain ubiquitin is markedly elevated in Alzheimer disease. Brain Res 566: 146–151. Wiltfang J, Esselmann H, Bibl M, et al. (2002). Highly conserved and disease-specific patterns of carboxyterminally truncated Abeta peptides 1–37/38/39 in addition to 1–40/ 42 in Alzheimer’s disease and in patients with chronic neuroinflammation. J Neurochem 81: 481–496. Yao Y, Clark CM, Trojanowski JQ, et al. (2005). Elevation of 12/15 lipoxygenase products in AD and mild cognitive impairment. Ann Neurol 58: 623–626. Zetterberg H, Wahlund LO, Blennow K (2003). Cerebrospinal fluid markers for prediction of Alzheimer’s disease. Neurosci Lett 352: 67–69.