Adenosine deaminase gene amplification in deoxycoformycin-resistant mammalian cells

Deoxycoformycin (dCF)-resistant mutants of rat hepatoma, mouse LMTK-, and Chinese hamster ovary (CHO) cells have been isolated and shown to overproduce adenosine deaminase (ADA). The overproduction of ADA was found to be due to ADA-gene amplification in rat and mouse cells but not in CHO cells. Deoxycoformycin-resistant rat hepatoma cells have large HSRs (homogeneously staining regions), mouse cells carry DMs (Double minutes), and CHO cells do not appear to have any gross chromosomal anomalies. When dCF-resistant rat hepatoma and mouse cells are selected by increasing the concentration of the inhibitor in small increments, there is a good correlation between the increase in ADA gene copy number and the increase in the level of expression of ADA, suggesting that all of the amplified genes are equally active in the expression of ADA.     

Tissue-dependent paired expression of miRNAs

It is believed that depending on the thermodynamic stability of the 5′-strand and the 3′-strand in the stem-loop structure of a precursor microRNA (pre-miRNA), cells preferentially select the less stable one (called the miRNA or guide strand) and destroy the other one (called the miRNA* or passenger strand). However, our expression profiling analyses revealed that both strands could be co-accumulated as miRNA pairs in some tissues while being subjected to strand selection in other tissues. Our target prediction and validation assays demonstrated that both strands of a miRNA pair could target equal numbers of genes and that both were able to suppress the expression of their target genes. Our finding not only suggests that the numbers of miRNAs and their targets are much greater than what we previously thought, but also implies that novel mechanisms are involved in the tissue-dependent miRNA biogenesis and target selection process.