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Mouse Dyskerin Mutations Affect Accumulation of Telomerase RNA and Small Nucleolar RNA, Telomerase Activity, and Ribosomal RNA Processing

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Mouse Dyskerin Mutations Affect Accumulation of Telomerase RNA and Small Nucleolar RNA, Telomerase Activity, and Ribosomal RNA Processing Mouse dyskerin mutations affect accumulation of telomerase RNA and small nucleolar RNA, telomerase activity, and ribosomal RNA processing Yuko Mochizuki*, Jun He*, Shashikant Kulkarni*, Monica Bessler*†, and Philip J. Mason*‡ *Department of Internal Medicine, Division of Hematology, Washington University School of Medicine, St Louis, MO 63110; and ‡Department of Haematology, Imperial College School of Medicine, Hammersmith Hospital, Du Cane Road, London W12 ONN, United Kingdom Edited by Joan A. Steitz, Yale University, New Haven, CT, and approved June 8, 2004 (received for review April 9, 2004) Dyskerin is a nucleolar protein present in small nucleolar ribonu- not clear whether the function of dyskerin as a pseudouridine (⌿) cleoprotein particles that modify specific uridine residues of rRNA synthase, its role in the telomerase complex, or both, is primarily by converting them to pseudouridine. Dyskerin is also a compo- responsible for the pathogenesis of DC. Although an autosomal nent of the telomerase complex. Point mutations in the human form of DC is caused directly by mutations in TERC (22), suggesting gene encoding dyskerin cause the skin and bone marrow failure that the disease is caused by defective telomerase activity, studies syndrome dyskeratosis congenita (DC). To test the extent to which from animal models suggest that defective rRNA processing may disruption of pseudouridylation or telomerase activity may con- cause some of the features of DC (23, 24). In this paper, we tribute to the pathogenesis of DC, we introduced two dyskerin approach this controversy by inducing DC-causing point mutations mutations into murine embryonic stem cells. The A353V mutation in the highly conserved Dkc1 gene of embryonic stem (ES) cells is the most frequent mutation in patients with X-linked DC, normally expressing high levels of dyskerin and telomerase activity. whereas the G402E mutation was identified in a single family. The We show that these two dyskerin mutations may affect both A353V, but not the G402E, mutation led to severe destabilization telomerase and pseudouridylation, and that the extent to which of telomerase RNA, a reduction in telomerase activity, and a significant continuous loss of telomere length with increasing these two processes are altered varies for the two mutations. numbers of cell divisions during in vitro culture. Both mutations Materials and Methods caused a defect in overall pseudouridylation and a small but Preparation of Targeting Construct, Generation of A353V and G402E detectable decrease in the rate of pre-rRNA processing. In addition, both mutant embryonic stem cell lines showed a decrease in the ES Cells, and Cell Culture. Targeting constructs were made by accumulation of a subset of H͞ACA small nucleolar RNAs, corre- oligonucleotide-directed mutagenesis using the Transformer lating with a significant decrease in site-specific pseudouridylation (Clontech) system and standard subcloning methods as de- efficiency. Interestingly, the H͞ACA snoRNAs decreased in the scribed (25). ES cells (RW-4, Siteman Cancer Center at Wash- G402E mutant cell line differed from those affected in A353V ington University School of Medicine) were transfected with mutant cells. Hence, our findings show that point mutations in A353V or G402E targeting vector and selected with Geneticin dyskerin may affect both the telomerase and pseudouridylation (Sigma). A353V and G402E ES clones were cultured on mito- pathways and the extent to which these functions are altered can mycin C-treated STO cell (ATCC CRL-2225) feeder layers in ES vary for different mutations. medium (DMEM, 17% FCS) supplemented with L-glutamine, nonessential amino acids, Hepes, leukemia inhibitory factor, and he nucleolar protein dyskerin is one of four proteins asso- 2-mercaptoethanol. Tciated with H͞ACA small nucleolar RNA (snoRNAs) in small nucleolar ribonucleoprotein particles that catalyze the RNA Isolation and Northern Blot Analysis. Total RNA was isolated pseudouridylation of specific uridine residues during the matu- from ES cells by using TRIzol (Invitrogen). For analysis of ration of rRNAs (1, 2). Dyskerin has similarities with yeast and snoRNAs, 20 ␮g of total RNA was electrophoresed through 6% bacterial pseudouridine synthases, suggesting that it is the active polyacrylamide͞7 M urea gels and transferred onto Zeta-probe enzyme (3–7). The H͞ACA snoRNAs act as guide RNAs that membranes (Bio-Rad) by using Transblot SD semidry blotting target the complex to specific residues via specific base pairing apparatus (Bio-Rad). The sequence of the oligonucleotides used (8, 9). Telomerase (10, 11), which spends part of its time in the to probe these blots is supplied in Table 1, which is published as nucleolus (12, 13), is a ribonucleoprotein complex whose core supporting information on the PNAS web site. components are the