Multiphoton-targeted photochemistry was used to selectively inactivate the expression of genes in vertebrate cells. A membrane permeable DNA-associating vital dye, ethidium bromide monoacetate (visible wavelength single photon absorption peak at 530 nm) was used to photosensitize chromosomes in dividing cells. A 100-ps infrared laser beam operating at 1.06 microns was focused onto a selected region of a mitotic chromosome corresponding to the sites of the nucleolar (ribosomal) genes. Individual cells followed through mitosis demonstrated a reduction in the number of nucleoli formed in daughter cells that corresponded to the number of nucleolar genes sites irradiated. These results demonstrate the ability to selectively manipulate genes by using the focal point specificity characteristic of multiphoton microscopy. This technique should have wide biotechnology applications both in vitro and in vivo.

The present investigation has been undertaken to examine the possibility that the cell nucleus, and specifically the genetic material, is a target site for photodynamic therapy. PTK2 and Hep-2 cells are pretreated with a medium containing 15 microg/ml (0.09 mM) 5-aminolevulinic acid (ALA). Individual fluorescence images are recorded for each selected cell using a cooled charge-coupled device (CCD). A laser microbeam system generating 630 nm is used for subcellular-region irradiation of specific targets: chromosomes, the mitotic spindle, the perispindle region and the peripheral cytoplasm. Nuclei of interphase cells are also irradiated. Data comparing the sensitivities of the different subcellular microirradiation sites in ALA-treated mitotic cells demonstrate that under the irradiation conditions used, the chromosome is the most sensitive subcellular target followed by the perispindle region, the peripheral cytoplasm and spindle, and, lastly, the interphase nucleus.