Researchers studying the natural history of Alzheimer’s disease are grappling with a puzzling group of volunteers—amyloid-free and cognitively normal older adults who show other biomarker evidence of neurodegeneration. Initially reported in 2011, sizable numbers of what were, perhaps wrongly, thought to be preclinical AD patients now turn up as having this suspected non-amyloid pathology (SNAP) in two new studies. In a longitudinal analysis of Minnesota seniors, 42 percent of those who eventually racked up high brain amyloid at baseline already had biomarkers suggestive of neurodegeneration even though their brain-amyloid scan was still negative. And a cross-sectional study by researchers in California found that many cognitively normal elderly had neurodegeneration biomarkers in brain regions typically affected by AD, yet lacked Aβ deposition. These California seniors were more likely than those with high brain Aβ to have cerebrovascular disease and perform poorly on cognitive tests. Reported in the October 16 Neurology and October 28 JAMA Neurology, the studies were led by Clifford Jack of Mayo Clinic, Rochester, Minnesota, and William Jagust of University of California, Berkeley, respectively. Together, the findings confirm that SNAP cases are fairly common, and suggest that age-related non-AD conditions can muddle the interpretation of data on AD biomarker progression.
There was no hint of SNAP in a study of Dominantly Inherited Alzheimer Network (DIAN) reported November 5 in the Proceedings of the National Academy of Sciences. Researchers led by John Morris at Washington University School of Medicine, St. Louis, analyzed amyloid and neurodegenerative biomarkers in adults with autosomal-dominant mutations years before they are expected to develop early onset AD. Because the participants are middle-aged, the study steers clear of the comorbidities that confound studies of older cohorts. In this longitudinal study, the researchers consistently found brain amyloid rising first, years before neurodegenerative abnormalities. However, the ordering and extent of neurodegenerative biomarker changes differed by brain region, raising questions as to why some brain structures appear to resist metabolic loss and atrophy better than others.
With the publication several years ago of new AD diagnostic criteria (see Apr 2011 news story), scientists have redoubled their efforts to define the fluid and neuroimaging biomarkers that map the preclinical stage preceding full-blown disease. Jack and colleagues outlined a hypothetical model whereby brain Aβ accumulates early, followed by changes in markers of neuronal metabolism and brain atrophy (see Jan 2010 ARF webinar; Feb 2013 conference story). Recent biomarker studies in healthy elderly seemed broadly consistent with this model, but found, to the surprise of some scientists, that about a quarter of participants had neurodegeneration markers in the absence of brain amyloid (Jack et al., 2012; Knopman et al., 2012).
Because researchers define preclinical AD as a transition from negligible to positive brain amyloid, the current work by Jack and colleagues addresses what happens in the brain at the onset of AD pathology. They measured amyloid deposition in healthy elderly people in the Mayo Clinic Study of Aging using positron emission tomography (PET) with the amyloid tracer Pittsburgh Compound B (PIB). They found that 123 of 207 participants were amyloid-free at baseline; of those, 26 became amyloid-positive during an average 1.3-year follow-up. Curiously, 11 of the 26 (42 percent) already had an established marker of neurodegeneration—low glucose metabolism or small hippocampi—at baseline, when brain amyloid still fell within the normal range.
In the JAMA Neurology study, first author Miranka Wirth and colleagues analyzed 72 nondemented older adults in the Berkeley Aging Cohort. Twenty-nine people (40 percent) had at least one abnormal neurodegenerative biomarker (hippocampal volume, glucose metabolism, or gray-matter thickness), and among those, 19 people were amyloid-negative by PIB PET. Those 19 amyloid-free neurodegeneration cases comprised 26 percent of the cohort—a figure similar to those in the 2011 studies that first reported SNAP.
Several issues make the SNAP group difficult to analyze. First, PET tracers only measure fibrillar Aβ. “You have to wonder if people have forms of amyloid we cannot measure,” Jagust said. “Soluble oligomeric Aβ could be causing the neurodegeneration we are calling ‘amyloid-negative.’” The absence of SNAP in DIAN would argue against that idea, since Aβ drives pathology in mutation carriers and yet none show the “neurodegeneration-first” phenotype. Another potential complication stems from the way cutoffs are defined, and whether amyloid burden should be measured as trajectories instead of binary positive/negative categories. While some cognitively normal elderly accumulate enough Aβ to cross a predetermined cutoff that defines amyloid positivity, others may have lower yet fast-rising amyloid levels. “Just because they didn’t hit some threshold doesn’t mean the amyloid isn’t causing trouble,” Jagust said. “We don’t know that the point at which amyloid causes problems is the point we define as ‘positive.’”
Second, the SNAP category may include people who have other non-AD pathologies before starting to accumulate brain amyloid. “The older the cohort, the less likely you are to have pure AD, and the more likely you are to have AD arising in conjunction with, or maybe after, other degenerative pathologies,” Jack told Alzforum, noting that the neurodegeneration biomarkers are not specific for AD. In support of this, a recent paper by Jagust and colleagues showed that gray-matter atrophy in cognitively normal seniors is much more closely associated with age than with amyloid (Oh et al., 2013). “There are likely many things driving what we call ‘neurodegeneration markers,’” Jagust said. “One is probably amyloid, another may be vascular disease, or other molecular processes associated with aging.”
Both studies found a number of SNAP cases even though they used different methods for collecting and analyzing data, Jagust said. His group used the distribution volume ratio (DVR) PIB PET approach, which requires longer scans but is believed to be quantitatively more robust than the quicker standardized uptake value ratio (SUVR) method used by Jack and colleagues. Jack’s team analyzed more participants, which helps clean up the noisy SUVR data. In addition, the two groups use different ways of defining amyloid positivity. Jack’s group defines the threshold based on AD cases, whereas Jagust and colleagues establish cutpoints as standard deviations above amyloid levels in healthy young adults. Despite these procedural differences, “the results are pretty consistent,” Jagust said. “SNAP seems to be a real phenomenon, at least across two cohorts.”
What defines this unusual group and how fast its members decline remains unclear. While some analyses suggest that people with SNAP deteriorate no faster than normal seniors who lack amyloid or neurodegenerative markers (see Vos et al., 2013), others paint a more confusing picture (see Aug 2013 conference story).
SNAP does not show up in familial AD. As reported in PNAS this week, first author Tammie Benzinger and colleagues analyzed 137 autosomal-dominant mutation carriers and 92 noncarriers from DIAN. Broadly speaking, their study confirmed the ordering of biomarkers proposed for sporadic AD—that is, amyloid deposition showing up early, followed by metabolic loss, and later, cortical thinning. “Everywhere we looked, we detected amyloid first,” Benzinger said. However, certain brain areas did not follow expected patterns of hypometabolism and atrophy. For instance, the hippocampus loses massive numbers of cells, despite having low amyloid burden compared to the Aβ amounts measured elsewhere. Other areas accumulate whopping amounts of amyloid yet do not succumb to hypometabolism until much later. Most importantly, the study confirms that “for cortical structures, amyloid deposition really does occur 15 to 20 years before symptoms,” she said. “That gives us a good therapeutic window.”—Esther Landhuis
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