title: A longitudinal study of gene expression in healthy individuals.
abstract: BACKGROUND
  The use of gene expression in venous blood either as a pharmacodynamic marker in clinical trials of drugs or as a diagnostic test requires knowledge of the variability in expression over time in healthy volunteers. Here we defined a normal range of gene expression over 6 months in the blood of four cohorts of healthy men and women who were stratified by age (22-55 years and > 55 years) and gender.
  
  
  METHODS
  Eleven immunomodulatory genes likely to play important roles in inflammatory conditions such as rheumatoid arthritis and infection in addition to four genes typically used as reference genes were examined by quantitative reverse transcription-polymerase chain reaction (qRT-PCR), as well as the full genome as represented by Affymetrix HG U133 Plus 2.0 microarrays.
  
  
  RESULTS
  Gene expression levels as assessed by qRT-PCR and microarray were relatively stable over time with approximately 2% of genes as measured by microarray showing intra-subject differences over time periods longer than one month. Fifteen genes varied by gender. The eleven genes examined by qRT-PCR remained within a limited dynamic range for all individuals. Specifically, for the seven most stably expressed genes (CXCL1, HMOX1, IL1RN, IL1B, IL6R, PTGS2, and TNF), 95% of all samples profiled fell within 1.5-2.5 Ct, the equivalent of a 4- to 6-fold dynamic range. Two subjects who experienced severe adverse events of cancer and anemia, had microarray gene expression profiles that were distinct from normal while subjects who experienced an infection had only slightly elevated levels of inflammatory markers.
  
  
  CONCLUSIONS
  This study defines the range and variability of gene expression in healthy men and women over a six-month period. These parameters can be used to estimate the number of subjects needed to observe significant differences from normal gene expression in clinical studies. A set of genes that varied by gender was also identified as were a set of genes with elevated expression in a subject with iron deficiency anemia and another subject being treated for lung cancer.
authors.affiliation: Department of Genomics and Oncology, Roche Molecular Systems, Pleasanton, CA, USA. chris.karlovich@roche.com.
authors.forename: Chris
authors.initials: C
authors.lastname: Karlovich
authors.forename: Guillemette
authors.initials: G
authors.lastname: Duchateau-Nguyen
authors.forename: Andrea
authors.initials: A
authors.lastname: Johnson
authors.forename: Patricia
authors.initials: P
authors.lastname: McLoughlin
authors.forename: Mercidita
authors.initials: M
authors.lastname: Navarro
authors.forename: Carole
authors.initials: C
authors.lastname: Fleurbaey
authors.forename: Lori
authors.initials: L
authors.lastname: Steiner
authors.forename: Michel
authors.initials: M
authors.lastname: Tessier
authors.forename: Tracy
authors.initials: T
authors.lastname: Nguyen
authors.forename: Monika
authors.initials: M
authors.lastname: Wilhelm-Seiler
authors.forename: John P
authors.initials: JP
authors.lastname: Caulfield