Katherine F. Vaeth

The subcellular localization of many mRNAs to neuronal projections allows neurons to efficiently and rapidly react to spatially restricted external cues. However, for most of these RNAs, the mechanisms that govern their localization are unknown. Here, using subcellular fractionation and single-molecule RNA FISH, we found that loss of TDP-43 results in increased accumulation of hundreds of mRNAs in neurites. Using high-throughput functional assays in cells and high-throughput binding assays in vitro, we subsequently identified specific regions within these mRNAs that mediate their TDP-43-dependent localization and interaction with TDP-43. We found that the same regions also mediated TDP-43-dependent mRNA instability, suggesting a mechanism by which TDP-43 regulates mRNA localization. ALS-associated mutations in TDP-43 resulted in similar mRNA mislocalization phenotypes as did TDP-43 loss in mouse dorsal root ganglia and human iPS-derived motor neurons. These findings establish TDP-43 as a direct negative regulator of mRNA abundance in neurites and suggest that mislocalization of specific transcripts may occur in ALS patients.

S4. Omacetaxine and Torin Synergize in MM Cell lines.

Protein homeostasis is critical to the survival of multiple myeloma cells. Although this is targeted with proteasome inhibitors, mRNA translation inhibition has not entered trials. Recent work illustrates broad sensitivity of multiple myeloma cells to the translation inhibitor omacetaxine. We hypothesized that understanding how multiple myeloma becomes resistant will lead to the development of drug combinations to prevent or delay relapse. We generated omacetaxine resistance in H929 and MM1S multiple myeloma cell lines and compared them with parental lines. Resistant lines displayed decreased sensitivity to omacetaxine, with EC50 > 100 nmol/L, compared with parental sensitivity of 24 to 54 nmol/L. As omacetaxine inhibits protein synthesis, we performed both RNA sequencing and ribosome profiling to identify shared and unique regulatory strategies of resistance. Transcripts encoding translation factors and containing a terminal oligopyrimidine sequence in their 5’ untranslated region were translationally upregulated in both resistant cell lines. The mTOR pathway promotes the translation of terminal oligopyrimidine motif–containing mRNAs. Indeed, mTOR inhibition with Torin 1 restored partial sensitivity to omacetaxine in both resistant cell lines. The combination was synergistic in omacetaxine-naïve multiple myeloma cell lines, and a combination effect was observed in vivo. Primary multiple myeloma cells from patient samples were also sensitive to the combination. These results provide a rational approach for omacetaxine-based combination therapy in patients with multiple myeloma, which have historically shown better responses to multiagent regimens.Implications:Through the use of ribosome profiling, our findings indicate mTOR inhibition as a novel combination therapy for partnering with the translation inhibitor omacetaxine in the treatment of multiple myeloma.

Supplemental Table 3. Myeloma Patient Characteristics

SigRNA Table. Genes with Significant RNA Differences between H929 and MM1S Parental and Omacetaxine Resistant Cells.

S1. Characterization of omacetaxine resistant MM cell lines.

SigTE Table. Genes with Significant Riboseq differences between H929 and MM1S Parental and Omacetaxine Resistant Cells.

S2. RNA Differences between H929 and MM1S.

S3. Riboseq differences in H929 and MM1S cells.

Protein homeostasis is critical to the survival of multiple myeloma cells. Although this is targeted with proteasome inhibitors, mRNA translation inhibition has not entered trials. Recent work illustrates broad sensitivity of multiple myeloma cells to the translation inhibitor omacetaxine. We hypothesized that understanding how multiple myeloma becomes resistant will lead to the development of drug combinations to prevent or delay relapse. We generated omacetaxine resistance in H929 and MM1S multiple myeloma cell lines and compared them with parental lines. Resistant lines displayed decreased sensitivity to omacetaxine, with EC50 > 100 nmol/L, compared with parental sensitivity of 24 to 54 nmol/L. As omacetaxine inhibits protein synthesis, we performed both RNA sequencing and ribosome profiling to identify shared and unique regulatory strategies of resistance. Transcripts encoding translation factors and containing a terminal oligopyrimidine sequence in their 5’ untranslated region were translationally upregulated in both resistant cell lines. The mTOR pathway promotes the translation of terminal oligopyrimidine motif–containing mRNAs. Indeed, mTOR inhibition with Torin 1 restored partial sensitivity to omacetaxine in both resistant cell lines. The combination was synergistic in omacetaxine-naïve multiple myeloma cell lines, and a combination effect was observed in vivo. Primary multiple myeloma cells from patient samples were also sensitive to the combination. These results provide a rational approach for omacetaxine-based combination therapy in patients with multiple myeloma, which have historically shown better responses to multiagent regimens. Implications: Through the use of ribosome profiling, our findings indicate mTOR inhibition as a novel combination therapy for partnering with the translation inhibitor omacetaxine in the treatment of multiple myeloma.