Ribosomal protein (RP) expression in higher eukaryotes is regulated translationally, through the 5′TOP sequence. This mechanism evolved to more rapidly produce RPs on demand in different tissues. Here we show that 40S ribosomes, in a complex with mRNA binding protein LARP1, selectively stabilize 5′TOP mRNAs, with disruption of this complex leading to induction of the impaired ribosome biogenesis checkpoint (IRBC) and p53 stabilization. The importance of this mechanism is underscored in 5q− syndrome, a macrocytic anemia caused by a large monoallelic deletion, which we found also encompasses the LARP1 gene. Critically, depletion of LARP1 alone in human adult CD34+ bone marrow precursor cells, leads to a reduction in 5′TOP mRNAs and the induction of p53. These studies identify a 40S ribosome function, independent of those in translation, which with LARP1 mediates the autogenous control of 5′TOP mRNA stability, whose disruption is implicated in the pathophysiology of 5q− syndrome.

Many oncogenes enhance nucleotide usage to increase ribosome content, DNA replication, and cell proliferation, but in parallel trigger p53 activation. Both the impaired ribosome biogenesis checkpoint (IRBC) and the DNA damage response (DDR) have been implicated in p53 activation following nucleotide depletion. However, it is difficult to reconcile the two checkpoints operating together, as the IRBC induces p21‐mediated G1 arrest, whereas the DDR requires that cells enter S phase. Gradual inhibition of inosine monophosphate dehydrogenase (IMPDH), an enzyme required for de novo GMP synthesis, reveals a hierarchical organization of these two checkpoints. We find that the IRBC is the primary nucleotide sensor, but increased IMPDH inhibition leads to p21 degradation, compromising IRBC‐mediated G1 arrest and allowing S phase entry and DDR activation. Disruption of the IRBC alone is sufficient to elicit the DDR, which is strongly enhanced by IMPDH inhibition, suggesting that the IRBC acts as a barrier against genomic instability.

The ability of ribosomes to translate genetic information into protein is a hallmark of evolution. However, during cancer progression, tumors often hijack and boost ribosome biogenesis in their favor, which they rely on for high rates of proliferation. In eukaryotes four distinct ribosomal RNAs (rRNAs) and approximately 80 different ribosomal proteins (RPs) are assembled into either the small 40S or large 60S subunit, while more than 300 additional factors participate in the complex process of ribosome biogenesis. Moreover, with the rise of multicellular organisms in higher eukaryotes a hierarchical translational control mechanism of cellular RP mRNAs has evolved. This mechanism has been put into place to guarantee that the building blocks of ribosomes are preferentially translated and integrated with those of the transcriptionally regulated rRNAs, in order to endow the cell with adequate protein synthetic capacity in different tissues upon cellular demand. In higher metazoans this translational mechanism is regulated through a cis regulatory element at the transcriptional start site of RP mRNAs, termed a 5'-terminal oligonucleotide pyrimidine (5' TOP) sequence. We previously demonstrated that 5' TOP mRNA expression at the translational levels is controlled by mTORC1 signaling, a central nexus of cellular growth and proliferation that is often exploited by tumor cells. Although it is thought that 5' TOP translation is mediated by mTORC1, new findings have suggested that their stability also may be selectively controlled by the 5' TOP element. Here we show that the 40S ribosomal subunit, in concert with RNA binding protein LARP1, take on this responsibility, through forming a novel complex with 5' TOP mRNAs. Importantly, we show that the repertoire of 5' TOPs mRNAs recognized by this new 40S complex encompasses categories of transcripts not only belonging to RP and translational factor mRNAs, but also to previously unidentified 5' TOPs involved in distinct cellular processes. LARP1 overexpression, like increased ribosome biogenesis is associated with many aggressive cancers. Thus the functional interaction between LARP1, the 40S subunit and 5' TOPs constitutes a cellular reservoir of RP mRNAs and other factors that can be translationally mobilized or suppressed upon cellular growth demands, particularly in tumors where mTOR signaling is hyperactivated. This abstract is also being presented as Poster A27. Citation Format: Antonio Gentilella, Francisco D. Morón-Duran, Guilherme Z. Rocha, Ferran Riaño-Canalias, Joffrey Pelletier, Albert Tauler, Sara Kozma, George Thomas. A novel 40S complex involved in the regulation of 5' TOP mRNA stability: A role in mTOR hyperactivated tumors? [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr PR02.