Failure to detect Chlamydia pneumoniae in the late-onset Alzheimer's brain

Abstract

Epidemiological studies have yet to identify a single cause for the most common late-onset form of Alzheimer's disease. The common respiratory pathogen Chlamydia pneumoniae recently has been implicated as a risk factor for this form of Alzheimer's disease. Were this true, there would be a dramatic shift in current paradigms of Alzheimer's disease research and treatment. In the absence of published confirmation, we obtained postmortem brain tissue from late-onset Alzheimer's disease patients (n = 15) and representative controls (n = 5) and extracted DNA from up to six separate brain regions in each instance, including those areas particularly relevant to Alzheimer's disease neuropathology. Each sample of DNA (n = 101) was assayed five times or more for the presence of C. pneumoniae DNA using a nested-PCR protocol targeting a species-specific gene sequence coding for the major outer membrane protein of this organism. We were unable unequivocally to detect C. pneumoniae in any of the 101 samples tested by PCR and failed to culture the organism from tissue samples. We conclude that C. pneumoniae is neither strongly nor uniquely associated with the neuropathology seen in late-onset Alzheimer's disease.

FIG. 1

Species specificity of PCR assay. Both the outer and nested primer pairs specifically amplify two strains of C. pneumoniae (C.pn) DNA but do not amplify DNA from C. trachomatis (C.t) or C. psittaci (C.ps). A 100-bp ladder was run (lanes labeled 100) alongside PCR products for size determination.

FIG. 2

Range of detection. Known amounts of C. pneumoniae DNA (10³ to 10⁻⁶ IFU) were spiked into a constant background of human genomic DNA (1 μg) and assayed by PCR. (A) The first round of amplifications with the outer primer pair demonstrates the ability of the assay to detect as few as 0.1 IFU. (B) The second round of amplifications using the nested primer pair extends this range to 0.001 to 0.0001 IFU. A 100-bp ladder was run (lanes labeled 100) alongside PCR products for size determination.

FIG. 3

PCR results. Example of PCR results for six hippocampal DNA samples. (A) First round of amplifications with outer primer pair; (B) second round of amplifications using the nested primer. A 100-bp ladder was run (lanes labeled 100) alongside PCR products for size determination.