RNA Polymerase I Is Uniquely Vulnerable to the Small-Molecule Inhibitor BMH-21

SIMPLE SUMMARY: Cancer cells upregulate RNA polymerase I (Pol I) activity to increase ribosome abundance in support of rapid cell growth and proliferation. During the last decade, Pol I has emerged as a promising anti-cancer target. Eukaryotes express three closely related RNA polymerases (Pols I, II, and III), responsible for synthesis of all the genome-encoded RNA required by the cell. Effective therapeutic development requires that the treatment be selective for Pol I, without inhibition of Pols II or III. This study evaluates the specificity of the compound BMH-21 on transcription by Pols I, II, and III using purified in vitro transcription assays. These results reveal that Pol I is uniquely sensitive to inhibition by BMH-21, compared to Pols II and III. These findings support ongoing preclinical development of BMH-21 and its derivatives for potential therapeutic applications. ABSTRACT: Cancer cells require robust ribosome biogenesis to maintain rapid cell growth during tumorigenesis. Because RNA polymerase I (Pol I) transcription of the ribosomal DNA (rDNA) is the first and rate-limiting step of ribosome biogenesis, it has emerged as a promising anti-cancer target. Over the last decade, novel cancer therapeutics targeting Pol I have progressed to clinical trials. BMH-21 is a first-in-class small molecule that inhibits Pol I transcription and represses cancer cell growth. Several recent studies have uncovered key mechanisms by which BMH-21 inhibits ribosome biosynthesis but the selectivity of BMH-21 for Pol I has not been directly measured. Here, we quantify the effects of BMH-21 on Pol I, RNA polymerase II (Pol II), and RNA polymerase III (Pol III) in vitro using purified components. We found that BMH-21 directly impairs nucleotide addition by Pol I, with no or modest effect on Pols II and III, respectively. Additionally, we found that BMH-21 does not affect the stability of any of the Pols’ elongation complexes. These data demonstrate that BMH-21 directly exploits unique vulnerabilities of Pol I.