Speech and language delay in a patient with WDR4 mutations

Pathogenic mechanism and therapeutic intervention of impaired N7-methylguanosine (m7G) tRNA modification

Mutations modification enzymes including the tRNA N7-methylguanosine (m7G) methyltransferase complex component WDR4 were frequently found in patients with neural disorders, while the pathogenic mechanism and therapeutic intervention strategies are poorly explored. In this study, we revealed that patient-derived WDR4 mutation leads to temporal and cell-type-specific neural degeneration, and directly causes neural developmental disorders in mice. Mechanistically, WDR4 point mutation disrupts the interaction between WDR4 and METTL1 and accelerates METTL1 protein degradation. We further uncovered that impaired tRNA m7G modification caused by Wdr4 mutation decreases the mRNA translation of genes involved in mTOR pathway, leading to elevated endoplasmic reticulum stress markers, and increases neural cell apoptosis. Importantly, treatment with stress-attenuating drug Tauroursodeoxycholate (TUDCA) significantly decreases neural cell death and improves neural functions of the Wdr4 mutated mice. Moreover, adeno-associated virus mediated transduction of wild-type WDR4 restores METTL1 protein level and tRNA m7G modification in the mouse brain, and achieves long-lasting therapeutic effect in Wdr4 mutated mice. Most importantly, we further demonstrated that both TUDCA treatment and WDR4 restoration significantly improve the survival and functions of human iPSCs-derived neuron stem cells that harbor the patient's WDR4 mutation. Overall, our study uncovers molecular insights underlying WDR4 mutation in the pathogenesis of neural diseases and develops two promising therapeutic strategies for treatment of neural diseases caused by impaired tRNA modifications.

Cilia defects upon loss of WDR4 are linked to proteasomal hyperactivity and ubiquitin shortage

The WD repeat-containing protein 4 (WDR4) has repeatedly been associated with primary microcephaly, a condition of impaired brain and skull growth. Often, faulty centrosomes cause microcephaly, yet aberrant cilia may also be involved. Here, we show using a combination of approaches in human fibroblasts, zebrafish embryos and patient-derived cells that WDR4 facilitates cilium formation. Molecularly, we associated WDR4 loss-of-function with increased protein synthesis and concomitant upregulation of proteasomal activity, while ubiquitin precursor pools are reduced. Inhibition of proteasomal activity as well as supplementation with free ubiquitin restored normal ciliogenesis. Proteasome inhibition ameliorated microcephaly phenotypes. Thus, we propose that WDR4 loss-of-function impairs head growth and neurogenesis via aberrant cilia formation, initially caused by disturbed protein and ubiquitin homeostasis.

Genetic Findings in Short Turkish Children Born to Consanguineous Parents

INTRODUCTION

The diagnostic yield of genetic analysis in the evaluation of children with short stature depends on associated clinical characteristics, but the additional effect of parental consanguinity has not been well documented.

METHODS

This observational case series of 42 short children from 34 consanguineous families was collected by six referral centres of paediatric endocrinology (inclusion criteria: short stature and parental consanguinity). In 18 patients (12 families, group 1), the clinical features suggested a specific genetic defect in the growth hormone (GH) insulin-like growth factor I (IGF-I) axis, and a candidate gene approach was used. In others (group 2), a hypothesis-free approach was chosen (gene panels, microarray analysis, and whole exome sequencing) and further subdivided into 11 patients with severe short stature (height <-3.5 standard deviation score [SDS]) and microcephaly (head circumference <-3.0 SDS) (group 2a), 10 patients with syndromic short stature (group 2b), and 3 patients with nonspecific isolated GH deficiency (group 2c).

RESULTS

In all 12 families from group 1, (likely) pathogenic variants were identified in GHR, IGFALS, GH1, and STAT5B. In 9/12 families from group 2a, variants were detected in PCNT, SMARCAL1, SRCAP, WDR4, and GHSR. In 5/9 families from group 2b, variants were found in TTC37, SCUBE3, NSD2, RABGAP1, and 17p13.3 microdeletions. In group 2c, no genetic cause was found. Homozygous, compound heterozygous, and heterozygous variants were found in 21, 1, and 4 patients, respectively.

CONCLUSION

Genetic testing in short children from consanguineous parents has a high diagnostic yield, especially in cases of severe GH deficiency or insensitivity, microcephaly, and syndromic short stature.

