Measuring tau and other neuronal proteins in cerebrospinal fluid gives insight into the state of the brain during its long run-up to Alzheimer’s dementia. However, cross-sectional studies yield but a snapshot, which can be misleading. Researchers need the “movie version,” i.e., serial measurements that show change over time. Now, just such a study from Anne Fagan and colleagues at Washington University in St. Louis finds that while CSF tau climbs in early stages of sporadic AD—as everyone knows by now—levels actually decline once symptoms set in. What’s more, the same is true for three other presumptive biomarkers of neuronal or synaptic injury, including the calcium sensor VILIP-1 and synaptic proteins neurogranin and SNAP-25. The findings appeared March 24 in Alzheimer’s and Dementia online.
- Cross-sectional studies don’t catch full picture for some biomarkers.
- Largest study of its kind looked at within-person changes in multiple CSF biomarkers of neuronal injury in late-onset AD.
- Phospho-tau, VILIP-1, SNAP-25, and neurogranin all increase up to symptom onset, then decline.
“We’ve needed these types of longitudinal studies with repeated sampling of individuals for a long time. This is the largest study with the broadest range of biomarkers to date, and it tells us some important things we didn’t know,” said Henrik Zetterberg, University of Gothenburg, Sweden.
The results emphasize the shortcomings of extrapolating biomarker curves from cross-sectional data, said Fagan. “We can’t be naïve about it anymore by saying you can estimate longitudinal change by looking at between-person levels of biomarkers in cross-sectional studies,” she told Alzforum.
In cross-sectional studies of CSF biomarkers, total tau (t-tau) and phospho-tau (p-tau) start to rise long before signs of dementia appear and continue to trend upward through to end-stage disease. However, in previous work, Fagan’s team analyzed longitudinal samples from a small number of cases of dominantly inherited AD and were surprised to find that p-tau levels in a given person actually dropped after that person had started developing symptoms (Mar 2014 news). The same study reported a similarly timed decrease in visinin-like protein-1, a calcium-sensing protein released from injured neurons. The WashU group had identified VILIP-1 as a potential CSF biomarker several years earlier (Mar 2012 news).
In the new study, first author Courtney Sutphen followed up this initial finding in DIAN by running a similar analysis in a larger group of people with late-onset sporadic AD from the Alzheimer’s Disease Neuroimaging Initiative. Sutphen identified 148 ADNI participants who had between two and seven CSF samples available for analysis, taken in a one- to seven-year span. At baseline, 56 of them were classified as cognitively normal, 76 had mild cognitive impairment, and 16 AD. Based on a CSF Aβ42 threshold of 192 pg/ml or less, 21 of the cognitively normal group, 58 with MCI, and all of the AD group were deemed amyloid-positive.
Sutphen measured VILIP-1, neurogranin, and SNAP-25 by ELISA, using antibodies developed in collaboration with Jack Ladenson at WashU. Like VILIP-1, both CSF neurogranin and SNAP-25 concentrations rise in people with AD, and are thought to signal neuronal injury or death. Rising neurogranin is associated with brain atrophy and reduced glucose uptake (Sep 2015 news; Jan 2015 news; Sep 2015 news). All the ELISAs showed good reproducibility, Sutphen said, with variances of 7 percent or less on repeat testing. She also analyzed Aβ42, t-tau and p-tau181 using Elecsys automated assays, which have been developed for those markers and are more reproducible than ELISAs.
At baseline, levels of all five injury markers were higher in the amyloid-positive MCI and AD groups than in cognitively normal people. Average concentrations of t-tau and p-tau in the AD group were twice those in cognitively normal, amyloid-negative controls, with the other three markers showing slightly less of an increase.
Spaghetti Time. Serial measures of CSF neurogranin show no change in cognitively normal people (left), or most people with MCI (middle), but a decline in AD (right). [Courtesy of Sutphen et al., Alzheimer’s & Dementia.]
When Sutphen looked at within-person changes, t-tau and p-tau showed consistent increases in the amyloid-positive CN and MCI participants, while p-tau fell significantly in those with AD. Their average change, a loss of 1.65 pg/ml/year, added up to a 3.9 percent yearly decrease in p-tau. Sutphen saw similar reductions in the other neuronal damage markers. After baseline, people with AD averaged 3.4, 2.5, and 6.9 percent annual declines in VILIP-1, SNAP-25, and neurogranin levels, respectively.
