11 January 2012. The longest study yet to correlate Alzheimer’s disease biomarkers in cerebrospinal fluid (CSF) with disease progression confirms that β amyloid levels bottom out at pathological levels up to 10 years before the clinical onset of disease. In contrast, CSF tau levels are higher the closer people are to developing dementia, implying that tau continues to rise as cognitive impairment progresses, report researchers led by Oskar Hansson at Skåne University Hospital, Malmö, Sweden. The data lend further support to current models of biomarker progression (see ARF Webinar). Writing in the January Archives of General Psychiatry, the authors also note that the ratio of β amyloid to phosphorylated tau in the CSF can predict with high accuracy which people will progress from mild cognitive impairment (MCI) to AD, making this biomarker potentially useful for selecting participants in clinical trials.
To study biomarker progression, first author Peder Buchhave analyzed a cohort of 137 patients with MCI who donated CSF samples at baseline. The authors previously reported on the first five years of data from this cohort (see Hansson et al., 2006); the new work extends the longitudinal data to nearly a decade. Clinical assessments on the cohort found that, at an average follow-up period of nine years, about 40 people remained stable, around 70 progressed to AD, and about 20 developed other forms of dementia.
At baseline, CSF Aβ42 was equally reduced in the MCI patients who developed AD within a year or two and in those who were diagnosed 10 years later, suggesting Aβ levels plateau long before disease develops. These low Aβ42 levels (around 350 ng/L) contrast with average levels of 600 ng/L in people who remained stable or developed other dementias, and around 700 ng/L in healthy controls. However, elevated total tau and phosphorylated tau at baseline distinguished people who converted to AD within the next five years from those who converted later. The baseline total tau levels hovered around 800 ng/L in early converters, versus 500 ng/L in late converters, and around 300 ng/L in controls and those who stayed stable. The data imply that CSF tau continues to rise in the years immediately before the onset of clinical symptoms, as progression models predict. Recent work by Cliff Jack and colleagues at the Mayo Clinic in Rochester, Minnesota, also provided evidence in support of this model (see ARF related news story
on Jack et al., 2011).
In addition, the authors report that an Aβ42:p-tau ratio of less than about 6 in MCI patients predicted the development of AD within nine years with a sensitivity and specificity of around 90 percent. To put it another way, MCI patients with pathological Aβ42:p-tau ratios developed AD with an incidence of almost 30 percent per year, compared to 2 percent per year in MCI patients with normal ratios. This marker could be useful in selecting patients for clinical trials, but is not good enough for clinical diagnosis, Hansson noted, pointing out that, for a useful clinical tool, accuracy would need to be nearly 100 percent.
“Not only do [the data] support the proposed trajectory of specific biomarker changes very early in the course of the disease, but they also provide support for the importance of assigning an underlying etiology to the construct of MCI,” Anne Fagan at Washington University School of Medicine, St. Louis, Missouri, wrote to ARF, noting that MCI with AD biomarkers should be considered early AD. “I predict that longer clinical follow-up in cognitively normal individuals who exhibit AD biomarker profiles will provide support for the utility of such biomarkers to identify individuals with the disease even earlier, before the onset of any clinical symptoms.” (See full comment below.)
For his part, Hansson is currently developing methods to detect Aβ oligomers in CSF, as oligomers are widely believed to be the neurotoxic form of Aβ. Hansson will investigate whether adding CSF oligomer levels to the equation will improve diagnostic accuracy. Monitoring changes in CSF oligomers might also provide a way to assess the effectiveness of anti-amyloid therapies, Hansson told ARF.—Madolyn Bowman Rogers.
Buchhave P, Minthon L, Zetterberg H, Wallin AK, Blennow K, Hansson O. Cerebrospinal fluid levels of β-amyloid 1-42, but not of tau, are fully changed already 5 to 10 years before the onset of Alzheimer’s dementia. Arch Gen Psychiatry. 2012 Jan;69(1):98-106. Abstract