Molecular approaches for analyzing differential gene expression: differential cDNA library construction and screening

Improved differential screening approach to analyse transcriptional variations in organized cDNA libraries

A cDNA library was generated from rat brain tissues and organized into 1536-well plates, using a fluorescence activated cell sorter (FACS), acting as a single cell deposition system. The organized library containing 10,000 clones, with 60% full-length cDNA inserts, allowed the generation of multiple identical membrane replicas. Each replica was hybridized with a complex probe obtained from a particular brain tissue or a given cultured cell. The signal intensity for each of the clones present on the membrane, quantified with a standard image-analysis software, is proportional both to the abundance of the corresponding mRNA in the probe and to the amount of plasmid template on the membrane. The latter value was thus used to normalize the signals produced with complex probes, to optimize the comparison of mRNA expression levels for the different systems under study. The construction of high-quality cDNA libraries, the generation of identical membrane replicas and comparable probes, and the utilization of an image-analysis software package, coupled with the normalization of the spot intensity by assaying plasmid quantity, significantly improves the differential screening approach. Altogether, these technical improvements open the possibility to compare a great number of different probes and, in consequence, to accumulate biological information for each clone present in an organized cDNA library. The functional information obtained should complement data from DNA sequencing projects.

Human cell lines as models for multidrug resistance in solid tumours

In spite of our expanding knowledge on the molecular biology of cancer, relatively little progress has been made in improving therapy for the solid tumours which are major killers, e.g., lung, colon, breast. Significant advances over the past 10-15 years in chemotherapy of some tumours such as testicular cancer and some leukaemias indicates that, in spite of the undesirable side-effects, chemotherapy has the potential to effect cure in the majority of patients with certain types of cancer. Multidrug resistance, inherent or acquired, is one important limiting factor in extending this success to most solid tumours. In vitro studies described in this review are now uncovering a diversity of possible mechanisms of cross-resistance to different types of drug. Sensitive methods such as immunocytochemistry, RT-PCR or in situ RNA hybridisation may be necessary to identify corresponding changes in clinical material. Only by classifying individual tumours according to their specific resistance mechanisms will it be possible to define the multidrug resistance problem properly. Such rigorous definition is a prerequisite to design (and choice on an individual basis) of specific therapies suited to individual patients. Since a much larger proportion of cancer biopsies should be susceptible to accurate analysis by the immunochemical and molecular biological techniques described above than to direct assessment of drug response, it seems reasonable to hope that this approach will succeed in improving results for cancer chemotherapy of solid tumours where other approaches such as individualised in vitro chemosensitivity testing have essentially failed. Results from clinical trials using cyclosporin A or verapamil are encouraging, but these agents are far from ideal, and reverse resistance in only a subset of resistant tumours. Proper definition of the other mechanisms of MDR, and how to antagonize them, is an urgent research priority.