A role for Lsmlp in response to ultraviolet-radiation damage in Saccharomyces cerevisiae.

A genome-wide screen in Saccharomyces cerevisiae identified LSM1 as a new gene affecting sensitivity to ultraviolet (UV) radiation. Lsmlp is a member of a cytoplasmic complex composed of Lsmlp-7p that interacts with the yeast mRNA degradation machinery. To investigate the potential role of Lsmlp in response to UV radiation, we constructed double mutant strains in which LSM1 was deleted in combination with a representative gene from each of three known yeast DNA repair pathways. Our results show that lsm1delta increases the UV-radiation sensitivity of the rad1delta and rad51delta mutants, but not the radl8delta mutant, placing LSM1 within the post-replication repair/damage tolerance pathway (PRR). When combined with other deletions affecting PRR, lsm1delta increases the UV-radiation sensitivity of the rev3delta, rad30delta and pol30-K164R mutants but not rad5delta. Furthermore, the UV-radiation sensitivity phenotype of lsmldelta is partially rescued by mutations in genes involved in 3' to 5' mRNA degradation, and mutations predicted to function in RNA interactions confer the most UV-radiation sensitivity. Together, these results suggest that Lsmlp may confer protection against UV-radiation damage by protecting the 3' ends of mRNAs from exosome-dependent 3' to 5' degradation as part of a novel RAD5-mediated, PCNA-K164 ubiquitylation-independent subpathway of PRR.

The RAD6/BRE1 histone modification pathway in Saccharomyces confers radiation resistance through a RAD51-dependent process that is independent of RAD18.

We examine ionizing radiation (IR) sensitivity and epistasis relationships of several Saccharomyces mutants affecting post-translational modifications of histones H2B and H3. Mutants bre1Delta, lge1Delta, and rtf1Delta, defective in histone H2B lysine 123 ubiquitination, show IR sensitivity equivalent to that of the dot1Delta mutant that we reported on earlier, consistent with published findings that Dot1p requires H2B K123 ubiquitination to fully methylate histone H3 K79. This implicates progressive K79 methylation rather than mono-methylation in IR resistance. The set2Delta mutant, defective in H3 K36 methylation, shows mild IR sensitivity whereas mutants that abolish H3 K4 methylation resemble wild type. The dot1Delta, bre1Delta, and lge1Delta mutants show epistasis for IR sensitivity. The paf1Delta mutant, also reportedly defective in H2B K123 ubiquitination, confers no sensitivity. The rad6Delta, rad51null, rad50Delta, and rad9Delta mutations are epistatic to bre1Delta and dot1Delta, but rad18Delta and rad5Delta show additivity with bre1Delta, dot1Delta, and each other. The bre1Delta rad18Delta double mutant resembles rad6Delta in sensitivity; thus the role of Rad6p in ubiquitinating H2B accounts for its extra sensitivity compared to rad18Delta. We conclude that IR resistance conferred by BRE1 and DOT1 is mediated through homologous recombinational repair, not postreplication repair, and confirm findings of a G1 checkpoint role for the RAD6/BRE1/DOT1 pathway.

Expression and silencing of the microtubule-associated protein Tau in breast cancer cells.

The microtubule-associated protein Tau has been reported to be a predictive factor for clinical response to taxanes in metastatic breast cancer. We generated a panel of eight taxane-resistant variants from four human breast cancer cell lines (MCF-7, T-47D, MDA-MB-231, and BT-549). Four variants had higher levels of Tau compared with their T-47D and MDA-MB-231 parental cells. Using isoform-specific primers, we found that Tau 0N, 1N, 2N, 3R, and 4R isoforms are overexpressed in the resistant variants, as is Tau exon 6 but not exons 4A or 8. To determine whether Tau overexpression produces resistance to taxanes, we derived three independent T-47D clones stably overexpressing Tau 3R and 4R isoforms. Tau overexpression did not result in taxane resistance compared with parental cells transfected with vector alone. We then knocked down Tau expression in three cell lines that expressed Tau constitutively (MCF-7 and ZR-75-1 breast cancer cells, and OVCAR-3 ovarian cancer cells). Lentivirus-mediated silencing of Tau expression in MCF-7 and OVCAR-3 cells did not result in increased taxane sensitivity compared with luciferase short hairpin RNA-infected cells and uninfected parental cells. Transient silencing using Tau-specific small interfering RNAs also did not alter taxane sensitivity relative to nontargeting controls in both MCF-7 and ZR-75-1 cells. These results show that neither overexpression nor depletion of Tau modulates cellular sensitivity to taxanes. Although Tau overexpression has been reported to be a predictive marker of taxane resistance, it is not likely to be a direct mechanism of taxane resistance in breast cancer.