"This is, to my knowledge, the first unbiased CSF proteomics study on people who carry mutations for familial AD," said Henrik Zetterberg, Sahlgrenska University Hospital in Molndal, Sweden, who was not involved in the study. "This is exactly what the field needs." Previous studies have looked for CSF protein markers for late-onset AD (see Zhang et al., 2008), but not familial early-onset disease. Testing subjects with autosomal dominant mutations has advantages, because researchers know with certainty that the participants will get the disease, and they know approximately when. That allows scientists to narrow the subject pool and approximate how long before disease onset a certain biomarker surfaces.

Ringman and colleagues analyzed the CSF protein content of 14 familial AD mutation carriers who ranged in age from 23-55 years old. Twelve of the mutation carriers had presenilin-1 gene mutations and two had amyloid precursor protein (APP) gene mutations. Overall, 10 were asymptomatic and ranged from five to 22 years younger than their family's median age of onset. Two participants were mildly symptomatic (but not demented), one had mild dementia, and one had moderate dementia. The age of people in this symptomatic group ranged from 12 years before to 12 years past their family's mean age at onset. Five related non-carriers (aged 29-55) served as the control group. CSF was collected before the Dominantly Inherited Alzheimer Network (DIAN) (see ARF related news story and ARF news story) had been established, but some of these participants have since joined the network. The Alzheimer's Prevention Initiative (see ARF related news story and ARF news story) is also studying people with FAD mutations, and the Alzheimer’s Disease Neuroimaging Initiative (ADNI) is conducting its own proteomic analysis in sporadic AD (see ARF related news story). New ADNI proteomics data based on multiplex immunoassays developed by the biotech company Myriad Rules Based Medicine became available on 3 January for analysis. More information can be found on the ADNI website.

An analysis of both non-symptomatic and symptomatic mutation carriers by high-resolution liquid chromatography-mass spectrometry revealed 56 proteins (46 more abundant and 10 less abundant) that differed in their CSF concentration relative to non-FAD controls. Previous studies had identified CSF differences in late-onset AD for a quarter of those proteins, including APP, afamin precursor, spondin 1, plasminogen, hemopexin, and neuronal pentraxin receptor. But this was the first study to report changes in the levels of other proteins—including calsyntenin-3, AMPA 4 glutamate receptor, CD99 antigen, di-N-acetyl-chitobiase, and secreted phosphoprotein 1—in familial AD mutation carriers. The team did a second analysis in which they compared just the presymptomatic FAD mutation carriers to controls. That analysis found 41 of the same proteins as in the first analysis, and turned up an additional 38.

"It was completely unknown that this large number of proteins was changed in early familial AD," said Zetterberg. Many of the markers fall within protein networks already suspected to play a role in Alzheimer's, so the type of hits did not come as a complete surprise, he added. "They fit well with the current thinking regarding Alzheimer's pathogenesis."

However, it was surprising that complement factors seemed to be affected so early on in the disease process, said Ringman. "We now know that inflammation is a very early event in these participants' central nervous systems," he said, adding that inflammation is one example of a potential future therapeutic target suggested by this type of study. Complement receptor 1 (CR1) and other genes associated with the innate immune system have popped up in GWAS analysis of late-onset AD (LOAD) (see ARF related news story on Biffi et al., 2010; ARF related news story on Naj et al., 2011; Hollingworth et al, 2011). Since the liquid chromatography-mass spectrometry method doesn't give precise quantitative information about the proteins, the team will need to confirm specific quantitative protein changes using other established methods, such as ELISA assays, he said. Further such studies within DIAN will build on these results, he added. The research group also plans to look at 33 more CSF samples to validate the findings, and perform similar analyses in participants with late-onset AD. Since the equivalent study has not been done for preclinical late-onset AD, researchers cannot be sure that these results hold true for the sporadic form of the disease, said Zetterberg.—Gwyneth Dickey Zakaib.

Reference:
Ringman JM, Schulman H, Becker C, Jones T, Bai Y, Immermann F, Cole G, Sokolow S, Gylys K, Geschwind DH, Cummings JL, Wan HI. Proteomic changes in cerebrospinal fluid of presymptomatic and affected persons carrying familial Alzheimer disease mutations. Arch Neurol. 2012 Jan. Abstract