Measurement of Aβ1-42 in cerebrospinal fluid is influenced by matrix effects.

Aβ1-42 measurement in CSF is an important biochemical marker for Alzheimer disease (AD). However, our understanding of why this biomarker is predictive and why it is often difficult to measure in a reproducible fashion is still lacking. To study these questions, the concentration of Aβ1-42 in CSF was compared before and after denaturation with 6M guanidine and reverse-phase HPLC. Measurement of the Aβ1-42 after denaturation and reverse-phase HPLC demonstrated that considerably more Aβ1-42 was present in CSF than revealed when assaying non-denatured CSF. A comparison of Aβ1-42 concentrations before and after HPLC in AD CSF with that in normal controls suggested that matrix interference may affect the differentiation between the diagnostic groups. A similar effect was observed with dilutions of crude CSF. Together, these results suggested that at least part of the mechanism by which low Aβ1-42 concentrations in CSF function as a biomarker of AD is related to matrix components which preferentially hide a portion of the Aβ1-42 from detection in AD CSF. In contrast, we show that the association of the APOEε4 allele with lower Aβ1-42 concentrations in CSF is preserved even after denaturation and HPLC. A similar relationship between the presence of the APOEε4 allele and lower concentrations of Aβ1-40 was also apparent, thereby generating similar ratios of Aβ1-42/ Aβ1-40 across the APOE genotypes. The results from the present study suggested that Aβ1-42 in CSF functions as a biomarker of AD in tandem with other CSF matrix components that are increased in AD CSF. Further studies are needed to identify which matrix factors (e.g. binding of Aβ to proteins) underlie the increased detection of Aβ1-42 concentrations after denaturation and HPLC. The data also suggested that denaturation and HPLC of CSF may be a useful approach for studies using Aβ1-42 as a pharmacodynamic marker or in other paradigms where measurement of total non-covalently bound Aβ1-42 is required.