Triple-negative breast cancer (TNBC) remains one of the most challenging breast cancer subtypes to treat due to the lack of effective therapeutic options. Ribosome biogenesis has recently emerged as a promising therapeutic target across various cancers. Despite the current targeting of ribosome biogenesis through RNA polymerase I (RNA Pol I) inhibition, we speculated that other factors essential for ribosome assembly, such as ribosomal RNA (rRNA) maturation factors, may also represent therapeutic targets in TNBC.

Ribosome biogenesis was evaluated in each breast cancer subtype using expression level of ribosome biogenesis factors from the UCSC XENA database. The sensitivity of TNBC cell to inhibition of ribosome biogenesis was evaluated on the TNBC cell lines MDA-MB-231 and BT-20, either using RNA Pol I inhibitors CX-5461 or BMH-21 or by knocking-down Fibrillarin (FBL) gene using an shRNA approach. Tumor cell growth and survival was monitored both in vitro and as xenografted tumors.

We demonstrate that ribosome biogenesis-related genes are significantly overexpressed in TNBC compared to other breast cancer subtypes, highlighting its potential role in TNBC progression. Accordingly, we show that RNA Pol I inhibition exerts potent anti-proliferative effects in pre-clinical models of TNBC, both in vitro and in vivo. However, the DNA-damaging activity of RNA Pol I inhibitors raises safety concerns, highlighting the need for alternative strategies to inhibit ribosome biogenesis. To this end, we show that targeting a downstream rRNA maturation step, specifically pre-rRNA cleavage, by inhibiting the maturation factor Fibrillarin, also inhibits tumor growth in TNBC models. Notably, ribosome biogenesis inhibition, through either RNA Pol I or Fibrillarin targeting, induces cell cycle arrest without triggering significant cell death.

These findings establish ribosome biogenesis as a therapeutic vulnerability in TNBC and identify rRNA maturation, and Fibrillarin in particular, as novel targets for potential therapeutic intervention.