Differential expression of the O6-alkylguanine-DNA alkyltransferase gene in normal human breast and skin tissue: in situ mapping of cell type-specific expression.

O6-Alkylguanine is a major toxic, mutagenic, and carcinogenic lesion in cellular DNA that is repaired by O6-alkylguanine-DNA alkyltransferase (ATase). The expression of this gene is directly related to the cellular sensitivity of alkylating agents, with levels of this protein varying widely among human organs, tumors, and cell types. To better understand specific cell-type responses to repairing O6-alkylguanine lesions in DNA, we used colorimetric in situ hybridization, with an ATase-specific antisense oligomer probe, to map the cellular distribution of ATase mRNA in tissue sections of normal adult human breast and neonatal foreskin tissues. This is the first report of mapping ATase gene expression directly in normal human breast and skin tissues. Paraffin-embedded tissue sections were hybridized with a digoxigenin-labeled, 39-base antisense ATase oligomer. Hybridization of the probe to cells expressing the ATase gene was visualized after immunodetection with an alkaline phosphatase-conjugated anti-digoxigenin antibody. After color development, we simultaneously identified tissue architecture and cell types and measured the expression of the ATase gene. There was no hybridization-specific color when sections were mock hybridized, hybridized with a sense probe, or treated with RNase. In the breast tissue, 93% of the cells in the loosely connective tissue and 84% of the myoepithelial cells expressed high levels of ATase mRNA. Most of the luminal ductal epithelial cells (61%) were devoid of stain, indicating undetectable levels of ATase mRNA. In skin dermis, 93% of the fibroblasts appeared to express high levels of ATase mRNA. Within the epidermis, approximately 64% of the basal and 65% of the granular epithelial cells expressed ATase mRNA. Expression was undetectable in the epithelial cells of the suprabasal layer of the epidermis. There was very little interindividual variation (< 17%) in the distribution of expression of ATase within the same cell types of different individuals. These data illustrate the differential potential of individual cell types within the organ matrix to repair O6-alkylguanine damage in cellular DNA. This data may provide insights into the understanding of cell type-specific responses to carcinogens.