Sweet 16: Novel APP Processing Pathway and a New Biomarker?
If you like thinking of amyloid precursor processing in Boolean logic terms, that is α- or β-secretase cleavage, then you may want to expand your logic board to include an “and.” Using mass spectrometry, researchers led by Erik Portelius and Kaj Blennow at the University of Gothenburg, Sweden, identified novel C-truncated forms of Aβ in human cerebrospinal fluid (CSF). Those small peptides (Aβ1-14, Aβ1-15, and Aβ1-16) are consistent with enzymatic APP cleavage by β- followed by α-, not γ-secretase. Supporting this idea, Portelius and colleagues, including Henrik Zetterberg, also at University of Gothenburg, found that neuroblastoma cells still produce these14- to 16-amino-acid peptides when γ-secretase is blocked (see Portelius et al., 2009). The existence of these peptides suggests that β- and α-secretase sequentially cleave APP, which goes counter to conventional wisdom. “I think it would be appropriate to say that APP can undergo three different processing pathways, and perhaps even more,” said Zetterberg. The peptides could also turn out to be useful biomarkers for γ-secretase inhibition.
In collaboration with Eric Siemers and Robert Dean at Eli Lilly, Indianapolis, Portelius and colleagues report in the March 29 Alzheimer’s Research & Therapy that Aβ1-16 is elevated in CSF of patients enrolled in Lilly’s Phase 2 clinical trial of LY450139 (semagacestat), a γ-secretase inhibitor. Siemers said he found the data compelling. “When Kaj asked for our spinal fluid samples, we thought it would be interesting to look at, although we weren’t convinced that we would be able to see an effect,” he told ARF. “But there was this very nice, dose-dependent, and statistically significant increase.”
The findings highlight these β/α fragments as potentially valuable biomarkers for following γ-secretase inhibition in vivo. Portelius and colleagues looked at CSF of 35 patients with mild to moderate AD who were randomized to placebo, 100 mg, or 140 mg of LY450139. The patients had a lumbar puncture at baseline and again 14 weeks after treatment began. Between sampling, CSF levels of the β/α fragments—Aβ1-14, 1-15, and 1-16—rose 57, 21, and 30 percent, respectively, in the 100 mg group and 74, 35, and 67 percent in the 140 mg group. Aβ42 levels were unchanged.
The field of clinical AD research is abuzz about biomarkers of late, though sorting out which biomarker best serves what purpose is an ongoing process (see ARF related news story). Evidence suggests CSF Aβ42 reduction is a sign of impending dementia (see Hansson et al., 2006), but that marker does not turn out to be a good reporter of γ-secretase inhibition. “The problem with Aβ42 is that the timing of the lumbar puncture has to be precise because its turnover is so rapid,” Siemers told ARF. “You get a reduction [in Aβ], but as soon as you remove the inhibitor it goes back to baseline again. Siemers speculates that Aβ1-16 might be a better marker for γ-secretase inhibition because its turnover is much slower; hence, the timing of the lumbar puncture would not have to be as precise.
In contrast, while Aβ1-16 might provide a good readout of γ-secretase inhibition, it seems a poor marker for impending dementia according to Zetterberg. This Swedish group found that the peptide is elevated about 15-30 percent in both familial and sporadic AD samples but that the elevation is not diagnostically useful. It is also not clear why Aβ1-16 is elevated in AD, given that it may be a sign of reduced γ-secretase activity. Zetterberg speculated that product inhibition due to the elevated levels of Aβ42 might suppress γ-secretase, leaving the β-secretase-derived C99 ripe for α-secretase to pick off Aβ1-16. “More research is needed on what these isoforms can tell us about the pathogenesis of AD,” he told ARF.
As for testing Aβ1-16 in their ongoing Phase 3 trial of semagacestat, Siemers said that was not built into the study, though he thought if he had to do the development all over again, the peptide might be a great biomarker. But he suggested this touches on a larger question Alzforum readers might debate. “When you get into Phase 3, your real mission is to show you’ve had an effect on cognitive function. How important is it to people to demonstrate biomarker changes, especially if they were already shown in Phase 2?” he asked. We welcome thoughts or suggestions.—Tom Fagan
- Portelius E, Price E, Brinkmalm G, Stiteler M, Olsson M, Persson R, Westman-Brinkmalm A, Zetterberg H, Simon AJ, Blennow K. A novel pathway for amyloid precursor protein processing. Neurobiol Aging. 2011 Jun;32(6):1090-8. PubMed.
- Hansson O, Zetterberg H, Buchhave P, Londos E, Blennow K, Minthon L. Association between CSF biomarkers and incipient Alzheimer's disease in patients with mild cognitive impairment: a follow-up study. Lancet Neurol. 2006 Mar;5(3):228-34. PubMed.
- Portelius E, Dean RA, Gustavsson MK, Andreasson U, Zetterberg H, Siemers E, Blennow K. A novel Abeta isoform pattern in CSF reflects gamma-secretase inhibition in Alzheimer disease. Alzheimers Res Ther. 2010;2(2):7. PubMed.
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Laval University Research Center
A critical question indeed. We should answer that. The evaluation of cognitive function may lack sensitivity, specificity and may be insufficiently quantitative. Measurement of a critical biomarker may be as helpful? At the moment, biomarkers are just surrogates of cognitive function, which remains the “real” clinical endpoint. However, with the advance of our knowledge on the pathogenesis of AD along with the development of pathogenesis-specific biomarkers, could biomarkers become the most relevant clinical endpoints?
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