M7G-related molecular subtypes can predict the prognosis and correlate with immunotherapy and chemotherapy responses in bladder cancer patients

BACKGROUND

N7-methylguanosine (m7G) is closely associated with tumor prognosis and immune response in many cancer types. The correlation between m7G and bladder cancer (BC) needs further study. We aimed to orchestrate molecular subtypes and identify key genes for BC from the perspective of m7G.

METHODS

RNA-seq and clinical data of BC patients were extracted from TCGA and GSE13507 datasets. The patients were subtyped by "ConsensusClusterPlus" and "limma." The clusters were validated by the Kaplan‒Meier curves, univariable and multivariate Cox regression models, the concordance index, and calibration curves. The immunotherapy response was evaluated by immune checkpoints, immune infiltration, TIDE score, and IMvigor210 cohort. Genomics of Drug Sensitivity in Cancer was utilized to predict the chemotherapy response between the clusters.

RESULTS

The m7G-related cluster was ultimately established by EIF4G1, NUDT11, NUDT10, and CCNB1. The independent prognostic value of the m7G-related cluster was validated by the TCGA and GSE13507 datasets. The cluster was involved in immune-associated pathways, such as neutrophil degranulation, antigen processing cross-presentation, and signaling by interleukins pathways. Meanwhile, cluster 2 was positively correlated with many immune checkpoints, such as CD274, CTLA4, HAVCR2, LAG3, PDCD1, and PDCD1LG2. The cluster 2 was significantly correlated with a higher TIDE score than the cluster 1. Furthermore, in the IMvigor210 cohort, patients in the cluster 1 had a higher response rate than those in the cluster 2. Patients in the cluster 2 were sensitive to many chemotherapies.

CONCLUSIONS

We successfully determined molecular subtypes and identified key genes for BC from the perspective of m7G, thereby providing a roadmap for the evolution of immunotherapy and precision medicine.

Wdr4 promotes cerebellar development and locomotion through Arhgap17-mediated Rac1 activation

Patients with mutations of WDR4, a substrate adaptor of the CUL4 E3 ligase complex, develop cerebellar atrophy and gait phenotypes. However, the underlying mechanisms remain unexplored. Here, we identify a crucial role of Wdr4 in cerebellar development. Wdr4 deficiency in granule neuron progenitors (GNPs) not only reduces foliation and the sizes of external and internal granular layers but also compromises Purkinje neuron organization and the size of the molecular layer, leading to locomotion defects. Mechanistically, Wdr4 supports the proliferation of GNPs by preventing their cell cycle exit. This effect is mediated by Wdr4-induced ubiquitination and degradation of Arhgap17, thereby activating Rac1 to facilitate cell cycle progression. Disease-associated Wdr4 variants, however, cannot provide GNP cell cycle maintenance. Our study identifies Wdr4 as a previously unappreciated participant in cerebellar development and locomotion, providing potential insights into treatment strategies for diseases with WDR4 mutations, such as primordial dwarfism and Galloway-Mowat syndrome.

Structures and mechanisms of tRNA methylation by METTL1-WDR4

Specific, regulated modification of RNAs is important for proper gene expression^1,2. tRNAs are rich with various chemical modifications that affect their stability and function^3,4. 7-Methylguanosine (m⁷G) at tRNA position 46 is a conserved modification that modulates steady-state tRNA levels to affect cell growth^5,6. The METTL1-WDR4 complex generates m⁷G46 in humans, and dysregulation of METTL1-WDR4 has been linked to brain malformation and multiple cancers^7-22. Here we show how METTL1 and WDR4 cooperate to recognize RNA substrates and catalyse methylation. A crystal structure of METTL1-WDR4 and cryo-electron microscopy structures of METTL1-WDR4-tRNA show that the composite protein surface recognizes the tRNA elbow through shape complementarity. The cryo-electron microscopy structures of METTL1-WDR4-tRNA with S-adenosylmethionine or S-adenosylhomocysteine along with METTL1 crystal structures provide additional insights into the catalytic mechanism by revealing the active site in multiple states. The METTL1 N terminus couples cofactor binding with conformational changes in the tRNA, the catalytic loop and the WDR4 C terminus, acting as the switch to activate m⁷G methylation. Thus, our structural models explain how post-translational modifications of the METTL1 N terminus can regulate methylation. Together, our work elucidates the core and regulatory mechanisms underlying m⁷G modification by METTL1, providing the framework to understand its contribution to biology and disease.

