Readers could be forgiven for doing a double-take at the title of a recent paper linking amyloid precursor protein (APP)—best known for its role in Alzheimer's disease—to amyotrophic lateral sclerosis (ALS). The authors, led by first author Petra Steinacker and senior author Johannes Brettschneider of the University of Ulm, Germany, report their results online in the August 15 PLoS One. They found that quantities of soluble fragments of APP released by α- (sAPPα) and β-secretases (sAPPβ) were lower than normal in the cerebrospinal fluid (CSF) of some people with ALS—those whose disease progresses rapidly. Thus, they suggest sAPP concentrations could help distinguish between people with fast-moving ALS and those whose disease will take a slower course.
I think that sAPPα and sAPPβ are among the most interesting new biomarkers for a range of neurodegenerative diseases, wrote Robert Perneczky of the Technical University in Munich, Germany, in an e-mail to ARF (see full comment below). Perneczky, who was not involved in the current work, recently reported on sAPP's potential as an AD biomarker (see ARF related news story on Perneczky et al., 2011). What makes the present report so interesting is that the results suggest that sAPP might serve as accurate predictors of disease course, he added. This is a highly relevant topic, since the velocity of disease progression, and therefore the survival rate, varies from a few months to several years.
Previously, Steinacker published a pilot study (Steinacker et al., 2009) that showed sAPP amounts helped distinguish between ALS and frontotemporal lobar degeneration, two sets of symptoms resulting from overlapping pathology (Geser et al., 2010). In the current cross-sectional study, the team recruited 68 people with ALS. They compared CSF from these subjects to that from 40 control participants who had tension headaches, but not neurodegenerative disease. In addition, the researchers included CSF from 20 people with Parkinson's disease, to help them tease out ALS-specific effects from those common to neurodegeneration in general.
At first glance, the results are disappointing—the scientists found no correlation between sAPP levels and ALS in general. However, sAPP did help them distinguish between subgroups of the ALS participants. Concentrations of both sAPPα and sAPPβ were unusually low in people with quickly progressing ALS. In people with rapid ALS, CSF sAPPα levels had a median of 21 ng/ml, compared to 27-28 ng/ml in the other subjects. Similarly, sAPPβ levels in people with fast-moving disease averaged 29 ng/ml, compared to 37-41 ng/ml in other participants.
The authors also observed that sAPP levels were slightly lower the longer a person had had ALS. In people who had been sick for only a few months, sAPPα levels ranged from approximately 15-50 ng/ml; by five years in, the values were between approximately five and 25. There was a similar change for sAPPβ. Scientists hope such markers might be useful to track progression during clinical trials.
The reduction of sAPP in ALS is probably due to the death of neurons that make the protein, Perneczky suggested. If that is correct, then sAPP loss is a surrogate marker for degenerating neurons, not a key part of the pathology. The change might only be apparent in ALS, not other neurodegenerative diseases, because motor neurons manufacture particularly high levels of APP, speculated Hui Zheng of the Baylor College of Medicine in Houston, Texas. Zheng was not part of the PLoS study team.
In addition, the study authors pointed out that sAPP is believed to be neuroprotective (see ARF related news story on Li et al., 2010; Kögel et al., 2003). Thus, declining sAPP production could mean that remaining neurons are more vulnerable to protein aggregation or other stresses.
Steinacker and colleagues also examined neurofilament heavy chain, a standard marker for axon damage (reviewed in Petzold, 2005) that is already under consideration as an ALS biomarker (Boylan et al., 2009). CSF neurofilament levels had a median of 15-18 pg/ml in control and PD subjects; 52 pg/ml in those with slow-moving ALS; and 108 pg/ml in people with rapidly progressing ALS.
How well do these markers separate swift disease from sluggish? Using neurofilament alone, the researchers calculated a test would have a sensitivity of 0.8 and specificity of 0.57. That is, the test would correctly identify 80 percent of rapidly progressing ALS cases, and 57 percent of more gradual cases. Adding sAPPα levels to the neurofilament analysis, the team raised the sensitivity and specificity slightly, to 0.84 and 0.6, respectively. Combining neurofilament with sAPPβ achieved similar rates.
Indeed, the best approach will likely be not one biomarker, but a set, Zheng said; researchers are pursuing this approach for several neurodegenerative conditions.—Amber Dance.
Reference:
Steinacker P, Fang L, Kuhle J, Petzoid A, Tumani H, Ludolph AC, Otto B, Brettschneider J. Soluble beta-amyloid precursor protein is related to disease progression in amyotrophic lateral sclerosis. PLoS One. 2011;6(8):e23600. Abstract
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