Inhibition of METTL3 Results in a Cell-Intrinsic Interferon Response That Enhances Antitumor Immunity

Therapies that enhance antitumor immunity have altered the natural history of many cancers. Consequently, leveraging nonoverlapping mechanisms to increase immunogenicity of cancer cells remains a priority. Using a novel enzymatic inhibitor of the RNA methyl-transferase METTL3, we demonstrate a global decrease in N6-methyladenosine (m6A) results in double-stranded RNA (dsRNA) formation and a profound cell-intrinsic interferon response. Through unbiased CRISPR screens, we establish dsRNA-sensing and interferon signaling are primary mediators that potentiate T-cell killing of cancer cells following METTL3 inhibition. We show in a range of immunocompetent mouse models that although METTL3 inhibition is equally efficacious to anti-PD-1 therapy, the combination has far greater preclinical activity. Using SPLINTR barcoding, we demonstrate that anti-PD-1 therapy and METTL3 inhibition target distinct malignant clones, and the combination of these therapies overcomes clones insensitive to the single agents. These data provide the mole-cular and preclinical rationale for employing METTL3 inhibitors to promote antitumor immunity in the clinic.

SIGNIFICANCE

This work demonstrates that METTL3 inhibition stimulates a cell-intrinsic interferon response through dsRNA formation. This immunomodulatory mechanism is distinct from current immunotherapeutic agents and provides the molecular rationale for combination with anti-PD-1 immune-checkpoint blockade to augment antitumor immunity. This article is featured in Selected Articles from This Issue, p. 2109.

Animal Hen1 2'-O-methyltransferases as tools for 3'-terminal functionalization and labelling of single-stranded RNAs

S-adenosyl-L-methionine-dependent 2′-O-methylati-on of the 3′-terminal nucleotide plays important roles in biogenesis of eukaryotic small non-coding RNAs, such as siRNAs, miRNAs and Piwi-interacting RNAs (piRNAs). Here we demonstrate that, in contrast to Mg²⁺/Mn²⁺-dependent plant and bacterial homologues, the Drosophila DmHen1 and human HsHEN1 piRNA methyltransferases require cobalt cations for their enzymatic activity in vitro. We also show for the first time the capacity of the animal Hen1 to catalyse the transfer of a variety of extended chemical groups from synthetic analogues of the AdoMet cofactor onto a wide range (22–80 nt) of single-stranded RNAs permitting their 3′-terminal functionalization and labelling. Moreover, we provide evidence that deletion of a small C-terminal region of the DmHen1 protein further increases its modification efficiency and abolishes a modest 3′-terminal nucleotide bias observed for the full-length protein. Finally, we show that fluorophore-tagged ssRNA molecules are successfully detected in fluorescence resonance energy transfer assays both individually and in a total RNA mixture. The presented DmHen1-assisted RNA labelling provides a solid basis for developing novel chemo-enzymatic approaches for in vitro studies and in vivo monitoring of single-stranded RNA pools.

Time-and-motion tool for the assessment of working time in tuberculosis laboratories: a multicentre study

SETTING

Implementation of novel diagnostic assays in tuberculosis (TB) laboratory diagnosis requires effective management of time and resources.

OBJECTIVE

To further develop and assess at multiple centres a time-and-motion (T&M) tool as an objective means for recording the actual time spent on running laboratory assays.

DESIGN

Multicentre prospective study conducted in six European Union (EU) reference TB laboratories.

RESULTS

A total of 1060 specimens were tested using four laboratory assays. The number of specimens per batch varied from one to 60; a total of 64 recordings were performed. Theoretical hands-on times per specimen (TTPS) in h:min:s for Xpert® MTB/RIF, mycobacterial interspersed repetitive unit-variable number of tandem repeats genotyping, Ziehl-Neelsen staining and manual fluorescence microscopy were respectively 00:33:02 ± 00:12:32, 00:13:34 ± 00:03:11, 00:09:54 ± 00:00:53 and 00:06:23 ± 00:01:36. Variations between laboratories were predominantly linked to the time spent on reporting and administrative procedures. Processing specimens in batches could help save time in highly automated assays (e.g., line-probe) (TTPS 00:14:00 vs. 00:09:45 for batches comprising 7 and 31 specimens, respectively).

CONCLUSIONS

The T&M tool can be considered a universal and objective methodology contributing to workload assessment in TB diagnostic laboratories. Comparison of workload between laboratories could help laboratory managers justify their resource and personnel needs for the implementation of novel, time-saving, cost-effective technologies, as well as identify areas for improvement.

Multiple epigenetic mechanisms and the piRNA pathway enforce LINE1 silencing during adult spermatogenesis

Transposons present an acute challenge to the germline, and mechanisms that repress their activity are essential for transgenerational genomic integrity. LINE1 (L1) is the most successful retrotransposon and is epigenetically repressed by CpG DNA methylation. Here, we identify two additional important mechanisms by which L1 is repressed during spermatogenesis. We demonstrate that the Piwi protein Mili and the piRNA pathway are required to posttranscriptionally silence L1 in meiotic pachytene cells even in the presence of normal L1 DNA methylation. Strikingly, in the absence of both a functional piRNA pathway and DNA methylation, L1 elements are normally repressed in mitotic stages of spermatogenesis. Accordingly, we find that the euchromatic repressive histone H3 dimethylated lysine 9 modification cosuppresses L1 expression therein. We demonstrate the existence of multiple epigenetic mechanisms that in conjunction with the piRNA pathway sequentially enforce L1 silencing and genomic stability during mitotic and meiotic stages of adult spermatogenesis.