Telomeres, the end fragments of chromosomes, play key roles in cellular proliferation and senescence. Here we characterize the genetic architecture of naturally occurring variation in leukocyte telomere length (LTL) and identify causal links between LTL and biomedical phenotypes in 472,174 well-characterized UK Biobank participants. We identified 197 independent sentinel variants associated with LTL at 138 genomic loci (108 new). Genetically determined differences in LTL were associated with multiple biological traits, ranging from height to bone marrow function, as well as several diseases spanning neoplastic, vascular and inflammatory pathologies. Finally, we estimated that, at the age of 40 years, people with an LTL >1 s.d. shorter than the population mean had a 2.5-year-lower life expectancy compared with the group with ≥1 s.d. longer LDL. Overall, we furnish new insights into the genetic regulation of LTL, reveal wide-ranging influences of LTL on physiological traits, diseases and longevity, and provide a powerful resource available to the global research community.

Polygenic basis and biomedical consequences of telomere length variation.

Loss of Human TGS1 Hypermethylase Promotes Increased Telomerase RNA and Telomere Elongation

Dyskerin and its homologs are ancient and conserved enzymes that catalyze the most common post-transcriptional modification found in cells, pseudouridylation. The resulting pseudouridines provide stability to RNA molecules and regulate ribosome biogenesis and splicing events. Dyskerin does not act independently-it is the core component of a protein heterotetramer, which associates with RNAs that contain the H/ACA motif. The variety of H/ACA RNAs that guide the function of this ribonucleoprotein (RNP) complex highlights the diversity of cellular processes in which dyskerin participates. When associated with small nucleolar (sno) RNAs, it regulates ribosomal (r) RNAs and ribosome biogenesis. By interacting with small Cajal body (sca) RNAs, it targets small nuclear (sn) RNAs to regulate pre-mRNA splicing. As a component of the telomerase holoenzyme, dyskerin binds to the telomerase RNA to modulate telomere maintenance. In a disease context, dyskerin malfunction can result in multiple detrimental phenotypes. Mutations in DKC1, the gene that encodes dyskerin, cause the premature aging syndrome X-linked dyskeratosis congenita (X-DC), a still incurable disorder that typically leads to bone marrow failure. In this review, we present the classical and most recent findings on this essential protein, discussing the evolutionary, structural, and functional aspects of dyskerin and the H/ACA RNP. The latest research underscores the role that dyskerin plays in the regulation of gene expression, translation efficiency, and telomere maintenance, along with the impacts that defective dyskerin has on aging, cell proliferation, haematopoietic potential, and cancer.

/pdf/dyskerin-an-essential-pseudouridine-synthase-with-en8lgx9wij.pdf

Dyskerin: an essential pseudouridine synthase with multifaceted roles in ribosome biogenesis, splicing, and telomere maintenance.

Genetics of human telomere biology disorders

Liquid-liquid phase separation (LLPS) brings together functionally related proteins through the intrinsic biophysics of proteins in a process that is driven by reducing free energy and maximizing entropy. The process of LLPS allows proteins to form structures, termed membrane-less organelles. These diverse, dynamic organelles are active in a wide range of processes in the nucleus, cytoplasm, mitochondria and synapse, and ranging from bacteria to plants to eukaryotes. RNA and DNA present long chained charged polymers that promote LLPS. Consequently, many RNA binding proteins (RBPs) and DNA binding proteins form membrane-less organelles. However, the highly concentrated phase separated state creates conditions that also promote formation of irreversible protein aggregates. Mutations in RNA and DNA binding proteins that increase the stability of irreversible aggregates also increase the accumulation of irreversible aggregates directly and from membrane-less organelles. Many of the RBPs that exhibit disease-linked mutations carry out cytoplasmic actions through stress granules, which are a pleiotropic type of RNA granule that regulates the translational response to stress. Phosphorylation and oligomerization of tau facilitates its interactions with RBPs and ribosomal proteins, affecting RNA translation; we propose that this is a major reason that tau becomes phosphorylated with stress. Persistent stress leads to the accumulation of irreversible aggregates composed of RBPs or tau, which then cause toxicity and form many of the hallmark pathologies of major neurodegenerative diseases. This pathophysiology ultimately leads to multiple forms of neurodegenerative diseases, the specific type of which reflects the temporal and spatial accumulation of different aggregating proteins.

The pathophysiology of neurodegenerative disease: Disturbing the balance between phase separation and irreversible aggregation.