Novel roles of METTL1/WDR4 in tumor via m7G methylation

As one of the prevalent posttranscriptional modifications of RNA, N⁷-methylguanosine (m⁷G) plays essential roles in RNA processing, metabolism, and function, mainly regulated by the methyltransferase-like 1 (METTL1) and WD repeat domain 4 (WDR4) complex. Emerging evidence suggests that the METTL1/WDR4 complex promoted or inhibited the processes of many tumors, including head and neck, lung, liver, colon, bladder cancer, and teratoma, dependent on close m⁷G methylation modification of tRNA or microRNA (miRNA). Therefore, METTL1 and m⁷G modification can be used as biomarkers or potential intervention targets, providing new possibilities for early diagnosis and treatment of tumors. This review will mainly focus on the mechanisms of METTL1/WDR4 via m⁷G in tumorigenesis and the corresponding detection methods.

N7-methylguanosine tRNA modification promotes esophageal squamous cell carcinoma tumorigenesis via the RPTOR/ULK1/autophagy axis

Mis-regulated RNA modifications promote the processing and translation of oncogenic mRNAs to facilitate cancer progression, while the molecular mechanisms remain unclear. Here we reveal that tRNA m⁷G methyltransferase complex proteins METTL1 and WDR4 are significantly up-regulated in esophageal squamous cell carcinoma (ESCC) tissues and associated with poor ESCC prognosis. In addition, METTL1 and WDR4 promote ESCC progression via the tRNA m⁷G methyltransferase activity in vitro and in vivo. Mechanistically, METTL1 or WDR4 knockdown leads to decreased expression of m⁷G-modified tRNAs and reduces the translation of a subset of oncogenic transcripts enriched in RPTOR/ULK1/autophagy pathway. Furthermore, ESCC models using Mettl1 conditional knockout and knockin mice uncover the essential function of METTL1 in promoting ESCC tumorigenesis in vivo. Our study demonstrates the important oncogenic function of mis-regulated tRNA m⁷G modification in ESCC, and suggest that targeting METTL1 and its downstream signaling axis could be a promising therapeutic target for ESCC treatment.

Deregulation of METTL1-mediated N7- methylguanosine tRNA modification can promote oncogenesis. Here, the authors report that this modification regulates the translation of proteins in both the mTOR and negative regulators of autophagy pathways, resulting in the progression of esophageal squamous cell carcinoma.

N7-methylguanosine tRNA modification promotes tumorigenesis and chemoresistance through WNT/β-catenin pathway in nasopharyngeal carcinoma

Treatment selections are very limited for patients with advanced nasopharyngeal carcinoma (NPC) experiencing disease progression. Uncovering mechanisms underlying NPC progression is crucial for the development of novel treatments. Here we show that N⁷-methylguanosine (m⁷G) tRNA modification enzyme METTL1 and its partner WDR4 are significantly elevated in NPC and are associated with poor prognosis. Loss-of-function and gain-of-function assays demonstrated that METTL1/WDR4 promotes NPC growth and metastasis in vitro and in vivo. Mechanistically, ARNT was identified as an upstream transcription factor regulating METTL1 expression in NPC. METTL1 depletion resulted in decreased m⁷G tRNA modification and expression, which led to impaired codon recognition during mRNA translation, therefore reducing the translation efficiencies of mRNAs with higher m⁷G codons. METTL1 upregulated the WNT/β-catenin signaling pathway and promoted NPC cell epithelial-mesenchymal transition (EMT) and chemoresistance to cisplatin and docetaxel in vitro and in vivo. Overexpression of WNT3A bypassed the requirement of METTL1 for EMT and chemoresistance. This work uncovers novel insights into tRNA modification-mediated mRNA translation regulation and highlights the critical function of tRNA modification in cancer progression.

METTL1/WDR4 mediated m7G tRNA modifications and m7G codon usage promote mRNA translation and lung cancer progression

Mis-regulated epigenetic modifications in RNAs are associated with human cancers. The transfer RNAs (tRNAs) are the most heavily modified RNA species in cells, however, little is known about the functions of tRNA modifications in cancers. In this study, we uncovered that the expression levels of tRNA N⁷-methylguanosine (m⁷G) methyltransferase complex components METTL1 and WDR4 are significantly elevated in human lung cancer samples and negatively associated with patient prognosis. Impaired m⁷G tRNA modification upon METTL1/WDR4 depletion resulted in decreased cell proliferation, colony formation, cell invasion and impaired tumorigenic capacities of lung cancer cells in vitro and in vivo. Moreover, gain-of-function and mutagenesis experiments revealed that METTL1 promoted lung cancer growth and invasion through regulation of m⁷G tRNA modifications. Profiling of tRNA methylation and mRNA translation revealed that highly translated mRNAs have higher frequencies of m⁷G tRNA decoded codons and knockdown of METTL1 resulted in decreased translation of mRNAs with higher frequencies of m⁷G tRNA codons, suggesting that tRNA modifications and codon usage play essential function in mRNA translation regulation. Our data uncovered novel insights on mRNA translation regulation through tRNA modifications and the corresponding mRNA codon compositions in lung cancer, providing new molecular basis underlying lung cancer progression.