None of the novel markers showed a significant movement up or down over time in the CN or MCI groups.
The inflammatory marker YKL-40 told a different story. It appeared to go up with age, but was highly variable both at baseline and over time. Its fluctuations appeared unrelated to amyloid status. Fagan speculated that YKL-40 reflects inflammation that is not specific to AD.
None of these biomarkers tracked with cognitive decline, hippocampal shrinkage, or cortical thinning, all of which accelerated as disease worsened.
Other investigators have noted declines in p-tau in late-onset AD patients (Aug 2017 conference news; Toledo et al., 2013; Seppälä et al., 2011). However, no one knows why tau, and now three other markers of neuronal injury, decline as disease advances. Fagan rules out the possibility that the proteins were simply diluted in a larger volume of CSF that results from brain shrinkage and ventricle enlargement with advancing disease. She said she still saw reductions after controlling for ventricular volume.
Alternatively, the drop in CSF concentrations could signal the slowing or ending of an acute neuronal die-off. Zetterberg said he’d like to see measures of neurofilament in these same samples, because as a generic marker of cell death it might shed light on how CSF tau and the other markers relate to neuronal loss.
The results highlight how little is known about the origins of CSF tau and other markers, and they left Fagan wondering what markers scientist might look for to track response to therapies. “It’s logical to assume that if you slow down neuronal injury, that would be accompanied by a reduction in tau,” Fagan said. “But if tau is already declining, would a positive therapeutic outcome mean the decrease would become less steep, or maybe tau would rise, like it did earlier in the disease process?”
Zetterberg sees a practical take-home from the study. “In clinical trials, the active arm and the placebo group will have to be carefully matched by disease stage. Minor differences in how far patients have deteriorated in the neurodegenerative process might translate into major differences in biomarker trajectories, and that could be misinterpreted as a treatment effect,” he warned.
One caveat of the study was the short follow-up times available, which averaged from 2.5 to four years. To get a fuller picture, Fagan plans to measure these biomarkers as people progress from asymptomatic through early symptomatic or MCI to end-stage AD, a process that takes 20 years or more. “That’s the only way to find out if we really do have elevations early on that then come down,” Fagan said. That’s one goal of WashU’s Adult Children Study, where CSF draws start at age 45.
Longitudinal studies might be easier if markers could be measured in blood. That’s the way forward, wrote Harald Hampel of the Sorbonne University in Paris in a comment (below). His group reported a longitudinal decline in CSF p-tau131 in patients with AD (Hampel et al., 2001). For future studies, a shift to blood-based biomarker panels seems the rational course, he wrote.—Pat McCaffrey
- DIAN Longitudinal Data Surprises With Late Drop in Tau
- Research Brief: VILIP-1 a Potential CSF Marker for AD?
- Cerebrospinal Fluid Neurogranin Correlates with Markers of Neurodegeneration
- New Biomarkers? Synaptic Proteins in Spinal Fluid Predict Cognitive Decline
- Paper Alert: Longitudinal Data Support CSF Neurogranin as an Early Synaptic Marker of Alzheimer’s Disease
- Longitudinal Data Say: Nope, CSF Markers Do Not Track Progression
- Toledo JB, Xie SX, Trojanowski JQ, Shaw LM. Longitudinal change in CSF Tau and Aβ biomarkers for up to 48 months in ADNI. Acta Neuropathol. 2013 Jun 29; PubMed.
- Seppälä TT, Koivisto AM, Hartikainen P, Helisalmi S, Soininen H, Herukka SK. Longitudinal changes of CSF biomarkers in Alzheimer's disease. J Alzheimers Dis. 2011;25(4):583-94. PubMed.
- Hampel H, Buerger K, Kohnken R, Teipel SJ, Zinkowski R, Moeller HJ, Rapoport SI, Davies P. Tracking of Alzheimer's disease progression with cerebrospinal fluid tau protein phosphorylated at threonine 231. Ann Neurol. 2001 Apr;49(4):545-6. PubMed.
No Available Further Reading
- Sutphen CL, McCue L, Herries EM, Xiong C, Ladenson JH, Holtzman DM, Fagan AM, ADNI. Longitudinal decreases in multiple cerebrospinal fluid biomarkers of neuronal injury in symptomatic late onset Alzheimer's disease. Alzheimers Dement. 2018 Jul;14(7):869-879. Epub 2018 Mar 23 PubMed.