Cajal bodies (CBs) are nuclear organelles concentrating two kinds of RNA--protein complexes (RNPs), spliceosomal small nuclear (sn), and small CB-specific (sca)RNPs. Whereas the CB marker protein coilin is responsible for retaining snRNPs, the tether for scaRNPs is not known. Here we show that Nopp140, an intrinsically disordered CB phosphoprotein, is required to recruit and retain all scaRNPs in CBs. Knockdown (KD) of Nopp140 releases all scaRNPs leading to an unprecedented reduction in size of CB granules, hallmarks of CB ultrastructure. The CB-localizing protein WDR79 (aka TCAB1), which is mutated in the inherited bone marrow failure syndrome dyskeratosis congenita, is a specific component of all scaRNPs, including telomerase. Whereas mislocalization of telomerase by mutation of WDR79 leads to critically shortened telomeres, mislocalization of telomerase by Nopp140 KD leads to gradual extension of telomeres. Our studies suggest that the dynamic distribution of telomerase between CBs and nucleoplasm uniquely impacts telomere length maintenance and identify Nopp140 as a novel player in telomere biology.

Nopp140-mediated concentration of telomerase in Cajal bodies regulates telomere length.

Background
The inherited bone marrow failure syndrome dyskeratosis congenita (DC) is most frequently caused by mutations in DKC1 (MIM# 300126), the gene encoding NAP57 (aka dyskerin). The typically missense mutations modulate the interaction of NAP57 with its chaperone SHQ1, but no DC mutations have been identified in SHQ1 (MIM# 613663). Here, we report on two compound heterozygous mutations in SHQ1 in a patient with a severe neurological disorder including cerebellar degeneration.

Methods
The SHQ1 mutations were identified by patient exome sequencing. The impact of the mutations was assessed in pulldown assays with recombinant NAP57.

Results
The SHQ1 mutations were the only set of mutations consistent with an autosomal recessive mode of inheritance. The mutations map to the SHQ1-NAP57 interface and impair the interaction of the recombinant SHQ1 variants with NAP57.

Conclusion
Intrauterine growth retardation and the neurological phenotype of the patient are reminiscent of the severe clinical variant of DC, the Hoyeraal-Hreidarsson syndrome (HH). Hence, SHQ1 screening may be warranted in patients with inherited bone marrow failure syndromes.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/mgg3.314

Inherited SHQ1 mutations impair interaction with NAP57/dyskerin, a major target in dyskeratosis congenita

Spliceosomal small nuclear RNAs (snRNAs) are modified by small Cajal body (CB)-specific ribonucleoproteins (scaRNPs) to ensure snRNP biogenesis and pre-mRNA splicing. However, the function and subcellular site of snRNA modification are largely unknown. We show that CB localization of the protein Nopp140 is essential for concentration of scaRNPs in that nuclear condensate; and that phosphorylation by casein kinase 2 (CK2) at ∼80 serines targets Nopp140 to CBs. Transiting through CBs, snRNAs are apparently modified by scaRNPs. Indeed, Nopp140 knockdown-mediated release of scaRNPs from CBs severely compromises 2'-O-methylation of spliceosomal snRNAs, identifying CBs as the site of scaRNP catalysis. Additionally, alternative splicing patterns change indicating that these modifications in U1, U2, U5, and U12 snRNAs safeguard splicing fidelity. Given the importance of CK2 in this pathway, compromised splicing could underlie the mode of action of small molecule CK2 inhibitors currently considered for therapy in cholangiocarcinoma, hematological malignancies, and COVID-19.

Nopp140-chaperoned 2'-O-methylation of small nuclear RNAs in Cajal bodies ensures splicing fidelity.

Spliceosomal small nuclear RNAs (snRNAs) are modified by small Cajal body (CB) specific ribonucleoproteins (scaRNPs) to ensure snRNP biogenesis and pre-mRNA splicing. However, the function and subcellular site of snRNA modification are largely unknown. We show that CB localization of the protein Nopp140 is essential for concentration of scaRNPs in that nuclear condensate; and that phosphorylation by casein kinase 2 (CK2) at some 80 serines targets Nopp140 to CBs. Transiting through CBs, snRNAs are apparently modified by scaRNPs. Indeed, Nopp140 knockdown-mediated release of scaRNPs from CBs severely compromises 2'-O-methylation of spliceosomal snRNAs, identifying CBs as the site of scaRNP catalysis. Additionally, alternative splicing patterns change indicating that these modifications in U1, U2, U5, and U12 snRNAs safeguard splicing fidelity. Given the importance of CK2 in this pathway, compromised splicing could underlie the mode of action of small molecule CK2 inhibitors currently considered for therapy in cholangiocarcinoma, hematological malignancies, and COVID-19.

/pdf/nopp140-chaperoned-2-o-methylation-of-small-nuclear-rnas-in-3q37wqz1qz.pdf

Jonathan Bizarro

Papers

Nopp140-mediated concentration of telomerase in Cajal bodies regulates telomere length.

Inherited SHQ1 mutations impair interaction with NAP57/dyskerin, a major target in dyskeratosis congenita

Nopp140-chaperoned 2'-O-methylation of small nuclear RNAs in Cajal bodies ensures splicing fidelity.

Nopp140-chaperoned 2'-O-methylation of small nuclear RNAs in Cajal bodies ensures splicing fidelity