Division of labor in epitranscriptomics: What have we learnt from the structures of eukaryotic and viral multimeric RNA methyltransferases?

The translation of an mRNA template into the corresponding protein is a highly complex and regulated choreography performed by ribosomes, tRNAs, and translation factors. Most RNAs involved in this process are decorated by multiple chemical modifications (known as epitranscriptomic marks) contributing to the efficiency, the fidelity, and the regulation of the mRNA translation process. Many of these epitranscriptomic marks are written by holoenzymes made of a catalytic subunit associated with an activating subunit. These holoenzymes play critical roles in cell development. Indeed, several mutations being identified in the genes encoding for those proteins are linked to human pathologies such as cancers and intellectual disorders for instance. This review describes the structural and functional properties of RNA methyltransferase holoenzymes, which when mutated often result in brain development pathologies. It illustrates how structurally different activating subunits contribute to the catalytic activity of these holoenzymes through common mechanistic trends that most likely apply to other classes of holoenzymes. This article is categorized under: RNA Processing > RNA Editing and Modification RNA Processing > Capping and 5' End Modifications.

Hypomodified tRNA in evolutionarily distant yeasts can trigger rapid tRNA decay to activate the general amino acid control response, but with different consequences

All tRNAs are extensively modified, and modification deficiency often results in growth defects in the budding yeast Saccharomyces cerevisiae and neurological or other disorders in humans. In S. cerevisiae, lack of any of several tRNA body modifications results in rapid tRNA decay (RTD) of certain mature tRNAs by the 5'-3' exonucleases Rat1 and Xrn1. As tRNA quality control decay mechanisms are not extensively studied in other eukaryotes, we studied trm8Δ mutants in the evolutionarily distant fission yeast Schizosaccharomyces pombe, which lack 7-methylguanosine at G46 (m7G46) of their tRNAs. We report here that S. pombe trm8Δ mutants are temperature sensitive primarily due to decay of tRNATyr(GUA) and that spontaneous mutations in the RAT1 ortholog dhp1+ restored temperature resistance and prevented tRNA decay, demonstrating conservation of the RTD pathway. We also report for the first time evidence linking the RTD and the general amino acid control (GAAC) pathways, which we show in both S. pombe and S. cerevisiae. In S. pombe trm8Δ mutants, spontaneous GAAC mutations restored temperature resistance and tRNA levels, and the trm8Δ temperature sensitivity was precisely linked to GAAC activation due to tRNATyr(GUA) decay. Similarly, in the well-studied S. cerevisiae trm8Δ trm4Δ RTD mutant, temperature sensitivity was closely linked to GAAC activation due to tRNAVal(AAC) decay; however, in S. cerevisiae, GAAC mutations increased tRNA loss and exacerbated temperature sensitivity. A similar exacerbated growth defect occurred upon GAAC mutation in S. cerevisiae trm8Δ and other single modification mutants that triggered RTD. Thus, these results demonstrate a conserved GAAC activation coincident with RTD in S. pombe and S. cerevisiae, but an opposite impact of the GAAC response in the two organisms. We speculate that the RTD pathway and its regulation of the GAAC pathway is widely conserved in eukaryotes, extending to other mutants affecting tRNA body modifications.

Novel PAK3 gene missense variant associated with two Chinese siblings with intellectual disability: a case report

BACKGROUND

Intellectual disability (ID) constitutes the most common group of neurodevelopmental disorders. Exome sequencing has enabled the discovery of genetic mutations responsible for a wide range of ID disorders.

CASE PRESENTATION

In this study, we reported on two male siblings, aged 4 and 2 years, with motor and mental developmental delays and mild dysmorphic facial features. To identify the genetic causes of these symptoms, we employed trio-whole exome sequencing for the proband. We found a novel hemizygous missense variant in the PAK3 gene (c.1112G > A, p.Cys371Tyr), which encodes the p21-activated kinase 3, in the proband, which inherited from mother. The younger brother also has the hemizygous variant, which was confirmed by Sanger sequencing. The variant is located in the kinase domain and was regarded as a likely pathogenic variant in this family.

