New Alzheimer's Blood Test Focuses on Lipids

Janis C. Kelly

March 14, 2014

The latest entry in the race for a predictive test for Alzheimer's disease (AD) is a 10-lipid panel.

Mark Mapstone, PhD, and colleagues found that the biomarker panel, which is used to test peripheral blood samples, was able to predict which participants, all of whom were cognitively normal at baseline, would develop amnestic mild cognitive impairment (aMCI) or AD over 2 to 3 years, with sensitivity of 90% and specificity of 90%.

According to the authors, test accuracy was similar to that from most studies of cerebrospinal fluid (CSF) biomarkers, while the new blood test is both less invasive and less expensive that those requiring CSF.

"The alterations in these lipids give us an early indication of the temporal course of the changes in preclinical disease before the symptoms manifest," Dr. Mapstone, who is associate professor of neurology at the University of Rochester Medical Center, New York, told Medscape Medical News. "The findings confirm that the disease process can be detected before symptoms start, without methods like lumbar puncture or brain imaging. They may also suggest a more widespread or systemic component to what has always been thought of as a brain disease."

Dr. Mapstone said that this study establishes proof-of-concept that such a blood test is possible, but he warned against assuming that it could quickly change clinical practice. "Clinicians need to know that we are reporting a biomarker of preclinical disease to be used primarily for clinical research. This is not a screening test suitable for the general population at this time," he said.

"We are optimistic that this can be translated to a screening test in the future, but until good options for treatment are available, there are limitations on how information from such a test might be used. If there were to be a test that could be ordered by a clinician to determine Alzheimer's risk, the patient, along with their family and clinician, would have to determine what's right for them (test or not test). The testing process would need to be managed and counselling needs to be available," Dr. Mapstone said.

Their findings were reported online as a letter in the March 9 in Nature Medicine.

Selecting Biomarkers

The researchers first enrolled 525 otherwise healthy community-dwelling participants aged 70 years or older into a 5-year observational study, by the end of which 74 had developed aMCI or mild AD. Forty-six of these 74 were incidental cases; 28 of 74 (the "converters") converted from nonimpaired memory status at entry (Converterpre) to aMCI or AD, over an average time of 2.1 years.

Cognitive impairment was defined by using a composite measure of memory performance that included the Multiple Assessment Inventory IADL Scale, the Multifactorial Memory Questionnaire, the Mini-Mental State Examination, the Geriatric Depression Scale-Short Form, the Wechsler Memory Scale-III Forward Digit Span, the Trail Making Test-Part A, the Wechsler Memory Scale-III Backward Digit Span, the Trail Making Test-Part B, Category Fluency (animals), the Boston Naming Test 60-Item version, the Rey Auditory Verbal Learning Test Learning, the Rey Auditory Verbal Learning Test Recall, the Rey Auditory Verbal Learning Test Retention, and the Hooper Visual Organization Test.

Normal controls with no memory impairment were selected from the study population and matched with the aMCI/AD group on the basis of age, education, and sex.

For development of the biomarker panel, 53 participants were selected in the third year of the study for metabolomic and lipidomic biomarker discovery, including 18 converters and 35 with aMCI/AD at entry. These were matched with 53 normal controls (NCs).

Plasma samples from these discovery-phase samples underwent untargeted analysis that yielded 2700 positive-mode features and 1900 negative-mode features, which were then run through least absolute shrinkage and selection operator analysis to identify markers that differentiated the control group from the preconversion samples of the Converterpre group. The binary comparisons identified 4 putative metabolite markers: phosphatidylinositol (18:0/0:0), proline-asparagine dipeptide, glucoursodeoxycholic acid, and malic acid.

More targeted analysis showed that Converterpre participants had significantly lower plasma levels of serotonin, phenylalanine, proline lysine, phosphatidylcholine (PC), taurine, and acylcarnitine (AC).

The authors write, "A notable finding of this targeted metabolomic and lipidomic analysis was the identification of a set of ten metabolites, comprising PCs, (PC diacyl (aa) C36:6, PC aa C38:0, PC aa C38:6, PC aa C40:1, PC aa C40:2, PC aa C40:6, PC acyl-alkyl (ae) C40:6), lysophophatidylcholine (lysoPC a C18:2), and acylcarnitines (ACs) (Propionyl AC (C3) and C16:1-OH) that were depleted in the plasma of the Converterpre participants but not in that of the NC group." These metabolites were similar to the levels in the aMCI/AD group.

