ABSTRACT: Cultured skin substitutes, prepared using keratinocytes, fibroblasts and biopolymers, can facilitate closure of massive burn wounds by increasing the availability of autologous tissue for grafting. However, because they contain only two cell types, skin substitutes cannot replace all of the functions of native human skin. To better understand the physiological and molecular differences between cultured skin substitutes and native skin, we undertook a comprehensive analysis of gene expression in native skin, cultured keratinocytes, cultured fibroblasts, and skin substitutes using Affymetrix gene chip microarrays. Goals: Our analysis focused on identifying gene signatures that were highly characteristic of each cell and tissue type, and those that are regulated by the formation of cultured skin substitute from the individual components. Normalization: We used a normalization and referencing strategy that consisted of BioConductor/RMA Express RMA processing of the entire series of cel files followed by a per gene normalization in which the median value of expression for each gene was derived from the cultured samples only, and this was used as a reference for all samples including the cultured skin substitute. This approach allowed for the identification of genes that were higher and lower-expressed in the cultured skin relative to the individual cell types that were also expressed strongly or weakly in normal skin relative to the median value established by the three cell types. Results Summary:We identified six major clusters of coordinately regulated genes that were the most differentially expressed between groups. These clusters correspond to biomarker pools representing expression signatures for native skin, fibroblasts, keratinocytes, and cultured skin. The expression analysis revealed that entire clusters of genes were either up-regulated or down-regulated upon combination of fibroblasts and keratinocytes in cultured skin grafts. Further, several categories of genes were overexpressed in cultured skin substitutes compared with native skin, including genes associated with hyperproliferative skin or activated keratinocytes. The observed pattern of expression indicates that cultured skin substitutes in vitro, which display a well-differentiated epidermal layer, exhibit skin-like differentiation relative to gene expression patterns in the individual cells. This consists of both the activation of normal skin signature genes and the suppression of keratinocyte and fibroblast signatures. There is also a signature consistent with a hyperproliferative phenotype similar to wounded native skin. Experiment Overall Design: The sample series consists of native human skin (NHS) samples isolated from female donors undergoing reduction mammoplasty (breast skin) or abdominoplasty (abdomen skin). Skin samples from donors that were used to establish cultures of fibroblasts (CF) and keratinocytes (CK) were assigned donor numbers in the order they were processed in the laboratory, for example: 633, 634, etc. An additional human skin sample (C-1-Ref) was used only to make RNA as a standard control, and was therefore not assigned a donor number. Cultured skin substitutes (CSS) were prepared using isogenic CF and CK from each donor, and were cultured for 2 weeks in vitro to permit development of a stratified and cornified epidermal layer (confirmed by histology). For microarray analysis, RNA was isolated from intact NHS, from CF and CK in monolayer cultures, and from CSS. Samples are labeled indicating the sample type and donor number; for example, CF633 represents cultured fibroblasts from donor 633. To control for variation between individuals, four donors (= biological replicates) were used for each sample type: NHS, CF, CK, and CSS. Efforts were made to have complete sets of 4 samples from each donor, but intact RNA was not obtainable from 2 of the NHS samples (donors 634 and 651); these were replaced with NHS RNA from similar donors (donors C-1-Ref and 636). To check the fidelity of the microarray analysis, 2 of the RNA samples (CK639 and CSS651) were analyzed in duplicate (= technical replicates)