CONCLUSION

We diagnosed two male siblings with developmental delays as having a PAK3 likely pathogenic variant. This finding expands the list of PAK3 gene mutations associated with neurodevelopmental disorders and provides further details on its clinical features.

An intellectual disability-associated missense variant in TRMT1 impairs tRNA modification and reconstitution of enzymatic activity

The human TRMT1 gene encodes an RNA methyltransferase enzyme responsible for catalyzing dimethylguanosine (m2,2G) formation in transfer RNAs (tRNAs). Frameshift mutations in TRMT1 have been shown to cause autosomal-recessive intellectual disability (ID) in the human population but additional TRMT1 variants remain to be characterized. Here, we describe a homozygous TRMT1 missense variant in a patient displaying developmental delay, ID, and epilepsy. The missense variant changes an arginine residue to a cysteine (R323C) within the methyltransferase domain and is expected to perturb protein folding. Patient cells expressing TRMT1-R323C exhibit a deficiency in m2,2G modifications within tRNAs, indicating that the mutation causes loss of function. Notably, the TRMT1 R323C mutant retains tRNA binding but is unable to rescue m2,2G formation in TRMT1-deficient human cells. Our results identify a pathogenic point mutation in TRMT1 that perturbs tRNA modification activity and demonstrate that m2,2G modifications are disrupted in the cells of patients with TRMT1-associated ID disorders.

Matching tRNA modifications in humans to their known and predicted enzymes

tRNA are post-transcriptionally modified by chemical modifications that affect all aspects of tRNA biology. An increasing number of mutations underlying human genetic diseases map to genes encoding for tRNA modification enzymes. However, our knowledge on human tRNA-modification genes remains fragmentary and the most comprehensive RNA modification database currently contains information on approximately 20% of human cytosolic tRNAs, primarily based on biochemical studies. Recent high-throughput methods such as DM-tRNA-seq now allow annotation of a majority of tRNAs for six specific base modifications. Furthermore, we identified large gaps in knowledge when we predicted all cytosolic and mitochondrial human tRNA modification genes. Only 48% of the candidate cytosolic tRNA modification enzymes have been experimentally validated in mammals (either directly or in a heterologous system). Approximately 23% of the modification genes (cytosolic and mitochondrial combined) remain unknown. We discuss these 'unidentified enzymes' cases in detail and propose candidates whenever possible. Finally, tissue-specific expression analysis shows that modification genes are highly expressed in proliferative tissues like testis and transformed cells, but scarcely in differentiated tissues, with the exception of the cerebellum. Our work provides a comprehensive up to date compilation of human tRNA modifications and their enzymes that can be used as a resource for further studies.

Early-onset infant epileptic encephalopathy associated with a de novo PPP3CA gene mutation

Epileptic encephalopathies are severe seizure disorders accompanied by intellectual disability. Whole-exome sequencing technology has enabled the discovery of genetic mutations responsible for a wide range of diseases, and severe epilepsy and neurodevelopmental diseases are often associated with rare de novo mutations. We identified a novel de novo frameshift mutation in the PPP3CA gene encoding calcium-dependent protein phosphatase (calcineurin) catalytic subunit A (c.1255_1256del, p.Ser419Cysfs*31) in an 11.5-mo-old female with early-onset refractory epilepsy and developmental delay. This finding expands the list of PPP3CA mutations associated with early-onset severe neurodevelopmental disease with seizures and provides further details on clinical features.

The emerging impact of tRNA modifications in the brain and nervous system

A remarkable number of neurodevelopmental disorders have been linked to defects in tRNA modifications. These discoveries place tRNA modifications in the spotlight as critical modulators of gene expression pathways that are required for proper organismal growth and development. Here, we discuss the emerging molecular and cellular functions of the diverse tRNA modifications linked to cognitive and neurological disorders. In particular, we describe how the structure and location of a tRNA modification influences tRNA folding, stability, and function. We then highlight how modifications in tRNA can impact multiple aspects of protein translation that are instrumental for maintaining proper cellular proteostasis. Importantly, we describe how perturbations in tRNA modification lead to a spectrum of deleterious biological outcomes that can disturb neurodevelopment and neurological function. Finally, we summarize the biological themes shared by the different tRNA modifications linked to cognitive disorders and offer insight into the future questions that remain to decipher the role of tRNA modifications. This article is part of a Special Issue entitled: mRNA modifications in gene expression control edited by Dr. Soller Matthias and Dr. Fray Rupert.