The proposed panel was validated in an independent, blinded, cross-validation using the remaining subset of 21 patients with aMCI/AD (which included 10 converters) and 20 matched NCs. Analysis showed that levels of the 10-metabolite panel were similar in these participants to those in the discovery samples.

The researchers then used the metabolomic data to build a linear classifier model to distinguish the aMCI/AD and Converterpre participants from NCs. "This model yielded a sensitivity of 90% and specificity of 90%, for classifying the Converterpre and NC groups in the validation phase," the authors write.

More Validation Needed

"The data look promising and raise our hopes for a blood biomarker signature becoming a reality for the early diagnosis of Alzheimer's. The weaknesses of this study are that the sample size is relatively small and no gold standard was undertaken to establish the accuracy of the clinical diagnosis, for example PET [positron emission tomography] amyloid imaging or CSF β-amyloid and CSF tau measurements," Ralph N. Martins, PhD, told Medscape Medical News.

Dr. Martins, who was not involved in the study, is principal investigator for the Australian Imaging Biomarker and Lifestyle Research Group and holds the Chair in Aging and Alzheimer's Disease at Edith Cowan University School of Medicine, Joondalup, Western Australia. He has done extensive work on the neuropathologic hallmarks of AD and was part of the collaborative team that first isolated and characterized β-amyloid and amyloid precursor protein.

Kaj Blennow, MD, PhD, professor of clinical neurochemistry at the Sahlgrenska Academy at University of Gothenburg Sahlgrenska University Hospital, Mölndal, Sweden, raised some concerns about how the biomarkers were selected. Dr. Blennow, who received the 2013 IFRADs (Fondation pour la Recherche sur Alzheimer) Grand Prize for research on AD, was not involved in the study.

Dr. Blennow told Medscape Medical News that the fact that the researchers started with 3600 peaks on mass spectrometry and made a statistical model based on 10 peaks raises the concern of a risk for "data overfitting," in view of the large variations for each molecule.

Dr. Blennow said, "It will be interesting to see if these biomarkers work in an independent set of samples, from another clinic, or even better, country. It will also be interesting to see if these biomarkers/blood lipids will change depending on what you have eaten (high-fat diet?), drugs (lipid-lowering?), body weight/BMI [body mass index] and 'standard' lipid levels in blood, other diseases with cell loss or degeneration, or kidney and liver function."

The researchers also considered the effect of the apolipoprotein E (APOE) genotype on the classification scheme. They found that presence or absence of the APOE ε4 allele added little to the classification accuracy.

"Furthermore, a classifier model using only APOE ε4 produced an AUC [area under the curve] of 0.54 for classifying the Converterpre and NC groups, implying virtually random classification," the authors write.

Biomarkers May Signal "Impending Dementia Cascade"

As a possible explanation for the accuracy of the 10-lipid panel, the researchers suggested that it may reflect the important structural and functional roles of PC and AC in the integrity of cell membranes.

"Decreasing plasma AC levels in the Converterpre participants in our study may indirectly signal an impending dementia cascade that features loss of these cholinergic neuronal populations. We posit that this ten–phospholipid biomarker panel, consisting of PC and AC species, reveals the breakdown of neural cell membranes in those individuals destined to phenoconvert from cognitive intactness to aMCI or AD and may mark the transition between preclinical states where synaptic dysfunction and early neurodegeneration give rise to subtle cognitive changes," the authors concluded.

Reisa Sperling, MD, professor of neurology, Harvard Medical School, and director of the Center for Alzheimer Research and Treatment at Brigham and Women's Hospital and Massachusetts General Hospital, Boston, would also like to see a larger validation study to determine whether the plasma phospholipids also predict the presence of imaging or CSF biomarkers of AD, in addition to predicting rapid cognitive decline.

Dr. Sperling, who was not involved in the study, told Medscape Medical News, "At this point, I think the blood test is mainly a research tool, but if validated, this type of test could become very clinically useful. I do think we will one day detect and treat Alzheimer's disease prior to clinically evident symptoms. But we have lots of research to do to get us to that point," Dr. Sperling said.

The study was funded by the US National Institutes of Health. The authors have disclosed no relevant financial relationships, nor have Dr. Martins, Dr. Blennow, and Dr. Sperling.

Nature Med. Published online March 9, 2014. Abstract

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