Inhibition of post-termination ribosome recycling at premature termination codons in UPF1 ATPase mutants

Recognition and rapid degradation of mRNA harboring premature translation termination codons (PTCs) serves to protect cells from accumulating non-functional and potentially toxic truncated polypeptides. Targeting of PTC-containing transcripts is mediated by the nonsense-mediated mRNA decay (NMD) pathway and requires a conserved set of proteins including UPF1, an RNA helicase whose ATPase activity is essential for NMD. Previously, we identified a functional interaction between the NMD machinery and terminating ribosomes based on 3’ RNA decay fragments that accrue in UPF1 ATPase mutants. Herein, we show that those decay intermediates originate downstream of the PTC and harbor 80S ribosomes that migrate into the mRNA 3’ UTR independent of canonical translation. Accumulation of 3’ RNA decay fragments is determined by both RNA sequence downstream of the PTC and the inactivating mutation within the active site of UPF1. Our data reveal a failure in post-termination ribosome recycling in UPF1 ATPase mutants.

Dosage-dependent requirements of Magoh for cortical interneuron generation and survival

Embryonic interneuron development underlies cortical function and its disruption contributes to neurological disease. Yet the mechanisms by which viable interneurons are produced from progenitors remain poorly understood. Here, we demonstrate dosage-dependent requirements of the exon junction complex component Magoh for interneuron genesis in mouse. Conditional Magoh ablation from interneuron progenitors, but not post-mitotic neurons, depletes cortical interneuron number through adulthood, with increased severity in homozygotes. Using live imaging, we discover that Magoh deficiency delays progenitor mitotic progression in a dosage-sensitive fashion, with 40% of homozygous progenitors failing to divide. This shows that Magoh is required in progenitors for both generation and survival of newborn progeny. Transcriptome analysis implicates p53 signaling; moreover, p53 ablation in Magoh haploinsufficient progenitors rescues apoptosis, completely recovering interneuron number. In striking contrast, in Magoh homozygotes, p53 loss fails to rescue interneuron number and mitotic delay, further implicating mitotic defects in interneuron loss. Our results demonstrate that interneuron development is intimately dependent upon progenitor mitosis duration and uncover a crucial post-transcriptional regulator of interneuron fate relevant for neurodevelopmental pathologies.This article has an associated 'The people behind the papers' interview.

ATP hydrolysis by UPF1 is required for efficient translation termination at premature stop codons

Nonsense-mediated mRNA decay (NMD) represents a eukaryotic quality control pathway that recognizes and rapidly degrades transcripts harbouring nonsense mutations to limit accumulation of non-functional and potentially toxic truncated polypeptides. A critical component of the NMD machinery is UPF1, an RNA helicase whose ATPase activity is essential for NMD, but for which the precise function and site of action remain unclear. We provide evidence that ATP hydrolysis by UPF1 is required for efficient translation termination and ribosome release at a premature termination codon. UPF1 ATPase mutants accumulate 3′ RNA decay fragments harbouring a ribosome stalled during premature termination that impedes complete degradation of the mRNA. The ability of UPF1 to impinge on premature termination, moreover, requires ATP-binding, RNA-binding and NMD cofactors UPF2 and UPF3. Our results reveal that ATP hydrolysis by UPF1 modulates a functional interaction between the NMD machinery and terminating ribosomes necessary for targeting substrates to accelerated degradation.

Nonsense-mediated mRNA decay (NMD) is a quality control pathway that recognizes and degrades transcripts harbouring nonsense mutations. Here the authors show that the ATPase activity of UPF1 mediates functional interactions between the NMD machinery and ribosomes required for efficient ribosome release at premature termination codons.

Abstract 786: Alternative RNA processing leads to decreased DNA topoisomerase IIα in etoposide (VP-16) resistant human leukemia K562 cells

Acquired resistance to the DNA topoisomerase II (topo II)-targeting agent, etoposide (VP-16) has been shown previously in this lab to be associated with decreased levels of topo II mRNA and protein in a K562 clone, K/VP.5. Utilizing exon junctional Taqman hydrolysis probes, real-time quantitative PCR (qPCR) revealed ∼2-fold decrease in topo IIα mRNA expression in K/VP.5 compared to parental K562 cells when probes were downstream of exon 19. However, use of probes upstream of exon 19 revealed a small but statistically significant increase in topo IIα mRNA in K/VP.5 compared to K562 cells. In contrast, expression of topo IIß mRNA was reduced in K/VP.5 compared to K562 cells in all 5’ and 3’ regions of the gene queried. RNA stability studies using the RNA polymerase II inhibitor DRB and 4 qPCR probes, regionally targeted to 3’ and 5’ domains, revealed similar topo IIα mRNA half-lives in K562 cells (6.55 hr [extreme 3’ probe] to 7.96 hr [extreme 5’ probe]) but a statistically significant increase in topo IIα mRNA half-lives in K/VP.5 cells (7.21 hr [extreme 3’ probe] to 11.86 hr [extreme 5’ probe (p=0.034)]). The differential RNA decay rate in K/VP.5 cells suggested alterations in exo- or endo-nuclease expression/function in resistant cells and/or alternative splicing events generating transcripts of different size and stability characteristics. Experiments using Rapid Amplification of cDNA Ends (3’-RACE) were consistent with loss of ∼3 kb of the total 5.8 kb topo IIα in K/VP.5 cells. PCR products derived from this truncated transcript were sequenced, revealing that the 5’ splice site of intron 19 is suppressed, resulting in the retention of ∼300 nt of intronic sequence in the mature mRNA, and the use of an alternative polyadenylation site. The retained portion of intron 19 contains an in-frame stop codon, as well as the consensus AAUAAA hexamer of the poly(A) signal. As a result of alternative 3’ end processing, exons 20-35 of the topo IIα gene are not included in the truncated transcript. Since the codon corresponding to the topo IIα active site tyrosine is contained in Exon 20 (Tyr 705), translation of the alternatively spliced transcript likely produces a non-functional protein. Surprisingly, the truncated transcript is also present in K562 cells, albeit at a greatly reduced level compared to K/VP.5, suggesting that alternative polyadenylation is a normal mechanism to regulate topo IIα expression. Taken together results indicate that posttranscriptional alterations in topo IIα may account, in part, for acquired resistance to VP-16, and potentially for regulation of topo II expression that may dictate intrinsic chemosensitivity as well.

Citation Format: Lucas D. Serdar, Ragu Kanagasabai, Jack C. Yalowich. Alternative RNA processing leads to decreased DNA topoisomerase IIα in etoposide (VP-16) resistant human leukemia K562 cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 786. doi:10.1158/1538-7445.AM2014-786