telomerase reverse transcriptase TERT and the telomerase RNA component TERC, which acts as a template Telomeric Repeat Amplification Protocol (TRAP) Assay. Telomerase for the synthesis of the TTAGGG telomere repeats at the ends activity was measured by using the TRAPeze telomerase detec- of chromosomes. These simple repeats, along with a number of tion kit (Intergen, Purchase, NY) according to the manufactur- associated proteins, protect the ends of chromosomes from er’s recommendations. Cell lysates were prepared by using the ͞ degradation and aberrant fusion events. TERC hasaHACA 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate Ј like domain at its 3 end (12) and is associated with the same four (CHAPS) lysis buffer provided in the kit and stored at Ϫ80°C. ͞ proteins as H ACA snoRNAs, including dyskerin (14–16). Be- Protein concentration was measured in each extract by using the cause no pseudouridylation target associated with telomerase Bio-Rad Protein assay (Bio-Rad). action has been identified, it is assumed that dyskerin has a structural role in the telomerase complex. X-linked dyskeratosis congenita (DC), caused by point mutations This paper was submitted directly (Track II) to the PNAS office. in the DKC1 gene that encodes dyskerin (17), is a rare, usually fatal, Abbreviations: DC, dyskeratosis congenita; snoRNA, small nucleolar RNA; ES, embryonic inherited skin and bone marrow failure syndrome in which patients stem; ⌿, pseudouridine; TRAP, telomeric repeat amplification protocol; IRES, internal develop mucocutaneous lesions, aplastic anemia, and an increased ribosome entry site. susceptibility to cancer (18, 19). Tissues with a rapid cell turnover †To whom correspondence should be addressed. E-mail: [email protected]. appear to be the main target of the DC pathobiology (20, 21). It is © 2004 by The National Academy of Sciences of the USA 10756–10761 ͉ PNAS ͉ July 20, 2004 ͉ vol. 101 ͉ no. 29 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0402560101 Downloaded by guest on September 26, 2021 Analysis of Telomere Lengths (26). Cells were isolated and embed- ded in agarose plugs following instructions from the manufac- turer (CHEF agarose plug kit; Bio-Rad). DNA embedded in the agarose plug was digested with MboI and electrophoresed through a 1% agarose gel for 23 h at 6 V͞cm at a constant pulse time of 5 s using the CHEF DR-III pulse-field system. The gel was analyzed by in-gel hybridization with [32P] 5Ј end-labeled oligonucleotide (CCCTAAA)4 probe. Western Blot Analysis. Primary antibody incubations were carried out by using the antibody to TOPOII␤ (H-286; dilution 1:200; Santa Cruz Biotechnology) and polyclonal antiserum to dyskerin raised against the peptides DCSNPLKREIGDYIR (amino acids 74–88) and GLLDKHGKPTDNTPAC (amino acids 382– 396ϩC; dilution 1:1,000) and supplied by Eurogentec (Brussels). Western blots were developed by using chemiluminescence (SuperSignal from Pierce). Metabolic Labeling and Analysis of rRNA Processing. ES cells were plated in 35-mm-diameter plates at 2.0 ϫ 106 cells per plate. The cells were incubated in medium with [3H]uridine (ICN Biomedi- ␮ ͞ ϭ cals; 5 Ci ml;1Ci 37 GBq) for 30 min and then incubated Fig. 1. Targeting the Dkc1 locus in ES cells. (A) The targeting construct was in nonradioactive medium for 2 h. a 6.5-kb EcoRI͞HindIII fragment in which an IRES-Neo element had been For [methyl-3H]methionine pulse–chase analysis, the cells cloned into a unique StuI site in the 3Ј untranslated region. The positions of were preincubated in methionine-free medium for 45 min, the probes are shown. The asterisk represents the position of the mutations before being pulse-labeled with 50 ␮Ci͞ml [methyl-3H]methi- A353V and G402E in exons 11 and 12, respectively. The positions of restriction onine (New England Nuclear), for 30 min and chased with cold enzyme sites used in confirmatory Southern blots (not shown) are indicated; ␮ ͞ B, BamHI; Bs, BspHI; H, HindIII; E, EcoRI. The Bs site in brackets is the site created methionine (30 g ml). Samples were taken every 15 min. Ј ϫ 6 with the A353V mutation. The probes were 5 , a single-copy 600-bp fragment Total RNA (from 1.0 10 cells) was separated on a 1% from Dkc1 intron 9; IRES, a 200-bp HindIII͞KpnI fragment from the IRES agarose formaldehyde gel and transferred to a nylon membrane. element; and 3Ј, a 500-bp BstX1͞HindIII fragment from Mpp1 cDNA. (B) 3 The membranes were sprayed with EN HANCE Spray (New Northern hybridization. RNA was extracted from ES cells containing the England Nuclear), dried, and exposed to x-ray films at Ϫ80°C. wild-type Dkc1 gene with an IRES-NEO cassette in the 3Ј untranslated region (WTϩNEO) or ES cell clones targeted to contain point mutations as well as the Analysis of Pseudouridylation in 28S and 18S rRNA. ES cells were IRES-NEO cassette (G402E and A353V). Twenty-five micrograms of total RNA pretreated in phosphate-free medium for 1 h and labeled for was loaded in each lane and hybridized sequentially with probes for Dkc1 and 3hwith[32P]orthophosphate (100 ␮Ci͞ml). Total RNA was ␤-actin.(C) Western blotting of nuclear extracts from ES cells. Ten micrograms electrophoresed through a 1% agarose formaldehyde gel.
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