Alterations in cell cycle regulation underlie cisplatin induced apoptosis of dorsal root ganglion neurons in vivo

Cisplatin is used in the treatment of ovarian and testicular cancer. Twenty percent of patients cannot be optimally treated because of sensory neurotoxicity. Human and animal studies demonstrate that the dorsal root ganglion neuron is the primary target of drug injury. We have previously demonstrated that cisplatin causes neuronal apoptosis in vitro. We now report a reproducible animal model of cell death induced by cisplatin. Drug was administered for 1 or 2 cycles of 5 days separated by 5 days. Total dose administered was 0, 5, 7.5, 10, or 15 mg/kg. Ganglia from 34 animals were processed and examined using in situ hybridization for cyclin D1 messenger RNA and digoxigenin coupled TUNEL staining. Overall, 2.9 ± 3.9% of neurons were TUNEL positive in treated rats compared with 0.2 ± 0.3% in controls (P <.005). There was a strong positive correlation (r²= 0.88; P = 0.018) between percentage of TUNEL stained DRG and cumulative dose of cisplatin. Two independent approaches to quantitation of in situ cyclin D1 hybridization were used; blinded grading by an observer and measurement of color density using digital image analysis. Both demonstrated dramatic upregulation of expression of cyclin D1 mRNA in treated compared with control rats. This demonstrates that apoptosis of neurons is preceded by aberrant reentry into G1 phase of the cell cycle in an animal model.