Triple helix-mediated inhibition of gene expression is increased by PUVA.

The combination of psoralens with UVA is used as PUVA therapy for psoriasis and other skin diseases. UVA-induced psoralen/DNA photoadducts act via suppression of DNA replication and cell proliferation, but do not sufficiently repress gene transcription. To explore whether PUVA may also be used for gene repression, psoralen was conjugated to a triplex-forming oligonucleotide (TFO) that targets a gene sequence of ICAM-1, a key molecule in cutaneous inflammation. Triplex formation between TFO and target sequence was detected by non-denaturing gel electrophoresis. UVA-irradiation induced psoralen cross-links at the triplex-duplex junction as verified by denaturing gel electrophoresis. When the target sequence was placed within the transcribed portion of the chloramphenicol acetyltransferase (CAT) gene, TFO inhibited CAT expression in A431 cells. Inhibition was sequence-specific, since a scrambled control oligonucleotide or mismatched or scrambled target sequences failed to inhibit CAT expression. Inhibition was not significant without UVA exposure, but was strongly enhanced by PUVA-mediated cross-links at the TFO target site. These results suggest that TFO may add a new quality to PUVA therapy by transcriptionally repressing pathogenically relevant genes, in addition to antiproliferative PUVA effects. TFO designed to repress only after PUVA activation may allow the development of a cutaneous organ specific strategy for gene repression.