New insights into HEATR1 functions

Human HEAT repeat containing 1 protein (HEATR1) is the structural and functional homolog of yeast U3 small nucleolar RNA-associated protein 10 (UTP 10) [1]. UTP 10 was originally identified as a protein required for processing of 18S rRNA precursors, acting in association with the U3 snoRNA in the small subunit (SSU) processome [1]. UTP 10 and its orthologues also play positive roles in pre-rRNA transcription, which led to their classification as transcription-UTP (t-UTP) proteins, suggesting that they may act as integrative hubs of pre-rRNA transcription and processing [1]. In a high-content RNA interference (RNAi) screen aiming to identify nucleolar proteins whose downregulation leads to increased p53 levels, Turi and co-workers identified HEATR1 as a potent hit [2]. Their further characterization of the effects of HEATR1 siRNA on ribosome biogenesis showed that HEATR1 co-localizes with RNA polymerase I (Pol I). Its downregulation almost completely abrogates Pol I-mediated transcription that is accompanied by perturbation of nucleolar structure, as documented by relocations of multiple markers – nucleophosmin, nucleolin and fibrillarin. This effect on ribosome biogenesis differs from previously published findings [1] that HEATR1 downregulation only partially inhibits Pol-I-mediated transcription and strongly impairs processing of 18S rRNA precursors. The discrepancy is most likely the result of a much higher depletion efficiency of HEATR1 in this present study compared to previous studies [1,2]. However, the underlying molecular mechanism by which HEATR1 regulates Pol I-mediated transcription still remains an open question. The observed activation of p53 upon depletion of HEATR1 is consistent with the previous demonstration that downregulation of the structurally different zebrafish homologue, bap28, triggers p53-dependent apoptosis in vivo [2,3]. Turi et. al. [2] showed that HEATR1 downregulation causes G1 cell cycle arrest, which is bypassed by genetic inactivation of p53 [2]. Importantly, they went on to demonstrate that the upregulation of p53 protein levels upon HEATR1 depletion is mediated by ribosomal protein uL18 (RPL5), which is a component of a ribosomal precursor complex that also contains nascent ribosomal protein uL5 (RPL11) and 5S rRNA [4]. Upon impairment of ribosome biogenesis, this complex is redirected from being assembled into ribosomes to binding Mdm2 leading to the stabilization and activation of p53 [4]. This specific signaling pathway was recently named the Impaired Ribosome Biogenesis Checkpoint (IRBC), to distinguish it from other p53-activating stress signaling pathways [5]. Recent research has clearly demonstrated the association between inherited and acquired abnormalities of ribosome biogenesis and increased risk of cancer [4]. The level of endogenous HEATR1 was much higher in human cancer cell lines compared with primary non-transformed cells and it was consistently associated with large nucleoli [2], which are being used by oncopathologists for many decades as one of the criteria of malignancy. These findings are consistent with increased expression of HEART in several cancer types [6]. It will be necessary to test whether overexpressed HEATR1 is sufficient to promote the accumulation of mature ribosomes, and to assess the potential role of this mechanism in cancer pathogenesis. Given that t-UTP 10 might also play a role in degrading defective rRNA precursors [7], it could be speculated that the accumulation of defective ribosomes as a result of HEATR1 dysregulation might alter the translation landscape of the cell and consequently lead to cancer pathogenesis. What would be the physiological role of the IRBC upon dysregulation of HEATR1? One may speculate that it prevents expansion of incipient cancer cells harboring ribosome aberrations. However, dysregulation of HEATR1 expression and/or function might potentially facilitate the malignant phenotype via mechanisms that are independent from its roles in ribosome biogenesis and the IRBC regulation as suggested previously [6]. Similar to HEATR1, Treacle ribosome biogenesis factor 1 (TCOF1) has been implicated in linking pre-rRNA transcription with pre-rRNA processing and its dysfunction also leads to aberrant p53 upregulation via the IRBC [1,4]. Since mutations in TCOF1 cause Treacher Collins syndrome (TCS), a congenital disorder which includes abnormal craniofacial development, it would be interesting to evaluate the mutational status of the HEATR1 gene in congenital disorders of unknown etiology [4]. Given the newly identified functions of HEATR1 in ribosome biogenesis and p53 regulation by Turi et. al. [2] we