This pilot study examined gene expression signatures in pathological gingival 
tissues of subjects with chronic or aggressive periodontitis, and explored 
whether new subclasses of periodontitis can be identified based on gene 
expression profiles. A total of 14 patients, seven with chronic and seven with 
aggressive periodontitis, were examined with respect to clinical periodontal 
status, composition of subgingival bacterial plaque assessed by checkerboard 
hybridizations, and levels of serum IgG antibodies to periodontal bacteria 
assayed by checkerboard immunoblotting. In addition, at least two pathological 
pockets/patient were biopsied, processed for RNA extraction, amplification and 
labeling, and used to study gene expression using Affymetrix U-133 A arrays. 
Based on a total of 35 microarrays, no significantly different gene expression 
profiles appeared to emerge between chronic and aggressive periodontitis. 
However, a de novo grouping of the 14 subjects into two fairly robust clusters 
was possible based on similarities in gene expression. These two groups had 
similar clinical periodontal status and subgingival bacterial profiles, but 
differed significantly with respect to serum IgG levels against the important 
periodontal pathogens Porphyromonas gingivalis, Tannerella forsythensis and 
Campylobacter rectus. These early data point to the usefulness of gene 
expression profiling techniques in the identification of subclasses of 
periodontitis with common pathobiology.